Insulation coordination for UHV AC systems.pdf

INSULATION COORDINATION FOR

UHV AC SYSTEMS WG C4.306

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Members  E. Zaima, Convenor (JP), T. Koboyashi, Secretary (JP), J. Takami, Asistant Secrery (JP), P. C.  Fernandez (BR), D. Peelo (CA), Q. Bui‐Van (CA), W. Chen (CN), A. Sabot (FR), F. Gallon (FR), E.  Kynast (DE), A. Pal (IN), R. N. Nayak (IN), S. Malgarotti (IT), T. Yamagiwa (JP), E. Shim (KR), A.  Lokhanin (RU), P. Tlhatlhetji (ZA), A. Amod (ZA), C. van der Merwe (ZA), U. Kruesi (CH), D.  Sologuren (CH), Y. Vachiratarapadorn (TH), A. J. F. Keri (US), A. Villa (VE), G. Carrasco (VE), H. Ito  (JP), T. Yokota (JP), Y. Shirasaka (JP), B. Richter (CH), U. Riechert (CH)   

Coordination with  P. Zhou (CN), J. Lin (CN), Z. Li (CN), K. Uehara (JP), Y. Ishizaki (JP), S. Okabe (JP), M. Miyashita (JP),  H. Kajino (JP) 

Copyright © 2011 “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 : (To be completed by CIGRE)

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ISBN : (To be completed by CIGRE)

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INSULATION COORDINATION FOR UHV AC SYSTEMS No Extra Cover page or “blank pages”  Use CIGRE abbreviations insofar as possible: Study Committee – SC, Technical Brochure – TB, Working Group – WG  Joint Working Group – JWG, Technical Committee – TC  Photos: must be of reasonable definition (preferably 300 dpi); all figures and tables must be titled, legible and numbered  with legends provided.   No Company logos… 

Table of Contents 1 Introduction .................................................................................................................6 2 Concept of recent practices on insulation coordination for the UHV and the 800kV

system ..........................................................................................................................8 2.1 Insulation coordination throughout substation and transmission line...................8 2.2 Reduction of insulation levels using overvoltage suppression measures .............16 References ..............................................................................................................19

3 Recent practice on insulation coordination for the UHV and the 800 kV system...........20 3.1 China UHV project .............................................................................................20 3.2 India UHV project ..............................................................................................29 3.3 TEPCO 1100 kV project .....................................................................................35 3.4 Brazil 800 kV project .........................................................................................46 3.5 China 750 kV AC Project....................................................................................60 3.6 India 800 kV project ..........................................................................................64 3.7 Korea 765 kV project.........................................................................................68 References ..............................................................................................................82

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4 Overvoltage in UHV range...........................................................................................83 4.1 Determination of stresses (TOV, switching overvoltage, lightning overvoltage and VFTO)......................................................................................................................83 4.2 TOV due to load rejection and ground fault .......................................................85 4.3 Switching overvoltages caused by closing and opening with ground fault overvoltage ............................................................................................................................... 93 4.4 Lightning overvoltages caused by back-flashover and direct lightning .............115 4.5 VFTO stress in GIS due to DS switching............................................................123 4.6 Influence of metal oxide surge arresters on circuit breaker TRVs .....................162 4.7 Conclusions ....................................................................................................168 5 Evaluation of overvoltage study results .....................................................................170 5.1 Overvoltage simulation tools and verification by measuring results..................170 References ............................................................................................................179 5.2 Main Characteristics of Metal-Oxide Surge Arresters (MOSAs) ..........................180 References ............................................................................................................188 5.3 Evaluation of waveform - Conversion in shape of field overvoltage to standard impulse waveform in determining representative overvoltages - ............................189 References ............................................................................................................200 5.4 Conclusions ....................................................................................................200 6 Switching Overvoltage Mitigation Measures for Future UHV Systems .........................201 6.1 Introduction ....................................................................................................201 6.2 Fast Insertion of Shunt Reactors ......................................................................202 6.3 Closing Resistors.............................................................................................202 6.4 Staggered Pole Closing ....................................................................................203 6.5 Line Surge Arresters ........................................................................................203 6.6 Controlled Closing...........................................................................................205 6.7 Comparison and Relevance to Future UHV Systems ..........................................207 6.8 Conclusions ....................................................................................................209 References ............................................................................................................209 7 Some aspect of insulation coordination of air gaps in the UHV range (phase-to-earth

and phase-to-phase insulation).................................................................................211 7.1 Introduction ....................................................................................................211 7.2 Air Gap Clearances chosen for UHV Projects in different countries ...................211 7.3 IEC “Minimum” Air gap clearance according to IEC 60071-1.............................212

