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Past, Present and Future of IEC And IEEE High-Voltage and High Current Testing Standards

Working Group D1.35

August 2014

PAST, PRESENT AND FUTURE OF IEC AND IEEE HIGH-VOLTAGE AND HIGH CURRENT TESTING STANDARDS WG D1.35 Contributors Y. Li, Convener (AU), J. Hällström, Secretary (FI) W. Larzelere (US), A. Bergman (SE), J. Rickmann (US), W. Hauschild (DE), R. Diaz (AR), F. Garnacho (EP), T. McComb (CA), S. Okabe (JP), Y. X. Zhang (US), A P. Elg (SE)

Copyright © 2014 “Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of use for personal purposes. Are prohibited, except if explicitly agreed by CIGRE, total or partial reproduction of the publication for use other than personal and transfer to a third party; hence circulation on any intranet or other company network is forbidden”.

Disclaimer notice “CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law”.

ISBN : 978-2-85873-292-0

Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Past, present and future of IEC and IEEE high-voltage and high current testing standards Table of Contents 1

INTRODUCTION ........................................................................................................................................ 5

2

HISTORY OF HIGH-VOLTAGE AND HIGH-CURRENT TEST STANDARDS ......................................... 6 2.1

History of DC high-voltage tests and measurements ............................................................................6

2.1.1

DC high-voltage test and measurement systems ............................................................................ 6

2.1.2

DC high-voltage supplies ................................................................................................................. 7

2.1.3

Future work on DC test and measurement systems ........................................................................ 7

2.2

History of AC test sources and measurement systems......................................................................... 7

2.2.1 2.3

Future work on AC test and measurement systems ........................................................................ 8

History of impulse voltage tests and measurements .............................................................................8

2.3.1

Early history and definition of impulse waveforms ........................................................................... 8

2.3.2

Definitions of lightning impulse peak voltage ................................................................................... 9

2.3.3

Introduction of test voltage function................................................................................................ 10

2.4

Use of sphere-gaps as measurement devices ....................................................................................10

2.5

Reference Measuring Systems ........................................................................................................... 11

2.6

History of IEEE Standard 4 - the sister Standard to IEC 60060 series ............................................... 13

3 OUTLINE OF MAJOR CHANGES MADE IN RECENT EDITIONS OF IEC AND IEEE HIGHVOLTAGE AND HIGH-CURRENT STANDARDS ...........................................................................................14 3.1

IEC 60060-1:2010................................................................................................................................ 14

3.2

IEC 60060-2:2010................................................................................................................................ 15

3.3

IEC 62475:2010 ................................................................................................................................... 15

3.4

IEC 61083-2:2013................................................................................................................................ 15

3.5

IEEE Std 4-2013 .................................................................................................................................. 15

4

DISCUSSION OF IMPORTANT DEFINITIONS AND REQUIREMENTS ................................................ 17 4.1

Lightning impulse test voltage and test voltage function .....................................................................17

4.2

Definitions of switching impulse........................................................................................................... 19

4.3

AC test voltage .................................................................................................................................... 21

4.4

Atmospheric correction factor .............................................................................................................. 22

4.4.1

Introduction ..................................................................................................................................... 22

4.4.2

Formulae of relevant parameters for atmospheric correction factor calculation ............................ 22

4.4.3

The converse iterative procedure ................................................................................................... 23

4.4.4

Iterative procedure for determining atmospheric correction factor at high altitudes ...................... 24

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards 4.4.5 4.5

Atmospheric corrections for AC voltages .......................................................................................30

Requirements for measurement systems ............................................................................................ 32

4.5.1

Structure of IEC 60060-2:2010....................................................................................................... 32

4.5.2

Calibration and estimation of measuring uncertainty ..................................................................... 32

4.5.3

System calibration by calibrations of components ......................................................................... 33

4.6

Measurement Uncertainty ................................................................................................................... 34

4.6.1

The approach in IEC 60060-2:2010 ............................................................................................... 34

4.6.2

Uncertainty Requirement Differences between IEC 60060-2:2010 and IEEE Std 4-2013 ............ 34

4.6.3

Relationship between uncertainty and tolerance ........................................................................... 36

5 DIFFERENCES BETWEEN THE LATEST REVISIONS OF IEC60060 SERIES AND IEEE STD 4 ................................................................................................................................................................ 37 5.1

General structure of IEEE Std 4-2013 ................................................................................................. 37

5.2

Summary of differences ....................................................................................................................... 37

5.2.1

Estimation of uncertainty ................................................................................................................ 37

5.2.2

Definition of AC peak voltage ......................................................................................................... 37

5.2.3

Linearity test ................................................................................................................................... 37

5.2.4

Highest frequency in impulse voltage test circuit ........................................................................... 37

5.2.5

Physical characteristics of reference impulse voltage dividers ...................................................... 37

5.3 6

Conclusion ........................................................................................................................................... 37 IMPROVEMENTS AND POSSIBLE ADDITIONS IN FUTURE REVISIONS........................................... 38

6.1

DC voltage ........................................................................................................................................... 38

6.2

AC voltage ........................................................................................................................................... 38

6.3

Lightning impulse voltage .................................................................................................................... 39

6.3.1

General considerations .................................................................................................................. 39

6.3.2

Further investigation of the test voltage function for SF6 ............................................................... 40

6.3.3

Further investigation of the test voltage function for oil .................................................................. 40

6.3.4

Further investigation of the test voltage function for air gaps ........................................................ 41

6.3.5

Test voltage function for multiple insulation materials.................................................................... 43

6.3.6

Generation and measurement of UHV lightning impulse ............................................................... 43

6.4

Improvement of atmospheric correction factor calculations ................................................................ 43

6.4.1

General remarks ............................................................................................................................. 43

6.4.2

Differences between IEC standards ............................................................................................... 44

6.4.3

Future work on atmospheric correction factors .............................................................................. 50

6.5

Review of voltage drop, AC and DC for future development .............................................................. 51

6.6

Waveforms of lightning impulse voltage and lightning impulse current............................................... 52

6.6.1

Introduction ..................................................................................................................................... 52

6.6.2

Observation of atmospheric lightning strike voltage waveforms .................................................... 52

6.6.3

Observation of atmospheric lightning strike current waveforms .................................................... 52

6.6.4

Conclusion ...................................................................................................................................... 53

6.7

Improvement of measurement systems .............................................................................................. 53

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards 6.7.1

Calibration of UHV impulse measurement systems ....................................................................... 53

6.7.2

Uncertainty and risk assessment ................................................................................................... 55

6.7.3

Examples of risk assessment ......................................................................................................... 55

6.7.4

Measurement software ................................................................................................................... 58

7

CONCLUSION .......................................................................................................................................... 58

8

REFERENCES ......................................................................................................................................... 59

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

1 INTRODUCTION Recently, IEC and IEEE have published a number of revised and new standards for high-voltage and highcurrent testing. These standards include IEC 60060-1:2010, IEC 60060-2:2010, IEC 61083-2:2013, IEC 62475:2010 and IEEE Std.4-2013. Significant changes and additions have been introduced to these revised and new standards. Many members of CIGRE WG D1.35 were involved in the revision and development of these standards as members of IEC TC 42 and IEEE PSIM Subcommittee HVTT, particularly in the development of the new techniques and new procedures that are now adopted in the standards. This Guide has been written by members of CIGRE WG D1.35 to give high-voltage test engineers a broader knowledge of how to apply the latest high-voltage and high-current testing standards. In the preparation of this Guide, the contributors have tried to point out areas of difficulty in interpretation of certain clauses of these standards that should be considered for future revisions to make HV testing standards more clear and user friendly. This Guide first presents a brief account of the history of these standards with the aim to allow readers to gain a better appreciation of the technical background. The following sections summarize the major changes made to the standards in their latest revisions to provide a general picture of the revisions. The individual sections provide detailed information on the important requirements and procedures that have now been incorporated into the standards. They also describe some of the specific technical background with a list of published references. Finally, some discussion is given on the practical implications of these changes. Practical examples are provided to illustrate some of the new techniques and new procedures. The guide also lists areas of possible improvements to the standards for future revisions.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

2 HISTORY OF HIGH-VOLTAGE AND HIGH-CURRENT TEST STANDARDS 2.1 History of DC high-voltage tests and measurements 2.1.1

DC high-voltage test and measurement systems

The requirements for DC test and measurement systems and DC test procedures have not changed significantly over last few decades. This is because the main objectives of DC tests have not fundamentally changed. Insulation systems most frequently tested with DC voltages are mass impregnated paper-oil insulated cables, DC insulators, DC bushings, DC converter assemblies and DC rectifier transformers. Testing insulators and bushings are performed under dry, wet and polluted conditions whereas most test objects are tested only under laboratory conditions. Recently, polymeric cables have been introduced for DC applications and have necessitated an expansion of DC testing capabilities to include partial discharge measurements. CIGRE WG D1.55 has been formed to study HVDC partial discharge testing. Now we also see interest in DC testing of Gas Insulated Switchgear and the new CIGRE Joint Working Group JWG D1/B3.57 has been recently formed (2014). The main test procedures for DC have remained more or less the same over a long period of time. The tests include: the withstand voltage test, the disruptive-discharge voltage test and the assured disruptivedischarge voltage test. Withstand tests are used for testing non-self-restoring insulation, such as oil-paper insulation in a high-voltage cable as well as self-restoring insulation, such as found with the insulating surfaces of insulators. The other tests are more specifically for self-restoring insulation. The main technical challenges for DC voltage generation equipment are: 1. To produce a test voltage with sufficiently low ripple voltage to meet the standards. Ripple voltage magnitude can be more prominent at the higher test voltages due to loading by streamer activity or high leakage currents. 2. To assure that the measuring system can record the test voltage and the ripple voltage magnitude accurately when short duration current pulses, or persistent repetitive current pulses from the load, are present. These current pulses are normally random and recurring and can adversely affect the voltage stability (i.e. cause voltage dips) of the test equipment supply. 3. To supply sufficiently high current values for charging long lengths of shielded power cable, in reasonable times, at very high voltages. 4. To supply large test current pulses, containing significant electric charge values (Q), such as those produced in pollution testing. The issue here is to avoid erroneous test results due to large voltage drops that may influence the flashover behavior. 5. To supply sufficiently high current when sudden changes of impedance in the test object occur, such as the sudden decrease of impedance prior to the breakdown of an insulator. The concern is to avoid excessive voltage drops that could affect the test results. 6. The introduction of polarity reversal tests in short time frames. 7. The introduction of DC partial discharge testing requirements. Because of the high instantaneous current magnitudes required for some DC voltage tests, the technical challenge for DC voltage measurement systems is not only its ability to measure the DC voltage with sufficient accuracy (or sufficiently low uncertainty), but also having adequate accuracy for measuring ripple and voltage drops due to the sudden change of test object impedance [1]. Unfortunately, the present standards have little to say specifically about how to deal with these problems, mainly because the current pulse demands are not specified. It should be noted that for most DC testing systems it is not necessary to have a dedicated ripple voltage measurement system built in if the test conditions are without corona discharges or time varying currents, such as those found during testing of polluted insulators. A type test of a new DC test system equipment will show the ripple characteristics for the steady state load current and if the test current is within the rating of the system for a given ripple voltage then the standards will be met. This also means that purely resistive dividers are adequate for these test systems to measure the DC voltage. If the equipment is used for tests where high magnitude, time-varying current pulses are anticipated and corona from test connections is likely, then a resistive/capacitive divider with a bandwidth of up to 10 kHz can provide transient data to actually measure voltage drops and ripple magnitude. The question remains: how much voltage drop for how long a time will affect the test results or performance of the apparatus being tested? These questions require input from the relevant apparatus committees who set test voltages and protocols.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards A DC test with a voltage polarity reversal in a short time has been increasingly performed in recent years for testing components in HVDC transmission systems since this is a normal operation of some DC links. This provides a challenge for the DC voltage generation and measurement systems because the energy of the test object must be discharged quickly and safely, and the sources be configured for the opposite polarity. This is often done with resistor networks and mechanically inverting diodes in the system. However, no test procedures and requirements for associated measurement systems have been specified in the latest editions of IEC 60060 series. Test procedures for polarity reversal tests are given in apparatus standards for converter transformers, cables and others. 2.1.2

DC high-voltage supplies

DC high-voltage supplies (generators) for high-voltage testing introduced in the early days did not have the advantage of modern silicon rectifiers, which can be connected in series for producing very high voltages. DC was produced using mercury vapor electron tubes that acted as diodes or by Kenotron tubes. Since the tubes required filaments that had to be supplied with power at high voltages, special, cumbersome isolation transformers had to be fitted to provide power at high voltage levels. This problem led to practical limitations on how high a voltage could be produced - especially if any significant current was required. With the introduction of solid-state rectifiers it became possible to make multiplier circuits for higher DC voltages. Voltage multiplier circuits invented by Heinrich Greinacher in 1914 were originally developed for physics research. These systems were not particularly well suited for capacitor charging applications or fast response, voltage regulated power supplies, due to the inherent high impedance of the circuit. Since the voltage multiplication was via transfer of energy through capacitors, these power supply designs have current limitations, based on the size of the capacitors used. In the mid 1970’s a new style of DC UHV power supplies was introduced into the market. The inventor was Stanley G. Peschel and the design was granted a United States Patent [2]. This design uses a cascade of low impedance transformers feeding a cascade of voltage doublers in series. This design overcame the limitations of capacitor–diode multiplier circuits and could provide stable, well-regulated DC sources at a very high voltage. This design, first used in high-voltage and high-current electron beam accelerators, has largely replaced multiplier circuits for HV apparatus testing, where large charging currents are required and they provide good voltage regulation in the presence of streamers. This use of cascade doubler circuits has the added advantage of being able to produce relatively low ripple voltages with high overload capacity for charging large capacitive loads such as installed power cables. Due to the modular nature of the design, systems for use in the UHV range are possible and are more compact than multiplier designs. 2.1.3

Future work on DC test and measurement systems

Future work now is directed to the following (see Section 6.1 for detailed discussions):

Better characterization of measuring systems for voltage drop and ripple; Techniques of Measuring partial discharges under DC stress and what impact it has on the source requirements; Recommendations of equipment ratings for UHV tests where pollution or wet testing is performed; Possible revisions of standards for normal dielectric tests in the UHV range.

2.2 History of AC test sources and measurement systems The history of AC test sources and measurement systems dates back to the turn of the 20th century when high-voltage AC was first used for transmission and distribution of electric energy. In the early days, conventional transformers of higher voltages than the service voltage were used to “proof” test AC system components. Insulation coordination studies over the years developed test levels for various apparatus in the system and higher safety factors which led to higher test equipment voltage ratings. Over time, increasingly larger capacitive test loads at higher voltages were produced, especially in the case of polymeric cables. Whereas early HV cables of paper-oil insulation could be tested with DC voltages due to the reasonably linear voltage distribution based on resistivity, voltage distribution of plastic cables is mainly dependent on the permittivity of the insulating materials. For testing these voltages, a variety of measurement devices were developed including inductive potential transformers, electrostatic voltmeters, capacitive dividers, compressed gas capacitors and standardized sphere gaps. Over time these devices became the standards for measurements with traceability established by comparisons or calculations or other low voltage means. In the late 1960’s new test systems were introduced with high power capability and test voltages went up to cover 800 kV class transmission. For testing equipment, HV sources used some form of inductive reactance

Page 7

Past, present and future of IEC and IEEE high‐voltage and high current testing standards in the test system to compensate for the capacitive reactance of the test load and limit test power demands. Limitations of expanding test transformers to very high voltage and current levels led to modular resonant systems that are seen everywhere today. Ferranti, LTD of the UK, using moving core inductor designs, first introduced series resonant systems. Unfortunately these designs had poor efficiency and still required significant power from the mains to energize a test object. In 1973, all of this changed with the introduction of the modern, high “Q” series resonant designs invented by Richard F Schutz and Stanley G Peschel [3]. These modern designs were 5 to 10 times more efficient than earlier designs. Modern variable inductance series resonant systems were also unique in that they could be constructed with very high voltage and power ratings. Variable inductance, high Q series resonant systems are now the industry standard for the majority of production and research testing and are supplied by a number of companies. Currently, test sources for AC are now being developed using variable frequency with fixed inductors as compared to variable inductors with fixed frequency. These designs have specific advantages for testing the new generation of very long installed lengths of high-voltage power cables. On the measurement side of UHV AC, the ability to make gas capacitors rated up to 1200 kV allowed direct calibrations for most applications. Use of sphere gaps declined due to inconvenience, insufficient accuracy for many applications and voltage limitations. Potential transformers are available for relatively high voltages but are expensive. Capacitive voltage dividers are readily available today and have been proven to be stable and sufficiently accurate (low uncertainty) for UHV measurements. Today, most AC test systems use capacitive voltage dividers to measure the voltage. In parallel with the development of modern AC test systems has been the development of higher voltage reference measurement systems with stable scale factors and demonstrable linearity. 2.2.1

Future work on AC test and measurement systems

Future work is now directed to the following (see Section 6.2 for detailed discussions):

Calibration of UHV level Reference Measuring Systems; Calibration of VLF test systems; Methods to check linearity in the UHV range; AC source requirements for the UHV range.

2.3 History of impulse voltage tests and measurements 2.3.1

Early history and definition of impulse waveforms

It is generally known that the practice of impulse voltage testing dates back to the early 20th century. According to HV measurements pioneer Nils Hylten Cavallius [4], the first dielectric tests with high impulse voltages, were performed by F. W Peek Jr. around 1915 [5]. The first measurement of the impulse shape, using a continuously pumped oscillograph, was performed by Gabor in 1927 [6]. It is generally considered correct that testing with simulated lightning and the measuring of voltage and currents that could affect the transmission system in a negative way were being studied and simulated in laboratories roughly 100 years ago. F. W Peek Jr. worked in Pittsfield, Massachusetts for General Electric Company and his book, Dielectric Phenomena in High Voltage Engineering, was first published in 1915. The book was a result of the need to explain the consequences of naturally occurring lightning on the apparatus being built for power transmission – especially transformers. In his third edition, published in 1929, the author refers to the tremendous amount of laboratory and field data that had already been accumulated by then and a separate chapter was devoted just for the benefit of engineers studying lightning phenomena. In Impulse-Voltage Testing, by W.G Hawley, published in 1959 [7], we find the results of 30 early years of work in studying the effects of a variety of impulse voltage dividers designs, impulse recorders and the mathematical analysis of dozens of test circuits. Clearly, much work was done by pioneers in high-voltage technology to determine how to best measure impulse voltages even in the early days. The question of standardizing high-voltage testing and measuring techniques, in general, has been discussed within IEC since 1922. The first work in this area was in connection with insulator testing since there were failures of these exposed devices and a solution was needed for the problem. In 1934, a subcommittee was set up to deal with impulse voltage testing leading to first edition of IEC 60, General specifications for impulse voltage tests. The second edition, High-voltage test techniques, was issued in 1962. The counting of editions was restarted, when the content of IEC 60 was distributed among four parts of a new IEC 60 series. First edition of part 1 of the series (High-voltage test techniques – General definitions

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards and test requirements) was first issued in 1973. Second edition was released in 1989, and now the third one in 2010. In the USA, similar work was ongoing and IEEE Std 4, High Voltage Test Techniques, was established to cover all HV testing. Std. 4 was originally issued by AIEE in 1928 and established guidelines for measurement and test methods. The later revisions of Std 4 were issued in 1953, 1969, 1978, and the seventh edition, in 1995. The eighth edition was prepared in parallel with IEC 60060-1 and was published in 2013. From the 1930’s to the early 1960’s national standards from Great Britain, Germany, USA, and internationally from IEC member countries defined different standard (lightning) impulse wave shapes. The German standards [8, 9] recommended front times 0.5 µs, 1 µs and 2.5 µs, all with time to half-value of 50 µs. On the USA side, impulses of 0.5/5 µs, 1/10 µs and 1.5/50 µs were selected, with preference of 1.5/50 µs. The method for evaluation of the front time also varied. For example, according to a British standard [10], the front time was calculated from T1 1.25t 90% t10% , whereas at the same time in the USA [11] a formula T1 2t 90% t 30% was used [12]. It seems that the IEC document [13] at that time defined T1 as time to peak, i.e., T1 t100% t 0% . An agreement on the standard lightning impulse voltage shape was found in 1960’s, when it was settled to be 1.2/50 µs in the second edition of IEC 60:1962 (High-voltage test techniques), and respectively in IEEE Std. 4, IEEE Standard Techniques for High Voltage Testing. The front time was essentially set half way between the German and USA practices, and the present definition for front time, T1 t90% t30% / 0.6 ,

was also introduced in IEC standard. Hyltén-Cavallius gives credit for this consensus to Mr. J. Hagenguth from USA [4]. Information on the early history of switching impulse is surprisingly scanty, Hyltén-Cavallius states from his own experience in revising IEC 60 in 1962 that: “But we missed at that time that the much longer front times as occurring in switching impulses were an important factor in the determination of the flashover voltage.” However, the same edition of IEC 60 did have the first definition of switching impulse - with the same time parameters as today. 2.3.2

Definitions of lightning impulse peak voltage

Over the years IEC HV standards have introduced many new concepts in measuring technique. IEC 60:1962 introduced the idea of removing the oscillations on the front of an impulse by drawing a mean curve according to graphical (non-mathematical) rules under certain conditions. The definition of peak value for lightning impulses in IEC 60:1962 reads: 6.1.3

Peak value, alternatively virtual peak value The peak value is normally the maximum value. With some test circuits oscillations or overshoot may be present on the voltage. If the amplitude of the oscillations is not greater than 5 per cent of the peak value and the frequency is at least 0.5 MHz (Mc/s), or alternatively, if the amplitude of the overshoot is not greater than 5 per cent of the peak value and the duration is not longer than 1 µs, then for the purpose of measurement a mean curve may be drawn, the maximum value of which is defined as the virtual peak value. (See Figure 8a, page 83).

The reasoning why the mean curve was introduced has not been documented, but one story has it that a gentleman from UK came to an IEC meeting with a set of HV test results, and he managed to convince the Technical Committee to introduce the requirement for a mean curve to remove high frequency oscillations with the reason that these fast events would not impact the performance of certain apparatus. The text remained more or less unchanged in the 1973 and 1989 editions of IEC 60-1. This is shown by the green line of Figure 1, i.e., oscillations with frequencies below 0.5 MHz should be included for evaluation of peak value and front time, and above that frequency they should be completely ignored. However, problems began when people found that with oscillations close to 500 kHz, completely different results could be calculated depending on identifying the frequency as above or below 500 kHz. This was accentuated when using computer routines, rather than human eye, to evaluate the oscillograms. The gentleman who proposed the method appeared to have been quite right, as recent experimental studies have shown that above a certain frequency, for many test objects, there is only small impact of minor high frequency oscillations on the insulation. The question of the magnitude of "minor" remains aside from the frequency, however the magnitude is almost always attenuated due to the circuit topology.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards 2.3.3

Introduction of test voltage function

The black dots in Figure 1 show the response of insulation to oscillations superimposed on a lightning impulse according to a recent study [14]. As a result, the new IEC 60060-1:2010 [15] defines a test voltage function [16]:

k f 1 1 2.2 f 2 , which is shown by the blue line in Figure 1. This formula is used to calculate the effective peak voltage as a function of the frequency of the oscillation where f is the frequency in MHz. The introduction of this function removes the problems related to the stepwise change at 0.5 MHz and makes a smooth transition that has a mathematical definition.

1.2 IEC 60:1962 1

Measured IEC 60060-1:2010

0.8 0.6 0.4 0.2 0 0.01

0.1

1

10

f [MHz]

Figure 1: Amplitude calculation factor for a range of oscillation frequencies superimposed on a lightning impulse, together with the old and new test voltage functions. Figure 1 shows that for oscillations below 100 kHz, the peak voltage is taken as the actual peak. For frequencies above 4 MHz, the oscillations are discarded and have no impact on the calculated peak voltage. The results of this recent experimental work have also led to the development of a well-defined new procedure for calculating the parameters of impulse test voltage. This procedure has now been adopted by IEC 60060-1:2010 and IEEE Std 4-2013. A more detailed discussion on this procedure is given in Section 4.1. Future work in this area involves proving that the dielectric stress produced by various superimposed oscillation frequencies on normal wave shapes can be normalized for various apparatus in terms of insulation performance and that the key parameters can be consistently calculated. See chapter 6.3 for detailed discussion on future work in this area.

2.4 Use of sphere-gaps as measurement devices The sphere-gap is a device that has been used for calibrating high-voltage measuring systems for over half a century [17, 18, 19, 20, 21, 22, 23]. However, due to the availability of economical voltage measuring devices with better measurement repeatability, reduced measurement uncertainty due to influence of atmospheric conditions and ease of use, the use of sphere-gap has been in decline. A sphere-gap does not provide sufficiently low values of uncertainty to qualify as a Reference Measurement System. Sphere-gaps also only give one piece of information — peak voltage value. Other measurement systems now provide information on time parameters of test voltages. There is still an IEC standard (IEC 60052) describing its use but it is only suited to measurement checks, not the calibration of measurement devices. It is intended that spheregaps will be used primarily as a device for Performance Checks of high-voltage AC and impulse

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards measurement systems approved in accordance with IEC 60060-2:2010 [24]. Sphere-gaps can also be used for calibrating measurement systems that do not require an uncertainty of voltage measurement of < 3 %, but today most users must meet this uncertainty requirement and most technical committees reference IEC 60060-2 for the uncertainty requirement and the requirement is < 3 % for the test voltages in most cases. Summary of Sphere Gap Considerations for Voltage Measurement 1. 2. 3. 4. 5. 6. 7. 8. 9.

Simple to construct but expensive for larger voltage ratings. Can only be set for one voltage at a time. Only measures peak voltage. Requires careful mechanical alignment. Range of operation for a given sphere size is limited. Can be influenced by particles in the air. Uncertainty of measurement > 3 % for AC & LI voltages. Requires little maintenance over long life time. Requires more test area space than voltage dividers.

Conclusion and Recommendations Sphere-gaps are useful devices for checking high-voltage measuring systems or to provide overvoltage protection as a protective gap during tests, but less convenient for almost any other HV testing needs.

2.5 Reference Measuring Systems The concept of using a Reference Measuring System for calibration is as old as the concept of measurement. However, the requirement to use a Reference Measuring System for calibration was not present in the earlier versions of IE 60 and IEEE Std. 4 before 1994. The calibration of an Approved Measuring System was primarily based on low-voltage components methods. In case of impulse dividers, as long as the total response time was less than 200 ns, the standards stated that the dividers would meet the measurement error requirements for the peak voltage and time parameters. In 1980s several researchers at different HV laboratories around the world independently found that the total response time had no definite relationship with the impulse parameter measurement errors. As a result, a few national metrology laboratories decided to build their own high precision reference dividers and conducted an international round-robin study with the dividers. The study formed the base of 1994 edition of IEC 60060-2 that required using reference dividers to calibrate approved impulse measuring system [25, 26, 27, 28]. This standard specified that the preferred method of calibrating a complete measuring system, including the divider, the transmission system, any secondary divider/attenuator, the measurement instrument and any measurement software, is by comparison against a Reference Measuring System. This concept is applicable to impulse, AC and DC voltages and the standard specified a minimum voltage rating for a reference divider of 20 % of the rating of the system being calibrated to determine its scale factor. Performance at levels higher than the calibration level was to be verified by a linearity test. The measurement uncertainty of a Reference Measuring System is normally significantly lower than that of an Approved Measuring System used in high-voltage testing. The low value of uncertainty of a Reference Measuring System is established through measurements traceable to National Standards. Comparisons among Reference Measuring Systems are also often conducted to ensure their measurement uncertainty and several international round robins have been conducted to confirm this [28, 29, 30, 31, 32, 33, 34]. Prior to the introduction of the requirement for the calibration of a complete measuring system against a Reference Measuring System, IEC 60-2 allowed the calibration of a high-voltage measuring system to be performed by calibration of its components. The method of component calibration was kept in the 1994 edition of IEC 60060-2, as well as in the 2010 edition of IEC 60060-2. However, the comparison and calibration against a Reference Measuring System or Approved Measuring System is specified as the preferred method of calibration. The Reference Measuring System or Approved Measuring System comparison calibration is generally much more efficient than evaluation by the component method. It should be noted that low voltage “check” methods have also been introduced for Performance Checks of measurement systems. The direct comparison calibration method against a Reference Measuring System rated for the full voltage of a system being calibrated is the best method and it is easier to demonstrate traceability to National Standards for scale factor and linearity.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards Table 1 is a summary of the requirements for measurement uncertainties of Reference Measuring Systems, along with the requirements for measurement uncertainties of Approved Measuring Systems, specified in the present edition of IEC 60060-2:2010 and IEC 62475:2010 [35].

