Report submitted to
EMPRIMUS - Critical Infrastructure Protection
Grid Impact of Neutral Blocking for GIC Protection
Impact of Neutral Grounding Capacitor on Switching Over Voltages
Prepared By:
Athula Rajapakse
Date: 28 June, 2013
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Third Party Disclaimer:
The content of this document is not intended for the use of, nor is it intended to be relied upon by any person, firm
or corporation, other than the authors of the report and EMPRIMUS LLC.
Authors of this report deny any liability whatsoever to any parties for damages or injury suffered by such third
party arising from the use of this document by the third party.
Confidentiality:
This document is restricted to the confidential use of the authors and EMPRIMUS LLC. Any retention,
reproduction, distribution or disclosure to third parties is prohibited without written authorization of EMPRIMUS
LLC.
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Table of Contents
1. Introduction ...................................................................................................................................... 1
2. Results
.......................................................................................................................................2
2.1 Scenario-1 .................................................................................................................................................. 2
2.2 Scenario-2 ................................................................................................................................................. 3
2.3 Scenario-3 ................................................................................................................................................. 5
2.4 Scenario-4................................................................................................................................................. 7
2.5 Scenario-5 ................................................................................................................................................ 10
2.6 Scenario-6................................................................................................................................................ 13
3. Concluding remarks ........................................................................................................................ 14
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1. Introduction
Results of the switching over voltage studies are presented in this report. The objective of the study is to
compare the magnitudes of the over voltages with and without the capacitive transformer neutral
grounding device for blocking geomagnetically induced currents (GIC). Therefore, only a selected set of
scenarios were simulated.
The network used for studies is shown in Figure 1. Over voltages due to energization of 500 kV lines
connected to bus-1 were studied using simulations carried out in PSCAD/EMTDC. The neutral of the 500
kV/345 kV autotransformer is assumed to be grounded through Solid Ground GIC blocking neutral
grounding system proposed by Emprimus. All simulations were conducted assuming two conditions: (i)
neutral is solidly grounded through the bypass switch, and (ii) neutral is grounded through the branch
consisting of the capacitor (1  at 60 Hz) and power resistor (1 ) in series.
Worst case for switching over voltages generally occurs when a line is energized with trapped charges. In
order to simulate this condition, the line was first kept energized, opened momentarily and then closed
again. Since the switching over voltages are dependent on the voltage phase angle at the point of switching,
statistical data are provided after conducting 100 simulation runs. In each simulation run, the line is
energized at a different point on the voltage cycle. The maximum, minimum, average, and standard
deviation of the peak over voltages are reported. The 2% and 98% peak over voltage levels based on a
normal distribution fitted to the 100 data points are also reported.
Figure 1 Network used for switching over voltage studies.
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2. Results
2.1 Scenario-1
In Scenario-1, line reactors and surge arresters are not included. One of the 500 kV lines between bus-1 and
bus-2 is switched. The other 500 kV parallel line is kept open from both ends. Energization with maximum
trapped charge on the line was simulated by initially keeping the line being switched energized from bus-1
end, opening it for a short period (50 – 67 ms depending on the point of switching on the wave) and then
closing. A table summarizing the observed over voltages is presented under each case. Typical switching
waveforms are shown in Figures 2 and 3. In the figures E1 is the voltage at bus-1, E2 is the voltage at the
open line end (near bus-2). Case-1 is with autotransformer neutral grounded with the GIC blocking device
and Case-2 is with autotransformer neutral solidly grounded.
Case-1: Autotransformer neutral grounded through capacitor
Table 1
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.250523328
2.246244086
1.773225418
0.292325416
1.172862406
2.373588431
E2 (pu)
1.425727418
4.260098654
2.983896809
0.884468141
1.167421302
4.800372318
Figure 2 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
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Case-2: Autotransformer neutral solidly grounded
Table 2
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.250416555
2.247348636
1.773254871
0.292678772
1.172166154
2.374343589
E2 (pu)
1.427694835
4.260668516
2.984421616
0.884704392
1.167460909
4.801382323
Figure 3 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
2.2 Scenario-2
Scenaro-2 is similar to Scenario-1 except for the status of parallel 500 kV line. In Scenario-2, circuit
breakers at both ends of the parallel line are kept closed. Line reactors and surge arresters are not
included. Results are presented in Tables 3 and 4, and Figures 4 and 5.