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7.4 Air Gap Clearance Calculation for SIWV ............................................................214 7.5 Background data on flashover characteristics for UHV air clearance .................220 7.6 Recent investigations on air gap clearance in the UHV range ............................227 7.7 Non standard switching impulse waveforms in the insulation coordination ......245 7.8 Conclusion ......................................................................................................248 7.9 References ......................................................................................................249 8 Selection of insulation levels.....................................................................................251 8.1 Procedure for Insulation coordination ..............................................................251 8.2 Determination process for LIWV and SIWV........................................................261 8.3 Consideration of VFTO for insulation coordination...........................................266 8.4 Power frequency (AC) voltage tests for substation equipments.........................276 REFERENCES ..........................................................................................................284 8.5 Conclusion ......................................................................................................285 9 Conclusion and Recommendation .............................................................................286 9.1 Recent Practices on insulation coordination for UHV and 800 kV system ..........286 9.2 Overvoltages in UHV range ..............................................................................286 9.3. Evaluation of overvoltages ..............................................................................287 9.4 Switching overvoltage mitigation measures for future UHV systems .................287 9.5 Review on insulation coordination of air gaps in the UHV range .......................288 9.6 Selection of insulation levels............................................................................288

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1 Introduction Different countries in the world are planning and realizing UHV AC systems with operating voltages exceeding 800kV. When planning a new power system, in particular at a new voltage level, insulation coordination is one of the most important subjects. The main task is the determination of stresses and the assessment of the strength of the system and the equipment installed. The general procedure of insulation coordination is described in IEC 60071-1 (2010). This standard does not give precise advice regarding new voltage levels although it provides insulation levels for Um values of both 1100 kV and 1200 kV. These insulation levels are based on both past experience available from former CIGRÉ work that also considered the 1000kV voltage level, and recent works in Japan, China and India. The research activities within previous CIGRÉ SC 33 in the topic of UHV transmission provided a good basis on overvoltages and air insulation performance to make possible the design of air insulation for both 1100kV and 1200kV highest voltage of equipment. Since 1990’s, metal oxide surge arresters have been applied to UHV substation design. Insulation coordination for UHV has been changed based on these arresters throughout substation and transmission line. Also, gas insulated switchgears (GIS, Hybrid-IS) have been generally applied to UHV substation design. Considering the above issues, CIGRÉ WG C4.306 has reviewed and discussed insulation coordination practice in the UHV AC range taking into account the state-of-the-art technology, with special reference to surge arresters. Such a review has been taken into account the accumulated knowledge of various CIGRÉ working bodies, and accomplished in collaboration with related CIGRÉ SC A3 and B3 (WG A3.22, A3.28, B3.22 and B3.29). Recommendation, for application guide IEC 60071-2 (1996) and IEC apparatus standards has been proposed. The task of CIGRÉ WG C4.306 is divided into four main sections dealing with (see Figure 1.1): ● Recent practice on insulation coordination for UHV system: - Insulation coordination throughout substation and transmission line - Reduction of insulation levels by application of higher performance surge arresters and other overvoltage suppression measures ● Overvoltage in UHV range: (especially focused on peculiarity to UHV AC system) - Determination of stresses (TOV, switching overvoltage, lightning overvoltage and VFTO) by simulation tools and verification by measuring results - TOV due to load rejection and ground fault - Switching overvoltages caused by closing and opening (fault-clearing) with ground fault overvoltage - Lightning overvoltage caused by back-flashover and direct lightning, VFTO stress in GIS due to disconnector switching (ref to CIGRE brochure "Monograph on GIS Very Fast Transients 1989) ● Review on insulation coordination of air gaps in the UHV range: - Phase-to-phase insulation ● Selection of insulation levels: - Coordination withstand voltages and safety factors for equipment - Selection of insulation levels for equipment and transmission lines

More than 250 written technical contributions have been prepared by 29 experts from 15 countries during the investigations.

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Recent practice of UHV insulation coordination

1.2

Collaboration with A3.22&28 and B3.22&29

IEC 60071-1 Ed 8.1 (2010) (UHV LIWV & SIWV)

U r/ 3

1. Recent practice on insulation coordination for UHV system 2. Overvoltage in UHV range (especially focused on peculiarity to UHV AC system) 3. Review on insulation coordination of air gaps in the UHV range 4. Selection of insulation levels

Proposal of recommendation for application guide IEC 60071-2 (1996) by the end of 2012

Figure 1.1 Task of CIGRÉ WG C4.306

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2 Concept of recent practices on insulation coordination for the UHV and the 800kV system The design of UHV and 800kV power system should achieve both economic efficiency and high reliability while being capable of heavily loaded, long-distance transmission. UHV transmission lines and substation equipment are inherently large, therefore they should be designed as compact as possible by applying effective insulation coordination. Metal oxide surge arrester, which has been applied since 1990’s, is the key technology for UHV insulation, and equipment can be designed optimally by applying them based on detailed overvoltage analysis. The insulation concept examined in this report is expected to reflect in IEC 60071. This chapter summarizes the concept of UHV recent practice and is related to the other chapters which mention each topic in detail.

2.1 Insulation coordination throughout substation and transmission line The general procedure of insulation coordination is described in IEC 60071-1 (Insulation co-ordination-Part1: Definitions, principles and rules), and IEC 60071-2 (Insulation co-ordination - Part2: Application guide). Insulation design of UHV system is required to achieve high reliability. UHV equipment sizes also tend to be large compared to lower-voltage equipment. Therefore economical and highly reliable transmission lines and substations with environmental considerations are essential in the UHV system. From these system requirements for UHV systems, reasonable specifications should be determined by analyzing overvoltage accurately, and applying sophisticated technologies, such as higher performance metal-oxide surge arrester (MOSA). The main characteristics of the higher performance surge arrester are described in 5.2.2.

