IEEE Std 1799-2012 -IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Windings

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

IEEE Power and Energy Society

Sponsored by the Electric Machinery Committee

IEEE 3 Park Avenue New York, NY 10016-5997 USA 30 November 2012

IEEE Std 1799™-2012

IEEE Std 1799™-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings Sponsor

Electric Machinery Committee of the

IEEE Power and Energy Society Approved 19 October 2012

IEEE-SA Standards Board

Abstract: The procedure for quality control testing of external discharges on stator coils, bars and windings of large air-cooled ac electric machines is described in this recommended practice. Keywords: ac, corona-imaging instrument, discharge inception voltage, electrical insulation, external discharges, IEEE 1799, stator winding, ultraviolet radiation •

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Participants At the time this IEEE recommended practice was completed, the P1799 Working Group had the following membership: Remi Tremblay, Chair Claude Hudon, Secretary David Agnew Kevin Alewine Raymond Bartnikas Kevin Backer Stefano Bomben Andy Brown Donald Campbell William Chen Doug Conley Ian Culbert Jeffrey Fenwick Shawn Filliben Nancy Frost Paul Gaberson

Michel Gagné Bal Gupta Gary Heuston Richard Huber Marcelo Jacob Aleksandra Jeremic Aleksandr Khazanoy Amir Khosravi Thomas Klamt Inna Kremza Laurent Lamarre Gerhard Lemesch Rimma Malamud William McDermid David McKinnon

Charles Millet Glenn Mottershead Beant Nindra Sophie Noel Ramtin Omranipour Howard Penrose Helene Provencher Emad Sharifi John Schmidt Jeffrey Sheaffer Reza Soltani Gregory Stone Chuck Wilson Hugh Zhu

The following members of the individual balloting committee voted on this recommended practice. Balloters may have voted for approval, disapproval, or abstention. Michael Adams David Agnew Martin Baur Thomas Bishop Stefano Bomben Steven Brockschink Chris Brooks Donald Campbell Weijen Chen Ian Culbert Matthew Davis Ray Davis Gary Donner Gary Engmann Jeffrey Fenwick Jorge Fernandez Daher Sudath Fernando Rostyslaw Fostiak Paul Gaberson Michel Gagné

Randall Groves Bal Gupta Werner Hoelzl Claude Hudon Innocent Kamwa Jim Kulchisky Chung-Yiu Lam Benjamin Lanz William Lockley Greg Luri Rimma Malamud William McBride William McCown William McDermid David McKinnon Don McLaren James Michalec G. Harold Miller Charles Millet Jerry Murphy

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Arthur Neubauer Michael S. Newman William Newman Sophie Noel Lorraine Padden Christopher Petrola Alvaro Portillo Iulian Profir Bartien Sayogo John Schmidt Jeffrey Sheaffer Gil Shultz Reza Soltani Gary Stoedter Gregory Stone James Timperley Remi Tremblay John Vergis Kenneth White Hugh Zhu

When the IEEE-SA Standards Board approved this recommended practice on 19 October 2012, it had the following membership: Richard H. Hulett, Chair John Kulick, Vice Chair Robert M. Grow, Past Chair Konstantinos Karachalios, Secretary Satish Aggarwal Masayuki Ariyoshi Peter Balma William Bartley Ted Burse Clint Chaplin Wael Diab Jean-Philippe Faure

Alexander Gelman Paul Houzé Jim Hughes Young Kyun Kim Joseph L. Koepfinger* John Kulick David J. Law Thomas Lee Hung Ling

Oleg Logvinov Ted Olsen Gary Robinson Jon Walter Rosdahl Mike Seavey Yatin Trivedi Phil Winston Yu Yuan

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons: Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative Julie Alessi IEEE Standards Program Manager, Document Development Malia Zaman IEEE Standards Program Manager, Technical Program Development

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Introduction This introduction is not part of IEEE Std 1799-2012, IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings.

External discharges in the end-windings are caused by inadequate workmanship for globally vacuumpressure impregnated (VPI) stators or problems on stators assembled on-site. Poorly finished lashes with an insufficient gap between bars produces coil-to-coil or bar-to-bar discharges. Misalignment between adjacent coils or bars may also reduce the gap distance and generate a high electric stress larger than the air breakdown strength. Sometimes misplaced resistance temperature detector (RTD) or air gap monitor leads have been seen to cause partial discharges (PDs) with high-voltage bars or coils. External discharges for the individual coil/bar could also be a result of improper design, improper material, or improper workmanship. After many years, the deterioration induces surface degradation that may lead, in the long run, to a phaseto-ground fault and reduce the overall reliability of the system. More detail on the theory of external discharges and their effects is given in Annex A. Some utilities have seen deterioration of the junction between the stress control coating and semiconducting slot coating of stator windings after only a few years of operation. Other secondary effects, such as the production of a large quantity of ozone, which may be deleterious to the equipment and dangerous to personnel, is also of concern. In addition, over the years, the ground-wall insulation thickness of stator coils and bars has been reduced to improve heat transfer through the ground-wall insulation. This optimization does, however, increase the dielectric stress on the insulation and on the end-winding stress grading system making them more susceptible to developing electrical discharges. In the current recommended practice, the term “semiconducting slot coating” is preferred to “semiconductive slot coating” often used in the industry. These coatings, composed of resin, varnish, enamels, or other compounds, are filled with carbon black powder, graphite, or other filler and should have electrical resistivity per unit of surface of 1 × 102 – 5 × 105 Ohms per square. The semiconducting slot coating applied on the insulation surface of the slot parts of winding must have uniform tight contacts with the grounded walls of the stator slot. This coating provides minimum voltage between the surface of the coil or bar and the grounded stator core. A stress control coating must be applied on the end turns of high-voltage stator winding and overlap the semiconducting slot coating to provide electrical contact between them. The stress control coating has a non-linear resistance with voltage. This recommended practice presents two methods for evaluating the quality of materials and design, factory workmanship, and on-site workmanship. The first one, the blackout test, has been used for many years. The second one, the corona-imaging inspection, is more recent and presents several advantages. Each method has its advantages and disadvantages. IEEE Std 1434 mentions these two inspection methods but with very little detail. The current recommended practice includes a more elaborate description of sample preparation, bench tests, test conditions, and acceptance criteria in the factory and on-site.

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Contents 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 2 1.2 Purpose ................................................................................................................................................ 2 2. Normative references.................................................................................................................................. 2 3. Definitions .................................................................................................................................................. 3 4. Test preparation and safety......................................................................................................................... 5 5. Test equipment and connections................................................................................................................. 5 5.1 Sensitivity of the corona-imaging instruments .................................................................................... 6 6. Quality control test of external discharges with corona-imaging instrument or blackout test.................... 7 6.1 Factory test on coils and bars............................................................................................................... 8 6.2 Stator model test ................................................................................................................................ 10 6.3 Test on fully assembled stator windings............................................................................................ 14 7. Data records.............................................................................................................................................. 20 7.1 How to fill the data logging tables..................................................................................................... 20 Annex A (informative) Theory of optical emissions from external discharges............................................ 24 Annex B (informative) Variability of discharge inception and extinction voltages ..................................... 27 Annex C (informative) Example of determination of the maximum voltage for a specific winding diagram........................................................................................................................................... 28 Annex D (informative) Example of correction factor to apply to the test voltage of a stator model and VPI stator for a machine which will operate at altitudes of more than 1000 m ..................................... 31 Annex E (informative) Example of operating-voltage table and bar/coil identification table used during test..................................................................................................................................................... 33 Annex F (informative) Bibliography ............................................................................................................ 35

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IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, health, or environmental protection, or ensure against interference with or from other devices or networks. Implementers of IEEE Standards documents are responsible for determining and complying with all appropriate safety, security, environmental, health, and interference protection practices and all applicable laws and regulations. This IEEE document is made available for use subject to important notices and legal disclaimers. These notices and disclaimers appear in all publications containing this document and may be found under the heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Documents.” They can also be obtained on request from IEEE or viewed at http://standards.ieee.org/IPR/disclaimers.html.

