Insulation coordination for GIS - New aspects

21, rue d’Artois, F-75008 Paris http://www.cigre.org

Session 1998

23-101

© CIGRÉ

INSULATION COORDINATION FOR GIS - NEW ASPECTS

M. P. Meirelles, R. Vaisman, R. Azevedo, A. J. S. Junqueira CEPEL BRAZIL

Summary Insulation coordination is one of the most important aspects to be considered during the planning of a substation. Properly done it minimises the risk of failures in the substation caused by overvoltages and such contributes to a high availability of the substation. A characteristic layout of a GIS substation is considered in order to analyse - by using a simulation model - the lightning overvoltages occurring in this substation caused by a back flash in the transmission line. The impact of the location of the surge arrester, the effect of the length of the GIS as well as the effect of other design parameters of a GIS substation on the lightning overvoltages in the substation are investigated. Based on the results obtained conclusions for insulation coordination studies are drawn. Key-word: insulation coordination, GIS, lightning overvoltage.

1 - Introduction Insulation coordination is one of the fundamental dimensioning criteria for all electric systems. Properly done it results in a restriction of insulation flashovers or breakdowns caused by overvoltages to those parts of a system where consequential damages are quite limited. Further improvement can be achieved by the correct application of surge arresters which effectively reduce the probability of costly damages. CEPEL - Cx. Postal 68007, 21944-970, Rio de Janeiro - Brazil Tel. +55 21 260 8937 Fax: +55 21 260 2236

M. Lacorte, W. Hofbauer ABB High Voltage Technologies SWITZERLAND

Based on many years experience general design rules were set up how to protect electric systems efficient and effective against damages caused by internally or externally generated overvoltages. Today software simulation tools are used which enable to calculate qualitatively and quantitatively the impact of insulation coordination measures on the propagation of overvoltage surges. An important aspect to be considered refers to the lightning surge injection into the substation [1]. This is an important factor especially when it comes to the overall optimisation of a GIS substation surge protection. Traditionally a GIS substation is protected by an outdoor arrester at the GIS entrance. Depending on the layout of the GIS, additional GIS arresters have to be used to achieve full protection of the GIS. This raises the question whether and under which conditions it is technically and economically reasonable to use just GIS arresters instead of additional outdoor arresters and vice versa. This question is analysed more in detail for a GIS substation model typically used in a 500 kV AC transmission system. The analysis is based on digital simulations performed using EMTP - Electromagnetic Transients Program - worldwide used for this purpose.

2 - GIS-Substation The electric system considered in this study consists of three sections which are characteristic for many GIS substations (Figure 1).

Stroken Tower 2km

300m

300m

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300m 15m 8m

6m Transformer GIS

Tower 5

Tower 4

Tower 3

Tower 2

Tower 1 CVT

GIS Bushing Outdoor Arrester

Supply System

Figure 1 - Schematic representation of the electrical system considered in the study 2.1 - Transmission System The rated voltage of the transmission system considered in the study is 500 kV / 50 Hz. The towers are modeled by a T equivalent diagram of surge impedances as shown in Figure 2. Typical values for the footing resistances of the towers are:

As only a single phase is considered when performing the overvoltage calculations inside the GIS, the other 2 phases of the transmission line are connected to earth at Tower 1 by lumped capacitances of 10 nF. Regarding the capacitive voltage transformer CVT, two different configurations are simulated : - no CVT; - CVT represented by 5 nF to ground.

- 5 Ω for Tower 1; - 10 Ω for the other Towers. Z

Z

Z

R footing

The outdoor surge arrester has a protection level of 1055 kV at 20 kA. It is simulated as a non linear resistance connected to the phase-wire by a 5 µH inductance representing the length of the connection leads plus the height of the arrester. A simulation with the surge arrester directly connected to the phase-wire is also performed in order to show the influence of the connection leads on the surge protection. 2.3 - GIS Section

Figure 2 - Tower geometry and tower EMTP model The equivalent diagram of the supply system used consists of a power frequency voltage source in series with a resistance. This resistance has the same value as the transmission line surge impedance in order to avoid reflections. 2.2 - Outdoor Section The outdoor connections between Tower 1 and GIS bushing (Figure 1) are simulated by a 0.5 µH/m inductance.

