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
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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
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 0.6596 16.3851K
2 2 0.6561 2.4788K
3 3 0.6559 2.3491K
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 0.7316 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.
V15S- shows the voltage to ground of bus-bar at 15S.
V17S- shows the voltage to ground of bus-bar at 17S.
35
V11UA- shows the voltage to ground of surge arrester at the end of
transformer unit1.
V12UA- shows the voltage to ground of surge arrester at the end of
transformer unit 3 and unit 4.
V13UA- shows the voltage to ground of surge arrester at the end of
transformer unit6.
VTR1- shows the voltage to ground of transformer at unit 1.
VTR3- shows the voltage to ground of transformer at unit 3.
VTR4- shows the voltage to ground of transformer at unit 4
VTR6- shows the voltage to ground of transformer at unit 6.
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.
Case-2: For 750 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 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
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. 3.10. 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
Very fast transient overvoltage VTR4 which is the voltage (V) to
ground of transformer at unit4 is shown in Fig.3.12. From Fig. 3.12, the
calculated rms value of VFTO at TR4 is 645.2 kV.
Fig.3.12 Voltage to ground of transformer at unit 4 when opening of
DS-50543 for 750 kV GIS
43
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
Voltage to ground ofsurge arrester (kV)
V11UA 748.0V12UA 860.1
Voltage to ground ofTransformer (kV)
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 1.57p.u; the maximum voltage to ground of surge arrester
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
Case-1: For 500 kV GIS
When the disconnect switch-50121 is opened before that the
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
Very fast transient overvoltage V14S which is the voltage (V) to
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
Very fast transient overvoltage V11UA which is the voltage (V) to
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.
Fig. 3.15 Voltage to ground of surge arrester at the end of transformer
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
calculated rms value of VFTO at TR1 is 493.9 kV.
Fig.3.16 Voltage to ground of transformer at unit 1 when opening of
DS-50121 for 500 kV GIS
Table 3.5 Values of VFTO at different points in 500kv GIS when opening
of DS-50121
Voltage to ground ofbus bar (kV)
V14S 712.5
Voltage to ground ofsurge arrester (kV)
V11UA 676.0
Voltage to ground oftransformer(kV)
VTR1 493.9
From Table 3.5, it has been observed that due to opening of DS-
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
Case-2: For 750 kV GIS
When the disconnect switch-50121 is opened before that the
switches DS-50122 and CB-5012 are already opened then the equivalent
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
DS-50121 for 750 kV GIS
Very fast transient overvoltage V11UA which is the voltage to
ground of surge arrester at the end of transformer unit 1 is shown in
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
ground of transformer at unit1 is shown in Fig. 3.19. From Fig. 3.19, the
calculated rms value of VFTO at TR1 is 673.5 kV.
Fig.3.19 Voltage to ground of transformer at unit 1 when opening of
DS-50121 for 750 kV GIS
49
Table 3.6 Values of VFTO at different points in 750kV GIS when opening
of DS-50121
From Table 3.6, it has been observed that due to opening of DS-
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
Case-1: For 500 kV GIS
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
Voltage to ground ofsurge arrester (kV)
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
Very fast transient overvoltage V14S which is the voltage (V) to
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.
Fig.3.21 Voltage to ground of bus-bar at 14S when opening of DS-50121
but DS-50122 is closed for 500 kV GIS
51
Very fast transient overvoltage V17S which is the voltage (V) to
ground of bus-bar is shown in Fig 3.22. From Fig. 3.22, the calculated
rms value of VFTO at 17S is 517.1 kV.
Fig.3.22 Voltage to ground of bus-bar at 17S when opening of DS-50121
but DS-50122 is closed for 500 kV GIS
Very fast transient overvoltage V12UA which is the voltage to
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
Very fast transient overvoltage V13UA which is the voltage to
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
Very fast transient overvoltage VTR1 which is the voltage (V) to
ground of transformer at unit1 is shown in Fig. 3.25. From Fig. 3.25, the
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
Very fast transient overvoltage VTR4 which is the voltage (V) to
ground of transformer at unit4 is shown in Fig. 3.26. From Fig. 3.26, the
calculated rms value of VFTO at TR4 is 457.1 kV.
Fig.3.26 Voltage to ground of transformer at unit 4 when opening of
DS-50121 but DS-50122 is closed for 500 kV GIS
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
Voltage to ground ofTransformer (kV)
VTR1 470.0VTR3 457.1VTR4 457.1VTR6 458.4
Case-2: For 750 kV GIS
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
Fig.3.27 Voltage to ground of bus-bar at 14S when opening of DS-50121
butDS-50122 is closed for 750 kV GIS
Very fast transient overvoltage V17S which is the voltage (V) to
ground of bus-bar is shown in Fig 3.28. From Fig. 3.28, the calculated
rms value of VFTO at 17S is 705.2 kV.
Fig.3.28 Voltage to ground of bus-bar at 17S when opening of DS-50121
but DS-50122 is closed for 750 kV GIS
55
Very fast transient overvoltage V12UA which is the voltage to
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.
Fig. 3.29 Voltage to ground of surge arrester at the end of transformer
unit 3&4for opening of DS-50121 but DS-50122 is closed for 750 kV GIS
Very fast transient overvoltage V13UA which is the voltage to
ground of surge arrester at the end of transformer unit 6 is shown in
Fig.3.30. From Fig. 3.30, the calculated rms value of VFTO at 13UA is
698.4 kV.
Fig. 3.30 Voltage to ground of surge arrester at the end of transformer
unit 6 when opening of DS-50121 but DS-50122 is closed for 750 kV GIS
56
Very fast transient overvoltage VTR1 which is the voltage (V) to
ground of transformer at unit1 is shown in Fig. 3.31. From Fig. 3.31, the
calculated rms value of VFTO at TR1 is 640.9 kV.
Fig. 3.31 Voltage to ground of transformer at unit 1 when opening of
DS-50121 but DS-50122 is closed for 750 kV GIS
Very fast transient overvoltage VTR4 which is the voltage (V) to
ground of transformer at unit 4 is shown in Fig.3.32. From Fig. 3.32, the
calculated rms value of VFTO at TR4 is 623.3 kV.
Fig. 3.32 Voltage to ground of transformer at unit 4 when opening of
DS-50121 but DS-50122 is closed for 750 kV GIS
\
57
Table 3.8 Values of VFTO at different points in 750kv GIS when
opening of DS-50121 but DS-50122 is closed
Voltage to ground ofbus bar (kV)
V14S 767.0V15S 757.7V17S 705.2
Voltage to ground ofsurge arrester (kV)
V11UA 736.9V12UA 753.8V13UA 698.4
Voltage to ground ofTransformer (kV)
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 1.58p.u; the maximum voltage to ground of surge arrester
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.
58
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
Fig.3.35 Voltage to ground of bus bar at 14S when residual charge
0.5 p.u. is considered for 500kV GIS
Table 3.10 Simulation result of residual charges influence