26 VFTOS IN 500 KV AND 750 KV GIS 3.0...

26

CHAPTER 3

VFTOS IN 500 KV AND 750 KV GIS

3.0 INTRODUCTION:

The quality of the simulation depends on the quality of the model

of each individual GIS component. In order to achieve reasonable results

even for longer time periods of some microseconds or for very complex

GIS structures, highly accurate models for each of the internal

equipment and also for components connected to the GIS are necessary.

Due to travelling wave nature of VFTO’s, modeling of GIS components

makes use of electrical equivalent circuits composed of lumped elements

and distributed parameters lines [1].

Switching operations or faults in Gas Insulated Substation lead to

malfunction of the substation equipments. These improper functioning

contain of malfunction of electronic equipments and deformation of

transformer winding and bushing failures. The reason of malfunction of

electronic equipments is due to coupling voltages on data and control

cables. The reasons of these problems are traveling waves which are

generated during switching operations or faults in the Gas Insulated

Substation (GIS).

The most adopted modeling of GIS components, to simulate very fast

transients by digital program, make use of electrical equivalent circuits

composed of lumped elements (of capacitances, inductances and

27

resistances) and distributed parameter lines derived from their surge

impedances and travel times.

Calculation of overvoltages is by no means easy because of the number of

bus bar sections and cables having distributed parameters where as

generators, transformers and cables capacitors are considered as lumped

elements.

3.1 MODELING CONCEPT

3.1.1 Power Transformer

Power transformer with bushing can be modeled by entrance

capacitance where entrance capacitance has been calculated in lightning

test. Here, the entrance capacitance of power transformer should be kept

as a 5000pF [1].

3.1.2 Transmission Line

The surge impedance of a transmission Line and travel time is

considered as 350Ω and 300m/µs respectively [1]. The GIS system

capacity is considered as 300MW and the basic formulae used in

modeling of transmission line are [4] as follows:

The surge impedance

60ln bZa

Ω (3.1)

Capacitance

2

lno rC ba

F (assume r=1) (3.2)

28

Inductance

ln

2

baL

H (assume μr =1) (3.3)

The surge impedance of transmission line is considered as [ Z1 ] = 350Ω

From equation 3.1

350 60ln ba

ln 5.8333ba

From equation 3.2, the Inductance is L1= 1.16 µH.

From equation 3.3, the capacitance is C1=9.53 pF.

3.1.3 GIS Bus bar

The GIS Bus Bar can be represented as a lossless π- line for a

50 Hz frequency. The surge impedance and travel time of GIS Bus Bar is

considered as [Z2] 80Ω and 231m/µs respectively [1].

From equation 3.1

80 60ln ba

ln 1.3333ba

From equation 3.2, the capacitance is C2=0.4172 pF.

From equation 3.3, the Inductance is L2=26.66 µH.

29

3.1.4 Cable

The Cable can also be represented as a lossless π- line for a 50 Hz

frequency. The surge impedance ZC and travel time of cable is considered

as 68.8 Ω and 103.8m/µs [1] respectively. The Calculations are as

follows:

From equation 3.1

68.8 60 ln ba

ln 1.1466ba

From equation 3.2, the capacitance is CC=0.4851 pF.

From equation 3.3, the inductance is LC=22.932 µH.

3.1.5 Surge Arrester

The Metal Oxide surge arresters are used to protect medium and

high voltage systems and equipment against lightning and switching

overvoltages [3].The calculations are represented as follows.

The Metal Oxide surge Arrester obeys the following equation

I KV ,α>1 (3.4)

Where

I current through arrester

V voltage across arrester

K ceramic constant (depending on arrester type),

α a nonlinearity exponent (measure of nonlinearity).

30

1

( ) ii ii

ref ref

V IKV I

(3.5)

3.1.5 A) For 420 kV Surge Arrester

The reference current of 420kV Surge Arrester is around 714.28 A

and its Characteristics are shown in table 3.1. [1]

Table 3.1 Characteristics of 420 kV surge arrester

i) For 500 kV Substation

The segment Characteristics of Metal Oxide Arrester used in

500kV GIS are obtained from the equations 3.4 and 3.5 as follows:

1 1 0.9375 27.5928K

2 2 0.9364 52.3847K

3 3 0.9165 3.3109K

ii) For 750 kV Substation

The segment Characteristics of Metal Oxide Arrester used in 750kV

GIS are obtained from the equations 3.4 and 3.5 as follows:

1 1 0.6936 32.8749K

2 2 0.7205 7.5369K , 3 3 0.6131 3.3175K

Current(A) Voltage (kV)0.008 594.0

20 674.510000 932.0

31

B) For 444 kV Surge Arrester

The reference current of 444 kV Surge Arrester is around 675.67 A and

Its Characteristics are shown in table 3.2

Table 3.2 Characteristics of 444 kV surge arrester

i) For 500 kV Substation



1 1 0.6596 16.3851K

2 2 0.6561 2.4788K

3 3 0.6559 2.3491K

ii) For 750 kV Substation



1 1 0.7316 35.5425K

2 2 0.6636 3.6337K

3 3 0.6618 3.6337K

Current(A) Voltage (kV)0.003 628.0

20000 1161.0

32

3.1.6 Transformer

The modeling values of transformer windings are designed by

using of the given formulae such as inductance and capacitance. These

values are useful to design the Transformer winding for high frequencies.

2ln 12To lL

R (3.6)

T r oWlCd (3.7)

For the following data, the inductance and capacitance are calculated by

using equation 3.6 and 3.7 respectively.

l= 1m, R= 0.05 m, εr = 1 in air cored material

W=0.05m, l=1m; d=0.5 mm, Frequency = 50 Hz

From equation 3.6, the inductance is L=0.08 µH.

From equation 3.7, the capacitance is C=0.06 nF.

The rated voltage of transformer for 500kV GIS= 500kV.

The rated voltage of transformer for 750kV GIS= 750kV.

33

3.2 EQUIVALENT DIAGRAM OF GIS

The equivalent diagram of GIS, used for simulation is shown in Fig.3.1

Fig 3.1 Equivalent diagram of GIS used for the simulation

In the above sections, the equivalent models for the each GIS

component i.e. Power transformer, GIS bus bar, cable, surge arrester

have been presented and the corresponding calculations for the

simulation also estimated.

34

3.3 RESULTS AND DISCUSSIONS

The equivalent diagram of a 500kV and 750kV Gas Insulated

Substation can be modified as a simulink model by using the modeling

components as already studied in previous sections 3.1[1] and 3.2.

The GIS system basically consists of circuit breakers, Surge arrestors;

disconnector switches, transformers, cables, etc.

When disconnector switch is opened or closed in GIS, there will

be an instantaneous change in voltage with in a very short rise time in

the range of 4 to 100 ns, and it is normally followed by oscillations

having frequencies in the range of 1 to 50MHz. It is due to restrikes or

prestrike between contacts and very short distance of GIS bus-bar in

which the transient voltages are generated and travels.

3.4 CALCULATION OF VFTO’S IN GAS INSULATED SUBSTATION

Against the difference of switch operation mode and their

position in GIS sub-station, three cases are considered to calculate Very

Fast Transient Over-voltages [VFTO] and the factors influencing VFTO

including residual charges, spark resistance and entrance capacitance of

Transformer have been studied.

The transient voltages are calculated from the equivalent modeling

diagram at different points of the GIS such as,

V14S- shows the voltage to ground of bus-bar at 14S.



35

V11UA- shows the voltage to ground of surge arrester at the end of

transformer unit1.


transformer unit 3 and unit 4.


transformer unit6.

VTR1- shows the voltage to ground of transformer at unit 1.


VTR4- shows the voltage to ground of transformer at unit 4


3.5 VFTO CAUSED BY OPERATION OF DS-50543

Case-1: For 500 kV GIS

When the disconnect switch-50543 is opened before that the

switches DS-50546 and CB-5054 are already opened then the equivalent

simulink model is shown in Fig.3.2. In this condition the calculated

VFTO level at different points discussed in section 3.5 is shown in Table

3.3. The System source voltage is taken as 500 kV [1].

When the disconnector switch is opened at 0.3 sec, the transients are

generated at different points in the system. The VFTO’s have been

calculated at nine points given byV14S, V15S, V17S, V11UA, V12UA,

V13UA, VTR1, VTR3 and VTR4.

36

Fig. 3.2 Equivalent simulink model of 500 kV GIS system when

opening of DS-50543

Very fast transient overvoltage V14S which is the voltage (V) to

ground of bus-bar at 14S is shown in Fig 3.3. From Fig 3.3, the

calculated rms value of VFTO at 14S is 535.1 kV.

37

Fig.3.3 Voltage to ground of bus bar at 14S when opening of DS- 50543

for 500 kV GIS

Very fast transient overvoltage V17S which is the Voltage (V) to ground of

bus-bar at 17S is shown in Fig 3.4. From Fig.3.4, the calculated rms

value of VFTO at 17s is 706.3 kV.