Table 1: Requirements for measurement uncertainties of Reference Measuring Systems Uncertainty limit of Reference Measuring Systems

Uncertainty limit of Approved Measuring Systems

Average value

1%

3%

Peak/√2 value

1%

3%

Test voltage (Ut))

1%

3%

Front time (T1)

5%

10 %

Time to-half-value (T2)

5%

10 %

Time to-chopping (Tc)

5%

10 %

Test voltage (Ut)

3%

5%

Time to-chopping (Tc)

5%

10 %

Test voltage (Ut)

1%

3%

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3%

10 %

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3%

10 %

Peak current value

Not specified

3%

Time parameters

Not specified

10 %

Type of test voltage

Measurement quantity

DC voltage AC Voltage

Full/Tail chopped lightning voltage

Front chopped lightning voltage

Switching impulse voltage

Impulse Currents (Exponential and rectangular)

Future work Future Work in the area of Reference Measuring Systems should be focused on demonstrating the uncertainty limits achievable, especially for calibrating equipment for testing in the UHV range.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

2.6 History of IEEE Standard 4 - the sister Standard to IEC 60060 series IEEE Std 4-1995 was the previous version of the general IEEE standard dealing with High Voltage Testing and Measurement Techniques. This standard was built on the work done by pioneers in the field of HV testing: Pete Bellaschi, Frank Creed, Nils Hylten Cavalius, Kurt Feser, Gianguido Carrara to name a few. The current revision, published in 2013, is the eighth edition of this document as a separate standard. The subject had been addressed in the earliest Standardization Report of the American Institute of Electrical Engineers (AIEE) in 1889 and had been substantially elaborated upon in the subsequent reports issued from 1902 to 1933. In 1922 it was decided to issue separate sections for measurement of test voltages by AIEE. The first edition of IEEE Std 4 was published in 1928. It is interesting that as we are going to higher transmission voltages for UHV many of technical problems solved for lower voltage systems must be investigated again for UHV. In the years prior to the last revision of the IEEE Std 4, which is referred to by many North American apparatus committees in their HV testing standards, much has changed in the global marketplace with respect to the power equipment manufacturing industry. Testing has become more automated, more precise and more informative about the quality of the insulation systems being evaluated. In addition, more awareness of the corresponding IEC standards has developed, namely IEC 60060 parts 1 and 2, “Highvoltage Test Techniques”. Now it is common for North American manufacturers to use both IEC and IEEE standards in testing their products, many of which enter the global marketplace. Increasingly data taken in one location may be instantly transmitted electronically to other facilities continents away for analysis and review. The need for standardization has never been more important to the reliability of components of the electrical grid. In preparing the latest revision for IEEE Standard 4, the committee members took every effort to try to align the revised technical content with the revised technical content of IEC 60060 parts 1 and 2 and the new standard IEC 62475 for High-current test techniques. This has been a difficult task as the three IEC standards (four including IEC on-site testing) have been evolving simultaneously with the IEEE revision. Fortunately, IEC revisions are now fixed and the two standards are aligned for the most critical requirements. Despite close relationships between IEEE and IEC organizations, there are still several small differences between IEEE and IEC that are noted in later sections.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

3 OUTLINE OF MAJOR CHANGES MADE IN RECENT EDITIONS OF IEC AND IEEE HIGH-VOLTAGE AND HIGH-CURRENT STANDARDS This section outlines the major changes in recent editions of high-voltage and high-current test standards. Some of these changes are described in detailed in Section 4. The relevant standards for this discussion are: IEC 60060-1 Ed. 3.0 (2010), High-voltage test techniques – Part 1: General definitions and test requirements. IEC 60060-2 Ed. 3.0 (2010), High-voltage test techniques – Part 2: Measuring systems. IEC 61083-2 Ed. 2.0 (2013), Instruments and software for measurement in high-voltage and high-current tests – Part 2: Requirements for software for impulse test. IEC 62475 Ed. 1.0 (2010), High-current test techniques - Definitions and requirements for test currents and measuring systems. IEEE Std 4-2013 High Voltage Test Techniques.

3.1 IEC 60060-1:2010 The definitions for basic lightning impulse voltage parameters, Up, T1 and T2, have not changed. The test voltage function (sometimes known as the k-factor) as described in the literature published in the period leading up to the approval of the standard, has been introduced to enable more accurate and consistent determination of the test voltage and time parameters of lightning impulses with superimposed oscillations of any frequency content. A number of new definitions related to this new procedure have been added, e.g., test voltage function, extreme value, relative overshoot and average rate of rise. The peak value of an alternating voltage is now defined as half of the peak-to-peak voltage. The earlier definition, “maximum value”, could lead to misinterpretation for cases where even harmonics of the test source are present. This harmonic distortion may lead to different positive and negative peak values. A maximum value of 2 % is allowed for the difference between positive and negative peak values. The test voltage value is the peak value divided by √2. No changes in the definition of switching impulse voltage parameters have been introduced in IEC 60060-1:2013, and the previous definition of time to peak has been retained, i.e. “time interval from the true origin to the time of maximum value of a switching-impulse voltage”. IEEE Std 4-2013 and IEC 60060-1:2010 differ slightly on evaluation of switching impulse. For the case of a standard switching impulse, with the time to peak being 250 µs ± 50 µs, IEC identifies a simplified method using a mathematical formula adopted from IEEE Std. 4-1995. This method is also given by IEEE Std 4-2013, but is stated as the definition. For non-standard switching impulses, IEC clearly states that other methods of evaluation should be used, e.g. “For non-standard impulses, the time to peak can be determined by various methods of digital curve fitting dependent on the actual shape”. Formulae have been introduced for the parameters of the atmospheric correction factor to make the computer calculation of the correction factor feasible. The iterative procedure for calculating the atmospheric correction factor of a test voltage is introduced, and is intended for voltage withstand tests. This procedure is intended to reduce the error of the correction factor due to the error in the estimated U50 (50 % probability breakdown voltage) that is needed for the calculation. The error becomes significant when the correction is significant. Only one wet test procedure has been retained. Test procedures for combined and composite voltages have been elaborated more in details. AC and DC Artificial pollution test procedures are removed as they are now specified in IEC 60507.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

3.2 IEC 60060-2:2010

Estimation of common components of uncertainties in high-voltage measurements are now specified in more details. Requirements on voltage linearity tests are more clearly specified. New requirements on dynamic performance, i.e., frequency response of power frequency AC voltage measurement systems, have been added. Step response parameter evaluation methods for characterization of voltage dividers and current shunts have been moved into an informative annex, with a number of definitions related to the step response measurement having been revised. An informative annex on evaluation of the dynamic performance of impulse measurement system by the convolution method has been added The standard sphere-gap is no longer accepted as a Reference Measuring Device but can still be used for Performance Checks. Calibration procedure for DC -systems using a rod/rod gap is removed. Measurement of impulse currents has been transferred to a new standard IEC 62475:2010, Highcurrent test techniques – Definitions and requirements for test currents and measuring systems.

3.3 IEC 62475:2010

The current measurement part of this standard covers more applications than the relevant sections in the obsolete 60060-2:1994, which covered only measurement of impulse current waves as used in arrestor testing. The new standard also covers requirements for testing with any type of high current as well as giving the requirements for a high-current measurement system. The types of high currents which have been added now include: steady-state direct current, steadystate alternating current, short-time direct current, short-time alternating current, and impulse current. The standard also covers current measurement in high-voltage dielectric testing. The standard has adopted a similar structure to that of IEC60060-2:2010 Estimation of measurement uncertainties is specified similarly to that in IEC 60060-2:2010.

3.4 IEC 61083-2:2013 IEC 61083 part 2 has been updated with a new Test Data Generator (TDG) to evaluate impulse measuring system software. The new version provides more impulse voltage waveforms, and includes waves with different overshoot amplitudes and frequencies. Frequencies are selected to prove performance around the transition frequency of 500 kHz. The new TDG helps users prove that their software is making correct evaluations of the key parameters in a consistent and comparable way. Reference values of lightning impulse waveforms have also been revised according to the new definition of the impulse test voltage in IEC 60060-1:2010 Waveforms of impulse currents are added to the TDG to represent a range of those used for arrestor testing that have been added over the last few years. More current waveforms, including lightning current impulses, are now included, in order to cover the range of current waveforms used in the new standard IEC 62475:2010. An annex on estimation of uncertainty contribution of software (waveform parameter calculation) has been added.

3.5 IEEE Std 4-2013

IEEE Std 4-2013 continues to cover both the requirements for testing and the requirements for measurement systems, which are covered separately by IEC 60060-1:2010 and IEC 60060-2:2010. Efforts have been made to harmonize IEEE Std 4-2013 with the two parts of IEC 60060, in terms of principles and fundamental requirements. Minor differences, however, still exist. A summary of the differences is given in Section 5.2. The new edition of IEEE Std 4 still contains a significant amount of tutorial information to give practical suggestions to the test engineer.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards The readers are also drawn to the attention of a closely related standard, IEC 60060-3:2006 [36] for on-site high-voltage testing. Many of techniques and procedures are similar to those specified in the above standards, but with variations to suit conditions and limitations of on-site tests.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

4 DISCUSSION OF IMPORTANT DEFINITIONS AND REQUIREMENTS This chapter discusses important definitions and requirements that have either undergone significant changes or deserve close attentions of users for correct interpretation and better use of the standards.

4.1 Lightning impulse test voltage and test voltage function As discussed in earlier sections, the impulse analysis procedure provided in IEC 60060-1:2010 and IEEE Std 4-2013 allows determination of impulse parameters without subjective determination of the existence or characteristics of overshoot and its frequency. The new procedure in the standards is also more precisely specified in order to eliminate discrepancies of impulse parameters obtained by different software packages. A summary of the major steps of the procedure is given in Figure 2. Evaluation of tail chopped impulse parameters requires the knowledge of the waveform of a full impulse taken in the same circuit configuration. This is because double exponential curve fitting will not produce a correct base curve for chopped impulses since the front is not really a perfect exponential and the tail does not exist on a chopped wave. A full impulse waveform is therefore needed to aid in the calculation. A full impulse from the same test circuit at a reduced voltage level is usually available in the impulse test of most apparatus before the tail chopped impulses. Therefore, for evaluation of tail chopped impulses, steps 1 and 2 in Figure 2 are performed using this full reference impulse; the base curve obtained is then scaled or normalized to match the amplitude of the tail chopped curve for the remaining steps of the procedure. For a front chopped lightning-impulse voltage, the test voltage curve is the recorded curve without further processing of the waveform. An integral part of requirements for impulse voltage tests in IEC 60060-1:2010 are the requirements for software specified in IEC 61083-2:2013, Instruments and software used for measurement in high-voltage and high-current tests - Part 2: Requirements for software for impulse tests. The Test Data Generator (TDG) of IEC 61083-2:2013 is a software package for generation of test data. The TDG produces digital records of a number of different impulse waveforms for testing an impulse measurement software package. IEC 61083-2:2010 specifies the reference values of test impulses generated from the TDG. The IEC 61083-2:2013 also provides error limits for acceptance of software being evaluated for measuring different types of impulses. After the new Test Voltage Function (k-factor) was proposed, a number of evaluation software programs were tested in round-robin tests. The work was first performed within CIGRE WG D1.33, but later the activity gradually moved to IEC TC 42 Maintenance Team (MT) 07. The main task of the latest work was to revise the original Test Data Generator to include more wave shapes for more comprehensive testing of impulse voltage and impulse current evaluation algorithms. Annex B and Annex C of IEC 60060-1:2010 are two of the products of MT07 work.

120

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60

U [kV]

1. Data starting from 20% on the front to 40% on the tail (pink) of the measured data (blue) are taken for curve fitting.

U [kV]

In addition to LI and SI impulse shapes in the TDG, IEC 61083-2:2013 also includes data sets for Oscillating Lightning Impulse waveforms (OLI) and Oscillating Switching Impulse waveforms (OSI) in accordance with IEC 60060-3:2006 (High-voltage test techniques - Part 3: Definitions and requirements for on-site tests) [36]. Impulse current waveforms defined in IEC 62475:2010 (High-current test techniques – Definitions and requirements for test currents and measuring systems) have been added to the TDG too.

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2. Double exponential base curve (green) is fitted to the data selected in step 1 (pink).

U [kV]

Past, present and future of IEC and IEEE high‐voltage and high current testing standards

40 20

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6. Test voltage curve (red) is shown together with the measured curve (blue). Up, T1 and T2 are calculated from the test voltage curve.

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Figure 2: Evaluation of the test voltage curve according to IEC 60060-1:2010.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

4.2 Definitions of switching impulse The definition of time to peak for switching impulse in IEC 60060-1:2010 is “time interval from the true origin to the time of maximum value of a switching-impulse voltage”. It has been found that it is often difficult to determine this time to-peak value, Tp, even from digital records of impulses with low uncertainty, due to the fact that the digitally recorded voltage values in the impulse peak region can be approximately equal for a long period of time, and many data points of the same amplitude will be found around the peak. To overcome the problem of calculating the Tp value, the empirical formula introduced in the previous edition of IEEE Std 4 and retained in IEEE Std 4-2013, is now included in IEC 60060-1:2010 for evaluation of switching impulses that conform to the standard waveform. The formula takes the following form: Tp = KTAB

(1)

where K is a dimensionless constant given by K = 2.42 – 2.08x10-3TAB + 1.51 x 10-4 T2 where TAB and T2 are in microseconds and TAB = t90 – t30 (see Figure 3) and the numerical constants 2.08 and 1.51 have dimension s-1. The difference between the Tp values calculated from formula (1) and the true Tp as defined in Clause 8.1.3 of IEC 60060-1:2010 for the standard switching impulse waveform of 250/2500 µs is negligible. Taking the TDG waveform SI-A1 of IEC 60183-2:2013 as an example, the difference between the value calculated using formula (1) and the reference value given by IEC 60183-2:2013 is less than 0.85 % (see Table 2). The reference values given in IEC 60183-2:2013 were determined by averaging the Tp values obtained in a round-robin test, by different software packages that calculate the Tp values in accordance with Clause 8.1.3 of IEC 60060-1:2010, i.e. the Tp value that corresponds to the maximum voltage of the impulse waveform. If the impulse parameters remain within the tolerance ranges of the standard switching impulse, i.e. Tp remains between 200 µs and 300 µs, and T2 remains between 1000 µs and 4000 µs, the maximum deviation of Tp values calculated, using formula (1), from the reference Tp value is less than 3 %, which is negligible for the purposes of high-voltage testing. However, since formula (1) was defined for standard switching impulses only, if the waveform deviates significantly from the standard switching impulse, the deviation of Tp values can be significantly larger that the permitted measurement uncertainty of Tp, which is ±10 % and for those cases other methods of evaluation must be used. However, for most testing situations this is not an issue.

U 1,0 0,9

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Td

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0 t

TAB T2

Figure 3: Illustration of defined switching impulse voltage parameters.

Page 19

Past, present and future of IEC and IEEE high‐voltage and high current testing standards Table 2: T p values given by TDG of IEC61083-2: 2010 and those calculated using formula (1) TDG Case No. IS-A1 SI-A2 SI-A3 SI-A5

TDG Tp (µs) 250.7 19.89 43.1 218

TDG T2 (µs) 2512 1321 3987 2407

Formula (1) Tp (µs) 248.8 15.30 35.99 221.8

Tp Error (% of TDG Tp) -0.76 -23.1 -16.5 +1.8

Note that waveforms S1-A2 and S1-A3 are non- standard switching impulses The errors of Tp values obtained by using formula (1) for some practical waveforms are shown in Figure 4 and Table 3. The reference values are obtained according to the definition in Clause 8.1.3 of IEC 60060-1:2010, using double exponential fitting to find the peak. Results in Table 2 and Table 3 show that when the standard switching impulse voltages are used, formula (1) is sufficiently accurate for meeting the uncertainty requirement of high-voltage testing.

Table 3: T p values obtained using formula (1) as compared to T p values by doubleexponential fitting Waveform No. 1 2 3 4 5 6

Tp by fitting (µs) 174.71 212.51 237.13 287.93 288.55 312.51

T2 by fitting (µs) 2481.95 1491.09 3998.58 4032.71 1504.95 2546.15

Formula (1) Tp (µs) 169.66 210.84 238.21 291.34 285.00 308.06

Tp Error (% of fitted Tp) 2.9 0.8 0.5 1.2 1.2 1.4

Figure 4: Errors of T p values of practical waveforms obtained using formula (1) with the T p values of fitted waveforms as the references.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

It has been found, however, that errors of Tp values obtained by other methods, for example, by curve fitting, would not be significantly lower, in some cases could be higher than those of formula (1). A number of curve fitting methods have been tested by various laboratories. One method was to fit the complete waveform from the origin of the impulse to a double exponential function. Another method [37] is to fit from the instant of 85 % of the peak value on the front to the instant of 95 % of the peak value on the tail (denoted as top fit). It was found that the difference in the Tp values obtained by these two fitting method for the last waveform in Table 3 is 1.7 %. Furthermore, when the time parameters are within the tolerance range of the standard switching impulse, the maximum errors of Tp values obtained by the fitting methods for the TDG waveforms are of similar magnitude to that of the formula (1), with reference to the reference values as given in IEC 61083-2:2013. Table 4 shows the errors of the two fitting methods.

Table 4: T p errors of two fitting methods for standard switching impulses

TDG Case No.

TDG Tp (µs)

IS-A1 SI-A5

250.7 218.0

Error of Tp (complete fit) (% of TDG Tp) +0.52 +3.2

Error of Tp (top fit) (% of TDG Tp) +0.53 -4.7

Error of Tp (Formula (1)) (% of TDG Tp) -0.76 +1.8

In conclusion, the difference of Tp values of standard switching impulses obtained by the fitting methods in accordance with Clause 8.1.3 of IEC 60060-1:2010 and the formula given in Clause 8.2.3.1 of IEC 60060-1:2010 and Clause 8.1 of IEEE Std 4-2013 are not significant for the purposes of high voltage testing. Therefore, for standard switching impulse measurement, formula (1) is recommended due to the simplicity of its implementation as no complicated nonlinear curve fitting and the related software development are needed. Non-Standard Switching Impulse Wave shapes The calculation errors using formula (1) can be significant for calibration of the measurement system systems using the non-standard switching impulses. For the non-standard switching impulses given in IEC 601083-2, the formula yields Tp values outside the acceptance limits of the reference values. Fitting methods may be utilized for these special waveforms by agreement between parties. In any case, these waveforms would generally not be acceptable for apparatus testing by relevant apparatus committees that reference IEC 60060 for standard wave shapes and their evaluation is left to those relevant apparatus committees to define.

4.3 AC test voltage A change in the new standards is for a new definition of the AC peak voltage. As mentioned earlier the AC peak voltage is now defined in IEC 60060-1:2010 as the “average of the magnitudes of the positive and negative peak values, as opposed to, “the maximum value” in the 1989 edition. The definition of peak voltage in IEEE Std 4-2013, remains the same as in its previous version, IEEE Std 4-1995, which is essentially identical to the corresponding definition in IEC 60060-1:1989. The practical impact of the difference between IEC 60060-1:2010 and IEEE Std 4--2013 is insignificant in the vast majority of AC tests, where the AC voltage waveforms are symmetrical to the zero voltage level. In rare cases, when the voltage waveform becomes unsymmetrical, e.g., with the presence of even harmonics produced by power electronics voltage sources, if a peak voltmeter is used to measure voltage, the difference between the peak voltage reading from a voltmeter that detects the maximum of the AC voltage and that from a voltmeter that reads the average of the positive peak and the negative peak can be different. IEC 60060-1:2010 requires that the difference between the positive peak and negative peak values shall be less than 2 %. For voltage sources that meet this 2 % requirement, the maximum difference in the measured peak voltage between IEC 60060-1:2010 and IEEE Std 4-2013 would be 1 % if there was an non-symmetrical AC shape due to electronic converter loads. Nearly all High-voltage AC testing produces symmetrical waveforms so this is not an issue for most testing.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

The requirement for the crest factor, which is defined as the ratio of the peak voltage to the rms value, remains the same for the new editions of both IEC 60060-1 and IEEE Std 4. The ratio between peak and rms should be within 5 % of √2. The crest factor has been used for some time as a determinant of waveform distortion especially in the peak area of the test voltage. With modern instruments being capable of more accurately measuring harmonics, the value of total harmonic distortion, THD, can now be easily measured. It is tempting to use this parameter to characterize the wave-shape, but it must however be considered, that THD does not directly relate to changes in the crest value and that THD is not a suitable measure of crest distortion. A voltage drop of up to 20 % is still considered acceptable in the revised standard during the AC tests such as the disruptive voltage test, the pollution test and the wet test.

4.4 Atmospheric correction factor 4.4.1

Introduction

The atmospheric correction factor is used for correcting the disruptive discharge voltage of insulation under a particular test atmospheric condition to a voltage under the standard atmospheric conditions. This section describes in detail some of the additions and changes in relation to atmospheric correction factor as defined in IEC 60060-1:2010. This chapter also lists the newly introduced formulae in IEC 60060-1:2010 that replaced the graphs in IEC 60060-1:1989. These formulae facilitate calculation of atmospheric correction factors by computer software. The atmospheric corrections defined in IEC 60060-1:2010 are valid for air-gaps and clean insulators. If atmospheric corrections are to be calculated for surface discharge tests, the results obtained with the method in IEC 60060-1:2010 have to be treated with caution. In IEEE Std 4-2013, two methods of calculating atmospheric correction factor are used. Method 1 is the same as IEC 60060-1:2010, and is recommended for new equipment. Method 2 has been used in the past and may be valuable for repeated tests on existing equipment designs. 4.4.2

Formulae of relevant parameters for atmospheric correction factor calculation

A number of formulae have been introduced for convenient implementation of computer calculation of atmospheric correction factor values. Changes have also been made to a few formulae as a result of the latest revision of IEC 60060-1. These changes are highlighted here to help users to make necessary changes to their calculation procedures. 4.4.2.1 Formulae of Exponents for air density correction and humidity correction In IEC 60060-1:1989, the value of the exponent for air density correction, m, and the value of the exponent for humidity correction, w, needed to be obtained from graphs. In IEC 60060-1:2010, formulae of these two exponents, as functions of parameter g, have been included to facilitate atmospheric correction factor calculations by computer programs. The formulae are especially useful when the iterative procedure (see 4.4.3 and 4.4.4) has to be used. The relationship between g and m, and that between g and w, are both described by piecewise functions listed in Table 1 of the IEC 60060-1 Ed.3.0, which is reproduced below as Table 5.

Table 5: Values of exponents, m for air density correction and w for humidity correction, as a function of the parameter g

g

Table 1 of IEC 60060-1:2010 m

w

2,0

g(g-0,2)/0,8 1,0 1,0 1,0

g(g-0,2)/0,8 1,0 (2,2-g)(2,0-g)/0,8 0

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

The definitions for g, m and w in IEC 60060-1:2010 remain the same as in IEC 60060-1:1989. The graphs for m and w are however still given for those who still want to use them. 4.4.2.2 The voltage type parameter k The parameter for voltage type, k, in the case of “direct voltage”, was expressed a linear function of “h/δ” in IEC 60060-1:1989 (See Figure 3 of IEC 60060-1:1989), where h is the absolute humidity of ambient air and δ is the air density. However, the function k in the “direct voltage” is slightly non-linear. The non-linear function has now been adopted in IEC 60060-1:2010. The other change is that the applicable range of humidity for k has been extended for the DC case and the impulse case. The extension of the humidity range was largely based on consensual decisions, with considerations given to the historical experimental data, practical issues and impact on the validity of the corrected test voltage. The changes related to parameter k are summarized below: IEC 60060-1:1989 Ed. 2.0: “Direct voltage:

k= 1 + 0,014(h/δ − 11) , for 1 g/m 3 < h/δ < 13 g/m 3 ”

“Impulse voltage:

k= 1 + 0,010(h/δ − 11) , for 1 g/m 3 < h/δ < 15 g/m 3 ”

IEC 60060-1:2010 Ed. 3.0: “DC:

for 1 g/m 3 < h/δ < 15 g/m 3 ”

k= 1 + 0,014(h/δ – 11) – 0,00022(h/δ – 11) 2

“Impulse:

for 1 g/m 3 < h/δ < 20 g/m 3 ”

k= 1 + 0,010(h/δ − 11)

In the AC voltage case, k remains unchanged. “AC:

for 1 g/m 3 < h/δ < 15 g/m 3 ”

k= 1 + 0,012(h/δ − 11)

4.4.2.3 Air humidity The calculation of the atmospheric correction factor requires determination of the absolute humidity of air, h. The value of h can be determined by the reading of a dry bulb thermometer and the reading of a wet bulb glass thermometer. However, IEC 60060-1:2010 also includes a conversion formula that allows calculation of the absolute humidity of air, h, from the relative humidity and the air temperature. This allows convenient measurement of h using electronic sensors measuring relative humidity.

h where

17 , 6t 243 t

6,11 R e , 0,4615 ( 273 t )

h is the absolute humidity in g/m3, R is the relative humidity in percent and t is the ambient temperature in °C.

The uncertainty of measurement of R using electronic sensors would normally be 1 % to 2 % relative humidity, and hence at least the same uncertainty in the value of h. Determination of absolute humidity using wet and dry thermometers is not expected to alter the uncertainty significantly (neglecting the uncertainty of the temperature measurement). The sensitivity of the uncertainty of atmospheric correction factor (and hence the corrected test voltage) to the uncertainty of h is low (see D.7.1.3 of IEEE Std 4-2013). As shown in Table D.2 (IEEE Std 4-2013), an uncertainty of 1 g/m3 in h would only lead to a relative uncertainty of the corrected test voltage of 1.1/559, which is approximately 0.2 %. 4.4.3

The converse iterative procedure

The atmospheric correction factor, Kt, is used for both breakdown tests of given breakdown probability and withstand tests. In most cases of product testing, atmospheric correction is performed for voltage withstand tests, with AC, DC, lightning impulse or switching impulse voltage, at an altitude less than the service altitude of the equipment under test.

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The procedure in IEC 60060-1:2010 is derived from experimental data of 50 % probability flashover voltage, however the correction often needed is for the corrected test voltage to be applied in a withstand test. The withstand test voltage is defined as the voltage with 10 % probability of disruptive discharge, U10. IEC 60060-1:2010 assumes the same correction factor applies to both U50 and U10. The calculation of the atmospheric correction factor requires a known value of U50 of the insulation under test. In a flashover test, U50 can be determined and therefore the value of U50 does not cause significant error in the correction factor if the experiment is performed correctly. However, U50 is usually unknown in a withstand test and has to be estimated for calculation of the correction factor. The iterative procedure introduced in IEC 60060-1:2010 (Annex E) is to reduce the error in the correction factor, hence the corrected test voltage, due to the error of the estimated U50. The error in the calculated correction factor is often significant in comparison with the required tolerance of the test voltage in cases where the correction is large, i.e., where the correction factor deviates from unity by more than the tolerance of the specified test voltage. The iterative procedure in IEC 60060-1:2010 recalculates U50 and U10 until both converge to constant values, and hence reduce the error caused by the initially estimated U50. Atmospheric corrections lately attracted more attention due to the introduction of UHV AC and DC systems, of which some are located at high altitudes. The corrections are necessarily large for test voltages to be used at high altitudes. The examples in section 4.4.4 give the calculated correction factors using the standard procedure, where a measured 50 % probability disruptive-discharge voltage in given conditions may be converted to the value which would have been obtained under standard reference atmospheric conditions. Calculations by the converse iterative procedure are also given to show the differences of the calculated correction factors to those obtained in accordance with standards used for insulation coordination or apparatus standards. It should be noted that the iterative procedure always leads to the lowest value in altitude correction factor (=1/Kt), which are closer to the altitude correction factors calculated using the methods of the other standards, IEC 60071-2:1996 (Insulation coordination: Application Guide) and IEC 61869-1:2007 (Instrument Transformers, General requirements). It should also be noted that for lightning impulse tests on some equipment, such as instrument transformers, no atmospheric corrections are to be applied according to the relevant standards. 4.4.4

Iterative procedure for determining atmospheric correction factor at high altitudes

When the converse procedure is used for determining the atmospheric correction factor, where a withstand test voltage is specified for standard reference atmosphere (Clause 4.3.1, IEC 60060-1:2010) and must be converted into the equivalent value under the test conditions, an iterative procedure described in IEC 60060-1:2010 may need to be used. The application of the iterative procedure is necessary if the correction factor Kt is lower than e.g. 0.95, for reducing the error of calculated correction factor for a high altitude test site. In the iterative calculation procedure, the correction factor Kt is calculated by iteration until it converges to be within a predetermined limit, i.e.: (i) = 1.1 × with

(i-1) = 1.1 ×

(i-1) ×

being the specified test voltage.

The iteration is continued until

1

, where i is the number of the iteration.

Figure 5 to Figure 8 show atmospheric correction factors Kt, calculated with the non-iterative procedure and the iterative procedure, keeping the humidity correction factor to unity. These results are then essentially the inverse of the values of the altitude correction factor used in the insulation coordination standard (IEC 60071-2:1996) and some equipment standards such as IEC 61689-1.

The calculations were performed for AC, lightning impulse (LI) and switching impulse (SI) voltages with gap distances of 3800 mm and 2000 mm and for DC voltage for 4000 mm and 2000 mm. The first number in the legend denotes the gap distance with the 2nd number being the specified test voltage level in kilovolt. For the 2000 mm gap, 806 kV is the AC flashover voltage level (the voltage level of assured disruptive discharge) and 651 kV is the AC withstand test voltage level. For the 3800 mm gap, 1245 kV is the AC flashover voltage level and 1047 kV is the AC withstand test voltage level. For AC (Figure 5) and switching impulse (Figure 8) voltages, the iterative procedure yields higher values of the atmospheric correction factor than the standard procedure. The atmospheric correction factors for tests

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at the withstand level are larger than those for tests at the flashover level. The atmospheric correction factors with the larger gap distance (3800 mm) are larger than those with the smaller gap (2000 mm). For DC voltages (Figure 6) the iterative procedure yields larger altitude correction factor values for tests only at the withstand voltage level. The other results are very similar. For lightning impulse voltages (Figure 7) the correction factors are the same irrespective of the altitude and the calculation procedures for the smaller gap (2000 mm). For the larger gap (3800 mm), the correction factors for tests at the flashover voltage level are the same for both calculation procedures and are the same as those of the smaller gap, only the correction factor at the withstand level obtained from the iterative procedure is larger.

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Figure 5: Atmospheric correction factors K t for AC voltage, obtained with the non-iterative procedure and the iterative procedure.

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Figure 6: Atmospheric correction factors K t for DC voltage, obtained with the non-iterative procedure and the iterative procedure.

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Figure 7: Atmospheric correction factors K t for lightning impulse voltage, obtained with the non-iterative procedure and the iterative procedure.

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Figure 8: Atmospheric correction factors K t for switching impulse voltage, obtained with the non-iterative procedure and the iterative procedure.

