
Abstract—This analysis focuses on a 500 kV gas-insulated substation (GIS) installation that has two types of double-break circuit breakers, one type containing 1400 pF grading capacitors and the other with 500 pF grading capacitors across each break.
The circuit breakers are connected to approximately 40 m of airinsulated buswork and 100 m of gas-insulated buswork, which includes existing and new installations of gas-insulated bus.
Voltage transformers (VTs) applied at the gas-insulated bus could cause ferroresonance conditions without a ferroresonance damping device (referred to as a “damping device” herein) connected to the 115 V secondary of the VT. This analysis verifies that a damping device adequately mitigates ferroresonance conditions. Specifically, the concern is for voltages on the VT primary and secondary, where excessive voltages could cause damage to equipment insulation and maloperation of relays without the damping device.
Index Terms—ferroresonance, voltage transformer, grading capacitor, gas-insulated substation
I. I NTRODUCTION
F erroresonance is a well-documented phenomena [1-11] where a resonance occurs between a saturable inductance and a capacitance connected in series. In this analysis, the concern is on voltages that occur because of the energy coupled, through grading capacitors of circuit breakers, from live gas-insulated bus to a de-energized section of bus where
VTs are connected. An industry-standard tool for electromagnetic transient modeling, the Electromagnetic
Transients Program (EMTP), was used to simulate various circuit conditions susceptible to ferroresonance. Digital programs such as EMTP can be used to simulate phenomena over a wide-range of frequencies, and for this analysis EMTP was used to simulate ferroresonance conditions in the multicycle to few seconds timeframe.
II. O BJECTIVES
The following are the specific objectives for the ferroresonance analysis of the 500 kV GIS:
(1) Identify potential ferroresonance circuits associated with the 500 kV GIS installation for cases with and without the damping device.
(2) Determine voltages associated with the equipment for the 500 kV GIS installation with and without a damping device for potential ferroresonance circuits
(a) Assess if concerns exist for the operation of relays
(b) Assess if concerns exist for the thermal degradation of equipment
D. Shoup, J. Paserba, D. Sullivan, P. Bolin, and R. Whiteside are with
Mitsubishi Electric Power Products, Inc. (MEPPI), Warrendale, PA 15086,
USA (e-mail: don.shoup@meppi.com).
III. EMTP M ODEL
Figure 1 shows the one-line diagram of the GIS for the limiting ferroresonance case. A total of 5 cases were analyzed, where the amount of floating buswork, i.e., buswork capacitance, was varied and different numbers of circuit breakers containing grading capacitors were in the circuit.
Figure 1 includes the gas-insulated buswork, air-insulated buswork, circuit breakers, grading capacitors, surge arresters,
VTs, transformer connections, and transmission line connections to the 500 kV GIS under study. Figure 2 shows the equivalent representation of Figure 1 modeled to examine ferroresonance voltages initiated by the energy exchange between the grading capacitance and VT.
The following is a listing of the key equipment modeled for the ferroresonance analysis:

VT and saturation characteristics

Damping device composed of a resistor (R
D
) and saturable reactor (L
D
)

Equivalent grading capacitance (C
1
) across circuit breaker contacts

Stray capacitance (C
2
) associated with gas-insulated buswork, air-insulated buswork, and equipment connected to the de-energized, or floating, buswork

Source voltage (V). The source voltage in EMTP was set based on the maximum continuous operating voltage
(MCOV), calculated as follows:
500000

1 .
10

2 / 3

449073 V
Table 1 lists VT data used for the EMTP model. Table 2 lists thermal and voltage insulation ratings for the VT and damping device. Thermal and voltage insulation ratings of the VT and damping device were properly coordinated with expected voltages and currents, verified by the analysis described here.
Figure 3 shows a comparison of the saturation characteristics of the saturable reactor and VT. Because of the low losses of the VT, ferroresonance conditions could be sustained for long periods of time and concerns could exist because of energy coupled into the circuit through the grading capacitors. The purpose of the damping device is to saturate before the VT, causing a damping resistor to be inserted into the circuit, which mitigates the ferroresonance conditions. Note that for the cases with the damping device modeled, the damping resistor was modeled as a lumped resistance of 0.47