Fig.3.4 Voltage to ground of bus bar at 17S when opening of DS- 50543

for 500kV GIS

Very fast transient overvoltage V12UA which is the voltage (V) to

ground of surge arrester at the end of transformer unit 3&4 is shown in

Fig 3.5. From Fig. 3.5, the calculated rms value of VFTO at 12UA is

630.7kv

38

Fig. 3.5 Voltage to ground of surge arrester at the end of transformer

Unit 3&4 when opening of DS-50543 for 500 kV GIS

Very fast transient overvoltage VTR1 which is the voltage (V) to ground of

transformer at unit1 is shown in Fig 3.6. From Fig. 3.6, the calculated

rms value of VFTO at TR1 is 472.5 kV.

Fig. 3.6 Voltage to ground of transformer at unit1 when opening of

DS-50543 for 500 kV GIS

Very fast transient overvoltage VTR4 which is the voltage (V) to

ground of transformer at unit4 is shown in Fig 3.7. From Fig. 3.7, the

calculated rms value of VFTO at TR4 is 473.2 kV.

39

Fig.3.7 Voltage to ground of transformer at unit4 when opening of

DS- 50543 for 500 kV GIS

Table 3.3 Values of VFTO at different points in 500kV GIS when

Opening of DS-50543

Voltage to ground ofbus-bar(kV)

V14S 535.1V15S 572.4V17S 706.3

Voltage to ground ofsurge arrester (kV)

V11UA 548.5

V12UA 630.7

Voltage to ground ofTransformer (kV)

VTR1 472.5VTR3 476.1VTR4 473.2

From Table 3.3, it has been observed that due to opening of DS-

50543, the maximum voltage to ground of bus bar near the switch

reaches 1.73p.u; the maximum voltage to ground of surge arrester

reaches 1.54p.u; and the maximum voltage to ground of transformer

reaches 1.16p.u.




simulink model is shown in 2. In this condition the calculated VFTO level

40

at different points discussed in section3.5 is shown in Table 3.4. The

System source voltage is considered as 750 kV.

When the disconnector switch is opened at 0.3 sec, the transients are

generated at different points in the system. The VFTO’s have been

calculated at nine points given byV14S, V15S, V17S, V11UA, V12UA,

V13UA, VTR1, VTR3 and VTR4.

Very fast transient overvoltage V14S which is the voltage (V) to ground of

bus-bar is shown in Fig. 3.8. From Fig. 3.8, the calculated rms value of

VFTO at 14S is 729.7 kV.

Fig. 3.8 Voltage to ground of bus-bar at 14S when opening

DS- 50543 for 750 kV GIS

Very fast transient overvoltage V17S which is the voltage (V) to ground

of bus-bar is shown in Fig. 3.9. From Fig.3.9, the calculated rms value of

VFTO at 17S is 963.1 kV.

41

Fig. 3.9 Voltage to ground of bus-bar at 17S when opening of DS- 50543

for 750 kV GIS


ground of surge arrester at the end of transformer unit 3&4 is shown in

Fig. 3.10. From Fig. 3.10, the calculated rms value of VFTO at 12UA is

860.1 kV.

Fig 3.10 Voltage to ground of surge arrester at the end of transformer

unit 3&4 when opening DS-50543 for 750 kV GIS.

Very fast transient overvoltage VTR1 which is the Voltage (V) to ground of

transformer at unit1 is shown in Fig. 3.11. From Fig. 3.11, the calculated

rms value of VFTO at TR1 is 644.3 kV.

42

Fig. 3.11 Voltage to ground of transformer at unit 1 when opening

of DS-50543 for 750 kV GIS


ground of transformer at unit4 is shown in Fig.3.12. From Fig. 3.12, the


Fig.3.12 Voltage to ground of transformer at unit 4 when opening of


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Table 3.4 Values of VFTO at different points in 750kV GIS when opening

of DS- 50543

Voltage to ground ofbus bar (kV)

V14S 729.7V15S 780.5V17S 963.1


V11UA 748.0V12UA 860.1


VTR1 644.3VTR3 649.3VTR4 645.2

From the Table 3.4, it has been observed that when opening of DS-

50543 the maximum voltage to ground of bus bar near the switch


reaches 1.40p.u and the maximum voltage to ground of transformer

reaches 1.06p.u.