1. Overview This quality control test is used to confirm that the insulation system of the stator winding of generator and motor operating in air, including the semiconducting slot and stress control coatings, are free of external discharges. Quality control of the semiconducting slot coating, stress control coating, and manufacturing process is best done in the factory. For stators assembled on-site, such as those for large hydro-generators, additional tests can be performed on the fully assembled generator in order to control the quality of the assembly and workmanship. This control includes: a)

evaluation of the spacing between end-arms and with the phase circuit rings or connections to the main phase terminals

b)

confirming proper alignment of the ground plane made by the semiconducting slot coating on the straight portion of the bar/coil with regard to the core pressure finger

c)

the positioning of all cables (RTD, air gap monitor) with respect to high voltage and

d)

inspection of imperfections that may have been introduced during assembly (presence of foreign objects, misplaced slot center filler, chips and scratches to bars or coils coating)

In the case of machines assembled in the factory, such as VPI machines, the complete quality control test can be done in the factory. However, special care should be taken so that no change in the machine’s

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

conditions occur during transportation (contamination by water and dust, or damage to end-arms during movement). The use of this recommended practice may eliminate the need for users to specify minimum clearances between bars/coils in the end-winding to avoid surface discharge activity.

1.1 Scope This recommended practice provides a procedure to detect external discharges in form-wound bars and coils and complete stator windings of rotating machines operating in air with a rated line-to-line voltage greater than 4200 V at power frequency. The recommended practice is applicable to bars, coils, and complete stator windings. The recommended practice covers two inspection methods: the visual blackout test, and the use of corona imaging instruments.

1.2 Purpose The purpose of this recommended practice is to suggest specimen preparation, test parameters, and procedures for detecting external discharges associated with bars, coils, and complete stator windings using the above mentioned methods. It also recommends acceptance criteria and a procedure for retest in the event of a test failure.

2. Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. IEC 60204-1, Safety of machinery—Electrical equipment of machines—Part 1: General requirements. IEC 61508, Functional safety of electrical/electronic/programmable electronic safety-related systems. IEEE Std 4, IEEE Standard Techniques for High-Voltage Testing. 1, 2 IEEE Std 4a, Amendment to IEEE Standard Techniques for High-Voltage Testing. IEEE Std 510-1983 (Withdrawn), Recommended Practice for Safety in High-Voltage and High-Power Testing. 3 ISO 14121-1, Safety of Machinery—Risk Assessment—Part 1: Principles. ISO/TR 14121-2, Safety of Machinery—Risk Assessment—Part 2: Practical Guidance and Examples of Methods 2.

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The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc. IEEE publications are available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 3 IEEE Std 510-1983 has been withdrawn; however, copies can be obtained from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 2

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

3. Definitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 4 blackout test: A test performed after eliminating all ambient light, specifically on energized electrical equipment, to detect external or surface discharges visible with the human eye (naked eye) after at least 15 min of acclimatization. conducting materials: Composition materials which usually have a dielectric binder and conductive filler (e.g., electrical insulation coating or compound filled with copper, silver powder, etc.). conductive materials: Solid materials which have a large number of free electrons that can easily be put into motion to create an electric current (e.g., metal [as steel, copper] sheet, copper foil, copper, silver powder, etc.). corona (air): A luminous discharge due to ionization of the air surrounding a conductor or insulated conductor caused by a voltage gradient exceeding a certain critical value. corona imaging instrument: An instrument used for visual detection of corona or external surface discharges on energized test objects in ambient light, frequently using ultraviolet radiation emitted by the discharge source. discharge extinction voltage (rotating machinery) DEV (ionization or corona-extinction voltage): The voltage at which discharge pulses that have been observed in an insulation system, using a discharge detector of specified sensitivity, cease to be detectable as the voltage applied to the system is decreased. discharge inception voltage (rotating machinery) DIV (ionization or corona inception voltage): The voltage at which discharge pulses in an insulation system become observable with a discharge detector of specified sensitivity as the voltage applied to the system is raised. external discharge: In rotating machines, external discharges may occur on the surface of bars/coils or in any air gap present between the bar/coil surface and the stator core, or in the end-winding of the stator. groundwall insulation: The main high-voltage electrical insulation that separates the copper conductors from the grounded stator core in motor and generator stator windings. high-potential test (power operations): A test that consists of the application of a voltage higher than the rated voltage for a specified time for the purpose of determining the adequacy against breakdown of high voltage insulation system and spacing under normal conditions. Syn: high pot; hipot. NOTE—The test is used as a proof test of new apparatus, a maintenance test on older equipment, or as one method of evaluating developmental insulation systems. 5

ionization: (A) A breakdown that occurs in parts of a dielectric when the electric stress in those parts exceeds a critical value without initiating a complete breakdown of the insulation system. (B) The process by which an atom or molecule receives enough energy (by collision with electrons, photons, etc.) to split into one or more free electrons and a positive ion. Ionization is a special case of charging. NOTE—Ionization can occur on both internal and external parts of a device. It is a source of radio noise and can damage insulation. 4 IEEE Standards Dictionary Online subscription is available at: http://www.ieee.org/portal/innovate/products/standard/standards_dictionary.html. 5 Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this standard.

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

noise: Unwanted disturbances superimposed on a useful signal that tend to obscure the signal’s information content. off-line testing (test, measurement, and diagnostic equipment): Testing of the unit under test removed from its operational environment or its operational equipment. Shop testing. ohms per square: A unit of surface resistivity used to characterize the resistance of a thin film material measured between two opposite sides of a square and is independent of the size of the square or its dimensional units. Surface resistivity can also be measured in a concentric ring fixture. partial discharge (PD): An electric discharge which only partially bridges the insulation between conductors and which may or may not occur adjacent to a conductor. NOTE—Partial discharges occur when the local electric-field intensity exceeds the dielectric strength of the dielectric involved, resulting in local ionization and breakdown. Depending on intensity, partial discharges are often accompanied by emission of light, heat, sound, radio influence voltage (with a wide frequency range) and oxidation if PD occurs in the presence of oxygen. “Corona” has also been used to describe partial discharges. This is a nonpreferred term since it has other unrelated meanings.

semiconducting materials: Composition materials which usually have dielectric binder and semiconductive filler (e.g., electrical insulation coating or compound filled with graphite, carbon black powder, SiC grains, etc.). semiconducting slot coating (rotating machinery): A coating, applied on the insulation surface of the slot parts of winding. The semiconducting coating, compound, or tape in which the powder filler or portion of powder filler is a semiconductive material and the electrical surface resistivity of this coating in such that, when converted into a semiconducting solid layer, is in the range of 1 × 102 – 5 × 105 Ohms per square. This semiconducting slot coating must have uniform tight contacts with the grounded walls of the stator slot. This coating provides minimum voltage between the surface of the coil or bar and the grounded stator core. (adapted from Younsi, K., Ménard, P., and Pellerin, J. [B29]) 6 NOTE—semiconductive slot coating: This alternative terminology, as well as “Slot Corona Protection” and “Conductive Armor” of the above definition is also used in the industry but will not be used in this document.

semiconductive materials: Solid materials which have limited free electrons and main conduction is carried by electron-hole conductivity (n-p transition) (e.g., graphite, carbon black powder, SiC grains, etc.). stress control coating: Coating used for external discharge suppression in the end turn parts of a winding. The semiconducting coatings, compounds, or tapes are often filled with semiconductive material such as silicon carbide grains. The resistivity of this composite is a non linear function of electric field, which modifies the surface resistance and consequently controls the surface potential gradient to a level that is lower than the breakdown strength of the surrounding media, or of the air, in air cooled machines. An overlap between the stress control coating and the semiconducting slot coating is made to provide electrical contact between the two coatings. NOTE—Other terms for this coating are “end grading system” and “stress grading protection” but are not used in this document.

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The numbers in brackets correspond to those of the bibliography in Annex F.