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In most cases of applying atmospheric corrections, these are used for withstand testing of either AC, DC, lightning impulse or switching impulse of equipment at an altitude less than the altitude the equipment will be used in service. Here the iterative procedure always leads to the lowest altitude correction factor (1/Kt). 4.4.5

Atmospheric corrections for AC voltages

Note 3 of clause 4.3.3 “Application of correction factors” in IEC 60060-1:2010 points out that the peak value has to be used in correcting power frequency voltages, because the discharge behavior is based on the peak value. Apparatus standards however specify RMS values based on the system voltage for which the equipment is to be used. If the specified RMS test voltage is not converted into the peak value, the calculation of the correction factors becomes erroneous and not consistent. Figure 9Figure 9 shows the effect of using the RMS value instead of the peak value when calculating atmospheric corrections using the standard procedure. The RMS voltage value of 570 kV and 460 kV correspond to a peak voltage value of 806 kV and 651 kV respectively for the 2000mm gap, 880 kV and 740 kV correspond to 1245 kV and 1047 kV respectively for the 3800 mm gap. For smaller gaps the atmospheric correction curves for the peak values of withstand and breakdown voltage converge at an altitude of 2500 m, but the difference between the correction factor using the peak value and the correction factor using the RMS value can be as large as 19 % at 4000 m. For larger gaps the correction factors become much larger when using the RMS value. The difference can be as large as 30 % as is the case for the 3800 mm gap at withstand level for an altitude of 4000 m; even at an altitude of 2000 m the difference is 8 %. For the iterative procedure at withstand level the trend is similar with smaller deviations, from 6 % at 2000 m to 12 % at 4000 m. This is an important note in IEC 60060-1:2010, which can easily be missed.

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Figure 9: Atmospheric correction factors K t for AC voltage, comparison of Peak voltage to RMS voltage.

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4.5 Requirements for measurement systems 4.5.1

Structure of IEC 60060-2:2010

Requirements for measurement systems are specified in IEC 60060-2:2010. The main structure of IEC 60060-2:2010 is similar to that of the previous edition IEC 60060-2:1994, although there have been significant additions of contents to some clauses, for example, the additional requirements in Clause 5 on estimation of measurement uncertainties. Table 6 compares the main clauses of Edition 2.0 and Edition 3.0 of IEC 60060-2. Term “AMS” in Table 6 denotes “Approved Measuring System” and “RMS” denotes “Reference Measuring System”. Both terms are defined in IEC 60060-2. Table 6: Comparison of the main clauses of editions 2.0 (1994) and 3.0 (2010) IEC 60060-2:1994, Ed. 2.0

IEC 60060-2:2010, Ed. 3.0

3. 4.

Definitions and symbols Qualification of AMS

3. 4.

Terms and definitions Qualification of AMS

5. 6.

Acceptance tests on components Performance tests on AMS

5.

Tests and test requirements for AMS, including the uncertainty estimation

7. 8. 9. 10.

Measurement of DC voltage Measurement of AC voltage Measurement of LI voltage Measurement of SI voltage

6. 7. 8. 9.

Measurement of DC voltage Measurement of AC voltage Measurement of LI voltage Measurement of SI voltage

11.

Measurement of impulse currents

12.

Reference Measuring Systems (RMS)

Annex A: Annex B:

Transferred to IEC 62475

Accreditation systems Record of performance

10.

Reference Measuring Systems (RMS)

Transferred to 4.1 and 4.4

Amendment 1/Annex H: Uncertainty estimation

Annex A: Uncertainty estimation (GUM) Annex B: Example for uncertainty calculation

Annex C:

Step response measurement

Annex C: Step response measurement Annex D: Convolution

Annex D: Annex E: Annex F:

Temperature rise of resistors RMS bibliography Summary of tests

Tables transferred to chapters 6 to 9

The two old clauses on acceptance and performance tests have been combined into the new Clause 5. This new clause gives detailed requirements on the determination of the measuring uncertainty. The summary tables for approving the HV measuring systems of different types of test voltages have been transferred from the old Annex F to the relevant clauses on individual voltage types. The old chapter on impulse current measurement has been transferred to the new standard IEC 62475:2010. The content of six of the seven annexes of the old standard have been, where appropriate, transferred to the main text of the new standard. Only the annex on step response measurement (Annex F, now C) remains and is complemented by an informative annex (Annex D) on convolution. 4.5.2

Calibration and estimation of measuring uncertainty

It is strongly expressed in IEC 60060-2:2010 that the preferred calibration procedure is the comparison method using a Reference Measuring System to qualify an Approved Measuring System. This often means that a calibration is performed by a certified calibration laboratory. The calibration procedure starts with the determination of the scale factor by which the recorded reading is multiplied to arrive at the actual test value. It also requires that the estimation of measurement uncertainty should follow ISO/IEC Guide 98-3, “Guide to the Expression of Uncertainty in Measurement” (also referred to

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as GUM in literature). Two new Annexes, annex A and annex B are added to IEC 60060-2:2010 to provide additional practical information and examples of uncertainty estimation in high-voltage measurements. Required measurement uncertainty limits for all types of voltages, for the purposes of high-voltage tests as specified in IEC 60060-1:2010, remain unchanged. An expanded measuring uncertainty of UM ≤ 3 % is required for the test voltage, whereas for time parameters of the full lightning impulse and the switching impulse, a value UM ≤ 10 % is required. For Reference Measuring Systems the values are UM ≤ 1 % for the test voltage and UM ≤ 5 % for time parameters. Only the voltage measurement of front-chopped LI impulse voltage is specified with higher uncertainties, with UM ≤ 5 % for an Approved Measuring System and for UM ≤ 3 % for a Reference Measuring System. Calibration by comparison should normally be performed at several voltage levels. When the rated voltage of the Reference Measuring System is sufficient to cover the assigned measurement range of the Approved Measuring System under calibration, then the number of voltage levels should be equal to or greater than 5 levels. In cases where a Reference Measuring System with sufficient operating voltage is not available, IEC 60060-2:2010, like the previous edition, allows the comparison calibration be performed with the highest calibration voltage being as low as 20 % of the assigned measurement range of the Approved Measuring System. In such cases, a voltage linearity test must be performed in addition to the comparison calibration or determination of scale factor. Several methods for determining linearity for various types of measurement systems are given for voltages up to the levels for 800 kV class equipment testing. It should be noted that specific information on suitable linearity test for DC is not given and may pose a problem. Recent development has however extended the DC calibration voltage available in the world to 1000 kV [38, 39]. IEC 60060-2:2010 provides specific requirements for the calibration and these are given in Clause 5.2.1.3, “Comparison over limited voltage range” and Clause 5.3, “Linearity test”, of the standard. The graphical illustration of this approach is given in Clause 5.2.1.3 and is reproduced in Figure 10 below. As can be seen from Figure 10, the total number of test levels for checking scale factor and linearity, should be at least 6. The lowest voltage level of the linearity test should be performed at the scale factor calibration voltage, which should be at least 20 % of the highest voltage of use for the Approved Measuring System.

Figure 10: Calibration over a limited voltage range 4.5.3

System calibration by calibrations of components

IEC 60060-2:2010 still retains the approach of calibration of a measuring system by calibrations of its components, with the requirements given in Clause 5.2.2. This approach is provided as the alternative method to the method of comparison of the complete measuring system with a Reference Measuring System, which is specified as the preferred method. When planning the calibrations and combining the results for the complete system, the interactions between the components and the influence of the transmission system (measurement cables), have to be considered to arrive at the correct values.

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4.6 Measurement Uncertainty 4.6.1

The approach in IEC 60060-2:2010

The uncertainty calculation has been significantly revised in the latest edition of IEC 60060-2:2010. The reasons for this major revision are mainly two fold. First, the revision is to provide testing personnel a simple and practical method of estimating measurement uncertainties that is consistent with ISO 98-3, Guide on Uncertainty of Measurement (ISO GUM). The method is intended to cover most common cases of high-voltage testing. Second, the approach described in the previous edition of IEC 60060-2 (Appendix H in IEC 60060-2:1994) is no longer considered consistent with the current edition of ISO Guide on Uncertainty of Measurement. That approach categorizes the uncertainty contributions as either “systematic” or “random”. Appendix H of IEC 60060-2:1994 also assumes that all systematic contributions can be covered by two types of distributions, the rectangular distribution and the Gaussian distribution. It also implies that systematic contributions can be considered to have infinite degrees of freedom. In fact, the use of “degrees of freedom” is omitted in Appendix H. These assumptions and omissions are now considered not adequate for many practical applications. Appendix H also omitted the concept “sensitivity coefficient”, another important concept that is used in the current ISO GUM and that has now been adopted in IEC 60060-2:2010. The major change advocated in the ISO GUM is to provide realistic estimates of measurement uncertainties, moving away from treating measurement uncertainty as a safe error limit, that is, treating measurement uncertainty as an estimate of maximum error that can possibly be expected for the measurement. By adopting the latest ISO GUM approach, it is possible to achieve measurement uncertainties that fit a specific testing purpose with less costly equipment and less time consuming procedures. In the 1994 edition of 60060-2, specified fixed limits were given for individual uncertainty components, for example, a 1 % limit was specified for non-linearity of voltage measurement systems. Fixed numbers of repeated measurements were also specified. An example of this is that the number of repeated applied impulses during an impulse voltage calibration was specified to be at least 10. With the adoption of the ISO GUM approach in the 2010 edition, these limits are no longer specified as long as the total expanded uncertainty (expanded uncertainty is a defined term) is within the required limit. The removal of these limits becomes possible because of the adoption of the statistically more rigorous approach of the ISO GUM. The 2010 edition of IEC 60060-2 also adopts an approach that is intended to provide practical help to users of the standard to better adapt to the new method of estimating measurement uncertainties. The latest IEC 60060-2 lists typical sources of uncertainty contributions in measuring systems. It also added two completely new Annexes, Annex A and Annex B, dedicated to the topic of measurement uncertainty. Annex A aims to provide an easy-to-understand explanation of the ISO GUM, assuming that a simplified procedure of the ISO GUM can be used for estimating measurement uncertainties for high-voltage tests in most cases. Annex B gives three examples of uncertainty calculation, all with the assumption the type B components often having high degrees of freedom. This practice is mainly for practical efficiency. It should be pointed out, type B components often do have low degrees of freedom and the “quality” of their estimates is often low. However, the degrees of freedom of individual components do not make a significant difference in the final calculated uncertainties. IEEE Std 4-2013 gives detailed explanation and examples how the degrees of freedom and sensitivity coefficients are determined and used. 4.6.2

Uncertainty Requirement Differences between IEC 60060-2:2010 and IEEE Std 4-2013

Table 7 below is a summary of the differences between the earlier editions and the latest editions of the standards in relation to estimation of measurement uncertainties. It should be emphasized that the differences in the two editions of IEC 60060-2 are significant, not only in details, but also in the general approach. The IEC 60060-2:2010 is very much harmonized with ISO Guide 98-3, while Annex H of IEC 60060-2:1994 was written before ISO Guide 98-3 was published and hence was not fully compatible with it. IEEE Std 4-2013 is also harmonized with ISO Guide 98-3. The main difference from IEC 60060-2: 2010 is that its uncertainty calculation examples contain more details.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards Table 7: Uncertainty estimation in two editions of IEC60060-2 and IEEE Std 4-2013

IEC 60060-2:1994

IEC 60060-2:2010

IEEE Std 4-2013

Uncertainty definition

Clause H.2: preISO/GUM of “uncertainty” is used.

The general term “uncertainty” as defined in IEC 60050 is given in 3.6.

The general term “uncertainty” is not given. This makes no impact to the uncertainty calculation.

Types of uncertainty contributions

H.2.1: Systematic contributions H.2.2: Random contributions

3.6.8: type A evaluation

Type A uncertainty and Type B uncertainty, as defined in ISO GUM is given in D2.

Term for uncertainty result

H.3: Overall uncertainty

3.3.6 Expanded uncertainty (ISO Guide 98-3)

In D2, and interpreted definition of “Expanded Uncertainty” as in “ISO Guide 98-3” is given.

Other relevant terms and definitions

Terms are used without being defined in the standard

Defined in 3.3.1 to 3.3.11, Including ISO/IEC definitions, such as “error”, “standard uncertainty”, “coverage factor” and “traceability”.

Definitions and terms similar to those in IEC 60060-2:2010 are given in Annex D

3.6.9: type B evaluation

Terms, such as “sensitivity coefficient”, “model function” and “effective degrees of freedom” are described in Annex A and Annex B Calculation procedures

Annex H.1 to H.5

5.2 to 5.11, estimation of uncertainty components from contributions such as calibration, dynamic performance, temperature effects, linearity and proximity effects.

Detailed procedure described in D.2 to D.6 of Annex D (informative). No procedure is given in the main text

Requirements for uncertainty components

Limits of individual components are specified, such as 1% for voltage linearity, proximity effect and effect of interference, with overall uncertainty also being specified.

No limits specified for individual components as long as the expanded uncertainty is within the specified value, e.g., 3% for test voltage and 10% for impulse time parameters. Also calculation with one dominant component is acceptable.

No limits specified for components as long as the expanded uncertainty is within the specified value, e.g., 3% for test voltage and 10% for impulse time parameters. Also, there should be at least 3 dominant components of comparable magnitude for achieving reasonable effective degrees of freedom for the expanded uncertainty.

Number of repeated measurements

No. of Measurements is specified for performance test

Not specified, as long as the expanded uncertainty is within the specified value, with the type A uncertainty achieved

Not specified, as long as the expanded uncertainty is within the specified value, with the type A uncertainty achieved

Principles

Limited information

Annex A: An informative annex explaining principles and concepts of ISO GUM

Terms are described Annex D

coverage factor determination

Assume to be 2

calculating coverage factor from effective degrees of freedom, assuming high degrees of freedom for individual type B components

Effective degrees of freedom for calculating coverage factor, estimating realistic degrees of freedom for individual type B components

Calculation examples

Annex H: two examples in H.6

Annex B: three detailed examples given.

Two very detailed examples in Annex D, demonstrating the uncertainty calculation steps and how intermediate values, such as standard uncertainty, degrees of freedom (type B component in particular), combined standard uncertainty, effective degrees of freedom and coverage factor, are determined.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards 4.6.3

Relationship between uncertainty and tolerance

Tolerance and uncertainty are different concepts that are specified in the standards for high-voltage and high-current testing. Uncertainty is a statistical quantity used to estimate the accuracy of measurements. The uncertainty depends in the first instance on the metrological quality of the measuring system, and often a range of other factors that influence the measurement. According to IEC 60060-2:2010 the uncertainty of a high-voltage measurement shall be expressed as an expanded uncertainty with a coverage probability of approximately 95 %, corresponding to a coverage factor k=2 under the assumption of a normal distribution. According to IEC 60060-1:2010, the tolerance constitutes the permitted difference between the measured value and the specified value. According to IEC 60060-1:2010, the uncertainty interval and the tolerance interval are considered separately. This is stated in terms 3.3.1 and 3.3.2 and associated notes. Typical tolerance is ±3 %, and the required expanded measurement uncertainty of test voltage for most high-voltage tests is 3 %. As long as both the tolerance and the uncertainty requirements are met, the test is considered valid. For example, an applied test voltage of 97.1 kV is considered just as valid as an applied voltage of 100.0 kV, for a test with a specified test voltage of 100 kV, as long as the expanded measurement uncertainty of the applied test voltage is within 3 % of the applied test voltage. The tolerance of the test voltage is necessary because it is not possible to set the test voltage exactly at the specified value due to technical and operational limitations. For example, the peak impulse voltage at a given charging voltage may vary to some extend depending on the individual impulse generators. During a 60 second AC voltage withstand test, the output voltage may vary around the initially set voltage due to fluctuation of the input voltage to the high-voltage test transformer. Every effort should be made to set the test voltage to the specified level as closely as possible. An operator should not purposely set the test voltage at the lower limit of the tolerance band. In the case of the tolerance of impulse voltage time parameters, it is considered good practice to use time parameter values that provide efficient testing with available wave shaping components, as long as the time parameters are within the specified tolerance limits. For example, the tolerance limits for the lightning impulse front time are 0.84 µs to 1.56 µs. The true value of a time parameter may fall outside the tolerance limits, for example, the true front time value for a measured front time of 0.85 µs with an uncertainty of 5 % may be less than 0.84 µs, however, the front time of the applied impulse is still considered meeting the requirements of IEC 60060-1:2010 and IEC 60060-2:2010.

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5 DIFFERENCES BETWEEN THE LATEST REVISIONS OF IEC60060 SERIES AND IEEE STD 4 5.1 General structure of IEEE Std 4-2013 IEEE Std 4 is the single standard equivalent of the combined IEC 60060 parts 1 and 2 and is used in North America and several other countries. This format and the practice of including tutorial sections have been kept in the 2013 edition. The current revisions of all these three standards were on-going in parallel for some years. While every attempt has been taken to make the standards compatible with IEC counterparts, there are some minor differences between them. The tutorial information included in this standard is to serve as a knowledge bridge between the novice engineer users and the experienced technical experts.

5.2 Summary of differences The following is a brief summary of the fundamental differences between the current versions of IEC and IEEE versions of High Voltage Testing Standards. There are many other differences such as the inclusion of treatment of methods for dielectric loss measurements in IEEE that do not appear in IEC. This list refers to items that overlap. 5.2.1

Estimation of uncertainty

Although IEEE uses the new term “uncertainty” there are still minor differences in implementation. For example, IEEE Std 4-2013 in Clause 5.7.6.8 does not make a distinction between uncertainty of the calibration and uncertainty in the situation of use. This relates to expanded uncertainty correlation and could result in some increased uncertainties in some cases by using the IEEE method of simplification but the differences should be small. The differences in uncertainty treatment are described in Section 4.6.2 of this document in more details. 5.2.2

Definition of AC peak voltage

Clause 6.1 of IEEE Std 4-2013 differs from IEC 60060-1:2010 definition in that it calls the peak the highest value. This is a small issue with IEC as it does not consider polarity of the highest value that could affect measurements in rare cases. 5.2.3

Linearity test

In Clauses 6.5.3.1 and 6.5.3.2 of IEEE Std 4-2013, there are slight differences from IEC 60060-2:2010 method, where linearity is covered in the calibration range. Extension of linearity from the upper limit of the calibration range is then done as a separate test. 5.2.4

Highest frequency in impulse voltage test circuit

IEEE Std 4-2013 still uses in Clause 8.3.3 the geometrical method for finding the highest frequency to be recorded in an impulse circuit. This has been modified in IEC but is retained in IEEE. In practice this has little effect since most users arbitrarily establish the lower level of oscillation frequency measurement – especially for circuits of large physical dimensions. This area requires more research for UHV measurement systems to balance the realistic performance of a large voltage divider and the need to detect small defects in insulation. 5.2.5

Physical characteristics of reference impulse voltage dividers

IEEE still contains the actual physical characteristics of a reference impulse voltage divider. Such a divider constructed to the requirements specified in the standard constitutes a reference divider without having actual high-voltage calibration of it performed at a National Measurement Institute.

5.3 Conclusion As can be seen above, the technical differences are relatively minor. The main changes for all of these standards relate to uncertainty measurements and new methods for calculating the values from impulse wave shapes. On these fundamental issues IEEE and IEC are in agreement and this will lead to easy harmonization of global testing.

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6 IMPROVEMENTS AND POSSIBLE ADDITIONS IN FUTURE REVISIONS 6.1 DC voltage As energy transmission using high voltage DC increases, testing for this application will become ever more important, leading to need of enhancement of test methods and measurements. Voltage levels have also been increasing to cope with the needs to support rapid urbanization in the world. DC testing, be it applied under clean and dry conditions but at high voltage, or under wet or polluted conditions at any voltage, will suffer from rapid dynamic load changes due to discharge phenomena. The limited available power in generators for DC will lead to transient voltage dips (voltage drops), which may lead to erroneous test results. The problem is two-fold, first to determine what limits on magnitude and duration of voltage dips that can be accepted, and secondly to define parameters for DC measuring systems that will ensure that the dips are properly assessed. The need for recommended ratings for DC test equipment will become more accentuated as the test voltages increase, especially for wet tests. The needs for pollution testing are more severe, but this is not in the domain of the IEC 60060 series. During the last revision of IEC 60060 Parts 1 and 2 and IEEE Std 4, it was found that there was some controversy about the definitions of test source suitability and questions about whether these current definitions could assure users of the ability to achieve the goal of performing repeatable tests in different test facilities. The partial discharge processes under DC are however less well understood and require further research. Work is currently under way in IEC TC 42 to codify present knowledge in this field in an amendment to IEC 60270, High-voltage test techniques - Partial discharge measurements. Further development is however expected in this field. Methods to investigate linearity of DC measuring systems are poorly described in the standards. Lately, however, facilities for calibration up to 1000 kV at high accuracy have been developed in Europe and in Australia. More work is needed on the extension of linearity measurements for extremely high DC voltage Questions for future work:

Is it necessary to define the shape and amplitude of current pulses we expect to see during DC dielectric and wet tests? Should requirements on DC measuring systems also have an option covering the needs of recording transient voltage drops? Is there a need to record of transient events during DC testing? Can we define the bandwidth of the measurement system for dielectric tests and wet tests and for tests to assure that voltage drop is controlled even in the face of massive corona discharges? Can we define the value of DC voltage drop that will result in correct testing? Why do we have different values of allowable AC and DC voltage Drop (20 % and 10 %)? Is there a technical reason? Is better characterization of measuring systems for voltage drop and ripple necessary? How to measure partial discharges under DC stress and what impact it has on the source requirements? Shall we recommend equipment ratings for UHV tests where pollution or wet testing is performed? Are the standards sufficient for normal dielectric tests in UHV range?

6.2 AC voltage Also for testing with AC voltages, concerns are found relating to voltage stability under clean and dry conditions but with high voltage, or under wet or polluted conditions at any voltage, due to rapid dynamic load changes caused by discharge phenomena. The limited available power in generating circuits will lead to transient voltage dips (or voltage drops), which may lead to erroneous test results. The problem is two-fold, first to determine what limits on magnitude and duration of voltage dips can be accepted, and secondly to define parameters for measuring systems for DC that will ensure that the dips are properly assessed. For calibration of UHV systems, presence of corona in the test circuit may change the scale factor of the Approved Measuring System, but can also affect the methods for proving linearity. E.g. a capacitive sensor

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relies on the size of the high voltage electrodes being precise. Corona activity will release ions into the air, which may distort the electrical field. Test and measurement standards on AC VLF (Very Low Frequency) may also need to be improved and expanded. VLF test systems at 1 Hz or lower were first developed to test primarily long length of cables. Because of the low frequency used, the requirement for the power supply is significantly lower than that for normal AC test source. Therefore, the test equipment is much smaller and lighter. VLF technique has also been used by utilities as a useful onsite diagnostic tool for cable and generator dielectric dissipation factor, power factor, and PD tests. Although some aspects of VLF test requirements have already been specified in IEC 60060-3:2006 [36] and a number of IEEE Standards [40,41], further standardization work is likely to be required to expanded test requirements to cover a wider range of equipment and test conditions as more research results become available. CIGRE working group D1.48 (Properties of Insulating Materials under VLF Voltages) is actively working in this area. Questions for future work:

Is it necessary to define the shape and amplitude of current pulses we expect to see during AC dielectric and wet tests? Is it necessary to define the bandwidth of the measurement system for dielectric tests and wet tests to assure that voltage drop is measured ? Why do we have different values of allowable AC and DC voltage Drop (20 % and 10 %)? Is there a technical reason? Is there a need to revise on-site test techniques as set out in IEC 60060-3? Is there a need to develop requirements on testing at Very Low Frequency today given in IEC 60060-3? How to calibrate UHV level Reference Measuring Systems? How to check linearity in the UHV range? What are the requirements for AC source in the UHV range?

6.3 Lightning impulse voltage Future work in this area involves proving that the dielectric stress produced by various superimposed oscillation frequencies on normal wave shapes can be normalized for various apparatus in terms of insulation performance and that the key parameters can be consistently calculated. 6.3.1

General considerations

The advent of ultra-high voltage (UHV) transmission systems, i.e. over 800 kV, necessitates testing at very high lightning impulse voltages. Both test methods and measurement techniques will have to be further studied. Issues are also open on applicability of calibration methods for UHV lightning impulse measuring systems. This work is largely the responsibility of the relevant apparatus committees Annex D of IEC 60060-1:2010 describes the technical background of the Test Voltage Function (k-factor) for evaluation of impulses with superimposed oscillations on the crest. The majority of tests for establishing that the Test Voltage Function had a basis in the physics of breakdown of different insulation systems were performed at voltages around 100 kV in the first European Project [42]. The tests studied various real test objects with a variety of real impulses with superimposed oscillations that could be adjusted. Additional tests to obtain the test voltage function (or k-factor function) for higher voltages than those used in [42] were carried out in recent research projects such as those conducted at TEPCO and LCOE [43, 44, 45, 46, 47, 48]. The aim of these research programs is to validate and improve the applicability of the test voltage function for future revision of the standards. Details of the test program and presentation of results is being prepared as Technical Brochure in CIGRE from Working Group D1.36. An overview is however presented below. Three different insulation systems, namely: SF6 gas insulation, oil insulation, and air insulated electrode gaps, have been studied using test voltages up to 1.8 MV. The test voltage function verification process is ongoing and improvements now relate to how the function can be fine-tuned for use with complex insulation systems for improved validity. Slight differences in the formulae can more closely track test data depending on the insulation type and the composition of multiple dielectric materials. The focus of the standards to date has been to have a single function definition for all test objects while it is clear that different frequencies of overshoot and oscillation can have different effects on the insulation system being used. This is due to the wide variation in the physical characteristics of those insulation systems. The sections below confirm that for

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

simple geometries and for single dielectric media, the currently published test voltage function (k-Factor) gives correct results. However, it is hoped that through further research in this area, a new universal test voltage function can be determined to suit testing of most insulation systems. 6.3.2

Further investigation of the test voltage function for SF 6

To investigate if the test voltage function in IEC 60060-1:2010 is applicable to a SF6 gas insulated dielectric medium at test voltages above 100 kV, a set of lightning impulse tests using quasi-homogenous field ( 0,45) were performed with negative polarity and test voltages, Ut, up to -1.0 MV. The parameters of these experiments are test voltage, Ut, oscillation frequency, f, of superimposed overshoot and relative overshoot amplitude, ’, according to [15] and the field uniformity according to Schwaiger coefficient, = Emean/Emax. The results were reported in [43, 44, 47] to compare with the k-factor values obtained earlier [42]. Figure 11 shows the compatibility between the results for SF6 dielectric medium obtained by the E.P. Project [42] and by TEPCO-Japan [43, 44, 47] up to -1.0 MV (symbols in color). It was concluded from the results that the experimental test voltage function values obtained for SF6 quasihomogenous field up to -1000 kV agreed with the earlier results of the European Project [42]. Overall, the results of SF6 obtained so far indicate that the test voltage function of IEC 60060-1:2010 is sufficiently accurate for the purposes of high-voltage testing, i.e.:

k( SF6 )

1 1 2.2 f 2

Figure 11: Experimental or test voltage function curve determined in the European Project for SF 6 gas insulation dielectric medium with quasi-homogenous field and further experimental test voltage function (k-factor) values at voltages up to -1000 kV. 6.3.3

Further investigation of the test voltage function for oil

Insulation oil samples with quasi-homogenous field configurations were also tested [43, 44] with lightning impulse, at voltages up to –1000 kV. Impulse voltages of –150 kV and –250 kV were also applied to the practical apparatus models . The parameters of these experiments are test voltage, Ut, oscillation frequency, f, of superimposed overshoot and relative overshoot amplitude, ’, according to IEC 60060-1:2010 and field uniformity according to Schwaiger coefficient, = Emean/Emax. For the oil samples, = 1 was used, whereas for the practical apparatus models, no unique could be established. The results (Figure 12) support the use of the test voltage function defined in IEC 60060-1: 2010. 47

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 12: Experimental test voltage curve (k-factor) determined in the European Project for oil dielectric medium and complementary experimental values for voltages up to -1.0 MV. 6.3.4

Further investigation of the test voltage function for air gaps

Lightning impulse tests at very high voltages are usually performed to prove performance of internal insulation, and therefore the need to evaluate for the characteristics of large air gaps is limited. In those cases switching impulse is usually the decisive test stress. However, for the cases where the external air insulation needs to be verified for lighting impulse performance, the test voltage function is important. Furthermore, at UHV voltage levels, physical size of the test circuits will limit the frequency of overshoot, with the result that the filtering effect of the test voltage function is small. A set of lightning impulse tests were performed with positive polarity for air gaps up to 1.8 MV. Tests with negative polarity were not considered because of the disruptive voltage level is higher than the level for positive polarity. The parameters of these experiments are test voltage, Ut, oscillation frequency, f, of superimposed overshoot and relative overshoot amplitude, ’, according to IEC 60060-1:2010 and field uniformity according to Schwaiger coefficient, , or according to air gap K factor of IEC 60071-2. Air gaps with homogenous field Figure 13 shows the comparison between the results for different air gaps; all of them with a quasihomogenous field obtained by the E.P. Project [42] and recently by LCOE [46] at voltages up to +0.8 MV. The overshoot frequency was limited to 0.25 MHz. Figure 13 shows the compatibility between the test voltage function of the present IEC 60060-1 standard and all the results for air gaps with homogenous field. The experimental test voltage function values are in agreement with the test voltage function of IEC 60060-1:2010.

k( AIR homogeneous

field )

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1 1 2.2 f 2

Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 13: Experimental test voltage function (k factor) for air dielectric medium in a homogenous field determined in the European Project (+98,5 kV) and the complementary experimental k-factor values obtained for voltages up to +800 kV. Air gaps with non-homogenous field

Experimental test voltage function (k-factor) values obtained for air gap spacings with non-homogenous field determined by the E.P. Project [42] and results recently obtained by TEPCO [43, 48] and LCOE [44] are shown in Figure 14. The test voltage functions (k-factor) for air gaps with a non-homogenous field depend on the air gap spacing, d, and on the non-homogeneity field given by the air-gap K factor parameter defined in IEC 60071-2. It is obvious from the test results that the test voltage function for air gaps can conceivably be quite different depending on the gap geometry. The applicability of one single test voltage curve may be questionable. Further research is on-going in CIGRE Working Group D1.36 and will be reported in a forthcoming Technical Brochure.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards k-factor; K=1 EP 0.15m

1

0.5m 0.8

k (pu)

1.0m 2.0m 2.5m

0.6

0.4

0.2

0 3 10

4

10

5

6

10

10

7

10

8

10

f (Hz)

Figure 14: Family of the test voltage function (k-factor) curves for rod plate configuration (K=1) for different air gap distances. 6.3.5

Test voltage function for multiple insulation materials

When multiple insulation media are involved in the same testing object the most restrictive test voltage function should be chosen in order to ensure that the equipment passes the lightning impulse test (e.g. the oil test voltage function should be used when both oil and SF6 insulations are involved in the test). 6.3.6

Generation and measurement of UHV lightning impulse

The impulse generator and associated circuits grow in size when the test voltage increases to UHV levels. From this follows that inductances in the circuit will increase. It is also observed that the capacitance of some test objects like cables, increase with the voltage level. Together these two physical realities mean that a standard waveform with a front time of 1.2 µs ± 30 % becomes difficult or even impossible to achieve. This matter is currently under study in CIGRE Working Group D1.36 and will be presented in a forthcoming Technical Brochure. Preliminary findings do however indicate that it will be necessary to relax the tolerance requirement on front. Also, the large circuit dimensions also limit the performance of UHV voltage dividers. A fundamental limit of the bandwidth of large devices may preclude their ability to measure standard lighting impulses. It is suspected, but not proven, that UHV dividers may be difficult to use in measurement of impulses approaching the lower tolerance limit for front time. Further research in this area is needed.

