Insulation coordination for UHV AC systems.pdf

(1)

INSULATION

COORDINATION

FOR

UHV AC

SYSTEMS

WG C4.306

(2)

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

“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”.

(3)

(4)

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…

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

(5)

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

(6)

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

(7)

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.

(8)

Figure 1.1 Task of CIGRÉ WG C4.306

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

1.2 Ur/ 3 Recent practice of UHV insulation coordination IEC 60071-1 Ed 8.1 (2010) (UHV LIWV & SIWV) Collaboration with

A3.22&28 and B3.22&29

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

(9)

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.

(10)

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)

(11)

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.

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.

Practical application of higher performance metal oxide surge arrester

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

LIWV

(Substation) (Substation) SIWV

Switching Overvoltage Insulation Design Level

(12)

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

Origin and classification of stressing voltages

Protective level of overvoltage limit ing devices Insulation characteristics Insulation characteristics Performance criterion Statistical distribution (+) Inaccuracy of input deta (+) (+) Effects combined in a co-ord ination factor Kc

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

Test conditions Test conversion factor Ktc Standard withstand voltages Rages of Um

System analysis

Representative voltages and overvoltage Urp (※1)

Selection of the insulation meet ing the performance criterion

Co-ordination withstand voltages Ucw (※2)

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

Required withstand voltage Urw (※3)

Selection of rated withstand voltages or standard rated withstand voltages

Uw fro m the lists

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

(13)

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.

(14)

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.

Figure 2.1.4 Relation between air insulation and switching impulse withstand

voltages

Transm ission line constructed this time Transmission line

constructed this time Expecting the application of 550kV

technology 0 500 1000 1500 2000 0 5 10

Air Insulation Distance (m)

W iths tan d V o lt ag e (kV )

3m

6m

9m

(550kV-Switching OV: 2.0pu)

(UHV-Switching OV: 1.7pu)

(15)

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.

(16)

LIWV SIWV TR 2250 kV 1800 kV SW 2400 kV 1800 kV

LIWV SIWV SW 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 LIWV SIWV TR 1950 kV 1425 kV SW 2250 kV 1550 kV (c) Jingmen substation (China) (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

(17)

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.

Italy Japan China India

Highest voltage (kV) 1050 1100 1100 1200

Type of substation GIS GIS GIS, Hybrid-IS AIS

Residual voltage (@20 kA) (kV) 1800 1620 1620 1700

Transformer 2250 1950 2250 2250 LIWV (kV)

GIS and others 2250 2250 2400 2400

Table 2.2.1 Protective performance of surge arresters in substation projects

(18)

Figure 2.2.2 Porcelain type

higher performance surge

arresters (China)

Figure 2.2.3 Porcelain type

higher performance surge

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) 1050 1100 1100 1200 Suppression of switching overvoltage MOSA Closing & opening R MOSA Closing & opening R MOSA Closing R MOSA Closing R

(19)

(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) 1050 1100 1100 1200

Type of substation GIS GIS GIS Hybrid-IS AIS

Pre-insertion resistor (Ω) 110 500 500 None None

Table 2.2.3 Application of pre-insertion resistor in international projects

1.52

1.42

1.60 1.83

(20)

M ovable

contact

Resistor

_{M ovable}

contact

Resistor

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

(21)

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

(22)

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

(23)

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.

(24)

(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.

(25)

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

(26)

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<6° for the plain area and α<-4° for the mountain area. (2) As for the double-circuit tower arrangement transmission lines: α<-3° for the plain area and α<-5° for the mountain area.

(3) As for the jumping-lines of the line strained angled towers: α≤6° for the single circuit transmission line in the plain area and α≤0° for the single circuit transmission line or the double-circuit tower arrangement transmission line

(27)

3.1.7 INSULATION COORDINATION

3.1.7.1 P

RINCIPLES OF INSULATION COORDINATION

By combining the research results on the China UHV overvoltage and internal and external insulation characteristics and according to the safety and economy principals, the insulation level may be determined. As for the China insulation coordination principle, some certain insulation safety margin has been required to reserve; and the internal and external insulation safety margins are 15 % and 5 %, respectively.