.
Table 3 contains an example for the determination of the C
2 capacitance used in Figure 2 corresponding to phase A for one of the five cases examined. As listed in Table 3, circuit breaker, disconnect switches, bushings, bus supports and posts, surge arresters, gas-insulated buswork, and air-

Wavetrap
(1-
 only )
Figure 1. One-line diagram of the GIS for the limiting case for the ferroresonance analysis.
V = 449073 V (MCOV, 550000*

2 /

3)
R
VTLOOP VTHIGH
Rated MAX. Volt.=
550000/

3 =
Figure 2. Equivalent representation of Figure 1 modeled to examine ferroresonance voltages and currents.
2

3 insulated buswork was considered for the stray capacitance connected in shunt on the high-side of the VT. The gasinsulated buswork had the largest impact on the magnitude of
C
2
.
The grading capacitors as shown in Figure 1 have a total capacitance of 700 pF per circuit breaker pole on the left-hand side of Figure 1 and a total capacitance of 250 pF per circuit breaker pole on the right-hand side of Figure 1.
Since ferroresonance is a non-linear phenomenon, it may not be appropriate to speculate on the combinations of C
1
(grading capacitance) and C
2
(capacitance to ground) that could trigger potential ferroresonance conditions of concern.
In this analysis, for the most part, greater concerns were identified for higher ratios of C
1
/C
2
compared to lower ratios of C
1
/C
2
. However, it is emphasized that because of the nonlinear characteristics of ferroresonance, lower C
1
/C
2
ratios could potentially be of greater concern than higher C
1
/C
2 ratios.
Rated MCOV
Table 1
VT Data Used for the EMTP Model
Parameter Value
449073 V
Model Designation
V
Rated low-side winding rms
Rated high-side winding rms
Winding ratio (Winding 2 / Winding 1)
Low-side winding resistance
High-side winding resistance
Low-side leakage inductance
High-side leakage inductance
Core loss (resistance) (low-side)
Magnetizing inductance (low-side)
Flux steady-state (low-side, peak)
115 V
287500 V
2500
0.01812 ohms
34420 ohms
0.06 mH
115000 mH
80 ohms
0.157 H (59.3 ohms)
0.43 V-s
Current steady-state (magnetizing, low-side, rms)
Current steady-state (resistive, low-side, rms)
Table 2
1.94 A
1.44 A
VT and Damping Device Ratings
VT thermal kVA rating
Winding 1
Winding 2
-
R
1
R
C
L
M
-
R
2
L
1
L
2
-
-
3 kVA
Damping resistor rated continuous power
Damping resistor rated maximum voltage (1 min)
Damping resistor insulation class
Damping resistor rated continuous current
200 W
161 V
600 V
26 A
IV. O VERALL S UMMARY OF F INDINGS
Table 4 lists a summary of simulation results for the analysis, organized by increasing amount of total grading capacitance for each case examined. Table 4 indicates sustained ferroresonance oscillations were observed without the damping device for total equivalent grading capacitances of 500 pF, 1200 pF, and 1950 pF, but not for 250 pF and 700 pF, implying that increasing C
1
/C
2
ratios are not in themselves indicative of ferroresonance concerns. C
2
, shown in Table 3 to be primarily composed of gas-insulated buswork, did not vary significantly from case to case and changes in the value of C
2 on the order of a few hundred pFs had a negligible impact on the results. Note that changes in C
2
for this analysis are caused by the amount of equipment in service for each circuit configuration associated with varying the number of circuit breaker bays connected to the bus and phase layout.
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0 200 400 600 800 1000 1200
Figure 3. Comparison of saturation characteristics for the saturable reactor and VT.
Table 3
Example for the Determination of C
2
Equipment of Floating Section
Capacitance
(each, pF)
(per phase)
Quantity
Total Capacitance
Value (pF)
(per phase)
Circuit breaker/disconnect switches/bushings
Bus supports/posts
Surge arresters
Gas-insulated buswork (phase A)
Air-insulated buswork (phase A)
75 to 250
75 to 120
100
54.2 pF / m
11.75 pF / m
4
3
1
92.1
37.5
570
230
100
4992
440
Total Capacitance to Ground for Floating Section (C
2
) (phase A)
6332
Figures 3 and 4 show an example of voltages on the floating bus section at the high-side of the VT for conditions with no damping device and with the damping device in the circuit, respectively, for Case 4 listed in Table 4.
The following summarizes the findings of the analysis based on the results listed in Table 4:
(1) Ferroresonance was observed for all cases examined without the damping device. With the damping device, ferroresonance conditions were suppressed.
(2) With the damping device, there is no cause for concern for equipment insulation or operation of relays. The following was observed without the damping device:
(a) Concern for degradation of equipment insulation because of sustained low-frequency voltages.
(b) Concern for relay operation based on relative magnitudes of voltages compared to relay set points.
(c) Note that operating concerns may also exist because of relatively higher voltages on floating buswork expected to be de-energized compared to conditions with the damping device.
V. R EFERENCES
[1] IEEE WG 15.08.09, “Modeling and Analysis of System Transients
Using Digital Programs,” IEEE PES Special Publication TP-133-0,
1998.
[2] Slow Transients Task Force of IEEE WG on Modeling and Analysis of
System Transients Using Digital Programs, “Modeling and Analysis
Guidelines for Slow Transients- Part III: The Study of Ferroresonance,”
IEEE Trans. on Power Delivery, vol. 15, no. 1, pp. 255-265, Jan. 2000.
[3] Andrei, R.G., Halley, B.R., “Voltage Transformer Ferroresonance From an Energy Standpoint,” IEEE Trans. on Power Delivery, vol. 4, no. 3, pp. 1773-1778, July 1989.