The GIS bushing is modeled by two surge impedances in series represented by Zbushing = Z1 + Z2 where:

Z1 = 200 Ω surge impedance, length = 3m. Z2 = 30 Ω surge impedance, length =1 m.

For comparison a simulation is also performed with the bushing modeled as a lumped capacitance of 150 pF [2]. The calculation results show that the two different models have a negligible effect on the overall overvoltages. Therefore the study is performed only

with one bushing model represented by two surge impedances in series. The GIS is simulated by a surge impedance of 60 ohms. The length of the GIS is varied in the range of 15 m to 150 m. The termination of the GIS is either a power transformer directly connected to the GIS and simulated by a 4 nF capacitance to ground or an open line-disconnector. The GIS surge arrester used in the study has a protection level of 1022 kV at 20 kA. It is simulated by a non linear resistance directly connected to the GIS.

4 - Simulations The main objective of insulation coordination studies for planned substations is to minimize the risk of failure due to overvoltages. Instead of applying probability concepts literature also suggests the use of a deterministic approach where a safety margin must be taken into account [1]. In this way, although the standardized BIL for 500 kV systems is 1550 kV, the lightning studies must lead to overvoltages not higher than BIL/1.2, where the safety margin of 20% is used to determine the calculated maximum overvoltage, that will be used as reference.

3 - Lightning Ucw → coordination withstand voltage According to the worldwide practice a lightning protection study must be performed considering two types of lightning strokes:

Ucw = BIL/1.2 = 1550/1.2 = 1292 kV

• a direct stroke; • a back flash.

A simulation with a safety margin of 15% (Ucw = BIL/1.15 = 1348 kV) is done in order to analyse the influence of this limit since this value is largely used [7].

3.1 - Direct Stroke This means a lightning stroke directly to a phase wire. 3.2 - Back Flash A back flash is caused by a direct lightning stroke to the ground wire or to a tower of a line, followed by a flashover across the insulation between the tower and a phase wire. For a back flash calculation a lightning stroke in a critical distance to the substation and a current level which is the highest to be expected once in a period of 400 years is considered [3]. In this study a lightning stroke to the second tower and a current magnitude of 300 kA is assumed representing a 0.014% probability of incidence [4, 5]. A simulation with 200 kA lightning current is done in order to analyse its influence on the insulation coordination result, since this value is rarely exceeded [5]. The back flash across the phase insulation of the struck tower is simulated by the so called Leader Progression Model [6] programmed in a subroutine TACS of EMTP. In general the overvoltage values resulting from a back flash are higher than those of a direct stroke to the phase wire. Therefore only lightning surge parameters of a back flash are considered in this study.

Four different surge protection concepts of the substation regarding arrester types and their positioning are simulated: • outdoor surge arrester positioned between the CVT and the GIS bushing; • GIS surge arrester positioned at the GIS bushing (GIS entrance); • GIS surge arrester positioned at half length of the GIS; • GIS surge arrester positioned at the transformer. 5 - Results 5.1 - GIS Length Effect Figures 3 to 5 show the calculated overvoltages at the CVT outside the GIS, at the GIS entrance and at the transformer respectively considering the four different locations of the arrester. The resulting overvoltages as a function of the GIS length is strongly non-linear which results from different reflections of the surge waves. Therefore no general statement can be made that the longer a GIS the higher the overvoltage. For any fixed reference node in the substation the local overvoltage occurring in general strongly depends on both the location of the surge arrester and the length of

the GIS. However, related to such a reference node there are certain ranges of the GIS length in which the effectiveness of the surge arrester is independent of its location.

For more than about 100 m effective length of a GIS surge arresters on two locations will be necessary in order to guarantee an adequate surge protection for the GIS substation.