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

ultraviolet radiation: In general, any radiant energy within the wavelength 10 nm to 380 nm (nanometers) is considered ultraviolet radiation. For power engineering purposes, the band of interest is the one of the emission spectrum of electrical discharges in air. The emission bands of nitrogen dominate the optical spectrum of discharges in air. Ninety percent of the total energy of the emitted optical spectrum of PD is in the ultraviolet region (280 nm–405 nm). The main part of the emission is invisible to the human eye. A relatively weak emission around 400 nm can be observed under conditions of absolute darkness. UN: Line-to-line voltage Uo: Line-to-ground voltage

4. Test preparation and safety WARNING The test voltages employed for the tests herein can cause personal injury, loss of life, or property damage. Accordingly, appropriate safety precautions are necessary to reduce the risk of such losses. The testing described in this document shall be carried out according to the safety procedures described by any relevant regulatory agencies and the safety procedures of the organization having control over activities at the test site. Preparation for the test should include, but not be limited to, the installation of warning signs and safety barriers around the test equipment and the machine to be tested. Grounds shall be installed as required by any relevant regulatory agencies and the local controlling authority. Other safety measures can be found in IEEE Std 510-1983, ISO 14121-1, ISO/TR 14121-2, IEC 60204-1, and IEC 61508. 7 All personnel involved in the test shall be thoroughly familiar with the test, the test equipment, the machine to be tested, and the hazards involved. When equipment is energized, no personnel shall infringe upon the minimum limits of approach described by any relevant regulatory agency or the local controlling authority.

5. Test equipment and connections Care must be taken in the selection of the ac power supply. The duration of the test, the test voltage, and the capacitance of the winding under test are the major factors to consider in the selection. Requirements for factory tests on bars or coils will be different with respect to the test object load and test time (duty cycle of the supply). The test should be done at 50 Hz or 60 Hz. It is hard to predict in advance the duration of quality control testing of external discharges on a stator winding. The duration of a particular test, if the three phases of the winding are tested separately, will typically take several minutes to an hour but could be longer when the number of discharge sites is large. It is then important to stay within the current limits and duty cycle of the power supply, especially when the load represented by the winding under test is close to the load rating of the power supply. In this condition, the power supply risks overheating rapidly and being seriously damaged if not ventilated properly. It is recommended that the test voltage be in the upper range of the output voltage range of the power supply.

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Information on references can be found in Clause 2.

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

When using a resonant test set, a measurement of the capacitance of the test object should be made at the test frequency before starting the test in order to adjust the test set properly. Do not depend on the hipot set metering; but rather use a calibrated voltage divider to accurately measure voltage during the test. Since the load of the winding under test is predominantly capacitive, it can be calculated using Equation (1): Pr = 2 π f C V 2

(1)

where: Pr = f= C= V=

Reactive power in VA Test frequency in hertz Total test load capacitance in farads Test voltage in volts

It is recommended to use a power supply providing reactive compensation to cancel out most of the capacitive load presented by the winding under test. A resonant or primary-compensated power supply should be used. It is recommended to connect both ends of the phase, line, and neutral ends to avoid surge voltages in the event of a sudden voltage interruption.

5.1 Sensitivity of the corona-imaging instruments The use of a corona-imaging instrument to enhance detection of UV radiation by external discharges may simplify and accelerate the test. Portability and the option to take a picture or video are features of interest. However, the main feature is the sensitivity, and not all corona-imaging instruments are equal with respect to UV detection. Moreover, specification sheets, which use different units (e.g., lux, watt/cm2, picocoulomb) not always related to the phenomenon of interest here, make it difficult to compare different instruments. In order to select a corona-imaging instrument, a simple test can be done to determine whether it answers the need for a quality control test. Instead of running a complex UV spectrum test that needs to be compared with the emission spectrum of the discharge activity and requires a specialized spectrometer, which is not available to most people in the electrotechnical industry, a comparison of the response of the corona-imaging instrument with the naked eye can be used. For many years, the naked eye was the reference for external-discharge detection during a blackout test. The sensitivity of the eye after several minutes in complete darkness is good and can make out the faint light of discharges extending to the lower visible wavelengths (typically more than 20 min to reach a good sensitivity. In the rest of the document 15 min is used for convenience, but longer time will lead to improved eye sensitivity. Here, since the goal is to qualify corona imaging equipment, a slightly longer time is used. Thus, any corona-imaging instrument performing equally well as, or better than, the eye in these conditions will be considered acceptable. This test, described below, can be used to qualify corona-imaging instruments before they are used in the field or in the factory. Standardization of the test configuration provides reproducibility; however, a corona-imaging instrument qualification test should also be easy without necessarily requiring an elaborate test facility such as a climatic room for atmospheric pressure, temperature, and humidity control. A simple way to check the sensitivity of the corona-imaging instrument is to create a non-uniform field with a needle plane configuration as shown in Figure 1 and to detect the discharge inception voltage (DIV) at the tip of the needle first with the naked eye in the dark (after 20 min). Then, test again with the instrument under evaluation. Since observation with the eye has been used with satisfaction in the blackout test for years, it can be used as a reference for the DIV of the setup. Thus, if the setup or ambient condition varies slightly from one user to the next, the minimum sensitivity requirement will always be determined with reference to the eye under the same conditions. 6

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Since there is inherent variability while performing such DIV tests for different consecutive trials, a margin of 1.0 kV can be tolerated. The final acceptance criterion is that the DIV detected with the corona-imaging instrument should be within 1 kV of the one observed with the naked eye after being in complete darkness for at least 20 min. The test with the corona-imaging instrument can be performed under normal room lighting, but reduced lighting can somewhat improve the sensitivity of PD light versus ambient light ratio. The use of incandescent light instead of UV emitting fluorescent light would also help. If the coronaimaging instrument does not respect this criterion, it is considered not sensitive enough in the spectrum of interest to be used for external-discharge detection. The dimensions in Figure 1 are given as guidelines, but other similar dimensions could be used, bearing in mind that this test is a comparative evaluation of the corona-imaging instrument against the sensitivity of the eye.

R 5.0 mm (0.197 in)

D 3.1 mm (0.122 in)

4.4 mm (0.173 in)

R 0.22 mm (0.008 in)

25.4mm

38.8° 11.2 mm (0.441 in)

(1.0 in)

25.4mm (1.0 in)

R 5.0 mm (0.197 in)

Figure 1 —Needle plane electrode configuration to create discharge activity in air It should be noted that a non-uniform electrical field is used in order to have a significant difference between the DIV and the breakdown voltage of the test gap. With the dimensions in Figure 1, at an atmospheric pressure of 101.3 kPa (1 atm), a temperature of 22 °C (71.6 °F) and relative humidity of 60%, the DIV is about 6.3 kV, and the discharge extinction voltage (DEV) is 5.8 kV. The typical intrinsic variability of the DIV and DEV from one trial to the next is presented in Table B.1. Under the same conditions, the breakdown voltage of this gap is 14.5 kV. Thus, if the voltage is raised slowly to the DIV, the risk of dielectric breakdown of the air gap while performing observation of the discharge activity at or close to the DIV is reduced. Alternatively, a single electrode using a needle sticking up in the air could be used to perform a similar comparative test between the eye and the corona-imaging instrument.

6. Quality control test of external discharges with corona-imaging instrument or blackout test The major advantage of using a corona-imaging instrument to observe the light emission from externaldischarge activity is that it extends the observation spectrum down to the UV range where the discharge spectrum is the most intense. Thus, the observation can be made without the need of a dark environment. Normal lighting in the factory or in the plant is an acceptable condition for performing the UV test with the corona-imaging instrument as long as a strong UV emitting lamp, such as a mercury vapor lamp, is not 7

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

used as room lighting. In some cases, especially in the factory, reduced lighting can enhance the UV/visible ratio. Locating the discharge sites will be easier with a corona-imaging instrument than with the blackout test because most instruments are also sensitive, to different extents, to visible light and thus both light emission from external discharges and the test object are visible at once. This will also accelerate the testing because the eye requires no time to acclimatize in obscurity as it does for the blackout test. In addition, working under lit conditions increases the safety of the personnel working in proximity to high voltages. For instance, during on-site testing in a power plant, the operator of the high-voltage source will have a direct view of the people doing the discharge observation. Instead of using a corona-imaging instrument for observation of external discharges, a blackout test can be performed on bars, coils, or entire machines. The first requirement for this test is to be able to achieve complete darkness in the room where the test is performed or have sufficient shielding against surrounding light to be able to observe external discharges with the naked eye. In many cases, especially in a power plant, just shutting off the lights would not be enough to let the eye become sufficiently sensitive to observe the smallest discharges. In some powerhouses, it will be possible to perform blackout tests only at nighttime to prevent daylight compromising the test. However, for safety’s sake, it is recommended to limit use of the blackout test to testing in the factory on bars and coils and on VPI machines where complete darkness is easier to achieve and where safety measures are easier to respect. For safety reasons, the use of a corona-imaging instrument is recommended for tests carried out on-site, on fully assembled stators. In addition, it is believed that inspection with the imaging instrument is better since the observer can see the stator and identify bars with respect to slot and know what portion of the stator has already been observed and which portion of the winding is not yet inspected.