3.6 VFTO CAUSED BY OPERATION OF DS-50121 WHEN DS-50122

OPEN



switches DS-50122 and CB-5012 (as shown in figure 3.1) are already

opened then the equivalent simulink model is shown in Fig 3.13. In this

condition the calculated VFTO level at different points discussed in

section3.5 is shown in Table 3.5.The System source voltage is 500 kV

44

Fig. 3.13 Equivalent simulink model of GIS system when opening of

DS-50121

45


ground of bus-bar at 14S is shown in Fig 3.14. From Fig. 3.14, the

calculated rms value of VFTO at 14S is 712.5 kV.

Fig.3.14 Voltage to ground of bus-bar at 14S when opening of DS-50121

for 500 kV GIS


ground of surge arrester at the end of transformer unit 1 is shown in

Fig.3.15. From Fig.3.15, the calculated rms value of VFTO at 11UA is

676 kV.


unit1 when opening DS-50121 for 500 kV GIS

46

Very fast transient overvoltage VTR1 which is the Voltage (V) to

ground of transformer at unit1 is shown in Fig. 3.16. From Fig. 3.16, the




Table 3.5 Values of VFTO at different points in 500kv GIS when opening

of DS-50121


V14S 712.5


V11UA 676.0

Voltage to ground oftransformer(kV)

VTR1 493.9


50121, the level of overvoltages is much higher due to few current shunts

circuit. The maximum voltage to ground of bus bar near the switch

reaches 1.75p.u; the maximal voltage to ground of surge arrester reaches

1.65p.u and the maxima voltage to ground of transformer reaches

1.20p.u.

47




simulink model is shown in Fig 3.13.

In this case the disconnector switch is opened at 0.2 sec; the transients

are generated at different points in the system. In this condition the

calculated VFTO level at different points is discussed in section 3.5 is

shown in Table 3.6. The System source voltage is considered as 750 kV.

Very fast transient overvoltage V14S which is the voltage to ground of

bus-bar at 14S is shown in Fig 3.17. From Fig. 3.17, the calculated rms

value of VFTO at 14S is 971.5 kV.

Fig.3.17 Voltage to ground of bus-bar at 14S when opening of


Very fast transient overvoltage V11UA which is the voltage to


48

Fig.3.18. From Fig. 3.18, the calculated rms value of VFTO at 11UA is

921.9 kV.

Fig.3.18 Voltage to ground of surge arrester at the end of transformer

unit1 when opening DS-50121 for 750 kV GIS

Very fast transient overvoltage VTR1 which is the Voltage (V) to





49

Table 3.6 Values of VFTO at different points in 750kV GIS when opening

of DS-50121


50121, the level of overvoltages is much higher due to few current shunts

circuit. The maximal voltage to ground of bus bar near the switch

reaches 1.58p.u; the maximal voltage to ground of surge arrester reaches

1.50p.u and the maximal voltage to ground of transformer reaches

1.09p.u.

3.7 VFTO CAUSED BY OPERATION OF DS-50121 WHEN DS-50122

CLOSED


When the disconnect switch-50121 is opened before that the CB-

5012 is already opened but DS-50122 is still closed then the equivalent

simulink model is shown in Fig. 3.20. In this condition the VFTO level at

different points discussed in section 3.5 is shown in Table 3.7. The

System source voltage is considered as 550 kV [1].

Voltage to ground of busbar (kV)

V14S 971.5


V11UA 921.9

Voltage to ground oftransformer(kV)

VTR1 673.5

50

Fig3.20 Equivalent simulink model of GIS system when opening of DS-

50121 but DS-50122 is closed


ground of bus-bar is shown in Fig 3.21. From Fig. 3.21, the calculated

rms value of VFTO at 14S is 562.5 kV.


but DS-50122 is closed for 500 kV GIS

51







ground of surge arrester at the end of transformer unit 3 &unit 4 is

shown in Fig.3.23. From Fig. 3.23, the calculated rms value of VFTO at

12UA is 552.8 kV

Fig. 3.23 Voltage to ground of surge arrester at end of transformer unit

3&4 when opening of DS-50121 but DS-50122 is closed for 500 kV GIS

52


ground of surge arrester at the end of transformer unit 6 is shown in Fig

3.24. From Fig. 3.24, the calculated rms value of VFTO at 13UA is 512.1

kV.

Fig.3.24 Voltage to ground of surge arrester at the end of transformer

unit 6 when opening of DS-50121 but DS-50122 is closed for 500 kV GIS



calculated rms value of VFTO at TR1 is 470 kV.