4
[4] Price, E.D., “Voltage Transformer Ferroresonance in Transmission
Substations,” 30 th
Annual Conference for Protective Relay Engineers,
Texas A&M University, April 1977.
[5] Ferracci, P., Ferroresonance, Cahier Technique Schneider n o 190,
ECT90, March 1998.
[6] Janssens, N., Even, A., Denoel, H., Monfils, P.A., “Determination of the
Risk of Ferroresonance in High Voltage Networks. Experimental
Verification on a 245 kV Voltage Transformer,” Sixth International
Symposium on High Voltage Engineering, New Orleans, LA, USA,
Aug. 28-Sept. 1, 1989.
[7] Jacobson, D.A.N., Swatek, D., Mazur, R., “Mitigating Potential
Ferroresonance in a 230 kV Converter Station,” IEEE T&D Conference,
Los Angeles, Sept. 1996.
[8] Graovac, M., Iravani, R., Wang, X., McTaggart, R.D., “Fast
Ferroresonance Suppression of Coupling Capacitor Voltage
Transformers,” IEEE Trans. on Power Delivery, vol. 18, no. 1, pp. 158-
163, Jan. 2003.
[9] Janssens, N., Craenenbroeck, Th.V., Dommelen, D.V., Meulebroeke,
F.V. De, “Direct Calculation of the Stability Domains of Three-Phase
Ferroresonance in Isolated Neutral Networks with Grounded-Neutral
Voltage Transformers,” IEEE Trans. on Power Delivery, vol. 11, no. 3, pp. 1546-1553, July 1996.
[10] Walling, R.A., Barker, K.D., Compton, T.M., Zimmerman, L.E.,
“Ferroresonant Overvoltages in Grounded Wye-Wye Padmount
Transformers with Low-Loss Silicon-Steel Cores,” IEEE Trans. on
Power Delivery, vol. 8, no. 3, pp. 1647-1660, July 1993.
[11] Swift, G.W., “An Analytical Approach to Ferroresonance,” IEEE Trans. on PAS, vol. PAS-88, no. 1, pp. 42-46, Jan. 1969.
VI. B IOGRAPHIES
Donald J. Shoup joined Mitsubishi Electric Power Products Inc.
(MEPPI), Warrendale, PA in July 2000. Prior to joining MEPPI, Mr. Shoup was with Robicon’s Research and Development Department in Pittsburgh,
PA, where he worked during the summers as an engineering assistant, beginning in 1998. In 2000, he earned a MS in Electric Power Engineering from Rensselaer Polytechnic Institute in Troy, NY. Prior to this, he earned his
BSEE from Gannon University in Erie, PA in 1999.
John J. Paserba earned his BSEE (‘87) from Gannon University, Erie,
PA, and his ME (‘88) from Rensselaer Polytechnic Institute, Troy, NY. Mr.
Paserba joined Mitsubishi Electric Power Products Inc. in 1998 after working for over 10 years at General Electric. He is currently the Vice-Chair for the
IEEE PES Power System Dynamic Performance Committee.
Daniel J. Sullivan joined MEPPI in April of 2002. He earned a BSEET degree from Pennsylvania State University in 1995, and is in the processing of earning an MSEE degree from the University of Pittsburgh. At MEPPI,