6.1 Factory test on coils and bars The purpose of this test is to validate that the bars/coils produced are not subject to external discharges. This test is to be done at the factory which manufactures the bars/coils in the presence of a user’s representative. Either the blackout test or visualization using a corona-imaging instrument may be used. Individual bars and coils are tested by subjecting them to voltage while resting on support with the semiconducting slot coating grounded carefully for the test. Additionally, in some cases, it may be desirable to test a group of bars in a stator model to reproduce their physical arrangement in the machine and thus control minimal spacing between end-arms and the core tightening system. This test is discussed in 6.2. 6.1.1 Sample size The specimens tested should be selected by the user’s representative and represent 5% of all bars/coils produced for a specific stator. The bars/coils chosen for this test should have successfully passed the routine dielectric tests and the final inspection. Particular attention should be paid to the cleanliness of the bars/coils. 6.1.1.1 Sample coils for globally VPI stator windings For globally VPI stator windings, sample coils must be VPI-treated and examined for external discharges. The number of sample coils may be limited to two to five coils as the complete stator winding can also be examined after the VPI treatment. The sample coils must have slot-simulating platens attached to them and receive a VPI treatment that resembles the VPI process used for treating the stator winding. It is recommended to process the sample coils in advance of the stator winding operation. This will allow for necessary remedial work on the stator coils prior to the winding operation and VPI treatment.

8

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

6.1.2 Test methods The test conditions should be selected from Table 1. Table 1 —Test parameters Test # 1 2

Environment Half-light Darkness

Observation with Corona-imaging instrument Naked eye

The end-user and manufacturer should agree upon the test method: test #1 or test #2. To perform test #1, half-light is needed to distinguish samples under test clearly with the corona-imaging instrument. Excessive light can create reflections on the sample surface that could be interpreted as corona activity. However, external discharges are usually intermittent whereas reflections tend to show constant light emission. In addition, unlike external discharges, reflection will be observed directly by the naked eye. If there is still any doubt, reduce the voltage until extinction of the corona. If there is no extinction at or close to zero voltage, it is not corona. To perform test #2, the test setup must be installed in a dark room. All sources of light should be turned off or masked, especially lights that could be within the field of vision during observation. 6.1.3 Test voltage on individual bars and coils The voltage chosen for a factory test should be selected so that no external discharges will occur in operation at the surface of the bars or coils, and the stress control coating at U0, and at operating temperature. Since the temperature is lower during factory testing than during operation, the temperature difference is often compensated by increasing the test voltage in the factory [B9]. It should also be pointed out that the maximum sustained voltage of the stator could be 5% to 10% above the nominal voltage rating. The increase in the test voltage is not to compensate for the aging of materials since each material ages differently and because the voltage distribution along the stress control coating depends on the voltage. A survey of industrial practices has shown that a range of test voltages is currently in use for this test. Based on experience, it is recommended that the factory test be performed at a voltage level within the range presented in Table 2. Note that this test is intended for bars and coils in their completed stage and should not be applied to VPI coils before impregnation (green coils). The exact voltage level at which to do the test must be determined by the user and supplier before starting the test. Voltages close to the minimum in Table 2 (this minimum is equal to 1.25 × Un/√3) are closer to the normal line-to-ground voltage but will compensate less for a temperature difference with operating conditions than the maximum proposed in the table. Table 2 —Recommended test voltage range for factory testing Minimum test voltage (xUN) Maximum test voltage (xUN) 0.72 1.15 NOTE—These voltage values are based on nominal voltage. They do not intend to take into account transient overvoltage during a fault or disturbances or overvoltage due to an ungrounded neutral.

6.1.4 Test procedure for individual bars and coils Bars/coils to be tested should be installed on supports, and the semiconducting slot coating should be grounded. ⎯

For bars, the high voltage should be applied to the bare copper, usually at one end of the bar

⎯

For coils, the high voltage is generally applied to both bare copper leads with the individual strands connected together 9

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Multiple bars/coils can be tested at the same time. The test setup should permit inspection of the four sides of the bars/coils. If necessary, the bars/coils can be examined in various positions; in which case, the voltage should be turned off and bars/coils shall be safely grounded before re-positioning. The ambient temperature, relative humidity, and atmospheric pressure should be recorded at the beginning of the test. Test #1 starts by applying to the bars/coils the test voltage selected from the range specified in Table 2. Observations may start immediately after reaching the test voltage and are initially focused on the endwinding area (end-turns or end-arms) where the stress control coating is applied, particularly at the bend, non-straight portions, and on both sides of the bar/coil. The semiconducting slot coating is then inspected for signs of external discharges. Test#2 is commenced by applying to the bars/coils the test voltage selected from Table 2. After at least 15 min in complete darkness, observation with the naked eye can be focused on the end-winding area where the stress control coating is applied to see any signs of external discharges. The semiconducting slot coating should also be inspected for signs of external discharges. 6.1.5 Acceptance criteria If none of the selected bars/coils exhibit external discharges during the test, then the production set for the stator is deemed to have met the requirements. If the user’s representative or the manufacturer has a doubt about any bars/coils during the selected test, the doubtful specimens could be re-inspected using the alternative test method described in this document. 6.1.6 Remedial actions and retest If one bar/coil exhibits external discharges during the selected test, this specimen should be repaired by the manufacturer. As a second verification, the test should be performed again on the repaired bar/coil and on a second batch of specimens representing 5% of all bars/coils produced for the machine. If one or more bars/coils present external discharges during the test on the second batch, then all the bars/coils of the machine should be tested, repaired if necessary, and retested.

6.2 Stator model test The purpose of this test is to validate that the clearance between one bar/coil and another or between bars/coils to ground when installed in a stator model (mock-up core) representing the stator, is not subject to external discharges. This test is optional but, if it is done, it should be carried out prior to manufacture of the complete winding or core because failures may require changes in the machine design or in the winding design as outlined in 6.2.6. This test is designed to test the assembly in the factory before the stator is fully assembled and provide a better line of view than on the fully assembled machine. Successfully passing the test on the complete stator is sufficient, but in the case of general clearance issue(s), testing on the stator model may facilitate necessary remedial action. This test is to be performed at the factory manufacturing the bars/coils in the presence of a user’s representative. Either the blackout test or visualization using a corona-imaging instrument may be used. It should be pointed out that if the test is performed in the factory for a machine to be installed at an altitude higher than 1000 m (3281 ft), standard spacing during the test will not ensure absence of discharges on site. Reduced pressure of the air at higher altitude will give a lower inception voltage than at sea level. For machines operating above 1000 m (3281 ft), correction of the factory test voltage will have to be agreed