As for the air clearance of UHV AC line and substation, discharge voltage test at power frequency voltage, lightning and switching impulse voltage have been relatively systematically and comprehensively carried out with the 1:1 tested objects in China and a series of discharge voltage curves have been obtained; as may provide the reasonable basis for the insulation coordination.

3.1.7.2 S

UBSTATION EQUIPMENT INSULATION LEVEL

The insulation levels of the main equipments in China UHV substation are shown in Table 3.1.2.

On the whole, the insulation levels of the main equipments in China UHV substation may be lower than those of the Russia UHV equipment, but they may be higher than those of the Japan UHV equipment. It is determined by combining the China UHV overvoltage level and the corresponding equipment manufacturing experiences; and it shall be suitable for the Chinese situation.

(Unit: kV)

Equipment Lightning impulse

withstand voltage

Switching impulse withstand voltage

Power-frequency short-duration withstand voltage Transformer and reactor

2250 (chopped impulse:

2400)

1800 1100 (5 min)

GIS (circuit breaker and

disconnecting switch) 2400 1800 1100 (1 min)

Post insulator and disconnector

(open type) 2550 1800 1100 (1 min)

Voltage transformer (CVT) 2400 1800 1300 (5 min)

Bushing (transformer and reactor)

2400 (chopped impulse:

2760)

1950 1200 (5 min)

Bushing (GIS) 2400 1800 1100 (1 min)

Switching device longitudinal

insulation 2400+900 1675+900 1100+635 (1 min)

(28)

3.1.7.2.1 T

RANSFORMER INSULATION LEVEL

The insulation level of China UHV transformers may be shown in Table 3.1.3.

(Unit: kV) Country Lightning impulse

withstand voltage Switching impulse withstand voltage Power-frequency withstand voltage China 2250 1800 1100 (5 min)

Table 3.1.3 The insulation level of China UHV transformers

The lightning impulse withstand voltage and the power frequency withstand voltage may play the decisive role for the UHV transformer insulation.

China UHV MOA is with good V-A curve saturation characteristics and the low lightning impulse protection level; the measure for lowering the maximum shielding failure lightning current at the entrance line section; the lightning overvoltage at the terminals of the transformer is not high. Therefore, the safety margin may be sufficient for selecting 2250 kV as the lightning impulse withstand voltage.

As shown in the operating experiences, the majority of transformer failures occurred under operating voltage. The power-frequency short-duration withstand voltage test shall be designed to inspect whether the partial discharge (being considered to be the precursor phenomenon of the dielectric breakdown) exists and verify the insulation strength as well as check the insulation aging characteristics to some extent. China experts advocate that the power frequency withstand voltage test should be considered severely; the rated power frequency withstand voltage should take as 1100 kV and the duration time for 5 minutes; it is more strict in comparison with the recommendation value (1 min) for the UHV transformer from IEC standard

The switching impulse withstand voltage of China UHV transformers is 1800 kV.

3.1.7.2.2 L

ONGITUDINAL INSULATION TESTING VOLTAGE OF THE CIRCUIT BREAKER AND THE DISCONNECTOR

The longitudinal insulation lightning impulse testing voltage of China UHV circuit breaker and disconnector should be 2250 + 900 (kV), in which 900 kV is the peak value of opposite polarity working voltage,1100× 2 / 3 =Un-m=

900 kV.

The amplitude of the opposite polarity power frequency component recommended in IEC60071-1 is 0.7×Um× 2 /

3 namely the amplitude of the working voltage should be multiplied with the coefficient 0.7.

In China national standard “Insulation Coordination for High-voltage Transmission and Distribution Equipment” (GB311.1), the coefficient is stipulated among 0.7-1.0, which should also be determined together with the electric power companies and the manufacturers.

If this coefficient is 0.7, in 1 cycle of the working voltage (the 360° phase), there is about 1/4 cycles not being included. Therefore, the guarantee probability is 0.75.

(29)

The A1' value is the minimum electrical distance of the substation conductor to the frame; the A1" value is the

minimum electrical distance of the substation equipment to the frame; and A2 value is the minimum electrical

distance between the phases in the substation.

As for the areas whose altitude being no more than 1000 m, the minimum air clearance for the 1000 kV substation are listed in Table 3.1.4.