Mr. Sullivan performs engineering studies associated with FACTS applications and EMTP studies associated with GIS equipment installations.
He is a licensed Professional Engineer in the state of Pennsylvania.
Phil Bolin graduated from Massachusetts Institute of Technology with a
Masters Degree ('68) and Bachelors Degree ('66). He currently is the General
Manager of the Substations Division with Mitsubishi Electric Power
Products, Inc, in Warrendale, PA. Prior to joining MEPPI, he was with ABB,
Westinghouse Electric, High Voltage Power Corporation, and Ion Physics
Corporation.
Ray Whiteside earned his BSEE from the University of Pittsburgh in
Pittsburgh, PA. Mr. Whiteside joined MEPPI in 2001 after working for
Westinghouse and ABB as a transmission and distribution engineer and is a licensed Professional Engineer in the state of Pennsylvania.
Table 4
Simulation Results for the Ferroresonance Analysis
No Ferroresonance Damper With Ferroresonance Damper
(1)
Case No.
(for ref.)
C1
(equivalent total grading capacitance)
(pF)
Equivalent Total Grading
Capacitance in pF and Number
of Circuit Breakers In Circuit
Listed in Parenthesis
(break-down of C1)
GIS
Connectivity
Resulting Voltage on Floating Section
(500 kV-side)
(kV rms)
Frequency
(Hz)
Resulting 60 Hz Voltage on Floating Section
(500 kV-side)
(kV rms)
Resulting Voltage on VT Secondary
(115 V-side)
(V rms)
Case 2 250 250 (1) 1 bay in-circuit 45
(1)
5 20.5
8.2
Case 2a
Case 1
Case 3
500
700
1200
250 (2)
700 (1)
700 (1)
250 (2)
2 bays in-circuit
Only new
CB/bus in-circuit
2 bays and new
CB/bus in-circuit
135
(2)
70.7
(1)
276.5
(2)
12
5
20
32
40
60
Case 4 1950
700 (1)
250 (5)
5 bays and new
CB/bus in-circuit
313
(2)
20 82.7
(1) Decaying voltage oscillations where simulated voltages were recorded at a time of approximately 2 seconds after breaker opening.
(2) Sustained, “undamped” or very lightly damped voltage oscillations.
3 D _ P K 3 B > V T L O W S ( T y p e 1 )
1 . 5
12.8
16.0
24.0
33.1
1 5 0
T im e ( m s )
-0 . 5
-1 . 0
1 . 0
0 . 5
0 . 0
1 5 0 0
T im e (m s )
Figure 3. Example of ferroresonance voltage on floating bus section at high-side of voltage transformer for case with no damping device (P.U. Base = 550 x

2 /

3 = 449073 V).
-1 . 5
0 5 0 0 1 0 0 0 1 5 0 0
T im e (m s )
2 0 0 0 2 5 0 0 3 0 0 0
T O P , T h e O u t p u t P ro c e s s o r® high-side of voltage transformer for case with damping device
(P.U. Base = 550 x

2 /

3 = 449073 V).