Fig. 3.25 Voltage to ground of transformer at unit 1 when opening of

DS-50121 but DS-50122 is closed for 500 kV GIS

53






Table 3.7 Values of VFTO at different points in 500kv GIS when

opening of DS-50121 but DS-50122 is closed

Voltage to ground ofbusbar (kV)

V14S 562.5

V15S 555.6V17S 517.1

Voltage to ground of surgearrester (kV)

V11UA 540.4V12UA 552.8V13UA 512.1


VTR1 470.0VTR3 457.1VTR4 457.1VTR6 458.4


When the disconnect switch-50121 is opened before that the CB-

5012 is already opened but DS-50122 is still closed then the equivalent

simulink model is shown in Fig. 3.20. In this condition the VFTO level at

54

different points discussed in section 3.5 is shown in Table 3.8. The

System source voltage is considered as 750 kV.

Very fast transient overvoltage V14S which is the voltage (V) to ground of

bus-bar is shown in Fig 3.27. From Fig. 3.27, the calculated rms value of

VFTO at 14S is 767 kv


butDS-50122 is closed for 750 kV GIS






55


ground of surge arrester at the end of transformer unit 3 & unit 4 is

shown in Fig.3.29. From Fig. 3.29, the calculated rms value of VFTO at

12UA is 753.8 kV.


unit 3&4for opening of DS-50121 but DS-50122 is closed for 750 kV GIS



Fig.3.30. From Fig. 3.30, the calculated rms value of VFTO at 13UA is

698.4 kV.


unit 6 when opening of DS-50121 but DS-50122 is closed for 750 kV GIS

56







ground of transformer at unit 4 is shown in Fig.3.32. From Fig. 3.32, the




\

57

Table 3.8 Values of VFTO at different points in 750kv GIS when

opening of DS-50121 but DS-50122 is closed


V14S 767.0V15S 757.7V17S 705.2


V11UA 736.9V12UA 753.8V13UA 698.4


VTR1 640.9VTR3 623.3VTR4 623.3VTR6 625.2

From Table 3.8, it can be observed that the level of overvoltages is

higher. The maximum voltage to ground of bus bar near the switch


reaches 1.50p.u and the maximum voltage to ground of transformer

reaches 1.08pu.

3.8 INFLUENCE OF RESIDUAL CHARGES ON THE LEVEL OF VFTO’S:

When disconnector switch-50543 opens on line, it has some

residual charges on the line that will influence the level of VFTO. The

values of residual charges considered for analyzing the effect of VFTO’s in

GIS are shown in Table 3.9.

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Fig. 3.33 Equivalent simulink model of GIS for influence of

residual charges on VFTO

The residual charges are the left over charges on the power equipment

which may be positive or negative in practical.

Table 3.9 Values of Residual charges

-1.0 p.u. 400pF.

-0.5 p.u. 200pF.0 p.u. Infinity0.5 p.u. 100pF.

59

Case- 1: For 500 kV GIS

When DS-50543 is opened then the equivalent model of GIS for

calculation of residual charges influence on VFTO is shown in Fig. 3.33

and the level of VFTO at different points discussed in section 3.5 is

shown in Table 3.10 and Table 3.11. The System source voltage is

considered as 550 kV [1].

When residual charge -1.0 p.u i.e. capacitance 400 pF is

considered,the voltage to ground of bus bar at 14S is shown in Fig.3.34.

From Fig. 3.34, the calculated rms value of VFTO at 14S is 535.1kV.

Fig.3.34 Voltage to ground of bus bar at 14S when residual charge

-1.0 p.u. is considered for 500kV GIS.

When residual charge 0.5 p.u i.e. capacitance 100 pF is considered,

the voltage to ground of bus bar at 14S is shown in Fig.3.35. From Fig.

3.35, the calculated rms value of VFTO at 14S is 464.4 kV.

60


0.5 p.u. is considered for 500kV GIS

Table 3.10 Simulation result of residual charges influence

on VFTO at bus bars in 500kV GIS.

Voltage to ground of Bus-bar (kV)

ResidualCharges(p.u.)

V14S V15S V17S

-1.0 535.1 572.4 690.6-0.5 511.5 539.4 632.60 488.0 506.5 568.7

0.5 464.4 473.7 504.7

From Table 3.10, it has been observed that the maximum voltage

to ground of bus-bar decreased when the residual charge is changed

from -1.0 p.u. to 0.5 p.u. At 14S bus-bar the voltage level is changed

from 535.1 kV to 464.4 kV. At 15S bus-bar the voltage level is changed

from 572.4 kV to 473.7 kV. At 17S bus-bar the voltage level is changed

from 690.6 kV to 504.7 kV. Therefore, the level of VFTO decreases as the

residual charges increases.