10

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

between user and manufacturer. An example of correction factors is proposed in Annex D and based on IEEE Std 4 and IEEE Std 4a. 6.2.1 Sample size The stator model should be large enough to accommodate a sufficient number of coils or half-coils in such a way that it represents one complete winding pitch. For two-pole and four-pole machines, half-coils can be used as long as the spacing between coil leads and coil knuckles can be tested. The sample size would remain the same for lap or wave windings, but additional support should be added to the end winding. 6.2.2 Test methods Test methods described in 6.1.2 for the factory test performed on individual coils or bars may be used on the stator model or mock-up core. The model could be made with wood. Conductive material such as steel or aluminum plates or wood covered with conductive foil should be used to simulate the core-tightening components at both ends of the stator model. If conductive foil is used, points or abnormal sharp edges should be avoided. Alternatively, a conducting coating can be used instead of foil to cover the wood. All conductive and conducting materials added to the model should be grounded. The minimum spacing between coils or bars in the model should be the same as in the stator. 6.2.3 Test voltage Testing on the stator model is mainly used to confirm that no external discharge will occur between coils/bars in the end-winding area before the assembly stage. This test will confirm that the spacing is sufficient to eliminate external discharges between coils/bars up to the maximum phase-to-phase voltage and at operating temperature. It could also be used to confirm that no external discharge activity occurs between the coils or the bars and the tightening system of the stator core laminations. Typically the temperature is lower in the factory than during operation, so the temperature difference should be compensated by increasing the test voltage in the factory. The increase in voltage is not intended to compensate for the aging of materials, as each material ages differently and because the voltage distribution along the stress control coating depends on voltage. It should be pointed out that not all the spacings are subjected to the full line-to-line voltage during machine operation. Moreover, several locations in the stator winding are only exposed to line-to-ground voltage, such as the junction between the semiconducting slot and the stress control coatings. It is thus recommended to perform this test at two voltage levels: the first one to test all ground clearance, and the second at higher voltage to test bar-to-bar or coil-to-coil clearances. It is recommended to test ground clearance to the voltage indicated in the left column in Table 3. This value corresponds to 115% (15% increase compensates both for the temperature difference between factory and operating conditions and the maximum allowable continuous voltage) of the maximum phaseto-ground voltage (1.15 × Uo = 0.66 × UN). Similarly, it is recommended to use a test voltage for bar-to-bar (or coil-to-coil) clearances based on the actual maximum voltage that appears at the clearances. For each location, the manufacturer will determine from the winding diagram the actual operating voltage (including the crossover region between top and bottom planes). This second test value is 115% of the maximum voltage found in the machine, as shown in the right-hand column of Table 3. Table 3 —Recommended voltage range for model test Test voltage of ground clearances Test voltage of bar-to-bar or coil-to-coil clearances 0.66 UN Maximum voltage based on winding diagram +15% NOTE—These voltage values are based on nominal voltage. They do not intend to take into account transient overvoltage during a fault, disturbance or overvoltage due to an ungrounded neutral.

It should be noted that for refurbished machines, the existing clearance from ground and between connections can be very different from one machine to another. When rewinding a stator with the existing 11

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

clearances, it may not be possible to make the test at, or even close to, the maximum voltage given in Table 3. In such a case, the exact voltage level at which to do the test must be determined by the user and supplier before starting the test. For new machines, the required test voltage may have an impact on the design choice; for example, deeper slots leading to a bigger stator, longer bars leading to a higher stator winding resistance, higher losses and so on. 6.2.4 Test procedure for a group of bars or coils in a stator model For the test in a stator model, coils or bars should be placed in the mock-up stator core having the same bore radius, the same slot size, and same core length as the actual stator core. When coils and bars are installed in the mock-up stator core, it should be possible to see if external-discharge activity occurs: ⎯

Between top and bottom coil legs or bars in the same slot at the junction of the semiconducting slot coating and stress control coatings

⎯

In the end-winding between adjacent top and bottom coils/bars

⎯

In the end-winding between top coils/bars and bottom coils/bars at crossovers

⎯

Between each of the coil knuckles and the lead of the adjacent coil

⎯

Between any coils or bars and the tightening system of the stator core laminations

The semiconducting slot coating of the coils or bars should be grounded. Coils and bars should be installed in the mock-up core slots using the same thickness of slot packing material and center slot filler that will be used in the stator slots. Some fiber blocks could be temporarily tied on the coil or bar end-turns to simulate the thickness of winding blocking. The mock-up stator core can be made as described in 6.2.2. The test voltage should be applied on only one coil/bar at a time with all the other coils/bars grounded. Each coil or bar installed in the mock-up core should be individually tested in relation to all the others. The test voltage to apply to individual coils/ bars should comply with the test voltage of bar-to-bar clearances defined in Table 3. When the clearance between coils or bars to ground is to be verified, all coils or bars in the mock-up core should be energized at the test voltage defined in Table 3 for test voltages of ground clearances. As PD could occur if steel elements are too close to the winding, the stator mock-up should also consider simulation of the fingers, the tightening plates, and the tightening studs. 6.2.5 Acceptance criteria No visible discharge should be observed at the various locations described in 6.2.4 at voltages up to and including the test voltage of bar-to-bar clearances defined in Table 3 for coil-to-coil or bar-to-bar clearance verifications and up to the test voltage of ground clearances defined in Table 3 for coil- or bar-to-ground clearance verifications. 6.2.6 Remedial actions and retest If visible discharges are found between coils or bars, different possible remedial actions could be implemented depending on the location where these discharges are found. The remedial action should be agreed upon by the manufacturer and the user. For some of these corrective actions, new coils or bars may have to be manufactured. Some of the characteristics of the following bullets are depicted in Figure 2. 12

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

a)

Visible discharges between top and bottom coil legs or bars in the same slot at the junction of the semiconducting slot and stress control coatings: Applying another coat of the stress control coating and/or improving the contact with the semiconducting slot coating could eliminate the discharges. Another solution is to increase the thickness of the center slot filler and/or modify the shape of the coils/bars outside of the core in order to provide more clearance at the junction of the semiconducting slot and stress control coatings between top and bottom coil legs/bars in the same slot. For the latter corrective action, new coils or bars will have to be manufactured. The test should be repeated to demonstrate the effectiveness of the implemented corrective action.

b)

Visible discharges in the end-winding between top and bottom coils or bars: Upon agreement between the manufacturer and the user, coils or bars could be placed in the mockup stator core in a different order from the original order in such a way that no visible discharges are found between coils or bars. If this remedial action is selected, all coils or bars will have to be placed in the mock-up core and tested to determine the order in which the coils or bars will have to be installed in the stator core. Alternatively, the shape of the coils or bars should be modified in the end-winding to provide more clearance between adjacent top and bottom coils or bars. For example, for coils, the developed length of the bottom end-winding of the coils between the core and the coil knuckle may have to be extended beyond its original length. For bars, the drop-back angle at the outside of the core on the bottom bars may have to be increased. For any one of these corrective actions, new coils or bars will have to be manufactured and the test should be repeated to demonstrate their effectiveness.

c)

Visible discharges in the end-winding between adjacent top coils/bars or between adjacent bottom coils/bars: Upon agreement between the manufacturer and the user, coils or bars could be placed in the mock-up stator core in a different order from the original order in such a way that no visible discharges are found between coils or bars. If this remedial action is selected, all coils or bars will have to be placed in the mock-up core and tested to determine the order in which the coils or bars will have to be installed in the stator core. Alternatively, the design of the end-winding of coils/bars could be changed to provide more clearance between adjacent top coils/bars and between adjacent bottom coils/bars. For this corrective action, new coils/bars will have to be manufactured and the test should be repeated in order to demonstrate their efficiency. To accomplish this, the length of the coil/bar end-winding may have to be increased.

d)

Visible discharges between each coil knuckle and the lead of the adjacent coil: Upon agreement between the manufacturer and the user, coils could be placed in the mock-up stator core in a different order from the original order in such a way that no visible discharges are found between each coil knuckle and the lead of the adjacent coil. If this remedial action is selected, all coils will have to be placed in the mock-up core and tested to determine the order in which the coils will have to be installed in the stator core. Alternatively, the shape of the coil lead leaving the coils between coil knuckles could be modified to provide more clearance at this location. For this corrective action, new coils will have to be manufactured and the test should be repeated in order to demonstrate their efficiency. Another alternative may consist in increasing the drop-back of the coil knuckles. The length of the coil end-winding may have to be increased for that. For this corrective action, new coils will have to be manufactured and the test should be repeated in order to demonstrate their efficiency. Ultimately, the width of the slot and/or the number of slots in the stator core could be revised. This corrective action requires a complete redesign of the stator core and its winding. Many generator parameters could be altered by the redesign, so consideration must be given to review all contractual requirements.

e)

Visible discharges between any coils/bars and the tightening system of the stator core laminations: The shape of the coils/bars should be modified in the end-winding to provide more clearance between coils or bars and the tightening system of the stator core laminations. For example, if the discharges are with the finger plates or with the tightening plates, the length of the straight part of 13

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

the coils or bars out of the stator core may have to be increased. To avoid discharge between top and bottom bars/coil legs, a thicker center filler could be used and/or the length of the slot corona protection can be increased. As another example, if the discharges are with the upper or lower air baffles or with the stator upper or lower brackets, the length of the coil/bar end-windings may have to be reduced. Other remedial actions could be agreed upon between the manufacturer and the user.