2.1.1 INSULATION COORDINATION RESEARCH IN UHV SYSTEM BY CIGRE AND IEC CIGRE had researched insulation coordination for UHV within previous CIGRE SC33 since 1970’s, and published Technical brochure No.32 (Final report of the UHV Ad Hoc Group, 1972), and Technical Brochure No.85 (Ultra High Voltage Technology, 1994). Rated insulation levels for UHV system are standardized in Amendment 1 of IEC 60071-1 Ed.8.1 (March 2011). The standard specifies rational insulation levels with the assumptions that several higher performance surge arresters are installed at adequate locations, and utilities can choose the reasonable insulation level to meet their own specifications. Table 2.1.1 shows the standard insulation levels in IEC 60071-1. LIWV for UHV system are 1950, 2100, 2250, 2400, 2550, 2700 kV and SIWV 1425, 1550, 1675, 1800, 1950 kV. But air insulation clearances described in IEC 60071-1 Ed.8.1 are under consideration as shown in Table 2.1.2, therefore the proposal reported in Chapter 7 of this report is very important. 1100 kV, 1200 kV were added as highest voltage of equipment when IEC 60038 was revised in June 2009. Table 2.1.3 shows the standard voltages. The UHV equipment and substation design were researched by CIGRE A3.22 and B3.22, and reported in Technical brochure No.362 (Technical Requirement for Substation Equipment exceeding 800 kV AC, 2010) and Technical brochure No.400 (Technical Requirement for Substation exceeding 800kV, 2010). In these technical brochures, more adequate technical requirements are stipulated by analysing, with the latest tool, suppression of overvoltages by higher performance arrester and the resistor insertion of disconnector. Detailed specifications of UHV circuit breakers and disconnectors and field tests have been discussed previously in CIGRE WG A3.28 and B3.29.

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Table 2.1.1 Standard insulation levels for UHV (IEC 60071-1 Ed.8)

Table 2.1.2 Standard rated switching impulse withstand voltage and minimum phase-to-phase clearance for UHV (IEC 60071-1 Ed.8.1)

Table 2.1.3 Standard voltage for UHV (IEC 60038)

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2.1.2 RECENT PRACTICE OF INSULATION COORDINATION FOR UHV AC TRANSMISSION SYSTEM Economical and highly reliable transmission lines and substations equipment with environmental considerations are essential in the UHV system. Therefore reducing the size of transmission lines and substation equipment are practical countermeasures. In Chinese, Indian, Japanese UHV projects, suppressing overvoltage by higher performance surge arresters is a common countermeasure, and additional countermeasures, such as suppressing overvoltage by the circuit breakers with closing and/or opening with pre-insertion resistors, are adopted in each project shown in Figure 2.1.1. In these projects, overvoltages are simulated by the latest analyzing technology such as EMTP.

Practical application of higher performance metal oxide surge arrester

Reliable circuit breaker with closing and/or opening resistor Rational Insulation Specification

LIWV (Substation)

SIWV (Substation)

Switching Overvoltage

Insulation Design Level (Transmission line)

Figure 2.1.1 UHV insulation coordination concept Figure 2.1.2 shows the flow chart of insulation coordination referred from IEC 60071-1. The basic concept has not been changed, but the concept is desirable to be reviewed with the latest point of view, by taking account of the analysis tool improvement, quality improvement, and safety factor which is included in analysis condition. To design the substation equipment rationally, detailed analysis is more recommended than just applying the insulation level based on LIPL (lightning impulse protective level of a surge arrester) and SIPL (switching impulse protective level of a surge arrester), which are calculated by simplified method in IEC 60071-2 (Ref. Chapter 8.2), because the insulation level has much influence on the construction cost in UHV design.

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Origin and classification of stressing voltages Protective level of overvoltage limit ing devices Insulation characteristics

System analysis

Representative voltages and overvoltage Urp (※1)

Insulation characteristics Performance criterion Statistical distribution (+) Inaccuracy of input deta (+) (+) Effects combined in a co-ord ination factor Kc

Selection of the insulation meet ing the performance criterion

Co-ordination withstand voltages Ucw (※2) Altitude correction factors Ka (or at mospheric correction factors

Equipment test assembly *) Dispersion in production *) Quality of installation *) Ageing in service *) Other unknown factors *) *) Effects combined in a safety factor Ks

Application of factors to account for the differences between type test conditions and actual service conditions

Required withstand voltage Urw (※3)

Test conditions Test conversion factor Ktc Standard withstand voltages

Selection of rated withstand voltages or standard rated withstand voltages Uw fro m the lists

Rages of Um Rated or standard insulation level : set of Uw

NOTE In brackets the subclauses reporting the definition of the term or the description of the action. Sided bo xes refer to required input Sided bo xes refer to performed actions Sided bo xes refer to obtained results