6.4 Improvement of atmospheric correction factor calculations 6.4.1

General remarks

Atmospheric corrections are performed for several purposes in high-voltage engineering.

To correct the disruptive discharge voltage of insulation under a particular test atmospheric condition to a voltage under the standard atmospheric conditions. To calculate test voltage levels to be utilized in testing when the testing is performed at an altitude significantly different from the site where the equipment will be installed. To provide altitude correction for correct choice of external insulation distances.

These calculations have evolved under different Technical Committees in Standardization and may lead to non-compatible corrections. As will be shown in the following sections, these differences tend to escalate

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards with higher voltage and/or higher altitudes. The inevitable conclusion is that these methods have to be reexamined and coordinated to provide reliable and compatible results 6.4.2

Differences between IEC standards

Atmospheric corrections are also performed for purposes such as insulation coordination defined in IEC 60071-2 and for altitude correction. The purpose is the determination of the insulation distance of equipment in relation to the altitude of its installation. An example of equipment altitude correction is the use of correction factor k in IEC 61968 1 (Instrument transformers, General requirements), where the required insulation distance is corrected for the service altitude using factor k. In these cases, only the correction due to altitude (hence the atmospheric pressure) is considered, while temperature and humidity are not considered in the correction factor. For example, the altitude correction factor in the insulation coordination standard IEC 60071-2- is defined as: where H is the altitude above sea level (in meters) and the value of m is as follows: m = 1.0 for lightning impulse voltages m = values as defined in Figure 15 (re-produced from Figure 9 of IEC 60071-2) for switching impulse withstand voltages m = 1.0 for short duration power-frequency withstand voltages of air-gaps and clean insulators

Figure 15: A reproduction of Figure 9 of IEC 60071-2. In IEC 61869-1:2007, it is specified that for installation at an altitude higher than 1000 m, the arcing distance under the standardized reference atmospheric conditions shall be determined by multiplying the withstand voltages required at the service location by a factor k in accordance with its Figure 1, reproduced in Figure 16 below.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 16: A reproduction of Figure 2 of IEC 61869-1:2007. In earlier international standards preceding IEC 61869-1:2007, such as IEC 186, the correction was performed by adding 10 % arcing distance per 1000 m increase in altitude for altitudes greater than 1000 m. Some IEEE standards specify the increase in the arcing distance by applying a dielectric strength correction factor of -0.1 per 1000 m for altitudes greater than 1000 m. In the first Edition of IEC 60060 1 published in 1973, the following formula for calculating the withstand voltages at higher altitudes was given

with b0 = 101.3 kPa, being the average air pressure at the sea level. The corresponding withstand voltage at the higher altitudes was given by

with Uwo the withstand voltage at pressure b0. The coefficient m is defined to be 1 for direct and lightning impulse voltages of any gap configuration and polarity, as well as for homogenous gaps with any kind of voltage. For larger gap distances, with approximate rod-rod and rod-plane gaps under AC voltage and switching impulse voltage, the coefficient m was defined as a function of the gap length shown in Figure 17.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 17: The coefficient m in the air density correction factor versus gap spacing d (IEC 60060-1:1973). Based on these definitions, the inverse of the atmospheric correction factor curves (1/kt) derived from IEC 60060-1:2010 presented above were compared with the altitude correction curves derived from these standards and the corrections derived using the correction method in the first edition of IEC 60060-1. The factor m for switching impulse voltages were taken for the different gaps, voltages and standards as following: m = 0.6 for 1300 kV, m = 0.64 for 1175 kV for 3800 mm gap distance and m = 0.78 for 800 kV, m = 0.83 for 700 kV for 2000 mm gap distance (IEC 60071-2) m = 0.88 for 3800 mm gap distance and m = 0.64 for 2000 mm gap distance (IEC 60060-1:1973) The curves calculated for IEC 60044 and IEEE were adjusted to 1000 m using the correction factor of IEC 60071-2 at 1000 m for the respective voltage. Figure 18 to Figure 20 show the comparison of the four different standards and the calculations according to the first edition of IEC 60060 1 for AC, LI and SI voltages, for withstand and flashover voltages of the 2000 mm and 3800 mm gaps. The curves for DC calculated using IEC 60060-1:2010 were not compared to IEC 60071-2, IEC 61869-1:2007 and the IEEE apparatus standards, since these standards do not define a method for DC voltages. Only comparisons to the values achieved with the iterative procedure in IEC 60060-1:2010 are shown, differences to the standard procedure for withstand voltages are shown in Table 8. Figure 18 and Table 8 show that for AC test voltages IEC 60060-1: 2010 will always give lower correction factors. The correction factors derived using the iterative procedure in IEC 60060-1:2010 result in larger differences to factors calculated using IEC 60071-2 and IEC 61869-1:2007. Above 1000 m the method in IEC 61869-1:2007 gives lower results than those derived from IEC 60071-2 due to the deduction of the first 1000 m from the actual altitude. The IEEE apparatus standards, which would correspond to the older procedure in IEC standards, give typically lower correction factors than IEC 60071 2 and IEC 61869-1:2007, but would still give larger correction factors than IEC 60060-1:2010. The correction method defined in IEC 60060-1:1973 gives results, which lay between the results derived with the methods of IEC 61869-1:2007 and some IEEE apparatus standards for the shorter gap but is closer to the results of IEC 60060-1:2010 for withstand levels at the longer gap. For longer gaps the differences of IEC 60060-1:2010 to the other standards are in general larger than for the shorter gap.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 18: Comparison of Atmospheric correction factors 1/K t for AC voltage, calculated according to different standards.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards Figure 19 gives the results for the calculated correction factors for LI voltages and shows good agreement between IEC 60060-1:2010 and IEC 60071-2, except for withstand levels for the 3800 mm gap. The correction factors derived using IEC 61869-1:2007 give the same results as for AC voltages. For apparatus design either the AC withstand voltage for shorter gaps, lower system voltages with Um < 300 kV, or the SI withstand voltage for larger gaps, higher system voltages with Um ≥ 300 kV, are relevant for the design of the external insulation distance. For testing of apparatus no correction for atmospheric conditions for lightning impulse testing is permitted, therefore these differences can be neglected. The correction factors used in IEEE standards typically give lower correction factors.

Figure 19: Comparison of Atmospheric correction factors 1/K t for LI voltage, calculated according to different standards.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards The calculated correction factors for SI voltages shown in Figure 20, have the results using the iterative procedure of IEC 60060-1:2010 as the boundaries between which the results of IEC 60071-2 and IEC 61869-1:2007 lie, the correction factors for flashover values being the upper limit and the correction factors for withstand levels giving the lower limit. The correction factors derived with the IEEE method and the corresponding older approach in IEC give in general higher correction factors than derived from the newer IEC standards.

Figure 20: Comparison of Atmospheric correction factors 1/K t for SI voltage, calculated according to different standards.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

For higher voltages, IEC 61869-1:2007 gives larger correction factors than IEC 60071-2:1996 since the fixed exponent m of 0.75 used in IEC 61869-1:2007 is larger than the one calculated for the two voltages according to IEC 60071-2, m = 0.6 for 1300 kV, m = 0.64 for 1175 kV. For lower voltages the exponent m is smaller for IEC 61869-1:2007, thus the calculated correction factors are smaller than for IEC 60071-2:1996. Table 8: largest calculated % differences in correction factors for AC and SI withstand voltages between the standards considered

AC

2000 mm gap

3800 mm gap

IEC 60060-1 to IEC 60071-2

-5.2 %

-10.5 %

IEC 60060-1(iterative) to IEC 60071-2

-14.9 %

-20.5 %

IEC 60060-1 to IEC 61869-1: 2007

-4.7 %

-9.2 %

IEC 60060-1(iterative) to IEC 61869-1: 2007

-11.7 %

-17.5 %

IEC 60060-1 to IEC 60071-2

+8.6 %

+19.3 %

IEC 60060-1(iterative) to IEC 60071-2

-4.4 %

-2.6 %

IEC 60060-1 to IEC 61869-1: 2007

+14.6 %

+16.6 %

IEC 60060-1(iterative) to IEC 61869-1: 2007

-2.6 %

-3.4 %

SI

Table 8 shows as a summary the largest calculated % difference between correction factors derived with IEC 60060-1:2010, IEC 60071-2:1996 and IEC 61869-1:2007 for AC and Switching Impulse voltage. For AC voltages the difference varies between -4.7 % comparing the results for the standard procedure in IEC 60060-1:2010 and IEC 61869-1:2007 for the 2000 mm gap and -20.5 % comparing results for the iterative procedure in IEC 60060-1:2010 and IEC 60071-2:1996 for the 3800 mm gap. For Switching Impulse voltages this difference varies between +19.3 % comparing the results for the standard procedure in IEC 60060-1:2010 and IEC 60071-2:1996 for the 3800 mm gap and -2.6 % comparing the results for the iterative procedure in IEC 60060-1:2010 and IEC 60071-2:1996 for the same gap. The iterative procedure in IEC 60060-1:2010 gives in principle smaller correction factors than the standard procedure, which get closer to the correction factors of both IEC 60071-2:1996 and IWC 61869-1:2007 for Switching Impulse voltages but the difference to these standards, gets larger for AC voltages when using the iterative procedure. 6.4.3

Future work on atmospheric correction factors

The differences and inconsistencies between standards shown above lead to the question of the correct procedure. Even though IEC 60071-2:1996 is related to insulation coordination and IEC 60060-1:2010 to testing of equipment, and therefore fulfills different purposes, the physical background for the correction factors is the same, the reduction in insulation strength of air insulation due to higher altitude respectively lower pressure. In order to further analyze these differences and determine atmospheric correction factors (temperature, pressure, absolute humidity) a JWG of IEC TC 28, TC 42, TC 36 and TC 115, JWG 22 “Atmospheric and altitude correction” and a new CIGRE WG D1.50 “Atmospheric and altitude correction factors for air gaps and clean insulators” were established. The results of these two groups may have an influence on the procedure to calculate correction factors in IEC 60060-1:2010. There is also a need to extend the humidity range of the atmospheric correction factors, Experimental results of a recent research work can be found in [49].

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

6.5 Review of voltage drop, AC and DC for future development The current IEEE and IEC standards for HV Testing include some details about the allowable voltage drop due to the interaction of the test object, the test circuit and the test source. These conditions are for normal dry or wet test situations but do not cover the specific demands of artificial pollution tests, which are covered by IEC 60507.

Summary of IEC Requirements for AC and DC testing

Test voltages for tests longer than 1 minute should be within ±3 % AC Transient voltage drops can be up to 20 % DC Transient voltage drops can be up to 10 % Bandwidth guidelines are given for voltage measurement systems but actual frequency values are not. Questions for future Revision of High Voltage Testing Standards with respect to dynamic load

Can we define the shape and amplitude of current pulses we expect to see during AC and DC dielectric and Wet Tests? Is this information important? Can we measure the current pulses accurately? Does it matter if we can do this as long as we require adequate measurement systems to record the transient voltage drop? Can we define the bandwidth of the measurement system for dielectric tests and wet tests and for tests where users allow massive corona discharges in their test circuits to assure that voltage drop is controlled? Can we define the value of AC and DC voltage drop that will result in correct testing? Why do we have different values of allowable AC and DC voltage Drop (20 % and 10 %)? Is there a technical reason?

Questions for Future Revision of High-voltage Testing Standards

Shall we specify a maximum voltage drop during AC and DC dry and wet tests of less than 10 % for transients of more than 1 second or 2 seconds for AC and more than 1 second or 2 seconds for DC? Or should we choose other time durations that relates to errors in testing? Shall we specify the upper frequency limit of the voltage measurement system for AC tests where streamers or wet conditions are anticipated to be greater than 7 times the fundamental frequency? Or should we choose a measurement bandwidth based on the source frequency? Shall we specify the time constant of voltage measurement systems for DC tests where streamers or wet conditions are anticipated to be less than 0.25 seconds? Or should we choose some other time period that relates to errors in testing. Shall we specify the voltage record length of tests with streamers or wet conditions to be equal to the withstand test time? Or should we sample intervals and compare sequential intervals? In the end, the relevant apparatus committees have to decide how much deviation from the test voltage can be accepted before the performance of the device being tested is in question. For dry testing indoors of normal proof or withstand tests, if the percentage of voltage cycles that have more than 10 % lower peak value are less than a few percent of the total test cycles, we can demonstrate compliance with a standard. For DC this could be a percentage of the time-on vs. the time where voltage drops are more than 10 %. Since there is no peak to evaluate, the dip will have to be more than a certain time, such as the charging cycle of the DC generator. For wet testing or pollution testing the same method could be used. With digital recorders it is possible to check each voltage cycle or in the case of DC each recharge period time of the DC generator and compare to the duration of the test for a percentage of time the peak voltage is down by 10 % or less.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

6.6 Waveforms of lightning impulse voltage and lightning impulse current 6.6.1

Introduction

The present standard waveforms of the lightning voltage and current impulses were derived based on lightning surge waveforms measured in the field many decades ago. Present power systems and equipment are characterized by various new factors, compared to those from the past. These factors include the wide use of gas-insulated switchgear (GIS) in substations, installation of overhead ground wires, use of highperformance surge arresters with improved characteristics, increase in ground electrostatic capacitance associated with a higher voltage and larger equipment capacity, and variation in the distance between substations. Because of the widespread changes it is necessary to re-assess the lightning impulse waveforms that are relevant to the modern electric power network. Observed results of studying lightning phenomena may differ depending on the country and regional situation. Papers [50-54] reported the recent observation results of lightning surges in the actual fields at some specific facilities. 6.6.2

Observation of atmospheric lightning strike voltage waveforms

The lightning strike voltages were observed at 10 substations, namely eight GIS substations and two airinsulated (AIS) substations of 500 kV and UHV designed (500 kV operation) transmission lines [50,51,52,53] for the purpose of setting the lightning current waveform and crest value in the lightning surge calculation reasonably and more accurately when calculating the lightning failure rate of transmission lines. On the basis of the observed data [52], Figure 21 presents the characteristics of the direct lightning strike waveforms observed at Switching Substations, denoted N- and M-, where UHV designed transmission lines are connected and at the Switching Substation denoted S-, where 500 kV transmission lines are connected. The parameters analyzed are the crest value vs. front time and the crest value vs. time to half-value. Here, the crest value indicates that of the lightning surge component, disregarding the power frequency component. 800 700

N-Switching Substation M-Switching Substation S-Switching Substation

700 600 Voltage (kV)

600 Voltage (kV)

800

N-Switching Substation M-Switching Substation S-Switching Substation

500 400 300

500 400 300

200

200

100

100

0

0

1

10

100

1000

1

(a) Crest value - front time relationship

10

100

1000

Time (μs)

Time (μs)

(b) Crest value - time to half-value relationship

Figure 21: Characteristics of the direct lightning surge waveform parameters. The 50 % value of the cumulative frequency of the crest values is 240 kV. For the front time and the time to half-value, the 50 % values of the cumulative frequency are 5.9 μs and 36 μs, respectively. Even within the present measurement, some waveforms are characterized depending on the measurement locations due to the differences in insulation voltage classes and system conditions. 6.6.3

Observation of atmospheric lightning strike current waveforms

The lightning strike current waveforms were also observed at the top of 60 towers of 500 kV and UHV designed (500 kV operation) transmission lines between 1994 and 2004 and 120 sets of data were obtained, including three cases exceeding 100 kA [54]. Figure 22 shows the cumulative occurrence frequency distribution of the front time and the time to half-value. The 50 % value of the front time was 4.8 μs and the

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

50 % value of the time to half-value was 37 μs. Large numbers of lightning current waveforms were previously observed and summarized in CIGRE paper [55].Even though that data was observed over a wide region, the observation results in Figure 22 showed good agreement. 99.9

Cumulative Probability(%) Probability (%) Cumulative

Cumulative Probability(%) Probability (%) Cumulative

99.9 99 95 90 80 70 60 50 40 30 20

99 95 90 80 70 60 50 40 30 20

10 5

10 5

1

1

0.1

1

0.1

10

1

10

Front duration(μs) Front time (μs)

100

1000

Stroke duration(μs) Time to half-value (μs)

(a) Front time

(b) Time to half-value

Figure 22: Cumulative frequency distribution of front time and time to half value of lightning current waveforms [54]. 6.6.4

Conclusion

These measurement results indicate that future standards for high-voltage and high-current testing may need to include the waveforms with these longer front times that have been observed in these modern power networks. This could be useful especially for UHV systems were it is difficult to generate short duration waveforms due to the large dimensions of the test equipment, test loop and test object.

6.7 Improvement of measurement systems 6.7.1

Calibration of UHV impulse measurement systems

More and more power apparatus at the ultra-high-voltage (UHV) level are being tested due to rapid expansion of UHV transmission network. There have been reported difficulties in establishing and documenting voltage linearity that is a required part of the calibration of the high-voltage dividers for impulse voltage testing [24]. The traditional and most efficient method of assessing the voltage linearity of impulse voltage dividers has been comparing the peak impulse voltage against the DC charging voltage of an impulse generator. However, it has been found that this method often does not yield satisfactory results although the divider may in fact be linear. The non-linearity of the impulse generator vs. charging voltage can be due to several factors: non-linearity of the DC charging voltage measurement, non-linearity of the generator output due to internal corona discharges, non-linearity of large moving sphere gaps with varying arc resistance, external corona, or voltage coefficient of the wave shaping resistors or capacitors. Figure 23 shows an example of non-linearity measured with this method. In this case, non-linearity exceeded the 3 % expanded uncertainty limit of the complete measurement system specified IEC 60060-2: 2010 [24]. There is also evidence that the change of the time parameters with voltage could reach the 10 % uncertainty limit specified in the standard (see Figure 24). It should be noted that not all impulse circuits have these problems and impulse generators with good linearity can be produced. New methods, such as field probes, need to be investigated. Field probes have been experimented with in the past and also have disadvantages and advantages. The main problem is sensitivity to discharge currents due to corona in the high-voltage circuit.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards The voltage linearity test of UHV impulse measurement systems is one of the topics that are considered by IEC TC42 WG 19, which was set up to accomplish the task of “Adaptation of TC 42 standards to UHV test requirements”.

Figure 23: Voltage linearity test of the 2800 kV divider with positive lightning impulse, with gradient of the line being the first ratio R 1.

Figure 24: Deviation of the time parameters from their values at the lowest test voltage, 2800 kV divider and impulse of positive polarity.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards 6.7.2

Uncertainty and risk assessment

IEC Publication 60060-2:2010 is based on GUM and defines the uncertainty as the expanded uncertainty corresponding to a 95 % level of confidence that the true value can be found within the range given by the measured value ± the expanded uncertainty. The conditions for the practical distribution to correspond to a normal distribution are given in IEC 60060-2, Clause 4.6 and when these conditions are fulfilled then the expanded uncertainty factor is twice the standard uncertainty. Satisfactory methods of determining contributions to the uncertainty and determining the standard uncertainty and the expanded uncertainty are given in IEC 60060-2, Clause 5. Risk assessment is outside the scope of IEC 60060-2 and users are referred to the relevant standards (e.g., ISO 31000:2009 Risk management – Principles and guidelines, and IEC 60050-903 (2013) International Electrotechnical Vocabulary – Part 903: Risk assessment). Users wishing to perform risk assessment should also take account of differences between measured values and the specified test value: these differences are required to be within the tolerances stated in IEC 60060-1. Note that the permitted tolerances should not be used but the actual differences between measured values and the specified test value. 6.7.3

Examples of risk assessment

When comparing a measurement result, with its associated uncertainty, with an acceptance criterion there are different possible evaluations, where the two most important can be denoted “positive proof” and the other “shared risk”. Positive proof poses very strict requirements on the measurement result, leading to very high confidence in the verdict, but it has the disadvantage that there is a risk that potentially good results are disqualified. Shared risk is so termed because the measurement provider and the equipment purchaser share the risk that the verdict may be wrong in the cases where the uncertainty overlaps the tolerance limit. 6.7.3.1 Important terms uncertainty (of measurement)

parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand. error

discrepancy between a measured value and the true, specified or theoretically correct value. tolerance

constitutes the permitted difference between the measured value and the specified value. Comment: Errors should be corrected for. In cases where this is not practical, a corresponding uncertainty contribution should be identified.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

6.7.3.2 Positive proof Let the measured value be V, its expanded uncertainty UV and the pass limits for the tolerance band be W ± a. Case 1: Non-compliance proved - the measured value and both uncertainty limits lie outside the tolerance band. W-a

W

W+a

V V-UV

V+UV

Case 2: Non-compliance not proved - the measured value and both uncertainty limits lie inside the tolerance band. W-a

W

W+a

V V-UV

V+UV

Case 3: Compliance not proved - the measured value lies inside the tolerance band, but the uncertainty lies outside. W-a

W

W+a

V V+UV

V-UV

Case 4: Compliance proved - the measured value lies outside the tolerance band, and the uncertainty lies inside. W-a

W

W+a

V V-UV

V+UV

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

6.7.3.3 Shared risk The risk of an incorrect decision can however be shared on a more equal basis between buyer and seller by accepting also values where uncertainty crosses the tolerance limit. With the same designations as above we have: Case 5: Non-compliance proved - the measured value and both uncertainty limits lie outside the tolerance band. W-a

W

W+a

V V-UV

V+UV

Case 6: Non-compliance proved on basis of shared risk - the measured value lies outside the tolerance band, but one uncertainty limit lies inside. W-a

W

W+a

V V-UV

V+UV

Case 7: Compliance proved on basis of shared risk - the measured value lies inside the tolerance band, but one uncertainty limit lies outside. W-a

W

W+a

V V+UV

V-UV

Case 8: Compliance proved - the measured value and both uncertainty limits lie inside the tolerance band. W-a

W

W+a

V V-UV

V+UV

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Case 9: Verdict not possible - the measured value lies clearly inside the tolerance limit, but the uncertainty limits both fall outside. It is recommended to require the measurement uncertainty to be low, e.g. less than 1/3rd of the tolerance limit in order to reduce the number of the cases where verdict is not possible. W-a

W

W+a

V V-UV

6.7.4

V+UV

Measurement software

Currently, requirements for software used for impulse measurement are specified in IEC 61083-2. This is the only IEC standard on software in the area of high-voltage test and measurement. However, more and more digital instruments equipped with measurement software are being used in all types of high-voltage tests, including AC and DC tests. These digital measurement systems have many advantages over the traditional analogue systems in that they allow measurement of not only the DC or AC signal, but also other high frequency signals, such as fast voltage changes and transient voltage drops due to pre-discharges in a pollution test. Since software is an important part of a measuring system, systematic testing of software to ensure its reliability and performance is often necessary. In light of this situation, IEC technical committee 42 is currently drafting a new standard on the requirements for software used for DC and AC measurements. This new standard is likely to be assigned as IEC61083-4

7 CONCLUSION Significant improvements have been made in the recently revised IEC and IEEE standards for high-voltage and high-current tests and measurements. These improvements reflect the change of industry needs, such as the testing in the UHV range, as well as advancement of technologies, such as digital measurement techniques. A much higher degree of harmonization has also been achieved between the corresponding IEC and IEEE standards, which would no doubt bring benefits to the power industry. Revision of standards is a continuing process. A number of areas that future revision of the standards should consider have also been identified in this document.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

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[2]

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[3]

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[4]

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[6]

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[7]

Impulse Voltage Testing, WG Hawley, Chapman & Hall LTD, 37 Essex Street, W.C.2, 1959

[8]

VDE 0450, Leitsätze für die Prüfung mit Spannungsstößen, 1933.

[9]

VDE 0450/XI, Leitsätze für die Erzeugung und Verwendung von Stoßspannungen für Prüfzwecke, 1939.

[10]

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[11]

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[12]

Ellesworth, G., “Some characteristics of double-exponential pulse-shaping networks in high-voltage impulse generators”, Proceedings of IEE, Monograph No. 231, Apr.1957.

[13]

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Garnacho, F., et al: Evaluation of lightning impulse voltages based on experimental results. Electra No. 204, October 2002.

[15]

IEC 60060-1:2010, High-voltage test techniques - Part 1: General definitions and test requirements.

[16]

Hällström, J., et al: “Applicability of different implementations of k-factor filtering schemes for the revision of IEC 60060-1 and -2”, Proceedings of the XIVth International Symposium on High Voltage Engineering, Beijing, 2005, paper B-32, p. 92.

[17]

CEI 52 (1953), Règles pour la mesure de la tension d´essai aux fréquences industrielles dans les essais diélectriques au moyen d´éclateurs á sphères.

[18]

IEC 60052 (2002), Voltage measurement by means of standard air gaps.

[19]

SEV 173 (1944), Regeln für Spannungsprüfungen.

[20]

VDE 0430 (1941), Regeln für Spannungsmessungen mit der Kugelfunkenstrecke.

[21]

IEEE Standard 4 (1978-1995-2001), Techniques for High-Voltage Testing.

[22]

Transactions AIEE Vol. 71 (1952), Part III, p.455.

[23]

ELECTRA No 136, June 1991, p.91-95.

[24]

IEC 60060-2:2010: High-voltage test techniques - Part 2: Measuring systems.

[25]

Qi , Q.C. and Zaengl, W.S., “Investigations of Errors Related to the Measured Virtual Front Time TA of Lightning Impulses,” IEEE Trans. Power App. Syst., Vol. PAS-102, pp. 2379-2390, 1983.

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[26]

Zhang,Y. X., McKnight, R. H. and Hebner, R.E., “Interactions between Two Dividers Used in Simultaneous Comparison Measurements”, IEEE Trans. on Power Delivery, Vol. 4, pp. 1586-1594, 1989.

[27]

McKnight, R. H., Lagnese, J. E. and Zhang, Y. X., “Characterizing Transient Measurements by Use of the Step Response and the Convolution Integral” IEEE Trans. on Instrumentation and Measurement, Vol.39, No. 2, April, 1990.

[28]

McComb T. R., Hughes, R.C., Lightfoot, H.A., Schon, K., Schulte, R., McKnight, R.H. and Zhang, Y. Z., “International comparison of HV impulse measuring systems”, IEEE Trans. on power delivery, Vol. 4, Issue 2, April 1989, pp. 906-915.

[29]

McComb, T. R., Dunn, J. G., Burgess, D. M., Rungis, J., Li, Y., Van Der Zwan, L. and Hoffman, D., "Inter-Laboratory Comparison of Transfer Standards for Direct Voltage Calibration of Industrial Measuring Systems", Proc. of 10th International Symposium on High Voltage engineering, pp.1-5, Vol.4, August 1997, Montreal, Canada.

[30]

Wakimoto, T., Ishii, M., Li, Y. and Kim, I. S., “Comparisons between impulse voltage calibrators and digitisers”, Trans. on Electrical and Electronics Engineering, IEEJ Trans. 1, No.3, September 2006, pp 226 – 232.

[31]

Li, Y., Rungis, J., McComb, T. R. and Lucas, W., “International Comparison of a Pulse Calibrator used in High Voltage Impulse Calibration”, IEEE Trans. On Instrumentation and Measurement, Vol.50 No.2, April 2001, p430-435.

[32]

Hällström, J, et al, “Worldwide Comparison of Lightning Impulse Voltage Measuring Systems at the 400-kV Level”, IEEE Trans. on instrumentation and measurement, vol. 56, issue 2, April 2007, pp388391.

[33]

Li, Y., Rungis, J., Jing, T., Su, T. H., Chen, I. P., Lee, D. and Shimizu, K, “International Comparison of Resistive Dividers at 100 kV DC”, IEEE Trans. On Instrumentation and Measurement, Vol.50 No.2, April 2001, p436-439.

[34]

Li, Y., Hällström, J. and Lucas, W., “ Comparison of Two Impulse Calibrators with a High-Resolution Digitizer”, IEEE Trans. on Instrumentation and Measurement, Vol. 54, No.2, April 2005, p608 – 611.

[35]

IEC 62475: 2010, High-current test techniques –Definitions and requirements for test currents and measuring systems.

[36]

IEC 60060-3:2006, High-voltage test techniques - Part 3: Definitions and requirements for on-site tests.

[37]

Nilsson, A., Bergman, A. and Hällström, J., “An Improved Method for Switching-Impulse Evaluation”, Conference Digest of 2012 Conference on Precision electromagnetic Measurements, Washington DC, 1-6 July 2012, p20-21.

[38]

Bergman, A., Dedeoğlu, S., Elg, A-P., Houtzager, E., Hällström, J., Klüss, J., Lehtonen, T., Lucas, W., Merev, A., Meisner, J., Suomalainen , E. P., Svensson, S. and Weber, C., “New references for HVDC metering”, paper A3-106, CIGRÉ session 2014, Paris.

[39]

Li, Y., Ediriweera, M.K., Emms, F.S. and Lohrasby, A., “Development of Precision DC High-Voltage Dividers”, IEEE Trans. on Instrumentation and Measurement, Vol. 60, Issue 7, pp 2211- 2216, July 2011.