Increase this angle as in bullet B. Location of discharges as in bullet B. Location of discharges as in bullet A.

Stress control coating

Overlap of stress control coating and semiconducting slot coating Top of stator core Core

Semiconducting slot coating

Increase thickness of center filler as in bullet A.

Figure 2 —Description of some of the material used as part of the insulating system of bars and coils and general location of some of the discharge sites

6.3 Test on fully assembled stator windings This is an off-line test where the stator winding is energized with an external voltage supply. The purpose of this test is to validate that fully assembled stator windings are not subject to external discharges in the end winding area. Discharges should be eliminated both at ground clearance and bar-to-bar or coil-to-coil clearance. The procedure and setup will depend on the type of test used: blackout or corona-imaging instrument. The use of a corona-imaging instrument is strongly recommended for tests performed on stators assembled on-site for safety reasons. This test should confirm that no external discharges will occur between bars or coils in the end-winding area due to insufficient spacing and will also indicate the quality of the assembly and the stress control junctions on all coils/bars of the stator winding. It should be pointed out that, during the test, all components of one phase winding are stressed at the same voltage, whereas in operation, only a portion of the winding is exposed to line-to-line voltage (between bars or coils), while most locations are exposed to much lower voltage. 6.3.1 Test setup For global VPI stators, it is preferable to test the stator winding upon completion of the VPI operation and prior to assembly of the machine. The test may be performed after assembly of the machine and completion of the performance (running) tests; however, it is recommended to remove the end-covers and the rotor to expose the entire stator end-winding. For stators wound on-site, it is preferable to carry out the test before the installation of the rotor. If the diameter of the machine allows, a platform should be installed inside the bore at a safe distance from any energized part (including end-arms). From this platform, there should be a direct line of view to both ends of the stator (CE: connection end, OCE: opposite connection end). A barrier can be installed to ensure that

14

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

no one gets too close to the winding. Another possibility for large generators is to use a nacelle with a crane for the observer to move around in the stator. For stators with a small diameter or horizontal machines of small core length, the observation could be done from both ends of the machine (CE, OCE) when it is not possible to stand at the center of the core. If a final coat of paint is to be applied to the winding for mechanical protection, the inspection should be made before application of this last coat. In the event that external discharge is observed, corrections will be possible if there is direct access to the semiconducting slot coating and to the stress control coating. When there are parallel circuits within each phase of the stator winding, all the circuits must be energized to avoid imposing voltage stresses and consequently producing external discharge activity in the air clearances (gaps) between the parallel circuits that belong to the same phase. It should be pointed out that some phase-neutral crossover locations in the same phase normally exposed to voltage in operation will not be stressed during the test. The number of such sites is larger for wave windings. When the neutral point is not accessible (internal Y connection), the test has to be performed on all phases connected together; and, in this case, no discharge can occur between phase windings under applied test voltage. Before the day of the test, obtain a table showing the voltage during normal operation, parallel circuits, and phase of all coils/bars of the winding. An example of such a table is given in Annex E. Before the test begins, number and mark the stator slots using a permanent marker or a temporary tag such as an adhesive-backed tape or magnetic strips; this will facilitate locating discharge activities during the tests. Ensure the temporary tag is removed upon completion of the test. Usually, marking down every tenth slot is sufficient. It is better to mark down both ends of the slots close to the end of the core. It is also recommended to identify the first, second, and third line-end coils for each parallel circuit per phase as these would be the most likely candidates to exhibit external discharges during operation in service. These positions can also be marked down on paper with a reference to a clock-like positioning for each of these coils at each end. It may be helpful to place marks (e.g., masking-tape tabs sticking up) in the core to serve as a reference point using 1 o’clock, 2 o’clock, etc. as reference markings. In addition, phase breaks, coil-to-coil spacing in the end-winding where the adjacent coils belong to different phases, could also be marked. 6.3.1.1 Blackout test on stators in the factory A “pitch-black” environment is required to perform this test. This is typically achieved by setting up an enclosure around the test setup including the stator and the hipot set. The enclosure material used must be impervious to light. Alternatively, the test may be carried out at night in an area where all the lights are switched off. The enclosure should be constructed with 1.8 m (6 ft) minimum clearance imposed from the end of stator winding (i.e., stator coil noses and circuit rings) at both ends. A fence or physical barrier should be installed around the ends of the coils so that a safe distance is maintained between the energized coils/bars. The distance between the fence and the ends of the coils/bars should be the minimum approach distance recommended by the controlling authority. Care must be taken with the enclosure entrance with regard to adequate material overlap, as any light leakage will compromise the test.

15

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

6.3.2 Test methods Two test voltage levels may be used: one to test ground clearances, and a second for testing phase-to-phase clearances as specified in Table 4. Every location showing external-discharge activity must be noted with the intensity on a relative scale (strong, intermediate, weak). In addition to the physical location of the discharge sites, the electrical position in the winding must also be determined. A different procedure is to determine the DIV and DEV for location(s) exhibiting external-discharge activity (see 6.3.4.3). On stators assembled on-site, observation of all end-arm locations with a direct line of view should be made with the corona-imaging instrument in reduced lighting if possible. If not, normal lighting is also acceptable but requires more attention to discriminate discharges from reflections and solar UV wave. For global VPI machines tested in the factory, both visualization with the corona-imaging instrument and the blackout test can be used. 6.3.3 Test voltage Since this test on the fully assembled stator winding is intended to confirm that no external discharges occur in operation between bars/coils of opposite phase or close to the tightening system in the endwinding area and at locations with ground clearances, either a single test voltage or two levels of voltage can be used to evaluate all conditions. The test helps confirm that the clearance is sufficient to eliminate external discharges up to nominal voltage during normal operating conditions and temperature. Since the temperature is lower during the test than under operating conditions, the temperature difference can be compensated by increasing the voltage during the test. The increase in voltage is not to compensate for aging of the material, as each material ages differently and because the voltage distribution along the stress grading system depends on voltage. Experience suggests performing the test first at the maximum test voltage shown in Table 4 to test bar-to-bar or coil-to-coil clearance. If no external discharges are observed anywhere in the machine, the test at the lower voltage would not be necessary. This voltage will be determined by the manufacturer based on the winding diagram of the machine. This value should correspond to the highest one seen in the machine including the crossover region between top and bottom planes +15%. A second test can be carried out to evaluate ground clearance and, as shown in Table 4, it is recommended to use a voltage of 0.66 × UN (which corresponds to (UN /√3) × 1.15 or U0 × 1.15). This test voltage would mainly test ground clearances. The 15% increase compensates for both the temperature difference between factory and operating conditions and the maximum allowable continuous voltage. Table 4 —Recommended test voltage range for fully assembled stator windings Test voltage of ground clearances Test voltage of bar-to-bar or coil-to-coil clearances 0.66 × UN Maximum voltage based on winding diagram +15% NOTE—These voltage values are based on nominal voltage. They do not intend to take into account transient overvoltage during a fault, disturbance, or overvoltage due to an ungrounded neutral.

If the test is done in the factory for a machine to be installed at an altitude higher than 1000 m, standard spacing during the test will not ensure the absence of discharges on-site. Reducing the pressure of the air at higher altitude will give a lower inception voltage than at sea level. For machines operating above 1000 m, a correction of the factory test voltage will have to be agreed upon by both user and manufacturer. An example of correction factors is proposed in Annex D. It should be pointed out that, for refurbished stators, the existing clearance with ground and between connections can be very different from one machine to another. When rewinding a stator with the existing clearances, it may not be possible to meet the test acceptance criteria at, or close to, the maximum voltage stated in Table 4. In such a case, the exact voltage level at which to do the test must be determined by the user and supplier before starting the test.