Case-1: Autotransformer neutral grounded through capacitor
Table 3
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
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E1 (pu)
1.191793081
1.898423551
1.554826334
0.221324115
1.100282167
2.0093705
E2 (pu)
1.624639185
4.031979979
2.712153001
0.800086878
1.068975422
4.35533058
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Figure 4 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
Case-2: Autotransformer neutral solidly grounded
Table 4
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.192648352
1.899086192
1.554943549
0.221567956
1.099898594
2.009988504
E2 (pu)
1.627321639
4.034962007
2.713478368
0.801028982
1.068365946
4.358590793
Figure 5 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
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2.3 Scenario-3
Scenaro-3 is similar to Scenario-1 except that surge arresters are included in the simulation. The 450 kV
rating surge arresters were modeled with typical V-I characteristics. Line reactors are not included. Results
are presented in Tables 5 and 6, and Figures 6 to 10. In the tables, Eng1 and Eng2 are the peak energy
dissipation at the arresters at bus-1 and bus-2 ends of the line respectively. Figures 6 and 9 show the
typical voltage waveforms. Figures 7 and 10 show the typical variations of the energy dissipated in the
arresters (all three phases are shown). Figure 8 shows the variation of the autotransformer neutral voltage
during the switching, when the neutral is grounded through the capacitor (Case-1).
Case-1: Autotransformer neutral grounded through capacitor
Table 5
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.248554539
1.93797121
1.664000339
0.206561595
1.239774682
2.088225997
E2 (pu)
Eng1 (kJ)
Eng2 (kJ)
1.380763385 27.36935 34.6689668
2.690856538 49.81951 1653.91309
2.125922499 34.974562 653.070103
0.335291889 7.9025631 523.77092
1.437317137
Na
Na
2.814527861
Na
Na
Figure 6 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
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Figure 7 Typical variation of energy dissipation in the surge arresters on the line
Figure 8 Typical variation of the neutral voltage of the autotransformer
Case-2: Autotransformer neutral solidly grounded
Table 6
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
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E1 (pu)
1.24845426
1.937953523
1.664068103
0.206718431
1.239520345
2.088615861
E2 (pu)
1.382425278
2.690418794
2.125829143
0.335201056
1.437410329
2.814247958
Eng1 (kJ)
27.404189
49.889641
35.015339
7.9080522
Na
Na
Eng2 (kJ)
34.7083779
1655.62569
653.424896
524.070337
Na
Na
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Figure 9 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
Figure 10 Typical variation of energy dissipation in the surge arresters on the line
2.4 Scenario-4
Scenaro-4 is similar to Scenario-3 except that line reactors (150 MVAr at each end) are included in addition
to surge arresters. Results are presented in Tables 7 and 8, and Figures 11 to 15. In the tables, Eng1 and
Eng2 are the peak energy dissipation at the arresters at bus-1 and bus-2 ends of the line respectively.
Figures 11 and 14 show the typical voltage waveforms. Figures 12 and 15 show the typical variations of the
energy dissipated in the surge arresters. Figure 13 shows the variation of the autotransformer neutral
voltage during the switching, when the neutral is grounded through the capacitor (Case-1).
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Case-1: Autotransformer neutral grounded through capacitor
Table 7
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.227094186
1.353696646
1.289611268
0.028826805
1.230408248
1.348814289
E2 (pu)
1.610992945
1.942831006
1.866237692
0.077737239
1.706584919
2.025890464
Eng1 (kJ)
0.8190415
1.9622335
1.2431699
0.2997393
Na
Na
Eng2 (kJ)
10.0296564
65.8822315
25.7555412
13.6799178
Na
Na
Figure 11 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
Figure 12 Typical variation of energy dissipation in the surge arresters on the line
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Figure 13 Typical variation of the neutral voltage of the autotransformer
Case-2: Autotransformer neutral solidly grounded
Table 8
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.227088482
1.353657243
1.289604528
0.028821728
1.230411934
1.348797122
E2 (pu)
1.611005673
1.942825815
1.866232846
0.07773853
1.706577421
2.02588827
Eng1 (kJ)
0.8188827
1.9614395
1.2430556
0.2995997
Na
Na
Eng2 (kJ)
10.0298209
65.8658984
25.7539349
13.6770694
na
Na
Figure 14 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
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Figure 15 Typical variation of energy dissipation in the surge arresters on the line
2.5 Scenario-5
Scenaro-5 is similar to Scenario-4 except that the double circuit line from bus-1 to bus-3 is taken out of
service. The line reactors (150 MVAr at each end) and surge arresters (450 kV rating) are included in the
simulation. Results are presented in Tables 9 and 10, and Figures 16 to 20.