[40]

IEEE Std. 400.2 -2013, IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)(less than 1 Hz)

[41]

IEEE Std. 433-2009, Recommended Practice for Insulation Testing of AC Electric Machinery with High Voltage at Very Low Frequency

[42]

Digital Measurement of parameters used for lightning impulse tests for high voltage equipment. European Project. Contract nº PL-951210- SMT- CT 96-2132, 17 Sept 1999.

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[43]

Okabe, S., Tsuboi, T. and Ueta, G., "Comprehensive Evaluation of the K-factor Values in the Lightning Impulse Voltage Test Techniques for UHV-class Electric Power Equipment", IEEE Trans. Dielectrics and Electrical Insulation, Vol. 19, No.3, pp. 812-820, 2012.

[44]

Ueta, G., Tsuboi, T., Takami, J. and Okabe, S., "Study on the K-factor Function in the Lightning Impulse Test for UHV-class Electric Power Equipment", IEEE Trans. Dielectrics and Electrical Insulation, Vol. 19, No.4, pp. 1383-1391, 2012.

[45]

Garnacho, F., Khamlichi, A., Valladolid, A., Pascual, S. and Valcarcel, M., ”k-factor Test Voltage Function for Oscillating Lightning Impulses in Non-homogenous Air Gaps “ IEEE Trans. Dielectrics and Electrical Insulation, D.O.I. 10.1109/TPWRD.2014.2300137. 0885-8977 © 2014 IEEE.

[46]

Garnacho, F., Khamlichi, A., Valladolid, A., Pascual, S. and Guirado, R., ”Procedures to Determine kFactor Function for Air Gaps “ IEEE Trans. Dielectrics and Electrical Insulation, Vol. 28, No.2, pp. 686692, 2013. D.O.I. 10.1109/TPWRD.2012.2228012.

[47]

Tsuboi, T., Ueta, G., Okabe, S., Miyashita, M. and Inami, K., "Insulation Breakdown Characteristics of UHV-class Gas Insulated Switchgear for Lightning Impulse Withstand Voltage Test Waveform - Kfactor Value and Front Time Related Characteristics", IEEE, Trans. Dielectr. Electr. Insul., Vol.18, pp. 1734-1742, 2011.

[48]

Ueta, G., Tsuboi, T., Okabe, S., Shimizu, Y. and Hino, E., "K-factor Value and Front Time Related Characteristics of UHV-class Air Insulation for Positive Polarity Lightning Impulse Test", IEEE Trans. Dielectr. Electr. Insul., Vol. 19, pp. , 2012.

[49]

Diaz, R. and Segovia, A., “Humidity corrections and front time tolerance for lightning impulse voltages th in metric air gaps”, Proceedings of the XVIII International Symposium on High Voltage Engineering, Seoul, Korea, 2013, paper PC02.

[50]

Okabe, S., Kan, M., and Kouno, T., “Analysis of Surges Measured at 550 kV Substations”, IEEE, Trans. Power Delivery, Vol. 6, No.4, pp. 1462-1468, 1991.

[51]

Takami, J., and Okabe, S., “Characteristics of Direct Lightning Strokes to Phase Conductors of UHV Transmission Line”, IEEE, Trans. Power Delivery, Vol. 22, No.1, pp. 537-546, 2007.

[52]

Okabe, S., Kan, M., and Kouno, T., “Analysis of Surges Measured at 550 kV Substations”, IEEE, Trans. Power Delivery, Vol. 6, No.4, pp. 1462-1468, 1991.

[53]

Taniguchi, S., Tsuboi, T. and Okabe, S., “Observation Results of Lightning Shielding for Large-scale Transmission Lines”, IEEE, Trans. Dielectric. Electr. Insul., Vol. 16, No.2, pp. 552-559, 2009.

[54]

Takami, J. and Okabe, S.,“Observational Results of Lightning Current on Transmission Towers”, IEEE, Trans. Power Delivery, Vol. 22, No.1, pp. 547-556, 2007.

[55]

Anderson, R. B. and Eriksson, A. J., “Lightning Parameters for Engineering Application”, CIGRE Electra No.69, pp. 65-102, 1980.

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Past, Present and Future of IEC And IEEE High-Voltage and High Current Testing Standards

Working Group D1.35

August 2014

PAST, PRESENT AND FUTURE OF IEC AND IEEE HIGH-VOLTAGE AND HIGH CURRENT TESTING STANDARDS WG D1.35 Contributors Y. Li, Convener (AU), J. Hällström, Secretary (FI) W. Larzelere (US), A. Bergman (SE), J. Rickmann (US), W. Hauschild (DE), R. Diaz (AR), F. Garnacho (EP), T. McComb (CA), S. Okabe (JP), Y. X. Zhang (US), A P. Elg (SE)

Copyright © 2014 “Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of use for personal purposes. Are prohibited, except if explicitly agreed by CIGRE, total or partial reproduction of the publication for use other than personal and transfer to a third party; hence circulation on any intranet or other company network is forbidden”.

Disclaimer notice “CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law”.

ISBN : 978-2-85873-292-0

Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Past, present and future of IEC and IEEE high-voltage and high current testing standards Table of Contents 1

INTRODUCTION ........................................................................................................................................ 5

2

HISTORY OF HIGH-VOLTAGE AND HIGH-CURRENT TEST STANDARDS ......................................... 6 2.1

History of DC high-voltage tests and measurements ............................................................................6

2.1.1

DC high-voltage test and measurement systems ............................................................................ 6

2.1.2

DC high-voltage supplies ................................................................................................................. 7

2.1.3

Future work on DC test and measurement systems ........................................................................ 7

2.2

History of AC test sources and measurement systems......................................................................... 7

2.2.1 2.3

Future work on AC test and measurement systems ........................................................................ 8

History of impulse voltage tests and measurements .............................................................................8

2.3.1

Early history and definition of impulse waveforms ........................................................................... 8

2.3.2

Definitions of lightning impulse peak voltage ................................................................................... 9

2.3.3

Introduction of test voltage function................................................................................................ 10

2.4

Use of sphere-gaps as measurement devices ....................................................................................10

2.5

Reference Measuring Systems ........................................................................................................... 11

2.6

History of IEEE Standard 4 - the sister Standard to IEC 60060 series ............................................... 13

3 OUTLINE OF MAJOR CHANGES MADE IN RECENT EDITIONS OF IEC AND IEEE HIGHVOLTAGE AND HIGH-CURRENT STANDARDS ...........................................................................................14 3.1

IEC 60060-1:2010................................................................................................................................ 14

3.2

IEC 60060-2:2010................................................................................................................................ 15

3.3

IEC 62475:2010 ................................................................................................................................... 15

3.4

IEC 61083-2:2013................................................................................................................................ 15

3.5

IEEE Std 4-2013 .................................................................................................................................. 15

4

DISCUSSION OF IMPORTANT DEFINITIONS AND REQUIREMENTS ................................................ 17 4.1

Lightning impulse test voltage and test voltage function .....................................................................17

4.2

Definitions of switching impulse........................................................................................................... 19

4.3

AC test voltage .................................................................................................................................... 21

4.4

Atmospheric correction factor .............................................................................................................. 22

4.4.1

Introduction ..................................................................................................................................... 22

4.4.2

Formulae of relevant parameters for atmospheric correction factor calculation ............................ 22

4.4.3

The converse iterative procedure ................................................................................................... 23

4.4.4

Iterative procedure for determining atmospheric correction factor at high altitudes ...................... 24

Page 2

Past, present and future of IEC and IEEE high‐voltage and high current testing standards 4.4.5 4.5

Atmospheric corrections for AC voltages .......................................................................................30

Requirements for measurement systems ............................................................................................ 32

4.5.1

Structure of IEC 60060-2:2010....................................................................................................... 32

4.5.2

Calibration and estimation of measuring uncertainty ..................................................................... 32

4.5.3

System calibration by calibrations of components ......................................................................... 33

4.6

Measurement Uncertainty ................................................................................................................... 34

4.6.1

The approach in IEC 60060-2:2010 ............................................................................................... 34

4.6.2

Uncertainty Requirement Differences between IEC 60060-2:2010 and IEEE Std 4-2013 ............ 34

4.6.3

Relationship between uncertainty and tolerance ........................................................................... 36

5 DIFFERENCES BETWEEN THE LATEST REVISIONS OF IEC60060 SERIES AND IEEE STD 4 ................................................................................................................................................................ 37 5.1

General structure of IEEE Std 4-2013 ................................................................................................. 37

5.2

Summary of differences ....................................................................................................................... 37

5.2.1

Estimation of uncertainty ................................................................................................................ 37

5.2.2

Definition of AC peak voltage ......................................................................................................... 37

5.2.3

Linearity test ................................................................................................................................... 37

5.2.4

Highest frequency in impulse voltage test circuit ........................................................................... 37

5.2.5

Physical characteristics of reference impulse voltage dividers ...................................................... 37

5.3 6

Conclusion ........................................................................................................................................... 37 IMPROVEMENTS AND POSSIBLE ADDITIONS IN FUTURE REVISIONS........................................... 38

6.1

DC voltage ........................................................................................................................................... 38

6.2

AC voltage ........................................................................................................................................... 38

6.3

Lightning impulse voltage .................................................................................................................... 39

6.3.1

General considerations .................................................................................................................. 39

6.3.2

Further investigation of the test voltage function for SF6 ............................................................... 40

6.3.3

Further investigation of the test voltage function for oil .................................................................. 40

6.3.4

Further investigation of the test voltage function for air gaps ........................................................ 41

6.3.5

Test voltage function for multiple insulation materials.................................................................... 43

6.3.6

Generation and measurement of UHV lightning impulse ............................................................... 43

6.4

Improvement of atmospheric correction factor calculations ................................................................ 43

6.4.1

General remarks ............................................................................................................................. 43

6.4.2

Differences between IEC standards ............................................................................................... 44

6.4.3

Future work on atmospheric correction factors .............................................................................. 50

6.5

Review of voltage drop, AC and DC for future development .............................................................. 51

6.6

Waveforms of lightning impulse voltage and lightning impulse current............................................... 52

6.6.1

Introduction ..................................................................................................................................... 52

6.6.2

Observation of atmospheric lightning strike voltage waveforms .................................................... 52

6.6.3

Observation of atmospheric lightning strike current waveforms .................................................... 52

6.6.4

Conclusion ...................................................................................................................................... 53

6.7

Improvement of measurement systems .............................................................................................. 53

Page 3

Past, present and future of IEC and IEEE high‐voltage and high current testing standards 6.7.1

Calibration of UHV impulse measurement systems ....................................................................... 53

6.7.2

Uncertainty and risk assessment ................................................................................................... 55

6.7.3

Examples of risk assessment ......................................................................................................... 55

6.7.4

Measurement software ................................................................................................................... 58

7

CONCLUSION .......................................................................................................................................... 58

8

REFERENCES ......................................................................................................................................... 59

Page 4

Past, present and future of IEC and IEEE high‐voltage and high current testing standards

1 INTRODUCTION Recently, IEC and IEEE have published a number of revised and new standards for high-voltage and highcurrent testing. These standards include IEC 60060-1:2010, IEC 60060-2:2010, IEC 61083-2:2013, IEC 62475:2010 and IEEE Std.4-2013. Significant changes and additions have been introduced to these revised and new standards. Many members of CIGRE WG D1.35 were involved in the revision and development of these standards as members of IEC TC 42 and IEEE PSIM Subcommittee HVTT, particularly in the development of the new techniques and new procedures that are now adopted in the standards. This Guide has been written by members of CIGRE WG D1.35 to give high-voltage test engineers a broader knowledge of how to apply the latest high-voltage and high-current testing standards. In the preparation of this Guide, the contributors have tried to point out areas of difficulty in interpretation of certain clauses of these standards that should be considered for future revisions to make HV testing standards more clear and user friendly. This Guide first presents a brief account of the history of these standards with the aim to allow readers to gain a better appreciation of the technical background. The following sections summarize the major changes made to the standards in their latest revisions to provide a general picture of the revisions. The individual sections provide detailed information on the important requirements and procedures that have now been incorporated into the standards. They also describe some of the specific technical background with a list of published references. Finally, some discussion is given on the practical implications of these changes. Practical examples are provided to illustrate some of the new techniques and new procedures. The guide also lists areas of possible improvements to the standards for future revisions.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

2 HISTORY OF HIGH-VOLTAGE AND HIGH-CURRENT TEST STANDARDS 2.1 History of DC high-voltage tests and measurements 2.1.1

DC high-voltage test and measurement systems

The requirements for DC test and measurement systems and DC test procedures have not changed significantly over last few decades. This is because the main objectives of DC tests have not fundamentally changed. Insulation systems most frequently tested with DC voltages are mass impregnated paper-oil insulated cables, DC insulators, DC bushings, DC converter assemblies and DC rectifier transformers. Testing insulators and bushings are performed under dry, wet and polluted conditions whereas most test objects are tested only under laboratory conditions. Recently, polymeric cables have been introduced for DC applications and have necessitated an expansion of DC testing capabilities to include partial discharge measurements. CIGRE WG D1.55 has been formed to study HVDC partial discharge testing. Now we also see interest in DC testing of Gas Insulated Switchgear and the new CIGRE Joint Working Group JWG D1/B3.57 has been recently formed (2014). The main test procedures for DC have remained more or less the same over a long period of time. The tests include: the withstand voltage test, the disruptive-discharge voltage test and the assured disruptivedischarge voltage test. Withstand tests are used for testing non-self-restoring insulation, such as oil-paper insulation in a high-voltage cable as well as self-restoring insulation, such as found with the insulating surfaces of insulators. The other tests are more specifically for self-restoring insulation. The main technical challenges for DC voltage generation equipment are: 1. To produce a test voltage with sufficiently low ripple voltage to meet the standards. Ripple voltage magnitude can be more prominent at the higher test voltages due to loading by streamer activity or high leakage currents. 2. To assure that the measuring system can record the test voltage and the ripple voltage magnitude accurately when short duration current pulses, or persistent repetitive current pulses from the load, are present. These current pulses are normally random and recurring and can adversely affect the voltage stability (i.e. cause voltage dips) of the test equipment supply. 3. To supply sufficiently high current values for charging long lengths of shielded power cable, in reasonable times, at very high voltages. 4. To supply large test current pulses, containing significant electric charge values (Q), such as those produced in pollution testing. The issue here is to avoid erroneous test results due to large voltage drops that may influence the flashover behavior. 5. To supply sufficiently high current when sudden changes of impedance in the test object occur, such as the sudden decrease of impedance prior to the breakdown of an insulator. The concern is to avoid excessive voltage drops that could affect the test results. 6. The introduction of polarity reversal tests in short time frames. 7. The introduction of DC partial discharge testing requirements. Because of the high instantaneous current magnitudes required for some DC voltage tests, the technical challenge for DC voltage measurement systems is not only its ability to measure the DC voltage with sufficient accuracy (or sufficiently low uncertainty), but also having adequate accuracy for measuring ripple and voltage drops due to the sudden change of test object impedance [1]. Unfortunately, the present standards have little to say specifically about how to deal with these problems, mainly because the current pulse demands are not specified. It should be noted that for most DC testing systems it is not necessary to have a dedicated ripple voltage measurement system built in if the test conditions are without corona discharges or time varying currents, such as those found during testing of polluted insulators. A type test of a new DC test system equipment will show the ripple characteristics for the steady state load current and if the test current is within the rating of the system for a given ripple voltage then the standards will be met. This also means that purely resistive dividers are adequate for these test systems to measure the DC voltage. If the equipment is used for tests where high magnitude, time-varying current pulses are anticipated and corona from test connections is likely, then a resistive/capacitive divider with a bandwidth of up to 10 kHz can provide transient data to actually measure voltage drops and ripple magnitude. The question remains: how much voltage drop for how long a time will affect the test results or performance of the apparatus being tested? These questions require input from the relevant apparatus committees who set test voltages and protocols.

Page 6

Past, present and future of IEC and IEEE high‐voltage and high current testing standards A DC test with a voltage polarity reversal in a short time has been increasingly performed in recent years for testing components in HVDC transmission systems since this is a normal operation of some DC links. This provides a challenge for the DC voltage generation and measurement systems because the energy of the test object must be discharged quickly and safely, and the sources be configured for the opposite polarity. This is often done with resistor networks and mechanically inverting diodes in the system. However, no test procedures and requirements for associated measurement systems have been specified in the latest editions of IEC 60060 series. Test procedures for polarity reversal tests are given in apparatus standards for converter transformers, cables and others. 2.1.2

DC high-voltage supplies

DC high-voltage supplies (generators) for high-voltage testing introduced in the early days did not have the advantage of modern silicon rectifiers, which can be connected in series for producing very high voltages. DC was produced using mercury vapor electron tubes that acted as diodes or by Kenotron tubes. Since the tubes required filaments that had to be supplied with power at high voltages, special, cumbersome isolation transformers had to be fitted to provide power at high voltage levels. This problem led to practical limitations on how high a voltage could be produced - especially if any significant current was required. With the introduction of solid-state rectifiers it became possible to make multiplier circuits for higher DC voltages. Voltage multiplier circuits invented by Heinrich Greinacher in 1914 were originally developed for physics research. These systems were not particularly well suited for capacitor charging applications or fast response, voltage regulated power supplies, due to the inherent high impedance of the circuit. Since the voltage multiplication was via transfer of energy through capacitors, these power supply designs have current limitations, based on the size of the capacitors used. In the mid 1970’s a new style of DC UHV power supplies was introduced into the market. The inventor was Stanley G. Peschel and the design was granted a United States Patent [2]. This design uses a cascade of low impedance transformers feeding a cascade of voltage doublers in series. This design overcame the limitations of capacitor–diode multiplier circuits and could provide stable, well-regulated DC sources at a very high voltage. This design, first used in high-voltage and high-current electron beam accelerators, has largely replaced multiplier circuits for HV apparatus testing, where large charging currents are required and they provide good voltage regulation in the presence of streamers. This use of cascade doubler circuits has the added advantage of being able to produce relatively low ripple voltages with high overload capacity for charging large capacitive loads such as installed power cables. Due to the modular nature of the design, systems for use in the UHV range are possible and are more compact than multiplier designs. 2.1.3

Future work on DC test and measurement systems

Future work now is directed to the following (see Section 6.1 for detailed discussions):

Better characterization of measuring systems for voltage drop and ripple; Techniques of Measuring partial discharges under DC stress and what impact it has on the source requirements; Recommendations of equipment ratings for UHV tests where pollution or wet testing is performed; Possible revisions of standards for normal dielectric tests in the UHV range.

2.2 History of AC test sources and measurement systems The history of AC test sources and measurement systems dates back to the turn of the 20th century when high-voltage AC was first used for transmission and distribution of electric energy. In the early days, conventional transformers of higher voltages than the service voltage were used to “proof” test AC system components. Insulation coordination studies over the years developed test levels for various apparatus in the system and higher safety factors which led to higher test equipment voltage ratings. Over time, increasingly larger capacitive test loads at higher voltages were produced, especially in the case of polymeric cables. Whereas early HV cables of paper-oil insulation could be tested with DC voltages due to the reasonably linear voltage distribution based on resistivity, voltage distribution of plastic cables is mainly dependent on the permittivity of the insulating materials. For testing these voltages, a variety of measurement devices were developed including inductive potential transformers, electrostatic voltmeters, capacitive dividers, compressed gas capacitors and standardized sphere gaps. Over time these devices became the standards for measurements with traceability established by comparisons or calculations or other low voltage means. In the late 1960’s new test systems were introduced with high power capability and test voltages went up to cover 800 kV class transmission. For testing equipment, HV sources used some form of inductive reactance

Page 7

Past, present and future of IEC and IEEE high‐voltage and high current testing standards in the test system to compensate for the capacitive reactance of the test load and limit test power demands. Limitations of expanding test transformers to very high voltage and current levels led to modular resonant systems that are seen everywhere today. Ferranti, LTD of the UK, using moving core inductor designs, first introduced series resonant systems. Unfortunately these designs had poor efficiency and still required significant power from the mains to energize a test object. In 1973, all of this changed with the introduction of the modern, high “Q” series resonant designs invented by Richard F Schutz and Stanley G Peschel [3]. These modern designs were 5 to 10 times more efficient than earlier designs. Modern variable inductance series resonant systems were also unique in that they could be constructed with very high voltage and power ratings. Variable inductance, high Q series resonant systems are now the industry standard for the majority of production and research testing and are supplied by a number of companies. Currently, test sources for AC are now being developed using variable frequency with fixed inductors as compared to variable inductors with fixed frequency. These designs have specific advantages for testing the new generation of very long installed lengths of high-voltage power cables. On the measurement side of UHV AC, the ability to make gas capacitors rated up to 1200 kV allowed direct calibrations for most applications. Use of sphere gaps declined due to inconvenience, insufficient accuracy for many applications and voltage limitations. Potential transformers are available for relatively high voltages but are expensive. Capacitive voltage dividers are readily available today and have been proven to be stable and sufficiently accurate (low uncertainty) for UHV measurements. Today, most AC test systems use capacitive voltage dividers to measure the voltage. In parallel with the development of modern AC test systems has been the development of higher voltage reference measurement systems with stable scale factors and demonstrable linearity. 2.2.1

Future work on AC test and measurement systems

Future work is now directed to the following (see Section 6.2 for detailed discussions):

Calibration of UHV level Reference Measuring Systems; Calibration of VLF test systems; Methods to check linearity in the UHV range; AC source requirements for the UHV range.

2.3 History of impulse voltage tests and measurements 2.3.1

Early history and definition of impulse waveforms

It is generally known that the practice of impulse voltage testing dates back to the early 20th century. According to HV measurements pioneer Nils Hylten Cavallius [4], the first dielectric tests with high impulse voltages, were performed by F. W Peek Jr. around 1915 [5]. The first measurement of the impulse shape, using a continuously pumped oscillograph, was performed by Gabor in 1927 [6]. It is generally considered correct that testing with simulated lightning and the measuring of voltage and currents that could affect the transmission system in a negative way were being studied and simulated in laboratories roughly 100 years ago. F. W Peek Jr. worked in Pittsfield, Massachusetts for General Electric Company and his book, Dielectric Phenomena in High Voltage Engineering, was first published in 1915. The book was a result of the need to explain the consequences of naturally occurring lightning on the apparatus being built for power transmission – especially transformers. In his third edition, published in 1929, the author refers to the tremendous amount of laboratory and field data that had already been accumulated by then and a separate chapter was devoted just for the benefit of engineers studying lightning phenomena. In Impulse-Voltage Testing, by W.G Hawley, published in 1959 [7], we find the results of 30 early years of work in studying the effects of a variety of impulse voltage dividers designs, impulse recorders and the mathematical analysis of dozens of test circuits. Clearly, much work was done by pioneers in high-voltage technology to determine how to best measure impulse voltages even in the early days. The question of standardizing high-voltage testing and measuring techniques, in general, has been discussed within IEC since 1922. The first work in this area was in connection with insulator testing since there were failures of these exposed devices and a solution was needed for the problem. In 1934, a subcommittee was set up to deal with impulse voltage testing leading to first edition of IEC 60, General specifications for impulse voltage tests. The second edition, High-voltage test techniques, was issued in 1962. The counting of editions was restarted, when the content of IEC 60 was distributed among four parts of a new IEC 60 series. First edition of part 1 of the series (High-voltage test techniques – General definitions

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards and test requirements) was first issued in 1973. Second edition was released in 1989, and now the third one in 2010. In the USA, similar work was ongoing and IEEE Std 4, High Voltage Test Techniques, was established to cover all HV testing. Std. 4 was originally issued by AIEE in 1928 and established guidelines for measurement and test methods. The later revisions of Std 4 were issued in 1953, 1969, 1978, and the seventh edition, in 1995. The eighth edition was prepared in parallel with IEC 60060-1 and was published in 2013. From the 1930’s to the early 1960’s national standards from Great Britain, Germany, USA, and internationally from IEC member countries defined different standard (lightning) impulse wave shapes. The German standards [8, 9] recommended front times 0.5 µs, 1 µs and 2.5 µs, all with time to half-value of 50 µs. On the USA side, impulses of 0.5/5 µs, 1/10 µs and 1.5/50 µs were selected, with preference of 1.5/50 µs. The method for evaluation of the front time also varied. For example, according to a British standard [10], the front time was calculated from T1 1.25t 90% t10% , whereas at the same time in the USA [11] a formula T1 2t 90% t 30% was used [12]. It seems that the IEC document [13] at that time defined T1 as time to peak, i.e., T1 t100% t 0% . An agreement on the standard lightning impulse voltage shape was found in 1960’s, when it was settled to be 1.2/50 µs in the second edition of IEC 60:1962 (High-voltage test techniques), and respectively in IEEE Std. 4, IEEE Standard Techniques for High Voltage Testing. The front time was essentially set half way between the German and USA practices, and the present definition for front time, T1 t90% t30% / 0.6 ,

was also introduced in IEC standard. Hyltén-Cavallius gives credit for this consensus to Mr. J. Hagenguth from USA [4]. Information on the early history of switching impulse is surprisingly scanty, Hyltén-Cavallius states from his own experience in revising IEC 60 in 1962 that: “But we missed at that time that the much longer front times as occurring in switching impulses were an important factor in the determination of the flashover voltage.” However, the same edition of IEC 60 did have the first definition of switching impulse - with the same time parameters as today. 2.3.2

Definitions of lightning impulse peak voltage

Over the years IEC HV standards have introduced many new concepts in measuring technique. IEC 60:1962 introduced the idea of removing the oscillations on the front of an impulse by drawing a mean curve according to graphical (non-mathematical) rules under certain conditions. The definition of peak value for lightning impulses in IEC 60:1962 reads: 6.1.3

Peak value, alternatively virtual peak value The peak value is normally the maximum value. With some test circuits oscillations or overshoot may be present on the voltage. If the amplitude of the oscillations is not greater than 5 per cent of the peak value and the frequency is at least 0.5 MHz (Mc/s), or alternatively, if the amplitude of the overshoot is not greater than 5 per cent of the peak value and the duration is not longer than 1 µs, then for the purpose of measurement a mean curve may be drawn, the maximum value of which is defined as the virtual peak value. (See Figure 8a, page 83).

The reasoning why the mean curve was introduced has not been documented, but one story has it that a gentleman from UK came to an IEC meeting with a set of HV test results, and he managed to convince the Technical Committee to introduce the requirement for a mean curve to remove high frequency oscillations with the reason that these fast events would not impact the performance of certain apparatus. The text remained more or less unchanged in the 1973 and 1989 editions of IEC 60-1. This is shown by the green line of Figure 1, i.e., oscillations with frequencies below 0.5 MHz should be included for evaluation of peak value and front time, and above that frequency they should be completely ignored. However, problems began when people found that with oscillations close to 500 kHz, completely different results could be calculated depending on identifying the frequency as above or below 500 kHz. This was accentuated when using computer routines, rather than human eye, to evaluate the oscillograms. The gentleman who proposed the method appeared to have been quite right, as recent experimental studies have shown that above a certain frequency, for many test objects, there is only small impact of minor high frequency oscillations on the insulation. The question of the magnitude of "minor" remains aside from the frequency, however the magnitude is almost always attenuated due to the circuit topology.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards 2.3.3

Introduction of test voltage function

The black dots in Figure 1 show the response of insulation to oscillations superimposed on a lightning impulse according to a recent study [14]. As a result, the new IEC 60060-1:2010 [15] defines a test voltage function [16]:

k f 1 1 2.2 f 2 , which is shown by the blue line in Figure 1. This formula is used to calculate the effective peak voltage as a function of the frequency of the oscillation where f is the frequency in MHz. The introduction of this function removes the problems related to the stepwise change at 0.5 MHz and makes a smooth transition that has a mathematical definition.

1.2 IEC 60:1962 1

Measured IEC 60060-1:2010

0.8 0.6 0.4 0.2 0 0.01

0.1

1

10

f [MHz]

Figure 1: Amplitude calculation factor for a range of oscillation frequencies superimposed on a lightning impulse, together with the old and new test voltage functions. Figure 1 shows that for oscillations below 100 kHz, the peak voltage is taken as the actual peak. For frequencies above 4 MHz, the oscillations are discarded and have no impact on the calculated peak voltage. The results of this recent experimental work have also led to the development of a well-defined new procedure for calculating the parameters of impulse test voltage. This procedure has now been adopted by IEC 60060-1:2010 and IEEE Std 4-2013. A more detailed discussion on this procedure is given in Section 4.1. Future work in this area involves proving that the dielectric stress produced by various superimposed oscillation frequencies on normal wave shapes can be normalized for various apparatus in terms of insulation performance and that the key parameters can be consistently calculated. See chapter 6.3 for detailed discussion on future work in this area.

2.4 Use of sphere-gaps as measurement devices The sphere-gap is a device that has been used for calibrating high-voltage measuring systems for over half a century [17, 18, 19, 20, 21, 22, 23]. However, due to the availability of economical voltage measuring devices with better measurement repeatability, reduced measurement uncertainty due to influence of atmospheric conditions and ease of use, the use of sphere-gap has been in decline. A sphere-gap does not provide sufficiently low values of uncertainty to qualify as a Reference Measurement System. Sphere-gaps also only give one piece of information — peak voltage value. Other measurement systems now provide information on time parameters of test voltages. There is still an IEC standard (IEC 60052) describing its use but it is only suited to measurement checks, not the calibration of measurement devices. It is intended that spheregaps will be used primarily as a device for Performance Checks of high-voltage AC and impulse

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards measurement systems approved in accordance with IEC 60060-2:2010 [24]. Sphere-gaps can also be used for calibrating measurement systems that do not require an uncertainty of voltage measurement of < 3 %, but today most users must meet this uncertainty requirement and most technical committees reference IEC 60060-2 for the uncertainty requirement and the requirement is < 3 % for the test voltages in most cases. Summary of Sphere Gap Considerations for Voltage Measurement 1. 2. 3. 4. 5. 6. 7. 8. 9.

Simple to construct but expensive for larger voltage ratings. Can only be set for one voltage at a time. Only measures peak voltage. Requires careful mechanical alignment. Range of operation for a given sphere size is limited. Can be influenced by particles in the air. Uncertainty of measurement > 3 % for AC & LI voltages. Requires little maintenance over long life time. Requires more test area space than voltage dividers.