Figure 2.1.2 Flow chart for the determination of rated or standard insulation level in IEC60071-1 NOTE: The definition of some terms in above figure as given in IEC60071-1 is summarized as follows; Urp: Representative overvoltages: Overvoltages assumed to produce the same dielectric effect on the insulation as overvoltages of a given class occurring in service due to various origins. They consist of voltages with the standard shape of the class. Ucw: Co-ordination withstand voltage: For each class of voltage, the value of the withstand voltage of the insulation configuration in actual service conditions, that meets the performance criterion Urw: Required withstand voltage: Test voltage that the insulation must withstand in a standard withstand voltage test to ensure that the insulation will meet the performance criterion when subjected to a given class of overvoltages in actual service conditions. Uw: Standard rated withstand voltage: Standard value of the rated withstand voltage as specified in this standard. The rated withstand voltage is value of the test voltage, applied in a standard withstand voltage test that proves that insulation complies with one or more required withstand voltages. It is a rated value of the insulation of an equipment. Kc: Co-ordination factor: Factor by which the value of the representative overvoltage must be multiplied in order to obtain the value of the co-ordination withstand voltage. Ks: Safety factor : Overall factor to be applied to the co-ordination withstand voltage, after the application of the atmospheric correction factor (if required), to obtain the required withstand voltage, accounting for all other differences in dielectric strength between the conditions in service during life time and those in the standard withstand voltage test

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2.1.3 OVERVOLTAGES SPECIFIC TO UHV AC TRANSMISSION SYSTEM Overvoltages which need to be considered in designing UHV transmission lines and substation equipment are classified into four categories from the voltage characteristics as shown in Figure 2.1.3 (See Chapter 4). Each shape of overvoltage is specified in IEC 60071 as shown Table 2.1.4.

Figure 2.1.3 Representative maxima of amplitude of overvoltages Um (pu)

Table 2.1.4 Classes and shapes of overvoltages, Standard voltage shapes and Standard withstand voltage tests (IEC 60071) 2.1.3.1 T EMPORARY

OVERVOLTAGES

(TOV)

TOV includes healthy phase overvoltages due to transmission line ground faults and load rejections. In the case of sudden load rejection on a heavily loaded, long line, such as a UHV system, the overvoltage is about 1.3 - 1.5 p.u. This TOV is required not only to cover the peak voltage in the system, but also to cover the overvoltage generated during their operation. Therefore power frequency withstand test was verified in both long time range and short time range, because the voltage stress is different from both range as described in Chapter 8.4.

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2.1.3.2 S LOW - FRONT

OVERVOLTAGES

(S WITCHING

OVERVOLTAGES )

The duration of wave front is about a few-hundred microseconds, such as the overvoltage in opening / closing transmission lines and ground fault. This switching overvoltage has much influence on the insulation design of towers, thus switching overvoltage is particular important for UHV systems because of the saturation effects of the air insulation distance on the switching impulse strength. As shown in Figure 2.1.4, for the 1100 kV voltage level, the flashover voltage of air-insulated gaps for switching overvoltage has a tendency to saturate. Therefore, extremely high tower is required for air insulation. On the contrary, to reduce the construction cost of UHV system, switching overvoltages can be reduced by adopting circuit breakers with closing and/or opening resistors and higher performance arrester. Figure 2.1.4 shows the relation between air insulation distance and switching overvoltage, and Figure 2.1.5 shows the comparison between the double-circuit tower design based on 2.0 p.u. and 1.7 p.u.

Withstand Voltage (kV)

2000

(UHV-Switching OV: 2.0pu (same as 550kV)) (UHV-Switching OV: 1.7pu)

1500

(550kV-Switching OV: 2.0pu)

1000

500

3m

0 0

6m

9m

5

10

Air Insulation Distance (m)

Figure 2.1.4 Relation between air insulation and switching impulse withstand voltages ExpectinTra g the nsmission ap plicaline tion of 550k V construtech ctednology this time

Tran sm ission lin e constru cted this time

Figure 2.1.5 Size reduction of 1100kV tower

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2.1.3.3 F AST - FRONT

OVERVOLTAGES

(L IGHTNING

OVERVOLTAGES )

Lightning strokes terminating on UHV transmission lines can generate overvoltages of several MV depending on the front-steepness of the overvoltage and the height of the tower. Shielding failures as well as back-flashovers have to be taken into account. Lightning overvoltage is the predominant factor for substation equipment design. Therefore, lightning overvoltages in the UHV substation are highly suppressed for size reduction within a rational level by installing several higher performance surge arresters at adequate locations.

2.1.3.4 V ERY

FAST TRANSIENT OVERVOLTAGES

(VFTO)

The GIS disconnector, when switching a charging current, repeats restriking and generates VFTO, which can reach up to approximately 3.0 p.u. At a UHV substation, lightning overvoltages are effectively suppressed by higher performance surge arresters. Disconnector switching overvoltages are likely to exceed the lightning overvoltage if no measures are taken to limit them. Therefore, the resistors can be a suppression measure for the VFTO.

2.1.4 SELECTION OF INSULATION LEVEL Insulation coordination of substations and transmission lines can be achieved to set a reasonable insulation level voltage without sacrificing supply reliability by installing higher performance surge arresters on specific locations in substations, adopting resistor-fitted switching schemes of disconnectors and circuit breakers, and comprehensive simulations and analysis of assumed overvoltage phenomenon. To select an appropriate insulation level and insulation requirements for equipment, it is necessary to evaluate technical data of equipment and set reasonable design margins to secure supply reliability while satisfying each project’s design constraints, such as substation type: open-air/Hybrid-IS/GIS type. Figure 2.1.6 shows substation designs and corresponding insulation levels (LIWV and SIWV) of Chinese and Japanese projects.