(Unit: kV) A1 value

The action voltage type

A1′ A1″

A2 value

Power frequency 4.2 6.8

Switching impulse 6.8 7.5

10.1 (ring - ring)

9.2 (four conductors - four sub-conductors)

11.3 (tubular bus - tubular bus)

Lightning impulse 5.0 5.5

Table 3.1.4 The minimum air clearance for the 1000 kV substation

3.1.7.4 L

INE INSULATION LEVEL

The contaminated insulator withstand voltage method or the specific creepage distance method may be used to determine the insulator number.

With comprehensively considering the insulation margin and the simplified design and other factors, the insulator configuration for the common lines in China UHV AC transmission test and pilot project may be listed as follows: (1) he suspension string: 54 double-shed disk-type insulators (for 300 kN) are used in the II grade polluted area; the composite insulator whose structural height is 9750 mm and creepage distance is 30300 mm are used in the III and IV grade polluted areas.

(2) he tension string: the 44, 54 and 60 disk-type insulators (550 kN and creepage distance is 700 mm) are used in II, III and IV grade polluted areas, respectively.

The minimum air clearance values for China 1000 kV lines are listed in Table 3.1.5.

As for the cup type-towers or cat-head type towers in the single-circuit line, the middle phase minimum air clearance is controlled by the switching overvoltage; on the other hand, the out phase minimum air clearance is controlled by the power frequency voltage under the weather condition of gale rather than the lightning overvoltage. Therefore, there shall be no any regulation on the minimum air clearance under the lightning impulse.

However, the towers of the UHV double-circuit tower arrangement transmission line may be very high so as to increase the attracting lightning area and weaken the conductor shielding effect from the earth; thus, the line may easily stricken by the lightning. Because too large lightning current is not able to cause the conductor shielding failure, so that the line maximum lightning shielding failure current amplitude is limited, the insulation level of the UHV double-circuit tower arrangement transmission line may be enhanced appropriately so as to effectively lower the lightning shielding failures. The main measure to enhance the line insulation level is to increase the distance from the conductor to its lower cross arm or the tower body; especially the former may significantly enhance the line insulation level. Therefore, the minimum clearance distance from the line lightning impulse voltage to the tower has been regulated for 1000 kV double-circuit tower arrangement transmission line.

While the discharge voltage within the air clearance under the switching impulse voltage is determined, 1000 kV line real type towers are tested in order to take fully account of the influence to the gap discharge voltage from the tower width; The wave front time of the switching impulse test voltage is 1000 μs.

(30)

The influence to the discharge voltage from the parallel connected number of gaps shall be taken into account in the insulation coordination and the entire line insulation flashover rate under the switching overvoltage has been calculated; the variation of the calculated switching overvoltage values at various locations along the line and the actual distribution shall be taken into account. The entire line insulation flashover rate under the switching overvoltage shall be no larger than 0.01 times/year.

(Unit: m)

The action voltage type The type of lines The minimum clearance/m

2.7 (altitude: 500 m) 2.9 (altitude: 1000 m) Power frequency voltage Single-circuit and 1-tower

double-circuit

3.1 (altitude: 1500 m)

Out phase: 5.9； middle phase: 6.7/7.9 (altitude: 500 m) Out phase: 6.2； middle phase: 7.2/8.0 (altitude: 1000 m) Single-circuit

Out phase: 6.4； middle phase: 7.9/8.1 (altitude: 1500 m) 6.0 (altitude: 500 m) 6.2 (altitude: 1000 m) Switching impulse 1-tower double-circuit 6.4 (altitude: 1500 m) Single-circuit No regulating

Plain area: 6.7 m; mountain area: 7.0 m (altitude: 500 m) Plain area: 7.1 m; mountain area: 7.4 m (altitude: 1000 m) Lightning impulse

1-tower double-circuit

Plain area: 7.6 m; mountain area: 7.9 m (altitude: 1500 m)