Case-1: Autotransformer neutral grounded through capacitor
Table 9
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
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E1 (pu)
1.266364745
1.519430444
1.416942675
0.060874406
1.291921929
1.541963421
E2 (pu)
1.485050548
1.664502169
1.585723322
0.044289267
1.494764287
1.676682357
Eng1 (kJ)
1.5163657
4.4233543
3.2268401
0.6833968
na
na
Eng2 (kJ)
9.27486115
26.4095769
18.0811048
4.47658735
Na
Na
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Figure 16 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
Figure 17 Typical variation of energy dissipation in the surge arresters on the line
Figure 18 Typical variation of the neutral voltage of the autotransformer
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Case-2: Autotransformer neutral solidly grounded
Table 10
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.26634688
1.519317804
1.416923614
0.060866542
1.291919017
1.541928211
E2 (pu)
Eng1 (kJ)
Eng2 (kJ)
1.485062616 1.5156152 9.27433319
1.66441599 4.4198833 26.4038388
1.585678192 3.2264337 18.0781873
0.044274433 0.683142 4.47503319
1.494749622
na
Na
1.676606762
na
Na
Figure 19 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
Figure 20 Typical variation of energy dissipation in the surge arresters on the line
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2.6 Scenario-6
Scenaro-6 simulates a capacitor bank switching event. The 150 MVAr capacitor bank connected to the
tertiary of the 500 kV/345 kV autotransformer connected between bus-1 and bus-4 is energized. During
the simulation, all 500 kV lines are kept open. The autotransformer is energized from the 345 kV side.
Results are presented in Tables 11 and 12, and Figures 21 to 23. In the tables and graphs, E3 is voltage at
the 345 kV bus (bus-4).
Case-1: Autotransformer neutral grounded through capacitor
Table 11
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.387696194
1.469155229
1.444020955
0.02305455
1.396672697
1.491369213
E3 (pu)
0.937673576
1.012660144
0.984975969
0.023625275
0.936455585
1.033496353
Figure 21 Typical voltage waveforms at bus-1 (E1) and bus-4 (E3)
Figure 22 Typical variation of the neutral voltage of the autotransformer
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Case-2: Autotransformer neutral solidly grounded
Table 12
Minimum:
Maximum:
Mean:
Std Dev:
2% Level:
98% Level:
E1 (pu)
1.387696194
1.469155229
1.444020955
0.02305455
1.396672697
1.491369213
E3 (pu)
0.937673576
1.012660144
0.984975969
0.023625275
0.936455585
1.033496353
Figure 23 Typical voltage waveforms at bus-1 (E1) and open end (E2) of the 500 kV line
3. Concluding remarks
Some key observations presented in Section-2 are summarized in the following tables, which compare the
cases of capacitor and direct grounding. Table 13 compares the peak over voltage observed at open line
end for Scenarios -1 to -5. Table 14 compares the peak over voltages observed at the energizing end (bus-1)
for Scenarios -1 to -5. Table15 presents the mean over voltages observed at open line end. Table 16
compares the estimated 98% value of the switching over voltage at the open line end for the same five
scenarios.
Based on the results of simulation studies, for a generally accepted worst case scenario, there is no
appreciable difference between the switching over voltages observed under the two cases considered: (i)
grounding of transformer neutral through capacitor/resistor combination and (ii) direct grounding of
transformer neutral.
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Table 13: Peak SOV at open line end
Scenario No.
Capacitor grounding
Direct grounding
4.260668516
1
4.260098654
4.031979979
4.034962007
2
2.690856538
2.690418794
3
1.942831006
1.942825815
4
1.664502169
1.66441599
5
Table 14: Peak SOV at energizing line end
Scenario No.
Capacitor grounding
Direct grounding
2.247348636
1
2.246244086
1.898423551
1.899086192
2
1.93797121
1.937953523
3
1.353696646
1.353657243
4
1.519430444
1.519317804
5
Table 15: Mean of SOVdistribution at open line end
Scenario No.
Capacitor grounding
Direct grounding
2.983896809
2.984421616
1
2.712153001
2.713478368
2
2.125922499
2.125829143
3
1.866237692
1.353657243
4
1.585723322
1.585678192
5
Table 16 : 98% value of SOV at open line end
Scenario No.
Capacitor grounding
Direct grounding
4.800372318
4.801382323
1
4.35533058
4.358590793
2
2.814527861
2.814247958
3
2.025890464
2.02588827
4
1.676682357
1.676606762
5
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