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TR SW

LIWV SIWV 2250 kV 1800 kV 2400 kV 1800 kV

SW

LIWV SIWV 2400 kV 1800 kV

(a) Jindongnan substation (China)

(b) Nanyang switching station (China)

LIWV SIWV TR 2250 kV 1800 kV SW 2400 kV 1800 kV (c) Jingmen substation (China)

LIWV SIWV TR 1950 kV 1425 kV SW 2250 kV 1550 kV (d) Shin-Haruna testing site (Japan)

Figure 2.1.6 Substation designs and corresponding insulation levels (LIWV and SIWV) of China and Japanese projects (TR: Transformer, SW: Switching equipment in above captions)

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2.2 Reduction of insulation levels using overvoltage suppression measures The higher performance surge arresters, high voltage shunt reactors, resistor-fitted switching schemes of disconnectors and circuit breakers have been utilized to suppress the overvoltages peculiar to UHV systems, and to reduce insulation design level of each project.

2.2.1 OVERVOLTAGE SUPPRESSION WITH HIGHER PERFORMANCE SURGE ARRESTERS The higher performance surge arrester, which has better protective performance (See Chapter 5.2), has been utilized to suppress LIWV and SIWV. The reliability of higher performance surge arrester was confirmed throughout its massive application in 550 kV systems, and it is recognized as an effective measure to suppress power system overvoltages. Recent UHV projects in China and Japan employ higher performance surge arresters with highest voltage of equipment of 1620 kV (1.80 p.u. at 20 kA) at 1100 kV system. On the other hand, a recent project in India is developing an arrester with highest voltage of equipment of 1700 kV (1.74 p.u. at 20 kA) at 1200 kV system. Typical locations of these higher performance arresters are transmission bays, busbars and transformer bays. Table 2.2.1 shows the LIWVs and protective performance of arresters in recent projects. Although each project adopts different insulation levels due to differences in location of arresters and substation types, all projects succeeded in reducing insulation voltage level ranges: 1950 kV-2250 kV for transformers and 2250 kV-2400 kV for switchgears.

Highest voltage (kV) Type of substation Residual voltage (@20 kA) (kV) Transformer LIWV (kV) GIS and others

Italy 1050 GIS 1800 2250 2250

Japan 1100 GIS 1620 1950 2250

China 1100 GIS, Hybrid-IS 1620 2250 2400

India 1200 AIS 1700 2250 2400

Table 2.2.1 Protective performance of surge arresters in substation projects

Figure 2.2.1 Dead tank-type higher performance surge arrester (Japan, China)

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Figure 2.2.2 Porcelain type

Figure 2.2.3 Porcelain type

higher performance surge

higher performance surge

arresters (China)

arresters (India)

2.2.2 RESISTOR-FITTED CIRCUIT BREAKERS To suppress the switching overvoltage, pre-insertion resistor is employed for UHV circuit breakers. Chinese and Indian UHV projects introduce resistor-closing technique, while Japanese project introduces resistor-closing / opening technique. Both techniques suppress switching overvoltages of transmission lines to below 1.7 p.u. The resistance of this switching scheme is usually between 500-700 Ω depending on the size of UHV system and its characteristics. Table 2.2.2 shows the insulation coordination of several UHV projects: (a) Closing overvoltage in Indian project and, (b) Opening overvoltage in Japanese project Figure 2.2.4 shows the study example of the relation between switching resistance and overvoltage suppression effects in Indian and Japanese project, and Figure 2.2.5 is an example of GCB with pre-insertion resistors.

Highest voltage (kV) Suppression of switching overvoltage Switching overvoltage insulation design level (p.u)

Italy 1050 MOSA Closing & opening R (500 Ω)

Japan 1100 MOSA Closing & opening R (700 Ω)

1.7

1.6 / 1.7

China 1100

India 1200

MOSA Closing R (600 Ω)

MOSA Closing R (600 Ω)

1.7

1.7

Table 2.2.2 Suppression methods and insulation designs of international projects [5]

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1.83 1.60 1.52 1.42

(a) Closing case in Indian project

(b) Opening case in Japanese project

Figure 0.4 Example of relation between resistance and overvoltage

Figure 2.2.5 GCB with pre-insertion resistors 2.2.3 THE DAMPING EFFECT OF THE RESISTOR-FITTED DISCONNECTORS EMPLOYED IN GIS AGAINST THE SWITCHING OVERVOLTAGES. In gas insulated substations, the resistor-fitted disconnectors are commonly utilized to suppress switching overvoltages. Examples of applications of resistor-fitted disconnectors are shown in the Table 2.2.3 below. The GIS system with fast-operating disconnectors can suppress the disconnectors’ overvoltage levels from 2.8 p.u. without the resistors to less than 1.3 p.u. with pre-insertion resistors.

Highest voltage (kV) Type of substation Pre-insertion resistor (Ω)

Italy 1050 GIS 110

Japan 1100 GIS 500

China 1100 GIS Hybrid-IS 500 None

India 1200 AIS None

Table 2.2.3 Application of pre-insertion resistor in international projects

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R esistor

M ovable contact

Figure 2.2.6 Disconnector with pre-insertion resistor 2.2.4 OTHER MEASURES FOR THE REDUCTION OF INSULATION LEVEL High voltage shunt reactors can be applied on long UHV transmission lines with adequate compensation degree (generally in the range from 70% to90%) to maintain reactive power balance and suppress TOV below 1.4p.u. Controlled switching and line arresters can be utilized as a mitigation measure for insulation level reduction, although they have not been commercially applied to UHV systems.