Table 3.1.5 The minimum air clearance in China 1000 kV lines

3.2 India UHV project

3.2.1 FEATURES OF POWERGRID’S 1200 KV TRANSMISSION SYSTEM

Major demand centres in Indian Power System are located towards Western and Northern parts of the country. On the other hand, generation pockets are located mainly in the Eastern part. In order to facilitate power transfer from Eastern to Western part of the country as well as to address right of way issue, high capacity East-West

transmission corridors comprising 765kV and 1200kV AC have been planned. Further, in the central part of India, Wardha is a gateway for power transfer towards Northern and Western part. In view of the above, high capacity 765kV transmission corridor has been planned upto Wardha from Eastern part, whereas beyond Wardha, a hybrid 1200kV and 765kV transmission corridor i.e, Wardha – Aurangabad – Padghe has been planned towards Western part. The 1200kV corridor includes Wardha – Aurangabad 1200kV one circuit (about 380km) in parallel with 4 nos. 765kV lines. However, initially due to less power transfer requirement over this corridor, the 1200kV line is

proposed to be operated at a 400kV double circuit i.e, line insulation at 1200kV level with terminal equipment at 400kV level, which is likely to be commissioned by 2013. Subsequently, with the increased power transfer requirement, this corridor shall be charged at rated voltage, sayby 2016-17. The power map showing proposed 1200kV transmission corridor is as under.

(31)

WR MAP SHOWING FUTURE 1200kV AND 800kV LINES

Figure 3.2.1 Power Map of Western Region showing proposed 1200kV corridor in

India

The connectivity diagram for the 1200kV corridor with 765kV system is shown in Figure 3.2.2.

(32)

3.2.2 INSULATION DESIGN FOR TRANSMISSION LINE

The nominal voltage of 1200kV system is considered as 1150kV. The line reactor size has been chosen as 660 MVAR (2 x 10 x 3) considering the maximum size of single phase reactor as 110 MVAR. This line reactor at each end of the line amounts to about 50% compensation of the reactive power. The remaining compensations are provided at the bus with 2 x 330 MVAR (2 x 3 x 110 MVAR, 1-ph) bus reactor.

The Switching Overvoltage studies are carried out under various source strength and the overvoltage value having 2% probability of being exceeded has been obtained as 1.7pu. To contain the overvoltage closing resistor of value 600 ohm has been considered which comes into the circuit initially for 10ms. The overvoltages are also contained with the help of line surge arrestors while the temporary faults are taken care by the single phase auto reclosing scheme.

The above information is tabulated below :

Design level (p.u.) 1.7 p.u.

Suppression measures for switching overvoltage Closing resistor of 600 Ù (10 ms insertion time) and line surge arrester

Switching overvoltage (maximum and / or 2 %)

(p.u.) and occurrence case 1.7 p.u. for line reenergization

Types of overvoltage to be studied

line energization/reenergization/ de-energization, line fault clearing, Reactor Switching, Transformer Energizing and tripping

on external fault as well as on no load

Shunt reactor: installed or not ? Yes

Reclosing scheme Single-phase reclosing

Table 3.2.1 Switching overvoltage design level of transmission line

3.2.3 “HIGHER PERFORMANCE” SURGE ARRESTER

In order to determine the lightning impulse withstand voltage level (LIWV) and switching impulse withstand voltage level (SIWV) for 1200 kV equipments, the Surge Arrester has been chosen with great care. Studies were

conducted for Voltage-Current (V-I) characteristics of ZnO blocks of the Surge arresters to achieve necessary protective margins as per IEC-60071-1&2. The rated voltage of the Surge Arrester for 1200 kV is chosen as 850 kVrms, the corresponding V-I characteristics are given in Table 3.2.2 below:

Sr. No. Surge Arrester Current (kA) Residual Voltage (kVp)

1 0.5 1380

2 1.0 1440

3 2.0 1500

4 10.0 1600

(33)

Rated voltage (kV) 850 kV

Switching impulse 1500 kV at 2 kA

Residual voltage (kV)

Lightning impulse 1700 kV at 20 kA

Table 3.2.3 Metal Oxide Surge Arresters (MOSA) main characteristics

3.2.4 INSULATION DESIGN FOR SUBSTATION

3.2.4.1 D

ETERMINATION PROCESS FOR

SIWV

AND

LIWV

The location of Surge Arresters is very critical in order to contain voltage rise due to Switching Surges as well as Lightning Surges. For adequate protection of Equipments against Lightening surges, it was decided to place Surge Arresters at Line Entrance and near Transformers/ Reactors. In addition, Surge Arresters in the Bus may also be considered necessary.