16

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

It should be recognized that for new machines the required test voltage may have an impact on the design choice, for example: deeper slots leading to a bigger stator, longer bars leading to a higher stator resistance, higher losses, and so on. 6.3.4 Test procedure on fully assembled stator windings Before application of the test voltage, record the environmental conditions including temperature, relative humidity, and barometric pressure. These parameters may be recorded at the beginning of the test for each phase winding, particularly when the test duration is relatively long. The start time, finish time, and date should also be recorded. The test can be started at the higher test voltage first, followed by lower voltage or the other way around. Alternatively, a single voltage test can be performed at the highest voltage of Table 4. Test procedures are explained in 6.3.4.1 and 6.3.4.2. For the test at the higher voltage level (single-voltage procedure), only the highest voltage in Table 4 is applied to each phase winding with the other two grounded while observing all parts of the end-windings. Finally, a procedure for the determination of DIV and DEV can be used, which is described in 6.3.4.3. 6.3.4.1 Test with corona-imaging instrument For testing phase-to-phase clearances, commence the test by energizing one phase of the machine at the full test voltage shown in Table 4 for phase-to-phase clearance evaluation with the other phases and the frame solidly grounded. When full test voltage (for phase-to-phase clearances) is achieved, begin examining the winding for indications of external discharges. Discharges from windings to grounded structures should be ignored during this test. The only areas to be evaluated are the gaps between energized phase groups and grounded phase groups. The gaps between phase groups include coil-to-coil vent spaces on the end-arms, gaps between top and bottom coil legs, and gaps between coil leads at the phase connections and in the parallel ring areas. Knowledge of which phase is energized and markings or tags previously located on the windings could expedite identification of the areas to be scanned for external discharges. Maintain a record of all locations showing external discharges as the test progresses. Details on how to fill out the recommended data tables are given in Clause 7. Information to be recorded should include bar or coil leg type (top or bottom bar), end of stator (connection end or opposite connection end), position on bar (left, right, front, back), and intensity of discharge (using a relative scale such as strong, intermediate, and weak). It is highly recommended that video recordings and/or pictures by the imaging equipment output be made when possible. The person recording the test data should have sufficient information available to determine that discharge sites identified are in fact from the phase currently energized because exact location is sometimes difficult (this can prevent the need to retest questionable areas). The observer has to be careful to distinguish PD emission from other sources of UV lights, such as reflection from fluorescent light. In order to view all locations of interest, a single observer will need to move around the stator and view both ends from multiple positions. When the examination is complete, reduce the voltage to zero and ground the winding. Repeat this process until all phases have been tested. NOTE—For three-phase windings, all three phases should be tested separately even though all phase-to-phase gaps will be exposed to voltage after energizing any two phases.

After completion of the phase-to-phase clearance tests, connect all phases together and energize the windings at the phase-to-ground voltage level identified in Table 4 to test ground clearances. At this test level the entire stator winding is viewed and evaluated. Also at this level there should be no external discharges anywhere in the machine. Particular attention should be given to the following areas:

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

⎯

The stress control coating’s junction with the semiconducting slot coating, all around the bar or coil and the gap to the core compression fingers

⎯

The end of the stress control coating

⎯

Bracing blocks and lashing, particularly where they reduce the air space

⎯

Loose threads from lashing or bristles lost from brushes during painting may cause external PD

⎯

Around instrument cables (RTDs, air gap, flux sensors, etc.)

⎯

Between circuit rings and support structures

Note that not all discharge sites have the same level of relevance in terms of risk for the machine. For instance, external PD from loose paintbrush bristles stuck on the stress control coating can usually be repaired in 100% of the cases, thus they are usually not as critical as the instance of PD caused by poor clearances. It is recommended that a minimum of three people be assigned to testing: one person to observe the external discharges with the corona-imaging instrument, a second to log results into the data tables, and a third to operate the high-voltage supply. If there is a large distance between personnel, the use of a communication system should be considered. The operator of the high-voltage supply should be in visual contact with the people performing the test at all times. If this cannot be done, additional safety precautions should be considered. For large-size machines, more than one observer can also reduce inspection time. It will typically take several minutes to an hour to complete the evaluation for each phase, depending on the size of the stator and the number of discharge sites. When a single observer is used, test durations of more than an hour are not unusual. Upon completion of the test, reduce the voltage to zero and ground the windings. Under well lit conditions, examine winding areas where external discharges were observed. Look for evidence of burning, scratches, chips in coatings, powder-like deposits, or the presence of foreign objects. Record any findings on the data sheets. 6.3.4.2 Blackout factory test It is important to allow approximately 15 min of conditioning time in total darkness for those who are going to make a visual examination of the stator winding. It is recommended that a minimum of three people be assigned to testing: one person to observe the external discharges, a second to log results into the data tables, and a third to operate the high-voltage supply. Personnel should not move about in the dark when the winding is energized at high voltage. Therefore, to expedite testing it is typical for more than one person to act as a visual observer. The use of flashlights with red filters can provide visual references without adversely affecting the UV sensitivity of the eyes of the observers. Listen intently as discharges will often emit a buzzing sound that may help with locating them. The operator of the high-voltage supply should be in verbal contact with the people performing the test at all times. If this cannot be done, additional safety precautions should be considered. After 15 min in the dark, commence the test by energizing one phase of the machine at the full test voltage shown in Table 4 for phase-to-phase clearance evaluation with the other phases and the frame solidly grounded. When full test voltage is reached, begin examining the winding for indications of external discharges. The discharge will initially appear as a faint violet or bluish glow in the end-winding, particularly between line-end coils. Should evidence of external discharges be observed, determine where it is occurring and report it to the person responsible for data logging.

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Alternatively, the voltage source may be switched off any time a location with discharge activity is identified and the location may be marked. The phase under test and the high-voltage supply must be securely grounded prior to approaching the winding. Use of a laser pointer and a flashlight (with red filter lens) will facilitate marking the exact location of the discharge activity. Attention should be given to the same areas as the bullets in the previous subclause. If it is necessary for any of the observers to change position during observation, it is recommended that the voltage be reduced to zero and not raised until everyone is again stationary. When the examination is complete, reduce the voltage to zero and ground the winding. Repeat this process until all phases have been tested. NOTE—For three-phase windings, all three phases should be tested separately even though all phase-to-phase gaps will be exposed to voltage after energizing any two phases.

After completion of the phase-to-phase clearance tests, connect all phases together and energize the windings at the phase-to-ground voltage level identified in Table 4 to test ground clearances. At this test level, the entire stator winding is viewed and evaluated. There should be no external discharges anywhere in the machine at this level. Upon completion of the test, reduce the voltage to zero and ground the windings. Under well lit conditions, examine winding areas where external discharges were observed. Look for evidence of burning, scratches, chips in coatings, powder-like deposits, or the presence of foreign objects. Record any findings on the data sheets. 6.3.4.3 Alternative procedure for determining DIV and DEV The specific voltages at which external discharges begin (discharge inception voltage or DIV) and terminate (discharge extinction voltage or DEV) can be determined with two tests. First, by slowly raising the test voltage from zero until discharges are observed, DIV can be determined and recorded. This process is simplified if testing per 6.3.4.1 or 6.3.4.2 is conducted first to identify areas where discharges are to be expected. After the DIV level is known, the DEV level (typically less than the DIV level) can be determined by slowly lowering the test voltage while observing the known discharge locations. CAUTION Do not exceed the maximum test voltages identified in Table 4. Prolonged exposure to voltages well in excess of the normal operating voltage may damage stator winding insulation. 6.3.5 Acceptance criteria No external discharges should be observed in the relevant areas of the stator at the applicable test voltage identified in Table 4. Alternatively, the DEVs measured in relevant areas should be greater than the pre-determined minimum acceptable DEV levels applicable to those areas. Two separate DEV levels should be used: one for areas exposed only to line-to-ground voltages, and another for areas exposed to line-to-line voltages. The minimum acceptable DEV levels should be determined by mutual agreement between manufacturer and user. Because the voltage distribution in the stator winding during a test is different from the distribution that exists during normal operation, some consideration may be given to discharges based on where they are located. Discharges located in areas known to have no voltage present during operation may be considered 19

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

non-relevant. As an example, the neutral end coils will normally have no potential to ground, so discharges occurring at the neutral could be considered non-relevant. However, if the design was required to support swapping the line and neutral connections for future unit life extension, then external discharges at the neutral would be relevant and would constitute a failure of the test. Designating discharges identified during testing as non-relevant based on location should be a subject of mutual agreement between manufacturer and user. 6.3.6 Remedial action When external discharges are detected, repairs may be performed. The winding should be connected to ground during all repair operations. It is beyond the scope of this specification to identify the types of repair procedures and processes that might be used. Once the repairs have been completed, the test must be repeated. Typically, only the repaired locations should be observed during the retest. Retesting should be performed only after all materials used, including insulating and semiconducting materials, are adequately cured.