Conclusion and Recommendations Sphere-gaps are useful devices for checking high-voltage measuring systems or to provide overvoltage protection as a protective gap during tests, but less convenient for almost any other HV testing needs.

2.5 Reference Measuring Systems The concept of using a Reference Measuring System for calibration is as old as the concept of measurement. However, the requirement to use a Reference Measuring System for calibration was not present in the earlier versions of IE 60 and IEEE Std. 4 before 1994. The calibration of an Approved Measuring System was primarily based on low-voltage components methods. In case of impulse dividers, as long as the total response time was less than 200 ns, the standards stated that the dividers would meet the measurement error requirements for the peak voltage and time parameters. In 1980s several researchers at different HV laboratories around the world independently found that the total response time had no definite relationship with the impulse parameter measurement errors. As a result, a few national metrology laboratories decided to build their own high precision reference dividers and conducted an international round-robin study with the dividers. The study formed the base of 1994 edition of IEC 60060-2 that required using reference dividers to calibrate approved impulse measuring system [25, 26, 27, 28]. This standard specified that the preferred method of calibrating a complete measuring system, including the divider, the transmission system, any secondary divider/attenuator, the measurement instrument and any measurement software, is by comparison against a Reference Measuring System. This concept is applicable to impulse, AC and DC voltages and the standard specified a minimum voltage rating for a reference divider of 20 % of the rating of the system being calibrated to determine its scale factor. Performance at levels higher than the calibration level was to be verified by a linearity test. The measurement uncertainty of a Reference Measuring System is normally significantly lower than that of an Approved Measuring System used in high-voltage testing. The low value of uncertainty of a Reference Measuring System is established through measurements traceable to National Standards. Comparisons among Reference Measuring Systems are also often conducted to ensure their measurement uncertainty and several international round robins have been conducted to confirm this [28, 29, 30, 31, 32, 33, 34]. Prior to the introduction of the requirement for the calibration of a complete measuring system against a Reference Measuring System, IEC 60-2 allowed the calibration of a high-voltage measuring system to be performed by calibration of its components. The method of component calibration was kept in the 1994 edition of IEC 60060-2, as well as in the 2010 edition of IEC 60060-2. However, the comparison and calibration against a Reference Measuring System or Approved Measuring System is specified as the preferred method of calibration. The Reference Measuring System or Approved Measuring System comparison calibration is generally much more efficient than evaluation by the component method. It should be noted that low voltage “check” methods have also been introduced for Performance Checks of measurement systems. The direct comparison calibration method against a Reference Measuring System rated for the full voltage of a system being calibrated is the best method and it is easier to demonstrate traceability to National Standards for scale factor and linearity.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards Table 1 is a summary of the requirements for measurement uncertainties of Reference Measuring Systems, along with the requirements for measurement uncertainties of Approved Measuring Systems, specified in the present edition of IEC 60060-2:2010 and IEC 62475:2010 [35].

Table 1: Requirements for measurement uncertainties of Reference Measuring Systems Uncertainty limit of Reference Measuring Systems

Uncertainty limit of Approved Measuring Systems

Average value

1%

3%

Peak/√2 value

1%

3%

Test voltage (Ut))

1%

3%

Front time (T1)

5%

10 %

Time to-half-value (T2)

5%

10 %

Time to-chopping (Tc)

5%

10 %

Test voltage (Ut)

3%

5%

Time to-chopping (Tc)

5%

10 %

Test voltage (Ut)

1%

3%

Time to-peak (Tp)

3%

10 %

Time to-half-value (T2)

3%

10 %

Peak current value

Not specified

3%

Time parameters

Not specified

10 %

Type of test voltage

Measurement quantity

DC voltage AC Voltage

Full/Tail chopped lightning voltage

Front chopped lightning voltage

Switching impulse voltage

Impulse Currents (Exponential and rectangular)

Future work Future Work in the area of Reference Measuring Systems should be focused on demonstrating the uncertainty limits achievable, especially for calibrating equipment for testing in the UHV range.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

2.6 History of IEEE Standard 4 - the sister Standard to IEC 60060 series IEEE Std 4-1995 was the previous version of the general IEEE standard dealing with High Voltage Testing and Measurement Techniques. This standard was built on the work done by pioneers in the field of HV testing: Pete Bellaschi, Frank Creed, Nils Hylten Cavalius, Kurt Feser, Gianguido Carrara to name a few. The current revision, published in 2013, is the eighth edition of this document as a separate standard. The subject had been addressed in the earliest Standardization Report of the American Institute of Electrical Engineers (AIEE) in 1889 and had been substantially elaborated upon in the subsequent reports issued from 1902 to 1933. In 1922 it was decided to issue separate sections for measurement of test voltages by AIEE. The first edition of IEEE Std 4 was published in 1928. It is interesting that as we are going to higher transmission voltages for UHV many of technical problems solved for lower voltage systems must be investigated again for UHV. In the years prior to the last revision of the IEEE Std 4, which is referred to by many North American apparatus committees in their HV testing standards, much has changed in the global marketplace with respect to the power equipment manufacturing industry. Testing has become more automated, more precise and more informative about the quality of the insulation systems being evaluated. In addition, more awareness of the corresponding IEC standards has developed, namely IEC 60060 parts 1 and 2, “Highvoltage Test Techniques”. Now it is common for North American manufacturers to use both IEC and IEEE standards in testing their products, many of which enter the global marketplace. Increasingly data taken in one location may be instantly transmitted electronically to other facilities continents away for analysis and review. The need for standardization has never been more important to the reliability of components of the electrical grid. In preparing the latest revision for IEEE Standard 4, the committee members took every effort to try to align the revised technical content with the revised technical content of IEC 60060 parts 1 and 2 and the new standard IEC 62475 for High-current test techniques. This has been a difficult task as the three IEC standards (four including IEC on-site testing) have been evolving simultaneously with the IEEE revision. Fortunately, IEC revisions are now fixed and the two standards are aligned for the most critical requirements. Despite close relationships between IEEE and IEC organizations, there are still several small differences between IEEE and IEC that are noted in later sections.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

3 OUTLINE OF MAJOR CHANGES MADE IN RECENT EDITIONS OF IEC AND IEEE HIGH-VOLTAGE AND HIGH-CURRENT STANDARDS This section outlines the major changes in recent editions of high-voltage and high-current test standards. Some of these changes are described in detailed in Section 4. The relevant standards for this discussion are: IEC 60060-1 Ed. 3.0 (2010), High-voltage test techniques – Part 1: General definitions and test requirements. IEC 60060-2 Ed. 3.0 (2010), High-voltage test techniques – Part 2: Measuring systems. IEC 61083-2 Ed. 2.0 (2013), Instruments and software for measurement in high-voltage and high-current tests – Part 2: Requirements for software for impulse test. IEC 62475 Ed. 1.0 (2010), High-current test techniques - Definitions and requirements for test currents and measuring systems. IEEE Std 4-2013 High Voltage Test Techniques.

3.1 IEC 60060-1:2010 The definitions for basic lightning impulse voltage parameters, Up, T1 and T2, have not changed. The test voltage function (sometimes known as the k-factor) as described in the literature published in the period leading up to the approval of the standard, has been introduced to enable more accurate and consistent determination of the test voltage and time parameters of lightning impulses with superimposed oscillations of any frequency content. A number of new definitions related to this new procedure have been added, e.g., test voltage function, extreme value, relative overshoot and average rate of rise. The peak value of an alternating voltage is now defined as half of the peak-to-peak voltage. The earlier definition, “maximum value”, could lead to misinterpretation for cases where even harmonics of the test source are present. This harmonic distortion may lead to different positive and negative peak values. A maximum value of 2 % is allowed for the difference between positive and negative peak values. The test voltage value is the peak value divided by √2. No changes in the definition of switching impulse voltage parameters have been introduced in IEC 60060-1:2013, and the previous definition of time to peak has been retained, i.e. “time interval from the true origin to the time of maximum value of a switching-impulse voltage”. IEEE Std 4-2013 and IEC 60060-1:2010 differ slightly on evaluation of switching impulse. For the case of a standard switching impulse, with the time to peak being 250 µs ± 50 µs, IEC identifies a simplified method using a mathematical formula adopted from IEEE Std. 4-1995. This method is also given by IEEE Std 4-2013, but is stated as the definition. For non-standard switching impulses, IEC clearly states that other methods of evaluation should be used, e.g. “For non-standard impulses, the time to peak can be determined by various methods of digital curve fitting dependent on the actual shape”. Formulae have been introduced for the parameters of the atmospheric correction factor to make the computer calculation of the correction factor feasible. The iterative procedure for calculating the atmospheric correction factor of a test voltage is introduced, and is intended for voltage withstand tests. This procedure is intended to reduce the error of the correction factor due to the error in the estimated U50 (50 % probability breakdown voltage) that is needed for the calculation. The error becomes significant when the correction is significant. Only one wet test procedure has been retained. Test procedures for combined and composite voltages have been elaborated more in details. AC and DC Artificial pollution test procedures are removed as they are now specified in IEC 60507.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

3.2 IEC 60060-2:2010

Estimation of common components of uncertainties in high-voltage measurements are now specified in more details. Requirements on voltage linearity tests are more clearly specified. New requirements on dynamic performance, i.e., frequency response of power frequency AC voltage measurement systems, have been added. Step response parameter evaluation methods for characterization of voltage dividers and current shunts have been moved into an informative annex, with a number of definitions related to the step response measurement having been revised. An informative annex on evaluation of the dynamic performance of impulse measurement system by the convolution method has been added The standard sphere-gap is no longer accepted as a Reference Measuring Device but can still be used for Performance Checks. Calibration procedure for DC -systems using a rod/rod gap is removed. Measurement of impulse currents has been transferred to a new standard IEC 62475:2010, Highcurrent test techniques – Definitions and requirements for test currents and measuring systems.

3.3 IEC 62475:2010

The current measurement part of this standard covers more applications than the relevant sections in the obsolete 60060-2:1994, which covered only measurement of impulse current waves as used in arrestor testing. The new standard also covers requirements for testing with any type of high current as well as giving the requirements for a high-current measurement system. The types of high currents which have been added now include: steady-state direct current, steadystate alternating current, short-time direct current, short-time alternating current, and impulse current. The standard also covers current measurement in high-voltage dielectric testing. The standard has adopted a similar structure to that of IEC60060-2:2010 Estimation of measurement uncertainties is specified similarly to that in IEC 60060-2:2010.

3.4 IEC 61083-2:2013 IEC 61083 part 2 has been updated with a new Test Data Generator (TDG) to evaluate impulse measuring system software. The new version provides more impulse voltage waveforms, and includes waves with different overshoot amplitudes and frequencies. Frequencies are selected to prove performance around the transition frequency of 500 kHz. The new TDG helps users prove that their software is making correct evaluations of the key parameters in a consistent and comparable way. Reference values of lightning impulse waveforms have also been revised according to the new definition of the impulse test voltage in IEC 60060-1:2010 Waveforms of impulse currents are added to the TDG to represent a range of those used for arrestor testing that have been added over the last few years. More current waveforms, including lightning current impulses, are now included, in order to cover the range of current waveforms used in the new standard IEC 62475:2010. An annex on estimation of uncertainty contribution of software (waveform parameter calculation) has been added.

3.5 IEEE Std 4-2013

IEEE Std 4-2013 continues to cover both the requirements for testing and the requirements for measurement systems, which are covered separately by IEC 60060-1:2010 and IEC 60060-2:2010. Efforts have been made to harmonize IEEE Std 4-2013 with the two parts of IEC 60060, in terms of principles and fundamental requirements. Minor differences, however, still exist. A summary of the differences is given in Section 5.2. The new edition of IEEE Std 4 still contains a significant amount of tutorial information to give practical suggestions to the test engineer.

Page 15

Past, present and future of IEC and IEEE high‐voltage and high current testing standards The readers are also drawn to the attention of a closely related standard, IEC 60060-3:2006 [36] for on-site high-voltage testing. Many of techniques and procedures are similar to those specified in the above standards, but with variations to suit conditions and limitations of on-site tests.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

4 DISCUSSION OF IMPORTANT DEFINITIONS AND REQUIREMENTS This chapter discusses important definitions and requirements that have either undergone significant changes or deserve close attentions of users for correct interpretation and better use of the standards.

4.1 Lightning impulse test voltage and test voltage function As discussed in earlier sections, the impulse analysis procedure provided in IEC 60060-1:2010 and IEEE Std 4-2013 allows determination of impulse parameters without subjective determination of the existence or characteristics of overshoot and its frequency. The new procedure in the standards is also more precisely specified in order to eliminate discrepancies of impulse parameters obtained by different software packages. A summary of the major steps of the procedure is given in Figure 2. Evaluation of tail chopped impulse parameters requires the knowledge of the waveform of a full impulse taken in the same circuit configuration. This is because double exponential curve fitting will not produce a correct base curve for chopped impulses since the front is not really a perfect exponential and the tail does not exist on a chopped wave. A full impulse waveform is therefore needed to aid in the calculation. A full impulse from the same test circuit at a reduced voltage level is usually available in the impulse test of most apparatus before the tail chopped impulses. Therefore, for evaluation of tail chopped impulses, steps 1 and 2 in Figure 2 are performed using this full reference impulse; the base curve obtained is then scaled or normalized to match the amplitude of the tail chopped curve for the remaining steps of the procedure. For a front chopped lightning-impulse voltage, the test voltage curve is the recorded curve without further processing of the waveform. An integral part of requirements for impulse voltage tests in IEC 60060-1:2010 are the requirements for software specified in IEC 61083-2:2013, Instruments and software used for measurement in high-voltage and high-current tests - Part 2: Requirements for software for impulse tests. The Test Data Generator (TDG) of IEC 61083-2:2013 is a software package for generation of test data. The TDG produces digital records of a number of different impulse waveforms for testing an impulse measurement software package. IEC 61083-2:2010 specifies the reference values of test impulses generated from the TDG. The IEC 61083-2:2013 also provides error limits for acceptance of software being evaluated for measuring different types of impulses. After the new Test Voltage Function (k-factor) was proposed, a number of evaluation software programs were tested in round-robin tests. The work was first performed within CIGRE WG D1.33, but later the activity gradually moved to IEC TC 42 Maintenance Team (MT) 07. The main task of the latest work was to revise the original Test Data Generator to include more wave shapes for more comprehensive testing of impulse voltage and impulse current evaluation algorithms. Annex B and Annex C of IEC 60060-1:2010 are two of the products of MT07 work.

120

120

100

100

80

80

60

60

U [kV]

1. Data starting from 20% on the front to 40% on the tail (pink) of the measured data (blue) are taken for curve fitting.

U [kV]

In addition to LI and SI impulse shapes in the TDG, IEC 61083-2:2013 also includes data sets for Oscillating Lightning Impulse waveforms (OLI) and Oscillating Switching Impulse waveforms (OSI) in accordance with IEC 60060-3:2006 (High-voltage test techniques - Part 3: Definitions and requirements for on-site tests) [36]. Impulse current waveforms defined in IEC 62475:2010 (High-current test techniques – Definitions and requirements for test currents and measuring systems) have been added to the TDG too.

40 20

40 20

0

0

-20

-20 -10

0

10

20

30

40

50

60

t [µs]

-2

0

2

4 t [µs]

Page 17

6

8

120

120

100

100

80

80

60

60

U [kV]

2. Double exponential base curve (green) is fitted to the data selected in step 1 (pink).

U [kV]

Past, present and future of IEC and IEEE high‐voltage and high current testing standards

40 20

40 20

0

0

-20

-20 -10

0

10

20

30

40

50

60

-2

0

2

120

120

100

100

80

80

60

60

40 20

0

-20

-20 0

10

20

30

40

50

60

-2

0

2

100

80

80

60

60

U [kV]

U [kV]

120

100

40 20

8

4

6

8

4

6

8

4

6

8

20

0

0

-20

-20 0

10

20

30

40

50

60

-2

0

2 t [µs]

120

120

100

100

80

80

60

60

U [kV]

U [kV]

6

40

t [µs]

40 20

40 20

0

0

-20

-20 -10

0

10

20

30

40

50

60

-2

0

2

t [µs]

t [µs]

120

120

100

100

80

80

60

60

U [kV]

U [kV]

4 t [µs]

120

-10

6. Test voltage curve (red) is shown together with the measured curve (blue). Up, T1 and T2 are calculated from the test voltage curve.

8

20

t [µs]

5. Filtered residual curve (brown) is added back to the base curve (green) to get the test voltage curve (red).

6

40

0

-10

4. Residual curve (orange) is filtered according to the test voltage function to get the filtered residual curve (brown).

4 t [µs]

U [kV]

3. Residual curve (orange) is obtained by subtracting the base curve (green) from the measured curve (blue).

U [kV]

t [µs]

40

40 20

20 0

0

-20

-20 -10

0

10

20

30

40

50

60

t [µs]

-2

0

2 t [µs]

Figure 2: Evaluation of the test voltage curve according to IEC 60060-1:2010.

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4.2 Definitions of switching impulse The definition of time to peak for switching impulse in IEC 60060-1:2010 is “time interval from the true origin to the time of maximum value of a switching-impulse voltage”. It has been found that it is often difficult to determine this time to-peak value, Tp, even from digital records of impulses with low uncertainty, due to the fact that the digitally recorded voltage values in the impulse peak region can be approximately equal for a long period of time, and many data points of the same amplitude will be found around the peak. To overcome the problem of calculating the Tp value, the empirical formula introduced in the previous edition of IEEE Std 4 and retained in IEEE Std 4-2013, is now included in IEC 60060-1:2010 for evaluation of switching impulses that conform to the standard waveform. The formula takes the following form: Tp = KTAB

(1)

where K is a dimensionless constant given by K = 2.42 – 2.08x10-3TAB + 1.51 x 10-4 T2 where TAB and T2 are in microseconds and TAB = t90 – t30 (see Figure 3) and the numerical constants 2.08 and 1.51 have dimension s-1. The difference between the Tp values calculated from formula (1) and the true Tp as defined in Clause 8.1.3 of IEC 60060-1:2010 for the standard switching impulse waveform of 250/2500 µs is negligible. Taking the TDG waveform SI-A1 of IEC 60183-2:2013 as an example, the difference between the value calculated using formula (1) and the reference value given by IEC 60183-2:2013 is less than 0.85 % (see Table 2). The reference values given in IEC 60183-2:2013 were determined by averaging the Tp values obtained in a round-robin test, by different software packages that calculate the Tp values in accordance with Clause 8.1.3 of IEC 60060-1:2010, i.e. the Tp value that corresponds to the maximum voltage of the impulse waveform. If the impulse parameters remain within the tolerance ranges of the standard switching impulse, i.e. Tp remains between 200 µs and 300 µs, and T2 remains between 1000 µs and 4000 µs, the maximum deviation of Tp values calculated, using formula (1), from the reference Tp value is less than 3 %, which is negligible for the purposes of high-voltage testing. However, since formula (1) was defined for standard switching impulses only, if the waveform deviates significantly from the standard switching impulse, the deviation of Tp values can be significantly larger that the permitted measurement uncertainty of Tp, which is ±10 % and for those cases other methods of evaluation must be used. However, for most testing situations this is not an issue.

U 1,0 0,9

B

Td

0,5

0,3

A

0 t

TAB T2

Figure 3: Illustration of defined switching impulse voltage parameters.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards Table 2: T p values given by TDG of IEC61083-2: 2010 and those calculated using formula (1) TDG Case No. IS-A1 SI-A2 SI-A3 SI-A5

TDG Tp (µs) 250.7 19.89 43.1 218

TDG T2 (µs) 2512 1321 3987 2407

Formula (1) Tp (µs) 248.8 15.30 35.99 221.8

Tp Error (% of TDG Tp) -0.76 -23.1 -16.5 +1.8

Note that waveforms S1-A2 and S1-A3 are non- standard switching impulses The errors of Tp values obtained by using formula (1) for some practical waveforms are shown in Figure 4 and Table 3. The reference values are obtained according to the definition in Clause 8.1.3 of IEC 60060-1:2010, using double exponential fitting to find the peak. Results in Table 2 and Table 3 show that when the standard switching impulse voltages are used, formula (1) is sufficiently accurate for meeting the uncertainty requirement of high-voltage testing.

Table 3: T p values obtained using formula (1) as compared to T p values by doubleexponential fitting Waveform No. 1 2 3 4 5 6

Tp by fitting (µs) 174.71 212.51 237.13 287.93 288.55 312.51

T2 by fitting (µs) 2481.95 1491.09 3998.58 4032.71 1504.95 2546.15

Formula (1) Tp (µs) 169.66 210.84 238.21 291.34 285.00 308.06

Tp Error (% of fitted Tp) 2.9 0.8 0.5 1.2 1.2 1.4

Figure 4: Errors of T p values of practical waveforms obtained using formula (1) with the T p values of fitted waveforms as the references.

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It has been found, however, that errors of Tp values obtained by other methods, for example, by curve fitting, would not be significantly lower, in some cases could be higher than those of formula (1). A number of curve fitting methods have been tested by various laboratories. One method was to fit the complete waveform from the origin of the impulse to a double exponential function. Another method [37] is to fit from the instant of 85 % of the peak value on the front to the instant of 95 % of the peak value on the tail (denoted as top fit). It was found that the difference in the Tp values obtained by these two fitting method for the last waveform in Table 3 is 1.7 %. Furthermore, when the time parameters are within the tolerance range of the standard switching impulse, the maximum errors of Tp values obtained by the fitting methods for the TDG waveforms are of similar magnitude to that of the formula (1), with reference to the reference values as given in IEC 61083-2:2013. Table 4 shows the errors of the two fitting methods.

Table 4: T p errors of two fitting methods for standard switching impulses

TDG Case No.

TDG Tp (µs)

IS-A1 SI-A5

250.7 218.0

Error of Tp (complete fit) (% of TDG Tp) +0.52 +3.2

Error of Tp (top fit) (% of TDG Tp) +0.53 -4.7

Error of Tp (Formula (1)) (% of TDG Tp) -0.76 +1.8

In conclusion, the difference of Tp values of standard switching impulses obtained by the fitting methods in accordance with Clause 8.1.3 of IEC 60060-1:2010 and the formula given in Clause 8.2.3.1 of IEC 60060-1:2010 and Clause 8.1 of IEEE Std 4-2013 are not significant for the purposes of high voltage testing. Therefore, for standard switching impulse measurement, formula (1) is recommended due to the simplicity of its implementation as no complicated nonlinear curve fitting and the related software development are needed. Non-Standard Switching Impulse Wave shapes The calculation errors using formula (1) can be significant for calibration of the measurement system systems using the non-standard switching impulses. For the non-standard switching impulses given in IEC 601083-2, the formula yields Tp values outside the acceptance limits of the reference values. Fitting methods may be utilized for these special waveforms by agreement between parties. In any case, these waveforms would generally not be acceptable for apparatus testing by relevant apparatus committees that reference IEC 60060 for standard wave shapes and their evaluation is left to those relevant apparatus committees to define.

4.3 AC test voltage A change in the new standards is for a new definition of the AC peak voltage. As mentioned earlier the AC peak voltage is now defined in IEC 60060-1:2010 as the “average of the magnitudes of the positive and negative peak values, as opposed to, “the maximum value” in the 1989 edition. The definition of peak voltage in IEEE Std 4-2013, remains the same as in its previous version, IEEE Std 4-1995, which is essentially identical to the corresponding definition in IEC 60060-1:1989. The practical impact of the difference between IEC 60060-1:2010 and IEEE Std 4--2013 is insignificant in the vast majority of AC tests, where the AC voltage waveforms are symmetrical to the zero voltage level. In rare cases, when the voltage waveform becomes unsymmetrical, e.g., with the presence of even harmonics produced by power electronics voltage sources, if a peak voltmeter is used to measure voltage, the difference between the peak voltage reading from a voltmeter that detects the maximum of the AC voltage and that from a voltmeter that reads the average of the positive peak and the negative peak can be different. IEC 60060-1:2010 requires that the difference between the positive peak and negative peak values shall be less than 2 %. For voltage sources that meet this 2 % requirement, the maximum difference in the measured peak voltage between IEC 60060-1:2010 and IEEE Std 4-2013 would be 1 % if there was an non-symmetrical AC shape due to electronic converter loads. Nearly all High-voltage AC testing produces symmetrical waveforms so this is not an issue for most testing.

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The requirement for the crest factor, which is defined as the ratio of the peak voltage to the rms value, remains the same for the new editions of both IEC 60060-1 and IEEE Std 4. The ratio between peak and rms should be within 5 % of √2. The crest factor has been used for some time as a determinant of waveform distortion especially in the peak area of the test voltage. With modern instruments being capable of more accurately measuring harmonics, the value of total harmonic distortion, THD, can now be easily measured. It is tempting to use this parameter to characterize the wave-shape, but it must however be considered, that THD does not directly relate to changes in the crest value and that THD is not a suitable measure of crest distortion. A voltage drop of up to 20 % is still considered acceptable in the revised standard during the AC tests such as the disruptive voltage test, the pollution test and the wet test.

4.4 Atmospheric correction factor 4.4.1

Introduction

The atmospheric correction factor is used for correcting the disruptive discharge voltage of insulation under a particular test atmospheric condition to a voltage under the standard atmospheric conditions. This section describes in detail some of the additions and changes in relation to atmospheric correction factor as defined in IEC 60060-1:2010. This chapter also lists the newly introduced formulae in IEC 60060-1:2010 that replaced the graphs in IEC 60060-1:1989. These formulae facilitate calculation of atmospheric correction factors by computer software. The atmospheric corrections defined in IEC 60060-1:2010 are valid for air-gaps and clean insulators. If atmospheric corrections are to be calculated for surface discharge tests, the results obtained with the method in IEC 60060-1:2010 have to be treated with caution. In IEEE Std 4-2013, two methods of calculating atmospheric correction factor are used. Method 1 is the same as IEC 60060-1:2010, and is recommended for new equipment. Method 2 has been used in the past and may be valuable for repeated tests on existing equipment designs. 4.4.2

Formulae of relevant parameters for atmospheric correction factor calculation

A number of formulae have been introduced for convenient implementation of computer calculation of atmospheric correction factor values. Changes have also been made to a few formulae as a result of the latest revision of IEC 60060-1. These changes are highlighted here to help users to make necessary changes to their calculation procedures. 4.4.2.1 Formulae of Exponents for air density correction and humidity correction In IEC 60060-1:1989, the value of the exponent for air density correction, m, and the value of the exponent for humidity correction, w, needed to be obtained from graphs. In IEC 60060-1:2010, formulae of these two exponents, as functions of parameter g, have been included to facilitate atmospheric correction factor calculations by computer programs. The formulae are especially useful when the iterative procedure (see 4.4.3 and 4.4.4) has to be used. The relationship between g and m, and that between g and w, are both described by piecewise functions listed in Table 1 of the IEC 60060-1 Ed.3.0, which is reproduced below as Table 5.

Table 5: Values of exponents, m for air density correction and w for humidity correction, as a function of the parameter g

g

Table 1 of IEC 60060-1:2010 m

w

2,0

g(g-0,2)/0,8 1,0 1,0 1,0

g(g-0,2)/0,8 1,0 (2,2-g)(2,0-g)/0,8 0

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The definitions for g, m and w in IEC 60060-1:2010 remain the same as in IEC 60060-1:1989. The graphs for m and w are however still given for those who still want to use them. 4.4.2.2 The voltage type parameter k The parameter for voltage type, k, in the case of “direct voltage”, was expressed a linear function of “h/δ” in IEC 60060-1:1989 (See Figure 3 of IEC 60060-1:1989), where h is the absolute humidity of ambient air and δ is the air density. However, the function k in the “direct voltage” is slightly non-linear. The non-linear function has now been adopted in IEC 60060-1:2010. The other change is that the applicable range of humidity for k has been extended for the DC case and the impulse case. The extension of the humidity range was largely based on consensual decisions, with considerations given to the historical experimental data, practical issues and impact on the validity of the corrected test voltage. The changes related to parameter k are summarized below: IEC 60060-1:1989 Ed. 2.0: “Direct voltage:

k= 1 + 0,014(h/δ − 11) , for 1 g/m 3 < h/δ < 13 g/m 3 ”

“Impulse voltage:

k= 1 + 0,010(h/δ − 11) , for 1 g/m 3 < h/δ < 15 g/m 3 ”

IEC 60060-1:2010 Ed. 3.0: “DC:

for 1 g/m 3 < h/δ < 15 g/m 3 ”

k= 1 + 0,014(h/δ – 11) – 0,00022(h/δ – 11) 2

“Impulse:

for 1 g/m 3 < h/δ < 20 g/m 3 ”

k= 1 + 0,010(h/δ − 11)

In the AC voltage case, k remains unchanged. “AC:

for 1 g/m 3 < h/δ < 15 g/m 3 ”

k= 1 + 0,012(h/δ − 11)

4.4.2.3 Air humidity The calculation of the atmospheric correction factor requires determination of the absolute humidity of air, h. The value of h can be determined by the reading of a dry bulb thermometer and the reading of a wet bulb glass thermometer. However, IEC 60060-1:2010 also includes a conversion formula that allows calculation of the absolute humidity of air, h, from the relative humidity and the air temperature. This allows convenient measurement of h using electronic sensors measuring relative humidity.

h where

17 , 6t 243 t

6,11 R e , 0,4615 ( 273 t )

h is the absolute humidity in g/m3, R is the relative humidity in percent and t is the ambient temperature in °C.

The uncertainty of measurement of R using electronic sensors would normally be 1 % to 2 % relative humidity, and hence at least the same uncertainty in the value of h. Determination of absolute humidity using wet and dry thermometers is not expected to alter the uncertainty significantly (neglecting the uncertainty of the temperature measurement). The sensitivity of the uncertainty of atmospheric correction factor (and hence the corrected test voltage) to the uncertainty of h is low (see D.7.1.3 of IEEE Std 4-2013). As shown in Table D.2 (IEEE Std 4-2013), an uncertainty of 1 g/m3 in h would only lead to a relative uncertainty of the corrected test voltage of 1.1/559, which is approximately 0.2 %. 4.4.3

The converse iterative procedure

The atmospheric correction factor, Kt, is used for both breakdown tests of given breakdown probability and withstand tests. In most cases of product testing, atmospheric correction is performed for voltage withstand tests, with AC, DC, lightning impulse or switching impulse voltage, at an altitude less than the service altitude of the equipment under test.