The values of SIWV and LIWV are obtained considering adequate margin over the values of SIPL and LIPL as obtained from V-I characteristics of the Surge Arrestor. The SIWV for 1200kV equipments has been considered as 1800kV with about 20% protective margin. The LIWV has about 35% protective margin for 1200kV equipments and about 25% protective margin for 1200kV transformer and reactors. The values of LIWV for 1200kV equipments and transformers/reactors have been considered as 2400kV and 2250kV respectively.

The following tables show us the LIWV and SIWV for 1200 kV equipments.

Substation type and MOSA layout

Substation Type (GIS, MTS, AIS) AIS & MTS

Line entrance Yes

Busbar Yes (location under consideration)

MOSA layout (unit)

Transformer Yes Determination process for LIWV

Representative

Value (kV) 1700 kV

Lightning overvoltage

Calculation method Simplified method based on protection level V20kA of surge arrester

Safety factor Insulation Coordination being carried out keeping IEC provisions in view

Transformer 2250 kV

LIWV (kV)

Other equipment 2400 kV

(34)

Transformer 1500 kV (1.53 p.u.) Representative Value

(Max and/or 2 % value)

(kV, p.u.) Other equipment 1500 kV (1.53 p.u.) Switching

overvoltage

Calculation method

Simplified method based on protection level V2kA of surge

arrester Safety factor

Insulation Coordination being carried out keeping IEC

provisions in view Atmospheric correction factor, altitude (m) NIL, Less than 1000 m

Transformer 1800 kV

SIWV (kV)

(phase-to-ground) _{Other equipment} _{1800 kV}

Transformer 2970 kV

SIWV (kV)

(phase-to-phase) _{Other equipment} _{2970 kV}

Phase-to-phase clearance of air gaps (m) 12.3 m (rod-cond.)

Table 3.2.5 Determination Process for SIWV

3.2.4.2 TOV

AND ENERGY ABSORPTION BY SURGE ARRESTER

For determination of temporary over voltages, preliminary studies have been carried out with single-line-to-ground fault followed by three phase interruption at receiving end. The studies have been carried out with different source strengths and reactive compensation as mentioned above. The study result showed a value of TOV as 1.33 p.u. with 10000 MVA short circuit level at the sending end. For the purpose of insulation coordination study the value of TOV is taken as 1.4 p.u..

To determine the discharge capability of the surge arrester, the most severe condition of operation has been considered. The series of events start with single line to ground fault followed by opening of local and remote end CB of the faulted phase. The single phase auto recloser becomes effective after a dead-time of 1000 ms. However the reclosing becomes unsuccessful and the breaker could not be opened due to struck breaker condition. The sequence of events is shown at Figure 3.2.3.

Steady SLG fault T=100ms Opening of local end CB of faulted T=120ms Closing of local end CB through SPAR Dead Time Opening of remote end CB of faulted Closing of remote end CB through Opening of CB due to permanent fault LBB Operates and clears the fault Stuck breaker T=1120ms T=1260 ms T=1560 ms 1000 ms T=1140 ms T=0ms

(35)

The discharge capability required for surge Arresters for above conditions is given below: Total energy = 2Long discharge (IEC Class-5) + TOV + Margins

= 25 MJ + 35 MJ + 10 MJ = 55 MJ

The information for TOV and Energy absorption by surge arrester is tabulated below:

Overvoltage on healthy phase in case of ground fault 1.4

Maximum TOV (p.u.) and its duration (sec) 1.4 p.u., duration 1 sec Case and condition for studies (corresponding to the energy

absorption of 55 MJ) Energy absorption of surge arrester (MJ) 1-Ph-Ground fault followed

by load rejection

Calculation method EMTP/ATP

Power frequency test voltage for substation equipment

Transformer 1200 kV

Assumed overvoltage condition for power frequency test

Other equipment 1200 kV

Analysis program EMTP/ATP

Table 3.2.6 TOV and Energy absorption by surge arrester

3.2.5 1200 KV TEST STATION

A number of countries have gone for development of UHV AC technology but UHV AC technology is still in evolving stage and not available commercially. Neither any International Standards for UHV AC system are available. Thus POWERGRID is setting up 1200kV Test Station at Bina (M.P.) in association with CEA (Central electricity Authority), CPRI (Central Power Research Institute) and Indian manufacturers to gain experience of field tests/ trials so that the results and feedback can be used for developing field proven equipment of 1200KV system in India as well as to gain initial operational experience.