61

Table 3.11 Simulation result of residual charges influence on

VFTO at Transformers in 500kV GIS


to ground of transformer decreased when the residual charge is changed

from -1.0 p.u. to 0.5 p.u. At TR1 transformer the voltage level is changed

from 472.5 to 448.6 kV. At TR3 transformer the voltage level is changed

from 472.2 to 448.7kV. At TR4 transformer the voltage level is changed

from 469.2 to 447.9 kV. Therefore, the level of VFTO decreases as the



When DS-50543 is opened then the equivalent model of GIS for

estimating the residual charges influence on VFTO is shown in Fig 3.33

and the level of VFTO at different points is shown in Table 3.12 and

Table 3.13. The System source voltage is considered as 750 kV.

When residual charge -1.0 p.u i.e. capacitance 400 pF is considered,the

voltage to ground of bus bar at 14S is shown in Fig.3.36. From Fig. 3.36,

the calculated rms value of VFTO at 14S is 729.7 kV.


VTR1[kV]

VTR3[kV]

VTR4[kV]

-1.0 472.5 472.2 469.2-0.5 464.6 464.3 462.1

0 456.6 456.5 455.00.5 448.6 448.7 447.9

62


-1.0 p.u. is considered for 750kV GIS.

When residual charge 0 p.u is considered, then the voltage to ground of

bus bar at 14S is shown in Fig.3.37. From Fig.3.37, the calculated rms

value of VFTO at 14S is 665.5 kV

Fig.3.37 Voltage to ground of bus bar at 14S when residual charge 0 p.u.

is considered for 750kV GIS


VFTO at bus bars in 750kV GIS


V14S[kV]

V15S[kV]

V17S[kV]

-1.0 729.7 780.5 949.9-0.5 697.5 735.5 862.60 665.5 690.7 775.5

0.5 633.2 646 688.2

63

From Table 3.12, it can be concluded that the maximum voltage to

ground of bus-bar decreased when the residual charge is changed from -

1.0 p.u. to 0.5 p.u. At 14S bus-bar the Voltage level is changed from

729.7 kV to 633.2 kV. At 15S bus-bar the voltage level is changed from

780.5 kV to 646 kV. At 17S bus bar the voltage level is changed from

949.9 kV to 688.2 kV. Therefore, the level of VFTO decreases as the



VFTO at Transformers in 750kV GIS

Voltage to ground of Transformer(kV)ResidualCharges(p.u.)

VTR1 VTR3 VTR4

-1.0 644.3 643.9 639.9-0.5 633.5 633.2 630.10 622.6 622.5 620.50.5 611.8 611.8 610.7


to ground of transformer decreased when the residual charge is changed

from -1.0 p.u. to 0.5 p.u. At TR1 transformer the voltage level is changed

from 644.3 kV to 611.8 kV. At TR3 transformer the voltage level is

changed from 643.9kV to 611.8kV. At TR4 transformer the voltage level

is changed from 639.9 kV to 610.7 kV. Therefore, the level of VFTO

decreases as the residual charges increases.

64

3.9 INFLUENCE OF SPARK RESISTANCE ON THE LEVEL OF VFTO’S

When restriking transient happens, spark resistance can have

effect on damping overvoltages. The equivalent simulink model for

analyzing the influence of spark resistance on the level of VFTO’s in GIS

is shown in Fig.3.38. The spark resistance is considered in the

disconnect switch-50543.

3.38 Equivalent simulink model of influence of spark resistance on VFTO

65

Case-1: For 500kV GIS

When DS-50543 is opened then the equivalent model of spark

resistance influence on VFTO is shown in Fig.3.38 and the calculated

level of VFTO at different points discussed in section 3.5 is shown in

Table 3.14 and Table 3.15. The System source voltage is considered as

500 kV [1].

When spark resistance 0.1 Ω is considered, then the voltage to

ground of bus bar at 14S is shown in Fig.3.39. From Fig.3.39, the

calculated rms value of the VFTO at 14S is 537.4 kV.

Fig.3.39 Voltage to ground of bus bar at 14S when spark resistance

0.1 Ω is considered for 500kV GIS

When spark resistance 100 Ω is considered, then the voltage to


calculated rms value of the VFTO at 14S is 477 kV.

66


100 Ω is considered for 500kV GIS

Table 3.14 Simulation result of spark resistance influence on



to ground of busbar is reduced if the spark resistance is changed from

0.1 to 200 Ω. At 14S, the voltage level is reduced from 537.4 kV to 419.4

kV. At 15S, the voltage level is reduced from 574.4 kV to 449.9 kV. At

17S, the voltage level is reduced from 701.3 kV to 436.7 kV. Therefore,

the level of VFTO decreases as the spark resistance increases.