7. Data records This clause contains three tables. The first three (Table 5 to Table 7) are for individual bar or coil factory tests, while Table 8 is for tests on fully assembled stators. For VPI stators, tests could be done in the factory and/or when the VPI stator is in its final position at the site. It is better to do in situ tests without the rotor in place, but with proper safety precautions this could be done with the rotor in place.

7.1 How to fill the data logging tables 7.1.1 Bar or coil factory tests The first column in sample Table 5 to Table 7 is for the identification number of tested bars or coils. Column 2 and column 3 are for specifying where the discharge occurs. On the connection side or the opposite connection side? On which side of the bar? For example, the top side in position #1. They also indicate if it occurs at the stress control junction or elsewhere. 7.1.2 Fully assembled stator The first column in Table 8 is to identify the slot number. The second column identifies the phase and the parallel circuit based on the winding diagram (e.g., A1, T11, E1-1, etc.). The third column shows the phase-to-ground operating voltage of the bar or coil leg. For simplification, in the case of coils, the same voltage is used for both legs. For voltage calculation, use the following equation:

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

The voltage of one bar is:

Vbar =

UN ⎛ P ⎞ ×⎜ ⎟ 3 ⎝N⎠

where UN is the nominal line-to-line voltage in kV, P is the position of the bar or coil with reference to neutral and

N=

k × N slot N phase× N parallel

where k=1 for coils and k=2 for bars Nslot is the number of stator slots Nphase is the number of phases Nparallel is the number of parallel circuits of each stator phase winding The fourth column is used to indicate if the discharge is detected on the top or bottom bar or coil leg. The fifth column will show if the discharge occurs on the connection end or opposite connection end. The sixth column shows the intensity of the discharge according to a relative scale (weak, intermediate, strong), and the seventh column indicates which type of discharges was observed (e.g., bar-to-bar, stress control junction, lashes, instrumentation cable, foreign object). Table 5 —Factory test—Roebel bars lap winding (diamond) POWER HOUSE:________________________________________________________ UNIT:_________

TEST VOLTAGE:___kV

MANUFACTURER:_______________________

Nominal voltage:

Ambient temperature:

Relative humidity

__kV Bar identification

___°C

Atmospheric pressure

__%

Position #1

__kPa Position #2

CE

OCE

OCE

Inspected by:______________________________________ Date:_______________ 21

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CE

IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Table 6 —Factory test—Roebel bars wave winding POWER HOUSE:________________________________________________________ UNIT:_________

TEST VOLTAGE:___kV

MANUFACTURER:_______________________

Nominal voltage:

Ambient temperature:

Relative humidity

__kV Bar identification

___°C

__%

Position #1 OCE

Atmospheric pressure __kPa

Position #2 CE

CE

OCE

Inspected by:______________________________________ Date:_______________ Table 7 —Factory test—Coils POWER HOUSE:________________________________________________________ UNIT:_________

TEST VOLTAGE:___kV

Nominal voltage:

Ambient temperature:

___kV

_°C

Coil identification

MANUFACTURER:_______________________ Relative humidity ___% Position #1

Top leg

Atmospheric pressure ___kPa Position #2

Top leg

Inspected by:_______________________________________ Date:_______________

22

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Table 8 —Global VPI stators or in situ tests of completely assembled stator on site POWER HOUSE:______________________________________________ UNIT:________ Test voltage:

Ambient temperature:

Relative humidity

Atmospheric pressure

__kV

___°C

__%

___kPa

Global VPI in factory:__

Slot

MANUFACTURER :_______________________ _

Nominal voltage: ___kV

Phase circuit

Operating voltage

Global VPI in situ:__

Top/Bottom coil leg or bar

Completely assembled stator on site:__

CE / OCE

Intensity

Type

TOTAL: Inspected by:_______________________________________ Date:_______________

23

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IEEE Std 1799-2012 IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Annex A (informative) Theory of optical emissions from external discharges As described in IEEE Std 1434, each time a partial discharge (PD) occurs, it is accompanied by a current pulse, radio frequency energy, an acoustic emission, and an optical emission. It results either from an electrical breakdown across a cavity within the insulation or at an air layer on the surface of a coil or bar. Under certain conditions, the discharge process within the cavities or air gaps may assume a pseudoglow or even a pulseless glow character, but will still give an optical emission. The only type of discharges that can be observed visually or with a UV enhancing instrument (named corona-imaging instrument in the current document) are those that are external to the insulation. For purposes of quality control of bars or coils in the factory, the locations that are susceptible to the occurrence of external discharges include the junction between the stress control coating and the semiconducting slot coating and along the slot coating. On fully assembled stators, only the ends of the stator windings are visually accessible. In addition to the stress control junction, any other locations with small spacing between bars in other phases are areas where light emissions due to external discharges can occur. Extensive discussions on the physics of electrical breakdown in gasses, and on partial discharge and corona can be found in [B2], [B13], and [B15]. The breakdown process in air occurs when free electrons in the air are accelerated by a local electric field above a critical value. If the electric field exceeds approximately 3 kV/mm in dry air at 100 kPa under room temperature, then some of the electrons will accelerate with enough energy to ionize gas molecules and atoms with which they collide. The resulting positive ion and the two electrons (the original electron plus the secondary electron) will also accelerate in the local field. Above the breakdown strength value, the number of secondary electrons produced will exceed the number of recombined electrons. Since billions of molecules and atoms may experience the ionizing collisions, the electric field across the air collapses due to the numerous free electrons and positive ions as a result of the increased conductivity of the affected region. There are two main processes by which light can be emitted in gasses undergoing electrical breakdown. In one process, the electrical breakdown of the air involves the formation of photons having various energies and frequencies. In this breakdown mechanism, many of the collisions between the electrons and molecules or atoms are non-ionizing. When an electron does not acquire sufficient energy between collisions, no secondary electron is ejected and the energy from the impacting electron raises the energy level of the electrons orbiting the atoms. After a certain time in this excited state, the energy level of the atom spontaneously returns to its stable unexcited lower energy or ground state, and the excess energy can be released in the form of a photon. The energy of the photon emitted depends on how much energy was transferred to the molecule or atom from the non-ionizing collision between the electron and the molecule or atom and the type of gas present. The energy of the photon is given by hf, where h is Plank’s constant and f is the frequency of the emitted light. Since the energy of the photon is proportional to the difference between the excited states and the lower, or ground, state of the atom or the atoms of a molecular gas, the frequency of the emitted light can be determined. A similar light emission process occurs when a positive ion (created by an ionizing collision) combines with a free electron. The emitted frequency spectrum due to electroluminescence is typically a line spectrum as opposed to that of a continuous spectrum, which is produced at elevated temperatures (e.g., from a heated filament in an incandescent lamp). Since there are many types of molecules in air (nitrogen, oxygen, carbon dioxide, etc.), and the range of original collision impact energies is wide, the resulting photons have a wide range of frequencies (wavelengths or colors). A measurement of the electromagnetic spectrum wavelength of light that accompanies electrical breakdown in air shows that the light varies from the visible region (lower visible wavelengths) to the ultraviolet (near UV) range. The UV frequency spectrum is situated between that of the visible violet light and long-wavelength X-rays; however, discharges in air do not contain the entire UV spectrum wavelength since the very far end of the UV spectrum with wavelengths