References [1] Eiichi Zaima, C.Neumann, “Insulation Coordination for UHV AC Systems based on Surge Arrester Application (CIGRE C4.306)”, IEC-CIGRE Second International Symposium on Standards for Ultra High Voltage Transmission [2] Guagfan Li, Jianbin Fan, “The experience of UHV substation facilities in China”, International Conference on Development of 1200kV National Test Station [3] IEC 60071-1 Ed. 8.1, “Insulation co-ordination - Part 1: Definitions, principles and rules”, 2011 [4] IEC 60038, “Standard voltages”, 2009 [5] CIGRE brochure No.400, “Technical Requirements for Substation exceeding 800kV”, 2010

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3 Recent practice on insulation coordination for the UHV and the 800 kV system 3.1 China UHV project 3.1.1 CHINA 1000 KV AC TRANSMISSION SYSTEM The schematic diagrams of UHV transmission systems are shown in Figure 3.1.1 and Figure 3.1.2. The UHV AC transmission test and pilot project (Jindongnan–Nanyang–Jingmen) was put into operation in January 2009, as shown in Figure 3.1.1. The planning construction Huainan-Shanghai UHV double-circuit tower arrangement transmission project is shown in Figure 3.1.2.

Figure 3.1.1 Wiring schematic diagram for the China 1000 kV AC transmission test and pilot project (single circuit) system

Figure 3.1.2 Schematic diagram for the China 1000 kV AC South Anhui-Shanghai double-circuit tower arrangement transmission system 3.1.2 POWER FREQUENCY TEMPORARY OVERVOLTAGE (TOV) AND THE PARAMETER SELECTION FOR METAL OXIDE ARRESTERS (MOA)

3.1.2.1 A MPLITUDE

AND DURATION OF

TOV

As for calculating the TOV, the following fault types shall be mainly taken into account.

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As for the single-circuit transmission lines, two kinds of failures shall be usually taken into account, namely, the load rejection under the normal operation and the load rejection in the case of the line single-phase grounding failure. As for the double-circuit tower arrangement transmission line, the double-circuit operation or one circuit out service /the other circuit operation shall be considered and the failures causing double-circuit 6-phase load rejection shall be taken into account. Most of China 1000 kV lines are relatively long and the high voltage shunt reactors are generally installed in the lines. The largest TOV may generally occur in the single-phase grounding fault case, in which the fault line circuit breaker at one line side shall be three-phase tripped and the circuit breaker at the other line side shall not be tripped. The above failure case may occur under the following two situations: (1) During the normal operation process, the line single-phase is grounded and the single phase reclosing is not successful the three-phase circuit breaker is tripped by relay protection. (2) During the line live working process, the single-phase reclosing shall be required to withdrawal; at this time, the single-phase grounding fault occurs and the line three-phase circuit breaker are also tripped. The main measure to limit the power frequency overvoltage is to install the line high voltage shunt reactor. The maximum TOV shall be allowed no more than 1.4 p.u. at the line side and the maximum TOV shall be allowed no more than 1.3 p.u. at the bus side in China. The TOV duration may play an important role in the choice of the arrester rated voltage and the equipment insulation level. The relay protection mode is adopted in which the UHV line both side circuit breakers are tripped synchronously, so as to shorten the duration of TOV and lower the energy absorbed by MOA. The maximum trip delay for the both side circuit breakers shall generally be controlled within 0.2 seconds; the TOV duration shall be no more than 0.5 seconds even if it is considered that the one side circuit breaker is failure to trip and the tripping shall be carried out by the back standby circuit breaker.

3.1.2.2 MOA

PARAMETER SELECTION

In the past, the traditional MOA rated voltage selection principle was Un≥TOV; whereas, the traditional MOA rated voltage selection principle has been broken through in China UHV project, namely, that Un is allowed to be less than TOV. The MOA rated voltage Un of the UHV system was selected as 828 kV, which is equivalent to 1.3 p.u. and is less than the maximum TOV (1.4 p.u.) of the UHV lines. Because the MOA is with the good short time power frequency overvoltage withstand capacity, the Un being lower than TOV for short time shall be permitted. According to the test data from the China MOA manufacturers, the TOV duration for MOA withstanding 1.4 p.u. is 10 seconds. The main electrical parameters for MOA (Un=828 kV) are listed in Table 3.1.1. The energy absorption allowable value is 40 MJ. The calculation results may show that the MOA actual maximum absorption energy shall be less than 10 MJ while the maximum TOV duration is 0.5 seconds and under the switching overvoltage cased by two times closing operation. Therefore, the Un is selected as 828 kV and there shall be relatively great margin for MOA absorption energy. The lower MOA rated voltage causes the MOA residual voltage to be lowered so as to lower the substation overvoltage amplitude and the requirement for the equipment insulation level; as plays some certain role for lowering the line overvoltage.

