Ferroresonance Analysis of a 500 kV
Gas-Insulated Substation
D. Shoup, Member, IEEE, J. Paserba, Fellow, IEEE, D. Sullivan, Member, IEEE
P. Bolin, Senior Member, IEEE, R. Whiteside, Senior Member, IEEE
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. INTRODUCTION
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. OBJECTIVES
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: [email protected]).
III. EMTP MODEL
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 (RD) and
saturable reactor (LD)
Equivalent grading capacitance (C1) across circuit
breaker contacts
Stray capacitance (C2) 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 C2
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-
2
500 kV Transmission Line
500 kV Line
Termination
Surge
Arrester
Gas-to-Air
Bushing
CCVT
Floating bus with
VT in circuit
Floating bus
Bus energized
at MCOV
500 KV AIS
VT (3-phases)
500 KV GIS
Open Circuit
Breaker
Equivalent total grading
capacitance = 700 pF
Equivalent total
grading
capacitance
= 250 pF per CB
(CBs A, B, C, D,
and XXX)
500 KV BUS
900 pF
CB X
Effective capacitance of AIS
equipment including bushing
= 900 pF per phase
Open Circuit Breaker
(Both associated disconnect
switches are closed)
Bus energized
at MCOV
CB A
CB B
CB XXX
CB C
CB D
CB XXX
Equivalent total grading
capacitance = 700 pF
500 KV BUS
500 kV Transmission Line
500 KV AIS
CB Y
VT (3-phases)
500 KV GIS
Wavetrap
(1- only )
Air-to-Oil
Bushing
Y
Surge
Arrester
Surge
Arrester
Surge
Arrester
Gas-to-Air
Bushing
Y
Gas-to-Air
Bushing
CCVT
Air-to-Oil
Bushing
500 kV Line
Termination
Y
Y
Figure 1. One-line diagram of the GIS for the limiting case for the ferroresonance analysis.
Circuit Values for Ferroresonance Case
(C1 and C2 were modified per case for different circuit connectivities)
V = 449073 V (MCOV, 550000* 2 / 3)
C1 = 1950 pF
C2 = 6332 pF (phase A), 6674 pF (phase B), 7173 pF (phase C)
R2 = 34420 ohms
L2 = 115 H
R1 = 0.01812 ohms
L1 = 0.06 mH
RC = 80 ohms (1.44 A rms or 2.04 A peak) (core resistance) (115 V-side)
RD = 0.47 ohms (variable)
Lm = 0.157 H (59.3 ohms) (magnetizing inductance) ( 115 V-side)
Flux steady-state = 0.43 V-s (peak) (115 V-side)
Current steady-state = 1.94 A rms or 2.74 A peak (EMTP input) (115 V-side)
C1
(CB grading capacitance)
Saturable Reactor
(115 V Saturation Characteristics)
(Estimated peak values)
550 kV VT
(115 V Saturation Characteristics)
(Estimated peak values)
Current (A)
2.740
8.485
21.213
43.841
89.095
176.777
353.553
707.107
Flux (V-s)
0.430
0.566
0.750
0.849
0.933
1.004
1.061
1.103
VT (Voltage Transformer)
Current (A)
0.0300
Flux (V-s)
0.0073
0.0856
0.0354
0.1696
0.2187
0.0976
0.1574
0.2909
0.2062
0.4442
0.3191
0.6407
1.1451
0.3941
0.4502
2.2600
0.4875
3.4068
0.5064
4.9634
15.8462
0.5253
0.5662
40.7614
0.5704
51.6993
63.6927
0.5741
0.5780
(Note 1)
VSOURC
VTLOOP
VSRCCB
CBSWIT
R2
VTHIGH
L2
Primary winding
resistance
and leakage
inductance
R1
L1
VTNODE
N=2500
(287500/115)
VTLOWS
Secondary winding
resistance and
leakage inductance
115 V rms
Ferroresonance
Damper
RD (Note 2)
(Damping
Resistance)
V
V
(Source voltage)
C2
(Effective
capacitance for
circuit connected
to VT)
Rated MAX. Volt.=
550000/3 =
317543 V
Vwind = 287500 V rms
VTBURD
I
V
LD (Note 3)
I
RC
(Saturable
Reactor)
(Equivalent
core-loss
resistance)
Figure 2. Equivalent representation of Figure 1 modeled to examine ferroresonance voltages and currents.