The test station contains two nos. 1200kV bays comprising 1200kV class equipment like Instrument transformers, Circuit Breakers and surge arresters and two nos. of 1000MVA transformer bank each comprising three single phase 400kV/1200kV, 333 MVA auto transformers. In addition to the above substation equipment, two nos. 1200kV AC test lines about a one km. long (one single ckt & one double ckt.) are being constructed and will be charged through these two 1200kV Bays to study their performance by conducting measurements of various line parameters. In the existing 400kV system through a Loop-in loop-out (LILO) arrangement, power flow through 1200kV test station shall be established.

(36)

Satna line 400kV Bina Bus To 400kV Satna line 1200kV line 1200/400kV Transformer 1200/400kV Transformer 400kV line 400kV line Satna line 400kV Bina Bus To 400kV Satna line 1200kV line 1200/400kV Transformer 1200/400kV Transformer 400kV line 400kV line

Figure 3.2.4 Power flow from Satna to Bina will be diverted via 1200kV test station

1200kV Equipment has been indigenously developed by Indian Manufacturers. 1200kV equipment will be field tested under full operating voltage for fine tuning and optimization of system parameters. Further, 1200kV Test station will help to learn safety, Quality & operational requirement for UHV AC Technology.

3.3 TEPCO 1100 kV project

3.3.1 FEATURES OF TEPCO’S UHV TRANSMISSION SYSTEM [1], [2]

Tokyo Electric Power Company (TEPCO) has been working to expand their 550 kV network since the mid-1970s, but it is difficult to obtain multiple power transmission routes in Japan. Countermeasures against the increase in short-circuit current due to network expansion were also required. To cope with these problems, TEPCO decided to introduce UHV (1100 kV) transmission system with a capacity of 3 to 4 times that of the 550 kV network.

By 1999, TEPCO had already constructed UHV-designed double-circuit transmission lines that ran 240 km from east to west and l90 km from north to south, totally 430 km, as shown in Figure 3.3.1. These lines are now operated at 550 kV and they are planed to be upgraded to UHV.

UHV systems are required to transmit very large power transmission (maximum 13 GW/route) and to ensure high reliability. Additionally, various new technologies had been developed for substations. To establish carefully these technologies towards UHV upgrading, field-testing of substation equipment has been carried out since 1996 connecting to the actual grid.

(37)

Figure 3.3.1 1100 kV transmission routes of TEPCO

3.3.2 CONCEPT OF INSULATION COORDINATION AND SPECIFICATIONS [3], [4]

For economical insulation design of transmission lines and substations, overvoltages generated in the system must be suppressed (limited) to a reasonable level. Fig. 3.4.2 shows the concept of insulation coordination on a TEPCO UHV system.

Overvoltage on transmission lines and in substations can be effectively controlled by the “higher performance” surge arresters (Refer to item 5.2 to see more details regarding “higher performance MOSA”). This metal oxide surge arrester (MOSA) is a key technology for UHV insulation coordination. As shown in Fig. 3.4.3, it has excellent protection characteristics with a residual voltage of 1620 kV (1.80 p.u.) at 20 kA (V20kA), flatter V-I characteristics

than conventional arresters, longer life under high-voltage stress, and higher discharging capability. Switching overvoltages on transmission lines should be limited to a value as low as possible because the

predominant factor to determine the size (dimensions) of a tower is the switching overvoltage. Closing and opening overvoltages are limited to the level of the ground-fault overvoltage, which can not be effectively controlled.

Practical application of high performance metal oxide surge arrester

Reliable circuit breaker with closing/opening resistance ・GIS: 2250kV ・Transformer: 1950kV LIWV (Substation) 1.5pu TOV 1.6～1.7pu Switching Overvoltage (Transmission line) 550kV　MOSA 1100kV　MOSA 　　　mA 　　　　　　　　10kA　20kA Higher Voltage Stress Flatter V-I　 Characteristics 1620kV (1.8pu) Vo lt ag e Current