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System nominal voltage

Installation location

Rated voltage (RMS)

Continuous operation voltage (RMS)

Switching impulse residual voltage under 30/60 μs and 2 kA

Lightning impulse residual voltage under 8/20 μs and 20 kA

1000

The line side, the bus side and the transformer side

828

638

≤1460

≤1620

Table 3.1.1 Switching overvoltage design level of transmission line 3.1.3 SWITCHING OVERVOLTAGE The following 3 categories of switching overvoltage shall mainly be taken into account: (1) The closing and reclosing no-load line overvoltage; (2) The ground fault overvoltage; (3) The clearing short-circuit fault overvoltage caused by circuit breakers tripping

3.1.3.1 C LOSING

AND RECLOSING NO - LOAD LINE OVERVOLTAGE

The closing and reclosing line overvoltage may play the control role to the insulation design of China 1000 kV lines. The main measures to limit the closing and reclosing line overvoltage are that the closing resistor is installed on line circuit breakers. The closing resistor is taken as 600 Ω and the closing resistor pre-insert time is 9.5±1.5 ms. The maximum phase-to-ground statistical switching overvoltage along the line shall be no more than 1.7 p.u. for China 1000 kV lines; the substation maximum phase-to-ground statistical switching overvoltage shall be no more than 1.6 p.u. and the maximum phase-to-phase statistical switching overvoltage shall be no more than 2.9 p.u. The maximum statistical switching overvoltage at the substation bus side shall be no more than 1.55 p.u. The front time of the 1000 kV line closing and reclosing line overvoltage shall generally be above 1000-3000 μs, which may greatly influence the air clearance selection of transmission line tower.

3.1.3.2 G ROUND

FAULT OVERVOLTAGE

The single-phase grounding fault type shall be taken into account for the calculation to the ground fault overvoltage of the China UHV systems; the overvoltage amplitude is relatively low; the maximum 2 % overvoltage shall be lower than 1.51 p.u.. As for the China UHV lines, the ground fault overvoltage shall not be the control factor to determine the line insulation level.

3.1.3.3 C LEARING

FAULT OVERVOLTAGE

This clearing fault overvoltage means the overvoltage occurred in the adjacent non-fault lines while the short-circuit fault in the fault line is cleared up. The fault type may significantly influence the overvoltage amplitude. The clearing single-phase grounding fault overvoltage amplitude shall be within the allowable range. The clearing 2-phase or 3-phase grounding fault overvoltage may be relatively high and the overvoltages in some lines may exceed the allowable values. The opening resistor may be installed in the line circuit breaker so as to lower the clearing fault overvoltage. The following situations shall be taken into account: (1) The opening resistor may absorb great energy and the operation may be complicated so that not only the cost of the circuit breaker may be increased but also the probability of the circuit breaker failure may be increased. (2) The probability of the 2-phase or 3-phase grounding fault shall be very low.

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(3) The maximum overvoltage may occur on lines rather than in substations. It may cause the line insulation flashover; however, the substation equipment may not generally be damaged by line overvoltage. As for the Jindongnan–Nanyang–Jingmen UHV transmission line, the opening resistor is not necessary to be installed.

3.1.3.4 E NERGIZING

UNLOADED TRANSFORMER OVERVOLTAGE

As shown in the research and field tests in China, the 500 kV circuit breaker is not equipped with the closing resistor for the energizing unloaded UHV transformer at the 500 kV side; there shall be no any relatively high resonance overvoltage. The inrush current and the overvoltage are within the allowable range. The possibility of the resonance overvoltage and the inrush current from the energizing unloaded UHV transformer at the 1000 kV side may be greater than that at the 500 kV side. The closing resistor may be adopted so as to be beneficial to lowering the resonance overvoltage and the inrush current; however, the closing resistor may not be valid for all system construction and operation modes; moreover, the closing resistor may increase the equipment cost and cause the switching operation mechanism to be complicated as well as lower the reliability. Under the normal circumstances, the energizing unloaded UHV transformer at the 500 kV side shall be provided.

3.1.4 VERY FAST FRONT TRANSIENT OVERVOLTAGE (VFTO) The GIS disconnector switching may generate the VFTO whose wave front is very steep and amplitude is very high and which may damage three types of equipment insulations: (1) GIS body; (2) equipment with winding, such as a transformer; (3) the secondary equipment. As for the GIS UHV substation, this problem may be more remarkable. Because the higher the system rated voltage is, the lower the ratio of the equipment lightning impulse withstand voltage LIWV and the system rated voltage Un. In comparison the 1000 kV GIS substation with the 500 kV GIS substation, the rated voltage is increased by 1 time, but the relative value of VFTO is basically the same; the absolute value of VFTO is proportionally increased by 1 time with the rated voltage; however, the insulation level (LIWV) of the 1000 kV GIS equipment is increased by 55 % in comparison with that of the 500 kV GIS equipment, which is not proportionally increased. Therefore, VFTO may do more greatly harm to UHV GIS equipment than 500 kV GIS equipment. Before the GIS rated withstand voltage is not determined under VFTO, we may temporarily adopt the GIS lightning impulse withstand voltage LIWV as the GIS rated withstand voltage under VFTO; The VFTO calculation research has been carried out by combining the UHV substation or switching station characteristics in China; it is thought that not only the VFTO at the initial GIS layout of the substation or switching station shall be calculated but also the VFTO at the long-term GIS layout; the substation or switching station GIS layout (such as the bus length) may greatly influence the VFTO amplitude. Thus, the following viewpoints may be put forward: (1) The shunt resistor (whose resistance is 500 Ω) is required to use in the GIS substation so as to effectively limit the VFTO. (2) The maximum VFTO caused by the disconnector switching may be 2.15 p.u. in the Hybrid-IS substation or switching station, which is not high and within the GIS insulation allowable range. Therefore, the disconnector of the Hybrid-IS station may not be equipped with the shunt resistor; the differential treatment is adopted so as to be safe and economical. Because the transformer in the China UHV substation is not directly connected with GIS, which is connected through the overhead lines and whose distance is far away; the VFTO is attenuated fast; the transformer VFTO amplitude is relatively low (the maximum peak value is 925 kV) and the wave front time is increased. The calculated wave front time is more than 1.5 μs. Thus, there shall be no any danger to the transformer insulation.