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The procedure in IEC 60060-1:2010 is derived from experimental data of 50 % probability flashover voltage, however the correction often needed is for the corrected test voltage to be applied in a withstand test. The withstand test voltage is defined as the voltage with 10 % probability of disruptive discharge, U10. IEC 60060-1:2010 assumes the same correction factor applies to both U50 and U10. The calculation of the atmospheric correction factor requires a known value of U50 of the insulation under test. In a flashover test, U50 can be determined and therefore the value of U50 does not cause significant error in the correction factor if the experiment is performed correctly. However, U50 is usually unknown in a withstand test and has to be estimated for calculation of the correction factor. The iterative procedure introduced in IEC 60060-1:2010 (Annex E) is to reduce the error in the correction factor, hence the corrected test voltage, due to the error of the estimated U50. The error in the calculated correction factor is often significant in comparison with the required tolerance of the test voltage in cases where the correction is large, i.e., where the correction factor deviates from unity by more than the tolerance of the specified test voltage. The iterative procedure in IEC 60060-1:2010 recalculates U50 and U10 until both converge to constant values, and hence reduce the error caused by the initially estimated U50. Atmospheric corrections lately attracted more attention due to the introduction of UHV AC and DC systems, of which some are located at high altitudes. The corrections are necessarily large for test voltages to be used at high altitudes. The examples in section 4.4.4 give the calculated correction factors using the standard procedure, where a measured 50 % probability disruptive-discharge voltage in given conditions may be converted to the value which would have been obtained under standard reference atmospheric conditions. Calculations by the converse iterative procedure are also given to show the differences of the calculated correction factors to those obtained in accordance with standards used for insulation coordination or apparatus standards. It should be noted that the iterative procedure always leads to the lowest value in altitude correction factor (=1/Kt), which are closer to the altitude correction factors calculated using the methods of the other standards, IEC 60071-2:1996 (Insulation coordination: Application Guide) and IEC 61869-1:2007 (Instrument Transformers, General requirements). It should also be noted that for lightning impulse tests on some equipment, such as instrument transformers, no atmospheric corrections are to be applied according to the relevant standards. 4.4.4

Iterative procedure for determining atmospheric correction factor at high altitudes

When the converse procedure is used for determining the atmospheric correction factor, where a withstand test voltage is specified for standard reference atmosphere (Clause 4.3.1, IEC 60060-1:2010) and must be converted into the equivalent value under the test conditions, an iterative procedure described in IEC 60060-1:2010 may need to be used. The application of the iterative procedure is necessary if the correction factor Kt is lower than e.g. 0.95, for reducing the error of calculated correction factor for a high altitude test site. In the iterative calculation procedure, the correction factor Kt is calculated by iteration until it converges to be within a predetermined limit, i.e.: (i) = 1.1 × with

(i-1) = 1.1 ×

(i-1) ×

being the specified test voltage.

The iteration is continued until

1

, where i is the number of the iteration.

Figure 5 to Figure 8 show atmospheric correction factors Kt, calculated with the non-iterative procedure and the iterative procedure, keeping the humidity correction factor to unity. These results are then essentially the inverse of the values of the altitude correction factor used in the insulation coordination standard (IEC 60071-2:1996) and some equipment standards such as IEC 61689-1.

The calculations were performed for AC, lightning impulse (LI) and switching impulse (SI) voltages with gap distances of 3800 mm and 2000 mm and for DC voltage for 4000 mm and 2000 mm. The first number in the legend denotes the gap distance with the 2nd number being the specified test voltage level in kilovolt. For the 2000 mm gap, 806 kV is the AC flashover voltage level (the voltage level of assured disruptive discharge) and 651 kV is the AC withstand test voltage level. For the 3800 mm gap, 1245 kV is the AC flashover voltage level and 1047 kV is the AC withstand test voltage level. For AC (Figure 5) and switching impulse (Figure 8) voltages, the iterative procedure yields higher values of the atmospheric correction factor than the standard procedure. The atmospheric correction factors for tests

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at the withstand level are larger than those for tests at the flashover level. The atmospheric correction factors with the larger gap distance (3800 mm) are larger than those with the smaller gap (2000 mm). For DC voltages (Figure 6) the iterative procedure yields larger altitude correction factor values for tests only at the withstand voltage level. The other results are very similar. For lightning impulse voltages (Figure 7) the correction factors are the same irrespective of the altitude and the calculation procedures for the smaller gap (2000 mm). For the larger gap (3800 mm), the correction factors for tests at the flashover voltage level are the same for both calculation procedures and are the same as those of the smaller gap, only the correction factor at the withstand level obtained from the iterative procedure is larger.

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Figure 5: Atmospheric correction factors K t for AC voltage, obtained with the non-iterative procedure and the iterative procedure.

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Figure 6: Atmospheric correction factors K t for DC voltage, obtained with the non-iterative procedure and the iterative procedure.

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Figure 7: Atmospheric correction factors K t for lightning impulse voltage, obtained with the non-iterative procedure and the iterative procedure.

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Figure 8: Atmospheric correction factors K t for switching impulse voltage, obtained with the non-iterative procedure and the iterative procedure.

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In most cases of applying atmospheric corrections, these are used for withstand testing of either AC, DC, lightning impulse or switching impulse of equipment at an altitude less than the altitude the equipment will be used in service. Here the iterative procedure always leads to the lowest altitude correction factor (1/Kt). 4.4.5

Atmospheric corrections for AC voltages

Note 3 of clause 4.3.3 “Application of correction factors” in IEC 60060-1:2010 points out that the peak value has to be used in correcting power frequency voltages, because the discharge behavior is based on the peak value. Apparatus standards however specify RMS values based on the system voltage for which the equipment is to be used. If the specified RMS test voltage is not converted into the peak value, the calculation of the correction factors becomes erroneous and not consistent. Figure 9Figure 9 shows the effect of using the RMS value instead of the peak value when calculating atmospheric corrections using the standard procedure. The RMS voltage value of 570 kV and 460 kV correspond to a peak voltage value of 806 kV and 651 kV respectively for the 2000mm gap, 880 kV and 740 kV correspond to 1245 kV and 1047 kV respectively for the 3800 mm gap. For smaller gaps the atmospheric correction curves for the peak values of withstand and breakdown voltage converge at an altitude of 2500 m, but the difference between the correction factor using the peak value and the correction factor using the RMS value can be as large as 19 % at 4000 m. For larger gaps the correction factors become much larger when using the RMS value. The difference can be as large as 30 % as is the case for the 3800 mm gap at withstand level for an altitude of 4000 m; even at an altitude of 2000 m the difference is 8 %. For the iterative procedure at withstand level the trend is similar with smaller deviations, from 6 % at 2000 m to 12 % at 4000 m. This is an important note in IEC 60060-1:2010, which can easily be missed.

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Figure 9: Atmospheric correction factors K t for AC voltage, comparison of Peak voltage to RMS voltage.

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4.5 Requirements for measurement systems 4.5.1

Structure of IEC 60060-2:2010

Requirements for measurement systems are specified in IEC 60060-2:2010. The main structure of IEC 60060-2:2010 is similar to that of the previous edition IEC 60060-2:1994, although there have been significant additions of contents to some clauses, for example, the additional requirements in Clause 5 on estimation of measurement uncertainties. Table 6 compares the main clauses of Edition 2.0 and Edition 3.0 of IEC 60060-2. Term “AMS” in Table 6 denotes “Approved Measuring System” and “RMS” denotes “Reference Measuring System”. Both terms are defined in IEC 60060-2. Table 6: Comparison of the main clauses of editions 2.0 (1994) and 3.0 (2010) IEC 60060-2:1994, Ed. 2.0

IEC 60060-2:2010, Ed. 3.0

3. 4.

Definitions and symbols Qualification of AMS

3. 4.

Terms and definitions Qualification of AMS

5. 6.

Acceptance tests on components Performance tests on AMS

5.

Tests and test requirements for AMS, including the uncertainty estimation

7. 8. 9. 10.

Measurement of DC voltage Measurement of AC voltage Measurement of LI voltage Measurement of SI voltage

6. 7. 8. 9.

Measurement of DC voltage Measurement of AC voltage Measurement of LI voltage Measurement of SI voltage

11.

Measurement of impulse currents

12.

Reference Measuring Systems (RMS)

Annex A: Annex B:

Transferred to IEC 62475

Accreditation systems Record of performance

10.

Reference Measuring Systems (RMS)

Transferred to 4.1 and 4.4

Amendment 1/Annex H: Uncertainty estimation

Annex A: Uncertainty estimation (GUM) Annex B: Example for uncertainty calculation

Annex C:

Step response measurement

Annex C: Step response measurement Annex D: Convolution

Annex D: Annex E: Annex F:

Temperature rise of resistors RMS bibliography Summary of tests

Tables transferred to chapters 6 to 9

The two old clauses on acceptance and performance tests have been combined into the new Clause 5. This new clause gives detailed requirements on the determination of the measuring uncertainty. The summary tables for approving the HV measuring systems of different types of test voltages have been transferred from the old Annex F to the relevant clauses on individual voltage types. The old chapter on impulse current measurement has been transferred to the new standard IEC 62475:2010. The content of six of the seven annexes of the old standard have been, where appropriate, transferred to the main text of the new standard. Only the annex on step response measurement (Annex F, now C) remains and is complemented by an informative annex (Annex D) on convolution. 4.5.2

Calibration and estimation of measuring uncertainty

It is strongly expressed in IEC 60060-2:2010 that the preferred calibration procedure is the comparison method using a Reference Measuring System to qualify an Approved Measuring System. This often means that a calibration is performed by a certified calibration laboratory. The calibration procedure starts with the determination of the scale factor by which the recorded reading is multiplied to arrive at the actual test value. It also requires that the estimation of measurement uncertainty should follow ISO/IEC Guide 98-3, “Guide to the Expression of Uncertainty in Measurement” (also referred to

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as GUM in literature). Two new Annexes, annex A and annex B are added to IEC 60060-2:2010 to provide additional practical information and examples of uncertainty estimation in high-voltage measurements. Required measurement uncertainty limits for all types of voltages, for the purposes of high-voltage tests as specified in IEC 60060-1:2010, remain unchanged. An expanded measuring uncertainty of UM ≤ 3 % is required for the test voltage, whereas for time parameters of the full lightning impulse and the switching impulse, a value UM ≤ 10 % is required. For Reference Measuring Systems the values are UM ≤ 1 % for the test voltage and UM ≤ 5 % for time parameters. Only the voltage measurement of front-chopped LI impulse voltage is specified with higher uncertainties, with UM ≤ 5 % for an Approved Measuring System and for UM ≤ 3 % for a Reference Measuring System. Calibration by comparison should normally be performed at several voltage levels. When the rated voltage of the Reference Measuring System is sufficient to cover the assigned measurement range of the Approved Measuring System under calibration, then the number of voltage levels should be equal to or greater than 5 levels. In cases where a Reference Measuring System with sufficient operating voltage is not available, IEC 60060-2:2010, like the previous edition, allows the comparison calibration be performed with the highest calibration voltage being as low as 20 % of the assigned measurement range of the Approved Measuring System. In such cases, a voltage linearity test must be performed in addition to the comparison calibration or determination of scale factor. Several methods for determining linearity for various types of measurement systems are given for voltages up to the levels for 800 kV class equipment testing. It should be noted that specific information on suitable linearity test for DC is not given and may pose a problem. Recent development has however extended the DC calibration voltage available in the world to 1000 kV [38, 39]. IEC 60060-2:2010 provides specific requirements for the calibration and these are given in Clause 5.2.1.3, “Comparison over limited voltage range” and Clause 5.3, “Linearity test”, of the standard. The graphical illustration of this approach is given in Clause 5.2.1.3 and is reproduced in Figure 10 below. As can be seen from Figure 10, the total number of test levels for checking scale factor and linearity, should be at least 6. The lowest voltage level of the linearity test should be performed at the scale factor calibration voltage, which should be at least 20 % of the highest voltage of use for the Approved Measuring System.

Figure 10: Calibration over a limited voltage range 4.5.3

System calibration by calibrations of components

IEC 60060-2:2010 still retains the approach of calibration of a measuring system by calibrations of its components, with the requirements given in Clause 5.2.2. This approach is provided as the alternative method to the method of comparison of the complete measuring system with a Reference Measuring System, which is specified as the preferred method. When planning the calibrations and combining the results for the complete system, the interactions between the components and the influence of the transmission system (measurement cables), have to be considered to arrive at the correct values.

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4.6 Measurement Uncertainty 4.6.1

The approach in IEC 60060-2:2010

The uncertainty calculation has been significantly revised in the latest edition of IEC 60060-2:2010. The reasons for this major revision are mainly two fold. First, the revision is to provide testing personnel a simple and practical method of estimating measurement uncertainties that is consistent with ISO 98-3, Guide on Uncertainty of Measurement (ISO GUM). The method is intended to cover most common cases of high-voltage testing. Second, the approach described in the previous edition of IEC 60060-2 (Appendix H in IEC 60060-2:1994) is no longer considered consistent with the current edition of ISO Guide on Uncertainty of Measurement. That approach categorizes the uncertainty contributions as either “systematic” or “random”. Appendix H of IEC 60060-2:1994 also assumes that all systematic contributions can be covered by two types of distributions, the rectangular distribution and the Gaussian distribution. It also implies that systematic contributions can be considered to have infinite degrees of freedom. In fact, the use of “degrees of freedom” is omitted in Appendix H. These assumptions and omissions are now considered not adequate for many practical applications. Appendix H also omitted the concept “sensitivity coefficient”, another important concept that is used in the current ISO GUM and that has now been adopted in IEC 60060-2:2010. The major change advocated in the ISO GUM is to provide realistic estimates of measurement uncertainties, moving away from treating measurement uncertainty as a safe error limit, that is, treating measurement uncertainty as an estimate of maximum error that can possibly be expected for the measurement. By adopting the latest ISO GUM approach, it is possible to achieve measurement uncertainties that fit a specific testing purpose with less costly equipment and less time consuming procedures. In the 1994 edition of 60060-2, specified fixed limits were given for individual uncertainty components, for example, a 1 % limit was specified for non-linearity of voltage measurement systems. Fixed numbers of repeated measurements were also specified. An example of this is that the number of repeated applied impulses during an impulse voltage calibration was specified to be at least 10. With the adoption of the ISO GUM approach in the 2010 edition, these limits are no longer specified as long as the total expanded uncertainty (expanded uncertainty is a defined term) is within the required limit. The removal of these limits becomes possible because of the adoption of the statistically more rigorous approach of the ISO GUM. The 2010 edition of IEC 60060-2 also adopts an approach that is intended to provide practical help to users of the standard to better adapt to the new method of estimating measurement uncertainties. The latest IEC 60060-2 lists typical sources of uncertainty contributions in measuring systems. It also added two completely new Annexes, Annex A and Annex B, dedicated to the topic of measurement uncertainty. Annex A aims to provide an easy-to-understand explanation of the ISO GUM, assuming that a simplified procedure of the ISO GUM can be used for estimating measurement uncertainties for high-voltage tests in most cases. Annex B gives three examples of uncertainty calculation, all with the assumption the type B components often having high degrees of freedom. This practice is mainly for practical efficiency. It should be pointed out, type B components often do have low degrees of freedom and the “quality” of their estimates is often low. However, the degrees of freedom of individual components do not make a significant difference in the final calculated uncertainties. IEEE Std 4-2013 gives detailed explanation and examples how the degrees of freedom and sensitivity coefficients are determined and used. 4.6.2

Uncertainty Requirement Differences between IEC 60060-2:2010 and IEEE Std 4-2013

Table 7 below is a summary of the differences between the earlier editions and the latest editions of the standards in relation to estimation of measurement uncertainties. It should be emphasized that the differences in the two editions of IEC 60060-2 are significant, not only in details, but also in the general approach. The IEC 60060-2:2010 is very much harmonized with ISO Guide 98-3, while Annex H of IEC 60060-2:1994 was written before ISO Guide 98-3 was published and hence was not fully compatible with it. IEEE Std 4-2013 is also harmonized with ISO Guide 98-3. The main difference from IEC 60060-2: 2010 is that its uncertainty calculation examples contain more details.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards Table 7: Uncertainty estimation in two editions of IEC60060-2 and IEEE Std 4-2013

IEC 60060-2:1994

IEC 60060-2:2010

IEEE Std 4-2013

Uncertainty definition

Clause H.2: preISO/GUM of “uncertainty” is used.

The general term “uncertainty” as defined in IEC 60050 is given in 3.6.

The general term “uncertainty” is not given. This makes no impact to the uncertainty calculation.

Types of uncertainty contributions

H.2.1: Systematic contributions H.2.2: Random contributions

3.6.8: type A evaluation

Type A uncertainty and Type B uncertainty, as defined in ISO GUM is given in D2.

Term for uncertainty result

H.3: Overall uncertainty

3.3.6 Expanded uncertainty (ISO Guide 98-3)

In D2, and interpreted definition of “Expanded Uncertainty” as in “ISO Guide 98-3” is given.

Other relevant terms and definitions

Terms are used without being defined in the standard

Defined in 3.3.1 to 3.3.11, Including ISO/IEC definitions, such as “error”, “standard uncertainty”, “coverage factor” and “traceability”.

Definitions and terms similar to those in IEC 60060-2:2010 are given in Annex D

3.6.9: type B evaluation

Terms, such as “sensitivity coefficient”, “model function” and “effective degrees of freedom” are described in Annex A and Annex B Calculation procedures

Annex H.1 to H.5

5.2 to 5.11, estimation of uncertainty components from contributions such as calibration, dynamic performance, temperature effects, linearity and proximity effects.

Detailed procedure described in D.2 to D.6 of Annex D (informative). No procedure is given in the main text

Requirements for uncertainty components

Limits of individual components are specified, such as 1% for voltage linearity, proximity effect and effect of interference, with overall uncertainty also being specified.

No limits specified for individual components as long as the expanded uncertainty is within the specified value, e.g., 3% for test voltage and 10% for impulse time parameters. Also calculation with one dominant component is acceptable.

No limits specified for components as long as the expanded uncertainty is within the specified value, e.g., 3% for test voltage and 10% for impulse time parameters. Also, there should be at least 3 dominant components of comparable magnitude for achieving reasonable effective degrees of freedom for the expanded uncertainty.

Number of repeated measurements

No. of Measurements is specified for performance test

Not specified, as long as the expanded uncertainty is within the specified value, with the type A uncertainty achieved

Not specified, as long as the expanded uncertainty is within the specified value, with the type A uncertainty achieved

Principles

Limited information

Annex A: An informative annex explaining principles and concepts of ISO GUM

Terms are described Annex D

coverage factor determination

Assume to be 2

calculating coverage factor from effective degrees of freedom, assuming high degrees of freedom for individual type B components

Effective degrees of freedom for calculating coverage factor, estimating realistic degrees of freedom for individual type B components

Calculation examples

Annex H: two examples in H.6

Annex B: three detailed examples given.

Two very detailed examples in Annex D, demonstrating the uncertainty calculation steps and how intermediate values, such as standard uncertainty, degrees of freedom (type B component in particular), combined standard uncertainty, effective degrees of freedom and coverage factor, are determined.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards 4.6.3

Relationship between uncertainty and tolerance

Tolerance and uncertainty are different concepts that are specified in the standards for high-voltage and high-current testing. Uncertainty is a statistical quantity used to estimate the accuracy of measurements. The uncertainty depends in the first instance on the metrological quality of the measuring system, and often a range of other factors that influence the measurement. According to IEC 60060-2:2010 the uncertainty of a high-voltage measurement shall be expressed as an expanded uncertainty with a coverage probability of approximately 95 %, corresponding to a coverage factor k=2 under the assumption of a normal distribution. According to IEC 60060-1:2010, the tolerance constitutes the permitted difference between the measured value and the specified value. According to IEC 60060-1:2010, the uncertainty interval and the tolerance interval are considered separately. This is stated in terms 3.3.1 and 3.3.2 and associated notes. Typical tolerance is ±3 %, and the required expanded measurement uncertainty of test voltage for most high-voltage tests is 3 %. As long as both the tolerance and the uncertainty requirements are met, the test is considered valid. For example, an applied test voltage of 97.1 kV is considered just as valid as an applied voltage of 100.0 kV, for a test with a specified test voltage of 100 kV, as long as the expanded measurement uncertainty of the applied test voltage is within 3 % of the applied test voltage. The tolerance of the test voltage is necessary because it is not possible to set the test voltage exactly at the specified value due to technical and operational limitations. For example, the peak impulse voltage at a given charging voltage may vary to some extend depending on the individual impulse generators. During a 60 second AC voltage withstand test, the output voltage may vary around the initially set voltage due to fluctuation of the input voltage to the high-voltage test transformer. Every effort should be made to set the test voltage to the specified level as closely as possible. An operator should not purposely set the test voltage at the lower limit of the tolerance band. In the case of the tolerance of impulse voltage time parameters, it is considered good practice to use time parameter values that provide efficient testing with available wave shaping components, as long as the time parameters are within the specified tolerance limits. For example, the tolerance limits for the lightning impulse front time are 0.84 µs to 1.56 µs. The true value of a time parameter may fall outside the tolerance limits, for example, the true front time value for a measured front time of 0.85 µs with an uncertainty of 5 % may be less than 0.84 µs, however, the front time of the applied impulse is still considered meeting the requirements of IEC 60060-1:2010 and IEC 60060-2:2010.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

5 DIFFERENCES BETWEEN THE LATEST REVISIONS OF IEC60060 SERIES AND IEEE STD 4 5.1 General structure of IEEE Std 4-2013 IEEE Std 4 is the single standard equivalent of the combined IEC 60060 parts 1 and 2 and is used in North America and several other countries. This format and the practice of including tutorial sections have been kept in the 2013 edition. The current revisions of all these three standards were on-going in parallel for some years. While every attempt has been taken to make the standards compatible with IEC counterparts, there are some minor differences between them. The tutorial information included in this standard is to serve as a knowledge bridge between the novice engineer users and the experienced technical experts.

5.2 Summary of differences The following is a brief summary of the fundamental differences between the current versions of IEC and IEEE versions of High Voltage Testing Standards. There are many other differences such as the inclusion of treatment of methods for dielectric loss measurements in IEEE that do not appear in IEC. This list refers to items that overlap. 5.2.1

Estimation of uncertainty

Although IEEE uses the new term “uncertainty” there are still minor differences in implementation. For example, IEEE Std 4-2013 in Clause 5.7.6.8 does not make a distinction between uncertainty of the calibration and uncertainty in the situation of use. This relates to expanded uncertainty correlation and could result in some increased uncertainties in some cases by using the IEEE method of simplification but the differences should be small. The differences in uncertainty treatment are described in Section 4.6.2 of this document in more details. 5.2.2

Definition of AC peak voltage

Clause 6.1 of IEEE Std 4-2013 differs from IEC 60060-1:2010 definition in that it calls the peak the highest value. This is a small issue with IEC as it does not consider polarity of the highest value that could affect measurements in rare cases. 5.2.3

Linearity test

In Clauses 6.5.3.1 and 6.5.3.2 of IEEE Std 4-2013, there are slight differences from IEC 60060-2:2010 method, where linearity is covered in the calibration range. Extension of linearity from the upper limit of the calibration range is then done as a separate test. 5.2.4

Highest frequency in impulse voltage test circuit

IEEE Std 4-2013 still uses in Clause 8.3.3 the geometrical method for finding the highest frequency to be recorded in an impulse circuit. This has been modified in IEC but is retained in IEEE. In practice this has little effect since most users arbitrarily establish the lower level of oscillation frequency measurement – especially for circuits of large physical dimensions. This area requires more research for UHV measurement systems to balance the realistic performance of a large voltage divider and the need to detect small defects in insulation. 5.2.5

Physical characteristics of reference impulse voltage dividers

IEEE still contains the actual physical characteristics of a reference impulse voltage divider. Such a divider constructed to the requirements specified in the standard constitutes a reference divider without having actual high-voltage calibration of it performed at a National Measurement Institute.

5.3 Conclusion As can be seen above, the technical differences are relatively minor. The main changes for all of these standards relate to uncertainty measurements and new methods for calculating the values from impulse wave shapes. On these fundamental issues IEEE and IEC are in agreement and this will lead to easy harmonization of global testing.

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6 IMPROVEMENTS AND POSSIBLE ADDITIONS IN FUTURE REVISIONS 6.1 DC voltage As energy transmission using high voltage DC increases, testing for this application will become ever more important, leading to need of enhancement of test methods and measurements. Voltage levels have also been increasing to cope with the needs to support rapid urbanization in the world. DC testing, be it applied under clean and dry conditions but at high voltage, or under wet or polluted conditions at any voltage, will suffer from rapid dynamic load changes due to discharge phenomena. The limited available power in generators for DC will lead to transient voltage dips (voltage drops), which may lead to erroneous test results. The problem is two-fold, first to determine what limits on magnitude and duration of voltage dips that can be accepted, and secondly to define parameters for DC measuring systems that will ensure that the dips are properly assessed. The need for recommended ratings for DC test equipment will become more accentuated as the test voltages increase, especially for wet tests. The needs for pollution testing are more severe, but this is not in the domain of the IEC 60060 series. During the last revision of IEC 60060 Parts 1 and 2 and IEEE Std 4, it was found that there was some controversy about the definitions of test source suitability and questions about whether these current definitions could assure users of the ability to achieve the goal of performing repeatable tests in different test facilities. The partial discharge processes under DC are however less well understood and require further research. Work is currently under way in IEC TC 42 to codify present knowledge in this field in an amendment to IEC 60270, High-voltage test techniques - Partial discharge measurements. Further development is however expected in this field. Methods to investigate linearity of DC measuring systems are poorly described in the standards. Lately, however, facilities for calibration up to 1000 kV at high accuracy have been developed in Europe and in Australia. More work is needed on the extension of linearity measurements for extremely high DC voltage Questions for future work:

Is it necessary to define the shape and amplitude of current pulses we expect to see during DC dielectric and wet tests? Should requirements on DC measuring systems also have an option covering the needs of recording transient voltage drops? Is there a need to record of transient events during DC testing? Can we define the bandwidth of the measurement system for dielectric tests and wet tests and for tests to assure that voltage drop is controlled even in the face of massive corona discharges? Can we define the value of DC voltage drop that will result in correct testing? Why do we have different values of allowable AC and DC voltage Drop (20 % and 10 %)? Is there a technical reason? Is better characterization of measuring systems for voltage drop and ripple necessary? How to measure partial discharges under DC stress and what impact it has on the source requirements? Shall we recommend equipment ratings for UHV tests where pollution or wet testing is performed? Are the standards sufficient for normal dielectric tests in UHV range?

6.2 AC voltage Also for testing with AC voltages, concerns are found relating to voltage stability under clean and dry conditions but with high voltage, or under wet or polluted conditions at any voltage, due to rapid dynamic load changes caused by discharge phenomena. The limited available power in generating circuits will lead to transient voltage dips (or voltage drops), which may lead to erroneous test results. The problem is two-fold, first to determine what limits on magnitude and duration of voltage dips can be accepted, and secondly to define parameters for measuring systems for DC that will ensure that the dips are properly assessed. For calibration of UHV systems, presence of corona in the test circuit may change the scale factor of the Approved Measuring System, but can also affect the methods for proving linearity. E.g. a capacitive sensor

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

relies on the size of the high voltage electrodes being precise. Corona activity will release ions into the air, which may distort the electrical field. Test and measurement standards on AC VLF (Very Low Frequency) may also need to be improved and expanded. VLF test systems at 1 Hz or lower were first developed to test primarily long length of cables. Because of the low frequency used, the requirement for the power supply is significantly lower than that for normal AC test source. Therefore, the test equipment is much smaller and lighter. VLF technique has also been used by utilities as a useful onsite diagnostic tool for cable and generator dielectric dissipation factor, power factor, and PD tests. Although some aspects of VLF test requirements have already been specified in IEC 60060-3:2006 [36] and a number of IEEE Standards [40,41], further standardization work is likely to be required to expanded test requirements to cover a wider range of equipment and test conditions as more research results become available. CIGRE working group D1.48 (Properties of Insulating Materials under VLF Voltages) is actively working in this area. Questions for future work:

Is it necessary to define the shape and amplitude of current pulses we expect to see during AC dielectric and wet tests? Is it necessary to define the bandwidth of the measurement system for dielectric tests and wet tests to assure that voltage drop is measured ? Why do we have different values of allowable AC and DC voltage Drop (20 % and 10 %)? Is there a technical reason? Is there a need to revise on-site test techniques as set out in IEC 60060-3? Is there a need to develop requirements on testing at Very Low Frequency today given in IEC 60060-3? How to calibrate UHV level Reference Measuring Systems? How to check linearity in the UHV range? What are the requirements for AC source in the UHV range?