3
1.6
1.4
VT Saturation Characteristics
1.2
1
Flux
Flux (V-s)
(V-s )
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
C2.
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 C1
(grading capacitance) and C2 (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 C1/C2 compared to lower ratios
of C1/C2. However, it is emphasized that because of the nonlinear characteristics of ferroresonance, lower C1/C2 ratios
could potentially be of greater concern than higher C1/C2
ratios.
0.6
VT Saturable Reactor
Saturation Characteristics
0.4
0.2
0
0
Rated MCOV
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)
Current steady-state (magnetizing, low-side, rms)
Current steady-state (resistive, low-side, rms)
Value
449073 V
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
1.94 A
1.44 A
Model Designation
V
Winding 1
Winding 2
R1
R2
L1
L2
RC
LM
-
Table 2
VT and Damping Device Ratings
VT thermal kVA rating
Damping resistor rated continuous power
Damping resistor rated maximum voltage (1 min)
Damping resistor insulation class
Damping resistor rated continuous current
3 kVA
200 W
161 V
600 V
26 A
IV. OVERALL SUMMARY OF FINDINGS
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 C1/C2 ratios are not in themselves
indicative of ferroresonance concerns. C2, 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 C2
on the order of a few hundred pFs had a negligible impact on
the results. Note that changes in C2 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.
200
400
600
800
1000
1200
Curre nt (A)
Current (A)
Figure 3. Comparison of saturation characteristics for the
saturable reactor and VT.
Table 3
Example for the Determination of C2
Table 1
VT Data Used for the EMTP Model
Parameter
0.8
Equipment of Floating Section
Circuit breaker/disconnect switches/bushings
Bus supports/posts
Surge arresters
Gas-insulated buswork (phase A)
Air-insulated buswork (phase A)
Total Capacitance to Ground
for Floating Section (C2) (phase A)
Capacitance
(each, pF)
(per phase)
Quantity
Total Capacitance
Value (pF)
(per phase)
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
-
-
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. REFERENCES
[1]
[2]
[3]
IEEE WG 15.08.09, “Modeling and Analysis of System Transients
Using Digital Programs,” IEEE PES Special Publication TP-133-0,
1998.
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.
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
Price, E.D., “Voltage Transformer Ferroresonance in Transmission
Substations,” 30th 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. 158163, 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.
[4]
VI. BIOGRAPHIES
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
With Ferroresonance Damper(1)
No 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
500
250 (2)
2 bays in-circuit
135(2)
12
32
12.8
Case 1
700
700 (1)
Only new
CB/bus in-circuit
70.7(1)
5
40
16.0
Case 3
1200
700 (1)
250 (2)
2 bays and new
CB/bus in-circuit
276.5(2)
20
60
24.0
Case 4
1950
700 (1)
250 (5)
5 bays and new
CB/bus in-circuit
313(2)
20
82.7
33.1
Decaying voltage oscillations where simulated voltages were recorded at a time of approximately 2 seconds after breaker opening.
Sustained, “undamped” or very lightly damped voltage oscillations.
(2)
3 N D _ P K 3 B > V T L O W S (T y p e 1 )
1 .5
3 D _ P K 3 B > V T L O W S (T y p e 1 )
1.5
3 N D _ P K 3 B > V T L O W S (T yp e 1 )
1 .5
(V
0 .5
0 .0
1.0
p u )
p u )
1 .0
- 0 .5
0 .5
( V
( V
p u )
1 .0
V o lt a g e
Zoom of First 300 ms
0.5
Frequency = 20 Hz
- 1 .5
0 .0
0
50
100
150
T im e ( m s )
200
E le c tr o te k C o n c e p ts ®
250
300
T O P , T h e O u tp u t P r o c e s s o r ®
V o lt a g e
V o lt a g e
- 1 .0
0.0
-0 . 5
-0 . 5
-1 . 0
-1 . 0
-1 . 5
-1 . 5
0
E le c t ro t e k C o n c e p t s ®
500
1000
1500
T im e (m s )
2000
2500
3000
T O P , T h e O u t p u t P ro c e s s o r®
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).
0
E le c t ro t e k C o n c e p t s ®
500
1000
1500
T im e (m s )
2000
2500
3000
T O P , T h e O u t p u t P ro c e s s o r®
Figure 4. Example of voltage on floating bus section at
high-side of voltage transformer for case with damping device
(P.U. Base = 550 x 2 / 3 = 449073 V).