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3.1.5 SUBSTATION LIGHTNING INVADING OVERVOLTAGE CALCULATION AND THE SUBSTATION ARRESTER LAYOUT As for the insulation design of the UHV substation equipment, the lightning overvoltage may be predominant. As for the calculation of the China UHV substation lightning invading overvoltage, the two relatively harsh connection modes shall be taken into account, which may be listed as follows: (1). The single-line mode and the line circuit breaker being tripped; (2). The single line + single bus + single transformer mode (as shown in Figure 3.1.3).

(a) single-line mode

(b) single line + bus + single transformer mode

Figure 3.1.3 Substation connection modes taken into account for the lightning invading overvoltage calculation

Figure 3.1.4 Ground wires have been adopted in the entrance line section of the UHV substation The substation maximum lightning overvoltage may be caused by the lightning shielding failure invading wave in the entrance line section of substation; two measures, namely, decreasing the maximum lightning shielding failure current in the entrance line section [the entrance line ground wire protection angle is decreased to ≤-4° and three ground wires have been adopted in the entrance line section (as shown in Figure 3.1.4). and optimizing the arrester layout, have been taken in China UHV AC transmission test and pilot project so as to decrease the lightning shielding failure invading overvoltage. Finally the scheme with the small quantity of arresters has been adopted: 1 group of MOAs is installed in each circuit entrance location; 1 group of MOAs is installed in each bus section; 1 group of MOAs is installed beside the transformer. The overvoltage values may be different for various substations. The typical values of the maximum lightning overvoltage of the equipment may be listed as follows: 2040 kV for

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GIS, 1854 kV for the shunt reactance and 1796 kV for the transformers. The lightning impulse withstand rated voltage of the transformer and the shunt reactance is 2250 kV and lightning impulse withstand rated voltage of other devices is 2400 kV in China. The allowable values of the equipment lightning impulse insulation levels shall be more than the maximum lightning invading overvoltage, which shall be meet the requirements of the internal insulation margin (15 %) and the external insulation margin (5 %). As for the single-line mode, the internal insulation margin may be lowered to 10 % because its occurrence probability is very small. As for the calculation of the substation lightning invading overvoltage, the interval statistics method may be used besides the deterministic method; moreover, the substation lightning MTBF (mean time between failures) shall be required to be more than 1500 years.

3.1.6 LINE LIGHTNING PROTECTION By taking the importance of the UHV lines as well as the characteristics of the UHV line high insulation level, the expected lightning trip rate for the 1000 kV lines shall be lower than that for the 500 kV lines (according to the 500 kV operation experiences, statistic lightning trip rate value is 0.14 times/100km·a), which may be 0.1 times/100km·a according to 70 % of the lightning trip rate for the 500 kV lines. As shown in the operating experiences, the line insulation level shall be increased along with the transmission line voltage level; the lightning back flash-over failure trip rate shall account for the less, of the total lightning trip rate, which may account for less than 10 % of the total lightning trip rate in China 500 kV transmission lines. As shown in the calculation results of the China 1000 kV line lightning protection, the lightning back flash-over of the line insulation may basically not occurred ; the main cause to give rise to the lightning flashover shall be lightning shielding failure. Therefore, the key for the UHV line lightning protection shall be against the lightning shielding failure. The main method for calculating the line trip rate is the improved electrical geometric model (EGM), in which the influencing factors such as the terrain along the line, the correction coefficient of the lightning striking distance to the earth as well as the probability distribution of the lightning leader incident angle shall be taken into account; at the same time, the study on the line lightning shielding failure trip-out rate calculated by utilizing the leader propagation model (LPM) has been carried out in China. Because some parameters and criteria used in the calculation by various units may be different, the calculation results may be quite different; moreover, these parameters and criteria have been lack of sufficient base for the time being. Therefore, the calculation result from the improved electrical geometric model (EGM) shall be the primary base; on the other hand, the calculation result from the leader propagation model (LPM) shall be a reference. The main measure for lowering the lightning shielding failure trip-out rate shall be to lower the ground wire protection angle α; and terrain along the line shall greatly influence the lightning shielding failure trip-out rate. Based on the relevant researches, the following regulations have been applied to the ground wire protection angle α in China 1000 kV lines, which may be listed as: (1) As for the single circuit transmission lines: α