1,60 OUTDOOR ARRESTER GIS ARRESTER AT GIS ENTRANCE GIS ARRESTER AT HALF LENGTH OF GIS

1,50

GIS ARRESTER AT TRANSFORMER

Peak Voltage [MV]

the surge protection of a GIS substation by a GIS arrester located directly at the GIS bushing (GIS entrance) is comparable with that of an outdoor arrester.

Ucw = 1292 kV

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5.3 - Capacitive Devices Effect

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The effect of the presence of capacitive devices (CVT) in a substation can be seen in figures 6 and 7 and demonstrated by the overvoltages occurring at the transformer.

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Figure 3 - Voltage at the CVT in function of the GIS length 1,60 OUTDOOR ARRESTER GIS ARRESTER AT GIS ENTRANCE GIS ARRESTER AT HALF LENGTH OF GIS GIS ARRESTER AT TRANSFORMER

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Due to the wave characteristic of lightning surges the effect of capacitive devices is not easily predictable. It could be expected that capacitive devices will damp surges. This might be correct for specific layouts of the substation but cannot be taken as a general rule. Therefore investigations have to be done individually for any layout and location of the surge arrester. 1,60 WITH CVT

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WITHOUT CVT

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Ucw = 1292 kV

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Peak Voltage [MV]

Peak Voltage [MV]

Ucw = 1292 kV

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Figure 4 - Voltage at the GIS entrance in function of the GIS length

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1,60 OUTDOOR ARRESTER

Figure 6 - Overvoltage at the transformer in function of the GIS length considering outdoor surge arrester

GIS ARRESTER AT GIS ENTRANCE

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GIS ARRESTER AT HALF LENGTH OF GIS GIS ARRESTER AT TRANSFORMER

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WITHOUT CVT

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Ucw = 1292 kV

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Peak Voltage [MV]

Ucw = 1292 kV

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Figure 5 - Voltage at the transformer in function of the GIS length

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5.2 - Surge Arrester Location Effect The effectiveness of the surge arrester at different locations can be seen also in Figures 3 to 5. In general

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Figure 7 - Overvoltage at the transformer in function of the GIS length considering GIS surge arrester located at the GIS bushing

an outdoor surge arrester (Figure 10) and a GIS surge arrester at the GIS bushing (Figure 11).

Figure 8 shows the influence of the connection lead between the phase wire and the outdoor surge arrester on the overvoltage protection. The longer the lead the less effective is the surge arrester which is again an effect of the traveling surge wave. 1,60 OUTDOOR ARRESTER WITH LEAD OUTDOOR ARRESTER WITHOUT LEAD

1,50

Peak Voltage [MV]

Ucw = 1292 kV

1,60 VOLTAGE AT CVT - 200kA VOLTAGE AT CVT - 300kA VOLTAGE AT THE TRANSFORMER - 200kA

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VOLTAGE AT THE TRANSFORMER - 300kA

Peak Voltage [MV]

5.4 - Connection Leads

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Ucw = 1292 kV

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Figure 10 - Overvoltage in function of the GIS length considering outdoor surge arrester

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VOLTAGE AT CVT - 200kA

Length [m]

VOLTAGE AT CVT - 300kA

5.5 - GIS Termination Effect The influence of the GIS termination on the overvoltage at the termination is analysed for the substation protected with an outdoor surge arrester and presented in Figure 9.

VOLTAGE AT THE TRANSFORMER - 200kA VOLTAGE AT THE TRANSFORMER - 300kA

Peak Voltage [MV]

Figure 8 - Voltage at coupling capacitor in function of the GIS length

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Ucw = 1292 kV

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Length [m] GIS WITH OPEN-END GIS WITH CAPACITIVE-END

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Ucw = 1292 kV

Peak Voltage [MV]

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Figure 11 - Overvoltage in function of the GIS length considering GIS surge arrester

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While for both locations of the arrester the current magnitude has a quite high influence on the overvoltage outside the GIS there is much less or nearly no influence at the transformer.