Voltage to ground of Bus-bar (kV)

SparkResistance(Ω) V14S V15S V17S0.1 537.4 574.4 701.3

25 535.1 572.4 696.710 528.9 566.4 683.450 505.9 541.9 631.5100 477.0 511.2 566.6200 419.4 449.9 436.7

67


VFTO at transformers in 500kV GIS


to ground of transformer is reduced if the spark resistance is changed

from 0.1 to 200 Ω. At TR1, the voltage level is reduced from 472.8 kV to

448.6 kV. At TR3, the voltage level is reduced from 472.7 kV to 445.3 kV.

At TR4, the voltage level at unit 4 is reduced from 478.4 kV to 445.9 kV.

Therefore, the level of VFTO decreases as the spark resistance increases.

Case -2: For 750kV GIS

When DS-50543 is opened then the equivalent model of spark

resistance influence on VFTO is shown in Fig.3.38 and the calculated

level of VFTO at different points discussed in section 3.5 is shown in

Table 3.15 and Table 3.16. The System source voltage is considered as

750 kV.

When spark resistance 0.1 Ω is considered, then the voltage to


calculated rms value of the VFTO at 14S is 732.8 kV.

Voltage to ground of Transformer(kV)

SparkResistance(Ω)

VTR1 VTR3 VTR4

0.1 472.8 472.7 478.425 472.5 472.2 469.310 471.4 470.9 468.250 466.6 465.5 463.5100 460.0 458.8 457.6200 448.6 445.3 445.9

68


0.1 Ω is considered for 750kV GIS

When spark resistance 100 Ω is considered, then the voltage to


calculated rms value of the VFTO at 14S is 650 kV.


100 Ω is considered for 750kV GIS

69



Voltage to ground of Bus-bar (kV)SparkResistance(Ω) V14S V15S V17S0.1 732.8 783.2 956.3

2.5 729.7 780.5 95010 721.3 772.4 931.950 689.8 738.8 861.1100 650 697.1 772.6200 571.9 613.4 595.5

From Table 3.16, it has been observed that the maximum voltage to

ground of busbar is reduced if the spark resistance is changed from 0.1

to 200 Ω. At 14S, the voltage level is reduced from 732.8 kV to 571.9 kV.

At 15S, the voltage level is reduced from 783.2 kV to 613.4 kV. At 17S,

the voltage level is reduced from 956.3 kV to 595.5 kV. So, the level of

VFTO reduces along with the increasing spark resistance.


VFTO at transformers in 750kV GIS

SparkResistance(Ω)

VTR1[kV]

VTR3[kV]

VTR4[kV]

0.1 644.7 644.6 652.425 644.3 643.9 639.910 642.8 642.2 638.450 636.3 634.8 632.0100 628.1 625.6 624.1

200 611.7 607.2 608.1

70


to ground of transformers is reduced if the spark resistance is changed

from 0.1 to 200 Ω. At TR1, the voltage level is reduced from 644.7 kV to

611.7 kV. At TR3, the voltage level is reduced from 644.6 kV to 607.2 kV.

At TR4, the voltage level is reduced from 652.4 kV to 608.1 kV. So, the

level of VFTO reduces along with the increasing spark resistance.

3.10 INFLUENCE OF ENTRANCE CAPACITANCE OF TRANSFORMER

WHEN DS-50543 OPENED

The entrance capacitance of transformer has influence on VFTO.

The equivalent simulink model for analyzing the influence of entrance

capacitance of transformer on the level of VFTO’s in GIS is shown in

Fig.3.43. The entrance capacitance is considered as the capacitance of

the transformer unit 6.

71

3.43 Equivalent simulink model of influence of entrance

capacitance of transformer on VFTO when DS-50543 opened

72


When DS-50543 is opened then the equivalent model of entrance

capacitance influence on VFTO is shown in Fig.3.43 and the calculated

level of VFTO at transformers is shown in Table 3.18. The System source

voltage is considered as 550 kV [1].

When the entrance capacitance 5000pF is considered, then the

voltage to ground of transformer at unit 1 is shown in Fig.3.44. From

Fig.3.44, the calculated rms value of the VFTO at TR1 is 472.4 kV.

