insulation coordination for gis - new aspects

8/10/2019 Insulation coordination for GIS - New aspects

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INSULATION COORDINATION FOR GIS - NEW ASPECTS

M. P. Meirelles, R. Vaisman, R. Azevedo, A. J. S. Junqueira M. Lacorte, W. Hofbauer

CEPEL ABB High Voltage Technologies

BRAZIL SWITZERLAND

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.

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).

21, rue dArtois, F-75008 Parishttp://www.cigre.org

Session 1998 CIGR23-101

CEPEL - Cx. Postal 68007, 21944-970, Rio de Janeiro - Brazil

Tel. +55 21 260 8937 Fax: +55 21 260 2236


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Supply System

Tower 5 Tower 4 Tower 3 Tower 2 Tower 1

15m

8m6m

Stroken Tower

Outdoor ArresterCVT

GIS

GIS Bushing

300m300m300m300m2km

Transformer

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:

- 5 for Tower 1;

- 10 for the other Towers.

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 GISbushing (Figure 1) are simulated by a 0.5 H/m

inductance.

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.

The outdoor surge arrester has a protection level of1055 kV at 20 kA. It is simulated as a non linear

resistance connected to the phase-wire by a 5 H

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

The GIS bushing is modeled by two surge impedances

in series represented by

Zbushing= Z1 + Z2

where: Z1 = 200 surge impedance, length = 3m.

Z2 = 30 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 overallovervoltages. Therefore the study is performed only

Z

Z

Z

R footing


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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 andsimulated 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.

3 - Lightning

According to the worldwide practice a lightning

protection study must be performed considering two

types of lightning strokes:

a direct stroke;

a back flash.

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 inorder 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 parametersof a back flash are considered in this study.

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 betaken 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.

Ucwcoordination withstand voltage

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

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].

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 onboth the location of the surge arrester and the length of


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5.4 - Connection Leads

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.

1,00

1,10

1,20

1,30

1,40

1,50

1,60

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Length [m]

PeakVoltage[MV]

OUTDOOR ARRESTER WITH LEAD

OUTDOOR ARRESTER WITHOUT LEAD

Ucw = 1292 kV

Figure 8 - Voltage at coupling capacitor in function of

the GIS length

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.

1,00

1,10

1,20

1,30

1,40

1,50

1,60

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Length [m]

PeakVoltage[MV]

GIS WITH OPEN-END

GIS WITH CAPACITIVE-END

Ucw = 1292 kV

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

an outdoor surge arrester (Figure 10) and a GIS surge

arrester at the GIS bushing (Figure 11).

1,00

1,10

1,20

1,30

1,40

1,50

1,60

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Length [m]

PeakVoltage[MV]

VOLTAGE AT CVT - 200kA


VOLTAGE AT THE TRANSFORMER - 200kA


Ucw = 1292 kV

Figure 10 - Overvoltage in function of the GIS length

considering outdoor surge arrester

1,00

1,10

1,20

1,30

1,40

1,50

1,60

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150Length [m]

PeakVoltage[MV]





Ucw = 1292 kV

Figure 11 - Overvoltage in function of the GIS length

considering GIS surge arrester

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.

5.7 - Safety Margin

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.


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1,00

1,10

1,20

1,30

1,40

1,50

1,60

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Length [m]

PeakVoltage[MV]

VOLTAGE AT CVT

VOLTAGE AT THE TRANSFORMER

Ucw = 1348 kV - Safety Margin of 15%


Figure 12 - Comparison of diferent safety margins

considering overvoltages with the outdoor arrester

1,00

1,10

1,20

1,30

1,40

1,50

1,60

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Length [m]

PeakVoltage[MV]

VOLTAGE AT CVT

VOLTAGE AT THE TRANSFORMER



Figure 13 - Comparison of diferent safety margins

considering overvoltages with the GIS arrester at GISbushing

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.

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.

The GIS termination has a strong impact on the

overvoltages at the termination. No generalstatement can be given that the overvoltages at an

open termination are the highest.

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.

7 - References

[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.

[2] - "Electric Transients and Insulation Coordination";

Salgado F.M., Vaisman R. et al, EDUFF/FURNAS -

Centrais Eltricas 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.

insulation coordination for gis - new aspects

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