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5.7 - Safety Margin

Length [m]

Figure 9 - Voltage at transformer in function of the GIS length No general statement can be made that the overvoltage at an open end will always be higher than at any other termination of the GIS as demonstrated for the transformer [7]. There is also a strong influence of the length of the GIS due to the wave characteristic of the surges. 5.6 - Lightning Current Magnitude Effect The effect of the lightning current magnitude on the overvoltages in different reference nodes is shown for

The adopted safety margin of 20%, which is a very restrict condition, can be reduced to 15% assuming that the substation has an accurate representation. Figures 12 and 13 shows the overvoltages at CVT and at the transformer considering the outdoor arrester and the GIS arrester positioned at the GIS bushing respectively. Considering this new margin, practically there is no restriction of using only one arrester to achieve full protection of all sections of a GIS substation, for GIS lengths less than 100m.

• The connection lead of an outdoor surge arrester has a significant impact on the effectiveness of the arrester. The longer the connection lead the higher the overvoltages in the GIS substation.

1,60 VOLTAGE AT CVT VOLTAGE AT THE TRANSFORMER

1,50

Ucw = 1348 kV - Safety Margin of 15%

Peak Voltage [MV]

Ucw = 1292 kV - Safety Margin of 20%

1,40

• The GIS termination has a strong impact on the overvoltages at the termination. No general statement can be given that the overvoltages at an open termination are the highest.

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Figure 12 - Comparison of diferent safety margins considering overvoltages with the outdoor arrester 1,60 VOLTAGE AT CVT VOLTAGE AT THE TRANSFORMER

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Ucw = 1348 kV - Safety Margin of 15%

• The lightning current magnitude has a strong impact on the overvoltages outside the GIS but it is minor inside the GIS. • Proper insulation coordination of any GIS substation requires an accurate digital simulation. Doing this, a safety margin of 15% can be adopted, which may lead to the use of only one arrester to protect the GIS substation for usual GIS lengths.

Peak Voltage [MV]

Ucw = 1292 kV - Safety Margin of 20%

1,40

7 - References

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[1] - "A Simplified Method for Estimating Lightning Performance of Transmission Lines"; IEEE WG on Lightning Performance of Transmission Lines, IEEE Trans. PAS-104, April/1985.

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Figure 13 - Comparison of diferent safety margins considering overvoltages with the GIS arrester at GIS bushing 6 - Conclusions Due to the wave characteristic of lightning surges a proper insulation coordination for a GIS substation has to consider many different aspects: • The length of the GIS has a significant influence on the overvoltages in the substation. No general statement can be given for common lengths of GIS that the longer the GIS the higher the overvoltages. • The location of a GIS arrester at the GIS bushing offers an equivalent lightning protection for the GIS substation as an outdoor surge arrester as long as the effective length of the GIS is less than 100 m. • For GIS lengths more than 100 m in minimum two surge arresters at different locations should be considered for an effective protection of the GIS substation. • The impact of capacitive devices in the GIS substation on the overvoltages is not predictable. Detailed analyses are required.

[2] - "Electric Transients and Insulation Coordination"; Salgado F.M., Vaisman R. et al, EDUFF/FURNAS Centrais Elétricas SA, 1987 (in Portuguese). [3] - "Lightning Overvoltage Protection of GIS"; GIS Technical Information; ABB Hochspannungstechnik AG, Doc. No.: HASV 685 401 E - AK 4G.1- A05/92. [4] - "Simplified Procedures for Determining Representative Substation Impinging Lightning Overvoltages"; Erikson A. and Weck K., CIGRE Paper no. 33-16, Paris, 1988. [5] - "Modeling Guidelines for Fast Front Transients"; IEEE WG on Modeling and Analysis of System Transients, IEEE Trans. PWRD, Vol. 11, No.1, January 1996. [6] - "Modeling of Transmission Line Exposure to Direct Lightning Strokes"; Farouk A.M. Rizk, IEEE 90 WM 084-4 PWRD, Atlanta, Georgia. [7] - "Re-evaluation of the Ligthning Impulse Level of Transformers and Shunt Reactors"; Vaisman R. et al, CIER Congress, Chile, 1987.