6.3 Lightning impulse voltage Future work in this area involves proving that the dielectric stress produced by various superimposed oscillation frequencies on normal wave shapes can be normalized for various apparatus in terms of insulation performance and that the key parameters can be consistently calculated. 6.3.1

General considerations

The advent of ultra-high voltage (UHV) transmission systems, i.e. over 800 kV, necessitates testing at very high lightning impulse voltages. Both test methods and measurement techniques will have to be further studied. Issues are also open on applicability of calibration methods for UHV lightning impulse measuring systems. This work is largely the responsibility of the relevant apparatus committees Annex D of IEC 60060-1:2010 describes the technical background of the Test Voltage Function (k-factor) for evaluation of impulses with superimposed oscillations on the crest. The majority of tests for establishing that the Test Voltage Function had a basis in the physics of breakdown of different insulation systems were performed at voltages around 100 kV in the first European Project [42]. The tests studied various real test objects with a variety of real impulses with superimposed oscillations that could be adjusted. Additional tests to obtain the test voltage function (or k-factor function) for higher voltages than those used in [42] were carried out in recent research projects such as those conducted at TEPCO and LCOE [43, 44, 45, 46, 47, 48]. The aim of these research programs is to validate and improve the applicability of the test voltage function for future revision of the standards. Details of the test program and presentation of results is being prepared as Technical Brochure in CIGRE from Working Group D1.36. An overview is however presented below. Three different insulation systems, namely: SF6 gas insulation, oil insulation, and air insulated electrode gaps, have been studied using test voltages up to 1.8 MV. The test voltage function verification process is ongoing and improvements now relate to how the function can be fine-tuned for use with complex insulation systems for improved validity. Slight differences in the formulae can more closely track test data depending on the insulation type and the composition of multiple dielectric materials. The focus of the standards to date has been to have a single function definition for all test objects while it is clear that different frequencies of overshoot and oscillation can have different effects on the insulation system being used. This is due to the wide variation in the physical characteristics of those insulation systems. The sections below confirm that for

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

simple geometries and for single dielectric media, the currently published test voltage function (k-Factor) gives correct results. However, it is hoped that through further research in this area, a new universal test voltage function can be determined to suit testing of most insulation systems. 6.3.2

Further investigation of the test voltage function for SF 6

To investigate if the test voltage function in IEC 60060-1:2010 is applicable to a SF6 gas insulated dielectric medium at test voltages above 100 kV, a set of lightning impulse tests using quasi-homogenous field ( 0,45) were performed with negative polarity and test voltages, Ut, up to -1.0 MV. The parameters of these experiments are test voltage, Ut, oscillation frequency, f, of superimposed overshoot and relative overshoot amplitude, ’, according to [15] and the field uniformity according to Schwaiger coefficient, = Emean/Emax. The results were reported in [43, 44, 47] to compare with the k-factor values obtained earlier [42]. Figure 11 shows the compatibility between the results for SF6 dielectric medium obtained by the E.P. Project [42] and by TEPCO-Japan [43, 44, 47] up to -1.0 MV (symbols in color). It was concluded from the results that the experimental test voltage function values obtained for SF6 quasihomogenous field up to -1000 kV agreed with the earlier results of the European Project [42]. Overall, the results of SF6 obtained so far indicate that the test voltage function of IEC 60060-1:2010 is sufficiently accurate for the purposes of high-voltage testing, i.e.:

k( SF6 )

1 1 2.2 f 2

Figure 11: Experimental or test voltage function curve determined in the European Project for SF 6 gas insulation dielectric medium with quasi-homogenous field and further experimental test voltage function (k-factor) values at voltages up to -1000 kV. 6.3.3

Further investigation of the test voltage function for oil

Insulation oil samples with quasi-homogenous field configurations were also tested [43, 44] with lightning impulse, at voltages up to –1000 kV. Impulse voltages of –150 kV and –250 kV were also applied to the practical apparatus models . The parameters of these experiments are test voltage, Ut, oscillation frequency, f, of superimposed overshoot and relative overshoot amplitude, ’, according to IEC 60060-1:2010 and field uniformity according to Schwaiger coefficient, = Emean/Emax. For the oil samples, = 1 was used, whereas for the practical apparatus models, no unique could be established. The results (Figure 12) support the use of the test voltage function defined in IEC 60060-1: 2010. 47

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 12: Experimental test voltage curve (k-factor) determined in the European Project for oil dielectric medium and complementary experimental values for voltages up to -1.0 MV. 6.3.4

Further investigation of the test voltage function for air gaps

Lightning impulse tests at very high voltages are usually performed to prove performance of internal insulation, and therefore the need to evaluate for the characteristics of large air gaps is limited. In those cases switching impulse is usually the decisive test stress. However, for the cases where the external air insulation needs to be verified for lighting impulse performance, the test voltage function is important. Furthermore, at UHV voltage levels, physical size of the test circuits will limit the frequency of overshoot, with the result that the filtering effect of the test voltage function is small. A set of lightning impulse tests were performed with positive polarity for air gaps up to 1.8 MV. Tests with negative polarity were not considered because of the disruptive voltage level is higher than the level for positive polarity. The parameters of these experiments are test voltage, Ut, oscillation frequency, f, of superimposed overshoot and relative overshoot amplitude, ’, according to IEC 60060-1:2010 and field uniformity according to Schwaiger coefficient, , or according to air gap K factor of IEC 60071-2. Air gaps with homogenous field Figure 13 shows the comparison between the results for different air gaps; all of them with a quasihomogenous field obtained by the E.P. Project [42] and recently by LCOE [46] at voltages up to +0.8 MV. The overshoot frequency was limited to 0.25 MHz. Figure 13 shows the compatibility between the test voltage function of the present IEC 60060-1 standard and all the results for air gaps with homogenous field. The experimental test voltage function values are in agreement with the test voltage function of IEC 60060-1:2010.

k( AIR homogeneous

field )

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1 1 2.2 f 2

Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 13: Experimental test voltage function (k factor) for air dielectric medium in a homogenous field determined in the European Project (+98,5 kV) and the complementary experimental k-factor values obtained for voltages up to +800 kV. Air gaps with non-homogenous field

Experimental test voltage function (k-factor) values obtained for air gap spacings with non-homogenous field determined by the E.P. Project [42] and results recently obtained by TEPCO [43, 48] and LCOE [44] are shown in Figure 14. The test voltage functions (k-factor) for air gaps with a non-homogenous field depend on the air gap spacing, d, and on the non-homogeneity field given by the air-gap K factor parameter defined in IEC 60071-2. It is obvious from the test results that the test voltage function for air gaps can conceivably be quite different depending on the gap geometry. The applicability of one single test voltage curve may be questionable. Further research is on-going in CIGRE Working Group D1.36 and will be reported in a forthcoming Technical Brochure.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards k-factor; K=1 EP 0.15m

1

0.5m 0.8

k (pu)

1.0m 2.0m 2.5m

0.6

0.4

0.2

0 3 10

4

10

5

6

10

10

7

10

8

10

f (Hz)

Figure 14: Family of the test voltage function (k-factor) curves for rod plate configuration (K=1) for different air gap distances. 6.3.5

Test voltage function for multiple insulation materials

When multiple insulation media are involved in the same testing object the most restrictive test voltage function should be chosen in order to ensure that the equipment passes the lightning impulse test (e.g. the oil test voltage function should be used when both oil and SF6 insulations are involved in the test). 6.3.6

Generation and measurement of UHV lightning impulse

The impulse generator and associated circuits grow in size when the test voltage increases to UHV levels. From this follows that inductances in the circuit will increase. It is also observed that the capacitance of some test objects like cables, increase with the voltage level. Together these two physical realities mean that a standard waveform with a front time of 1.2 µs ± 30 % becomes difficult or even impossible to achieve. This matter is currently under study in CIGRE Working Group D1.36 and will be presented in a forthcoming Technical Brochure. Preliminary findings do however indicate that it will be necessary to relax the tolerance requirement on front. Also, the large circuit dimensions also limit the performance of UHV voltage dividers. A fundamental limit of the bandwidth of large devices may preclude their ability to measure standard lighting impulses. It is suspected, but not proven, that UHV dividers may be difficult to use in measurement of impulses approaching the lower tolerance limit for front time. Further research in this area is needed.

6.4 Improvement of atmospheric correction factor calculations 6.4.1

General remarks

Atmospheric corrections are performed for several purposes in high-voltage engineering.

To correct the disruptive discharge voltage of insulation under a particular test atmospheric condition to a voltage under the standard atmospheric conditions. To calculate test voltage levels to be utilized in testing when the testing is performed at an altitude significantly different from the site where the equipment will be installed. To provide altitude correction for correct choice of external insulation distances.

These calculations have evolved under different Technical Committees in Standardization and may lead to non-compatible corrections. As will be shown in the following sections, these differences tend to escalate

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards with higher voltage and/or higher altitudes. The inevitable conclusion is that these methods have to be reexamined and coordinated to provide reliable and compatible results 6.4.2

Differences between IEC standards

Atmospheric corrections are also performed for purposes such as insulation coordination defined in IEC 60071-2 and for altitude correction. The purpose is the determination of the insulation distance of equipment in relation to the altitude of its installation. An example of equipment altitude correction is the use of correction factor k in IEC 61968 1 (Instrument transformers, General requirements), where the required insulation distance is corrected for the service altitude using factor k. In these cases, only the correction due to altitude (hence the atmospheric pressure) is considered, while temperature and humidity are not considered in the correction factor. For example, the altitude correction factor in the insulation coordination standard IEC 60071-2- is defined as: where H is the altitude above sea level (in meters) and the value of m is as follows: m = 1.0 for lightning impulse voltages m = values as defined in Figure 15 (re-produced from Figure 9 of IEC 60071-2) for switching impulse withstand voltages m = 1.0 for short duration power-frequency withstand voltages of air-gaps and clean insulators

Figure 15: A reproduction of Figure 9 of IEC 60071-2. In IEC 61869-1:2007, it is specified that for installation at an altitude higher than 1000 m, the arcing distance under the standardized reference atmospheric conditions shall be determined by multiplying the withstand voltages required at the service location by a factor k in accordance with its Figure 1, reproduced in Figure 16 below.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 16: A reproduction of Figure 2 of IEC 61869-1:2007. In earlier international standards preceding IEC 61869-1:2007, such as IEC 186, the correction was performed by adding 10 % arcing distance per 1000 m increase in altitude for altitudes greater than 1000 m. Some IEEE standards specify the increase in the arcing distance by applying a dielectric strength correction factor of -0.1 per 1000 m for altitudes greater than 1000 m. In the first Edition of IEC 60060 1 published in 1973, the following formula for calculating the withstand voltages at higher altitudes was given

with b0 = 101.3 kPa, being the average air pressure at the sea level. The corresponding withstand voltage at the higher altitudes was given by

with Uwo the withstand voltage at pressure b0. The coefficient m is defined to be 1 for direct and lightning impulse voltages of any gap configuration and polarity, as well as for homogenous gaps with any kind of voltage. For larger gap distances, with approximate rod-rod and rod-plane gaps under AC voltage and switching impulse voltage, the coefficient m was defined as a function of the gap length shown in Figure 17.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 17: The coefficient m in the air density correction factor versus gap spacing d (IEC 60060-1:1973). Based on these definitions, the inverse of the atmospheric correction factor curves (1/kt) derived from IEC 60060-1:2010 presented above were compared with the altitude correction curves derived from these standards and the corrections derived using the correction method in the first edition of IEC 60060-1. The factor m for switching impulse voltages were taken for the different gaps, voltages and standards as following: m = 0.6 for 1300 kV, m = 0.64 for 1175 kV for 3800 mm gap distance and m = 0.78 for 800 kV, m = 0.83 for 700 kV for 2000 mm gap distance (IEC 60071-2) m = 0.88 for 3800 mm gap distance and m = 0.64 for 2000 mm gap distance (IEC 60060-1:1973) The curves calculated for IEC 60044 and IEEE were adjusted to 1000 m using the correction factor of IEC 60071-2 at 1000 m for the respective voltage. Figure 18 to Figure 20 show the comparison of the four different standards and the calculations according to the first edition of IEC 60060 1 for AC, LI and SI voltages, for withstand and flashover voltages of the 2000 mm and 3800 mm gaps. The curves for DC calculated using IEC 60060-1:2010 were not compared to IEC 60071-2, IEC 61869-1:2007 and the IEEE apparatus standards, since these standards do not define a method for DC voltages. Only comparisons to the values achieved with the iterative procedure in IEC 60060-1:2010 are shown, differences to the standard procedure for withstand voltages are shown in Table 8. Figure 18 and Table 8 show that for AC test voltages IEC 60060-1: 2010 will always give lower correction factors. The correction factors derived using the iterative procedure in IEC 60060-1:2010 result in larger differences to factors calculated using IEC 60071-2 and IEC 61869-1:2007. Above 1000 m the method in IEC 61869-1:2007 gives lower results than those derived from IEC 60071-2 due to the deduction of the first 1000 m from the actual altitude. The IEEE apparatus standards, which would correspond to the older procedure in IEC standards, give typically lower correction factors than IEC 60071 2 and IEC 61869-1:2007, but would still give larger correction factors than IEC 60060-1:2010. The correction method defined in IEC 60060-1:1973 gives results, which lay between the results derived with the methods of IEC 61869-1:2007 and some IEEE apparatus standards for the shorter gap but is closer to the results of IEC 60060-1:2010 for withstand levels at the longer gap. For longer gaps the differences of IEC 60060-1:2010 to the other standards are in general larger than for the shorter gap.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Figure 18: Comparison of Atmospheric correction factors 1/K t for AC voltage, calculated according to different standards.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards Figure 19 gives the results for the calculated correction factors for LI voltages and shows good agreement between IEC 60060-1:2010 and IEC 60071-2, except for withstand levels for the 3800 mm gap. The correction factors derived using IEC 61869-1:2007 give the same results as for AC voltages. For apparatus design either the AC withstand voltage for shorter gaps, lower system voltages with Um < 300 kV, or the SI withstand voltage for larger gaps, higher system voltages with Um ≥ 300 kV, are relevant for the design of the external insulation distance. For testing of apparatus no correction for atmospheric conditions for lightning impulse testing is permitted, therefore these differences can be neglected. The correction factors used in IEEE standards typically give lower correction factors.

Figure 19: Comparison of Atmospheric correction factors 1/K t for LI voltage, calculated according to different standards.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards The calculated correction factors for SI voltages shown in Figure 20, have the results using the iterative procedure of IEC 60060-1:2010 as the boundaries between which the results of IEC 60071-2 and IEC 61869-1:2007 lie, the correction factors for flashover values being the upper limit and the correction factors for withstand levels giving the lower limit. The correction factors derived with the IEEE method and the corresponding older approach in IEC give in general higher correction factors than derived from the newer IEC standards.

Figure 20: Comparison of Atmospheric correction factors 1/K t for SI voltage, calculated according to different standards.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

For higher voltages, IEC 61869-1:2007 gives larger correction factors than IEC 60071-2:1996 since the fixed exponent m of 0.75 used in IEC 61869-1:2007 is larger than the one calculated for the two voltages according to IEC 60071-2, m = 0.6 for 1300 kV, m = 0.64 for 1175 kV. For lower voltages the exponent m is smaller for IEC 61869-1:2007, thus the calculated correction factors are smaller than for IEC 60071-2:1996. Table 8: largest calculated % differences in correction factors for AC and SI withstand voltages between the standards considered

AC

2000 mm gap

3800 mm gap

IEC 60060-1 to IEC 60071-2

-5.2 %

-10.5 %

IEC 60060-1(iterative) to IEC 60071-2

-14.9 %

-20.5 %

IEC 60060-1 to IEC 61869-1: 2007

-4.7 %

-9.2 %

IEC 60060-1(iterative) to IEC 61869-1: 2007

-11.7 %

-17.5 %

IEC 60060-1 to IEC 60071-2

+8.6 %

+19.3 %

IEC 60060-1(iterative) to IEC 60071-2

-4.4 %

-2.6 %

IEC 60060-1 to IEC 61869-1: 2007

+14.6 %

+16.6 %

IEC 60060-1(iterative) to IEC 61869-1: 2007

-2.6 %

-3.4 %

SI

Table 8 shows as a summary the largest calculated % difference between correction factors derived with IEC 60060-1:2010, IEC 60071-2:1996 and IEC 61869-1:2007 for AC and Switching Impulse voltage. For AC voltages the difference varies between -4.7 % comparing the results for the standard procedure in IEC 60060-1:2010 and IEC 61869-1:2007 for the 2000 mm gap and -20.5 % comparing results for the iterative procedure in IEC 60060-1:2010 and IEC 60071-2:1996 for the 3800 mm gap. For Switching Impulse voltages this difference varies between +19.3 % comparing the results for the standard procedure in IEC 60060-1:2010 and IEC 60071-2:1996 for the 3800 mm gap and -2.6 % comparing the results for the iterative procedure in IEC 60060-1:2010 and IEC 60071-2:1996 for the same gap. The iterative procedure in IEC 60060-1:2010 gives in principle smaller correction factors than the standard procedure, which get closer to the correction factors of both IEC 60071-2:1996 and IWC 61869-1:2007 for Switching Impulse voltages but the difference to these standards, gets larger for AC voltages when using the iterative procedure. 6.4.3

Future work on atmospheric correction factors

The differences and inconsistencies between standards shown above lead to the question of the correct procedure. Even though IEC 60071-2:1996 is related to insulation coordination and IEC 60060-1:2010 to testing of equipment, and therefore fulfills different purposes, the physical background for the correction factors is the same, the reduction in insulation strength of air insulation due to higher altitude respectively lower pressure. In order to further analyze these differences and determine atmospheric correction factors (temperature, pressure, absolute humidity) a JWG of IEC TC 28, TC 42, TC 36 and TC 115, JWG 22 “Atmospheric and altitude correction” and a new CIGRE WG D1.50 “Atmospheric and altitude correction factors for air gaps and clean insulators” were established. The results of these two groups may have an influence on the procedure to calculate correction factors in IEC 60060-1:2010. There is also a need to extend the humidity range of the atmospheric correction factors, Experimental results of a recent research work can be found in [49].

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

6.5 Review of voltage drop, AC and DC for future development The current IEEE and IEC standards for HV Testing include some details about the allowable voltage drop due to the interaction of the test object, the test circuit and the test source. These conditions are for normal dry or wet test situations but do not cover the specific demands of artificial pollution tests, which are covered by IEC 60507.

Summary of IEC Requirements for AC and DC testing

Test voltages for tests longer than 1 minute should be within ±3 % AC Transient voltage drops can be up to 20 % DC Transient voltage drops can be up to 10 % Bandwidth guidelines are given for voltage measurement systems but actual frequency values are not. Questions for future Revision of High Voltage Testing Standards with respect to dynamic load

Can we define the shape and amplitude of current pulses we expect to see during AC and DC dielectric and Wet Tests? Is this information important? Can we measure the current pulses accurately? Does it matter if we can do this as long as we require adequate measurement systems to record the transient voltage drop? Can we define the bandwidth of the measurement system for dielectric tests and wet tests and for tests where users allow massive corona discharges in their test circuits to assure that voltage drop is controlled? Can we define the value of AC and DC voltage drop that will result in correct testing? Why do we have different values of allowable AC and DC voltage Drop (20 % and 10 %)? Is there a technical reason?

Questions for Future Revision of High-voltage Testing Standards

Shall we specify a maximum voltage drop during AC and DC dry and wet tests of less than 10 % for transients of more than 1 second or 2 seconds for AC and more than 1 second or 2 seconds for DC? Or should we choose other time durations that relates to errors in testing? Shall we specify the upper frequency limit of the voltage measurement system for AC tests where streamers or wet conditions are anticipated to be greater than 7 times the fundamental frequency? Or should we choose a measurement bandwidth based on the source frequency? Shall we specify the time constant of voltage measurement systems for DC tests where streamers or wet conditions are anticipated to be less than 0.25 seconds? Or should we choose some other time period that relates to errors in testing. Shall we specify the voltage record length of tests with streamers or wet conditions to be equal to the withstand test time? Or should we sample intervals and compare sequential intervals? In the end, the relevant apparatus committees have to decide how much deviation from the test voltage can be accepted before the performance of the device being tested is in question. For dry testing indoors of normal proof or withstand tests, if the percentage of voltage cycles that have more than 10 % lower peak value are less than a few percent of the total test cycles, we can demonstrate compliance with a standard. For DC this could be a percentage of the time-on vs. the time where voltage drops are more than 10 %. Since there is no peak to evaluate, the dip will have to be more than a certain time, such as the charging cycle of the DC generator. For wet testing or pollution testing the same method could be used. With digital recorders it is possible to check each voltage cycle or in the case of DC each recharge period time of the DC generator and compare to the duration of the test for a percentage of time the peak voltage is down by 10 % or less.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

6.6 Waveforms of lightning impulse voltage and lightning impulse current 6.6.1

Introduction

The present standard waveforms of the lightning voltage and current impulses were derived based on lightning surge waveforms measured in the field many decades ago. Present power systems and equipment are characterized by various new factors, compared to those from the past. These factors include the wide use of gas-insulated switchgear (GIS) in substations, installation of overhead ground wires, use of highperformance surge arresters with improved characteristics, increase in ground electrostatic capacitance associated with a higher voltage and larger equipment capacity, and variation in the distance between substations. Because of the widespread changes it is necessary to re-assess the lightning impulse waveforms that are relevant to the modern electric power network. Observed results of studying lightning phenomena may differ depending on the country and regional situation. Papers [50-54] reported the recent observation results of lightning surges in the actual fields at some specific facilities. 6.6.2

Observation of atmospheric lightning strike voltage waveforms

The lightning strike voltages were observed at 10 substations, namely eight GIS substations and two airinsulated (AIS) substations of 500 kV and UHV designed (500 kV operation) transmission lines [50,51,52,53] for the purpose of setting the lightning current waveform and crest value in the lightning surge calculation reasonably and more accurately when calculating the lightning failure rate of transmission lines. On the basis of the observed data [52], Figure 21 presents the characteristics of the direct lightning strike waveforms observed at Switching Substations, denoted N- and M-, where UHV designed transmission lines are connected and at the Switching Substation denoted S-, where 500 kV transmission lines are connected. The parameters analyzed are the crest value vs. front time and the crest value vs. time to half-value. Here, the crest value indicates that of the lightning surge component, disregarding the power frequency component. 800 700

N-Switching Substation M-Switching Substation S-Switching Substation

700 600 Voltage (kV)

600 Voltage (kV)

800

N-Switching Substation M-Switching Substation S-Switching Substation

500 400 300

500 400 300

200

200

100

100

0

0

1

10

100

1000

1

(a) Crest value - front time relationship

10

100

1000

Time (μs)

Time (μs)

(b) Crest value - time to half-value relationship

Figure 21: Characteristics of the direct lightning surge waveform parameters. The 50 % value of the cumulative frequency of the crest values is 240 kV. For the front time and the time to half-value, the 50 % values of the cumulative frequency are 5.9 μs and 36 μs, respectively. Even within the present measurement, some waveforms are characterized depending on the measurement locations due to the differences in insulation voltage classes and system conditions. 6.6.3

Observation of atmospheric lightning strike current waveforms

The lightning strike current waveforms were also observed at the top of 60 towers of 500 kV and UHV designed (500 kV operation) transmission lines between 1994 and 2004 and 120 sets of data were obtained, including three cases exceeding 100 kA [54]. Figure 22 shows the cumulative occurrence frequency distribution of the front time and the time to half-value. The 50 % value of the front time was 4.8 μs and the

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

50 % value of the time to half-value was 37 μs. Large numbers of lightning current waveforms were previously observed and summarized in CIGRE paper [55].Even though that data was observed over a wide region, the observation results in Figure 22 showed good agreement. 99.9

Cumulative Probability(%) Probability (%) Cumulative

Cumulative Probability(%) Probability (%) Cumulative

99.9 99 95 90 80 70 60 50 40 30 20

99 95 90 80 70 60 50 40 30 20

10 5

10 5

1

1

0.1

1

0.1

10

1

10

Front duration(μs) Front time (μs)

100

1000

Stroke duration(μs) Time to half-value (μs)

(a) Front time

(b) Time to half-value

Figure 22: Cumulative frequency distribution of front time and time to half value of lightning current waveforms [54]. 6.6.4

Conclusion

These measurement results indicate that future standards for high-voltage and high-current testing may need to include the waveforms with these longer front times that have been observed in these modern power networks. This could be useful especially for UHV systems were it is difficult to generate short duration waveforms due to the large dimensions of the test equipment, test loop and test object.

6.7 Improvement of measurement systems 6.7.1

Calibration of UHV impulse measurement systems

More and more power apparatus at the ultra-high-voltage (UHV) level are being tested due to rapid expansion of UHV transmission network. There have been reported difficulties in establishing and documenting voltage linearity that is a required part of the calibration of the high-voltage dividers for impulse voltage testing [24]. The traditional and most efficient method of assessing the voltage linearity of impulse voltage dividers has been comparing the peak impulse voltage against the DC charging voltage of an impulse generator. However, it has been found that this method often does not yield satisfactory results although the divider may in fact be linear. The non-linearity of the impulse generator vs. charging voltage can be due to several factors: non-linearity of the DC charging voltage measurement, non-linearity of the generator output due to internal corona discharges, non-linearity of large moving sphere gaps with varying arc resistance, external corona, or voltage coefficient of the wave shaping resistors or capacitors. Figure 23 shows an example of non-linearity measured with this method. In this case, non-linearity exceeded the 3 % expanded uncertainty limit of the complete measurement system specified IEC 60060-2: 2010 [24]. There is also evidence that the change of the time parameters with voltage could reach the 10 % uncertainty limit specified in the standard (see Figure 24). It should be noted that not all impulse circuits have these problems and impulse generators with good linearity can be produced. New methods, such as field probes, need to be investigated. Field probes have been experimented with in the past and also have disadvantages and advantages. The main problem is sensitivity to discharge currents due to corona in the high-voltage circuit.

Page 53

Past, present and future of IEC and IEEE high‐voltage and high current testing standards The voltage linearity test of UHV impulse measurement systems is one of the topics that are considered by IEC TC42 WG 19, which was set up to accomplish the task of “Adaptation of TC 42 standards to UHV test requirements”.

Figure 23: Voltage linearity test of the 2800 kV divider with positive lightning impulse, with gradient of the line being the first ratio R 1.

Figure 24: Deviation of the time parameters from their values at the lowest test voltage, 2800 kV divider and impulse of positive polarity.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards 6.7.2

Uncertainty and risk assessment

IEC Publication 60060-2:2010 is based on GUM and defines the uncertainty as the expanded uncertainty corresponding to a 95 % level of confidence that the true value can be found within the range given by the measured value ± the expanded uncertainty. The conditions for the practical distribution to correspond to a normal distribution are given in IEC 60060-2, Clause 4.6 and when these conditions are fulfilled then the expanded uncertainty factor is twice the standard uncertainty. Satisfactory methods of determining contributions to the uncertainty and determining the standard uncertainty and the expanded uncertainty are given in IEC 60060-2, Clause 5. Risk assessment is outside the scope of IEC 60060-2 and users are referred to the relevant standards (e.g., ISO 31000:2009 Risk management – Principles and guidelines, and IEC 60050-903 (2013) International Electrotechnical Vocabulary – Part 903: Risk assessment). Users wishing to perform risk assessment should also take account of differences between measured values and the specified test value: these differences are required to be within the tolerances stated in IEC 60060-1. Note that the permitted tolerances should not be used but the actual differences between measured values and the specified test value. 6.7.3

Examples of risk assessment

When comparing a measurement result, with its associated uncertainty, with an acceptance criterion there are different possible evaluations, where the two most important can be denoted “positive proof” and the other “shared risk”. Positive proof poses very strict requirements on the measurement result, leading to very high confidence in the verdict, but it has the disadvantage that there is a risk that potentially good results are disqualified. Shared risk is so termed because the measurement provider and the equipment purchaser share the risk that the verdict may be wrong in the cases where the uncertainty overlaps the tolerance limit. 6.7.3.1 Important terms uncertainty (of measurement)

parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand. error

discrepancy between a measured value and the true, specified or theoretically correct value. tolerance

constitutes the permitted difference between the measured value and the specified value. Comment: Errors should be corrected for. In cases where this is not practical, a corresponding uncertainty contribution should be identified.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

6.7.3.2 Positive proof Let the measured value be V, its expanded uncertainty UV and the pass limits for the tolerance band be W ± a. Case 1: Non-compliance proved - the measured value and both uncertainty limits lie outside the tolerance band. W-a

W

W+a

V V-UV

V+UV

Case 2: Non-compliance not proved - the measured value and both uncertainty limits lie inside the tolerance band. W-a

W

W+a

V V-UV

V+UV

Case 3: Compliance not proved - the measured value lies inside the tolerance band, but the uncertainty lies outside. W-a

W

W+a

V V+UV

V-UV

Case 4: Compliance proved - the measured value lies outside the tolerance band, and the uncertainty lies inside. W-a

W

W+a

V V-UV

V+UV

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

6.7.3.3 Shared risk The risk of an incorrect decision can however be shared on a more equal basis between buyer and seller by accepting also values where uncertainty crosses the tolerance limit. With the same designations as above we have: Case 5: Non-compliance proved - the measured value and both uncertainty limits lie outside the tolerance band. W-a

W

W+a

V V-UV

V+UV

Case 6: Non-compliance proved on basis of shared risk - the measured value lies outside the tolerance band, but one uncertainty limit lies inside. W-a

W

W+a

V V-UV

V+UV

Case 7: Compliance proved on basis of shared risk - the measured value lies inside the tolerance band, but one uncertainty limit lies outside. W-a

W

W+a

V V+UV

V-UV

Case 8: Compliance proved - the measured value and both uncertainty limits lie inside the tolerance band. W-a

W

W+a

V V-UV

V+UV

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

Case 9: Verdict not possible - the measured value lies clearly inside the tolerance limit, but the uncertainty limits both fall outside. It is recommended to require the measurement uncertainty to be low, e.g. less than 1/3rd of the tolerance limit in order to reduce the number of the cases where verdict is not possible. W-a

W

W+a

V V-UV

6.7.4

V+UV

Measurement software

Currently, requirements for software used for impulse measurement are specified in IEC 61083-2. This is the only IEC standard on software in the area of high-voltage test and measurement. However, more and more digital instruments equipped with measurement software are being used in all types of high-voltage tests, including AC and DC tests. These digital measurement systems have many advantages over the traditional analogue systems in that they allow measurement of not only the DC or AC signal, but also other high frequency signals, such as fast voltage changes and transient voltage drops due to pre-discharges in a pollution test. Since software is an important part of a measuring system, systematic testing of software to ensure its reliability and performance is often necessary. In light of this situation, IEC technical committee 42 is currently drafting a new standard on the requirements for software used for DC and AC measurements. This new standard is likely to be assigned as IEC61083-4

7 CONCLUSION Significant improvements have been made in the recently revised IEC and IEEE standards for high-voltage and high-current tests and measurements. These improvements reflect the change of industry needs, such as the testing in the UHV range, as well as advancement of technologies, such as digital measurement techniques. A much higher degree of harmonization has also been achieved between the corresponding IEC and IEEE standards, which would no doubt bring benefits to the power industry. Revision of standards is a continuing process. A number of areas that future revision of the standards should consider have also been identified in this document.

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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

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