Fig.3.44 Voltage to ground of transformer at unit1 when entrance

capacitance 5000pF is considered for 500kV GIS

When entrance capacitance 15000pF is considered, then the

voltage to ground of transformer at unit 1 is shown in Fig.3.45 From




73

Table 3.18 Simulation result of entrance capacitance influence

on VFTO at transformers in 500kV GIS when DS-50543 opened

Voltage to ground of Transformer(kV)TransformerEntranceCapacitance(pF)

VTR1 VTR3 VTR4

5000 472.4 472.2 469.210000 455.8 461.2 458.915000 453.3 458.2 456.620000 450.9 455.1 454.225000 448.4 452.0 451.9


to ground of transformers is reduced if the entrance capacitance of

transformer is changed from 5000 to 25000 pF. At TR1, the voltage level

is reduced from 453.3 kV to 448.4 kV. At TR3, the voltage level is

reduced from 458.2 kV to 452 kV. At TR4, the voltage level is reduced


entrance capacitance of transformer increases.


When DS-50543 is opened then the equivalent model of entrance

capacitance influence on VFTO is shown in Fig.3.43 and the level of

VFTO at transformers is shown in Table 3.19. The System source voltage

is considered as 750 kV.

When entrance capacitance 5000pF is considered, then the voltage to

ground of transformer at unit 1 is shown in Fig.3.46. From Fig.3.46, the

calculated rms value of the VFTO at TR1 is 644.2 kV.

74








75

Table 3.19 Simulation result of entrance capacitance influence on

VFTO at transformers in 750kV GIS when DS-50543 opened

From Table 3.19, it has been observed that the maximum voltage to

ground of transformers is reduced if the entrance capacitance of



reduced from 643.9 kV to 616.4 kV. At TR4, the voltage level is reduced



3.11 INFLUENCE OF ENTRANCE CAPACITANCE OF TRANSFORMER

WHEN DS-50121 OPENED

The entrance capacitance of transformer also has some

influence on VFTO. The equivalent simulink model is for analyzing the

influence of entrance capacitance of transformer on VFTO level in GIS

shown in Fig.3.48. The entrance capacitance is considered as the

capacitance of the transformer unit 6.

Voltage to ground ofTransformer(kV)

TransformerEntranceCapacitance(pF)

VTR1 VTR3 VTR4

5000 644.2 643.9 639.810000 621.5 628.9 625.815000 618.2 624.8 622.620000 614.8 620.6 619.425000 611.5 616.4 616.2

76

Fig.3.48 Equivalent simulink model of influence of entrance capacitance

of transformer on VFTO when DS-50121 opened


When DS-50121 is opened but DS-50122 is closed then the

equivalent model of entrance capacitance influence on VFTO is shown in

77

Fig.3.48 and the level of VFTO at transformers is shown in Table 3.20.

The System source voltage is considered as 550 kV.



calculated rms value of the VFTO at TR1 is 470 kV.


capacitance 5000pF is considered for 500kV GIS when DS-50121 is

opened






opened

78

Table 3.20 Simulation result of entrance capacitance influence on





is reduced from 470 kV to 452 kV. At TR3, the voltage level is reduced

from 457.1 kV to 441.1 kV. At TR4, the voltage level is reduced from

457.1 kV to 441.2 kV. At TR6, the voltage level is reduced from 458.4 kV

to 443.8 kV. Therefore, the level of VFTO decreases as the entrance

capacitance of transformer increases.


When DS-50121 is opened but DS-50122 is closed then the

equivalent model of entrance capacitance influence on VFTO is shown in


VTR1 VTR3 VTR4 VTR6

5000 470.0 457.1 457.1 458.4

10000 458.9 449.7 449.7 451.2

15000 456.6 447.0 446.9 448.7

20000 454.3 444.2 444.0 446.3

25000 452.0 441.4 441.2 443.8

79

Fig.3.48 and the calculated level of VFTO at different transformers is

shown in Table 3.21. The System source voltage is considered as 750 kV.






opened






opened

80

Table 3.21 Simulation result of entrance capacitance Influence on






reduced from 623.3 kV to 601.9 kV. At TR4, the voltage level is reduced

from 623.3 kV to 601.6 kV. At TR6, the voltage level is reduced from

625.1 kV to 605.2 kV. Therefore, the level of VFTO decreases as the


The various factors influencing the VFTO levels in GIS have been

discussed in above sections by developing the simulink models obtained

from the equivalent models of each GIS component.


VTR1 VTR3 VTR4 VTR6

5000 640.9 623.3 623.3 625.110000 625.8 613.3 613.2 615.3

15000 622.7 609.5 609.4 611.9

20000 619.5 605.7 605.5 608.6

25000 616.3 601.9 601.6 605.2

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