Siemens Energy, Inc.
Power Technology
Issue 111
Modeling Metal Oxide Varistors (MOV) in Short Circuit Calculations
Krishnat Patil
Staff Software Engineer
krishnat.patil@siemens.com
Carlos Grande-Moran
Principal Consultant
carlos.grande@siemens.com
Introduction
The ability to model Metal Oxide Varistor (MOV) protection for series compensated lines in short circuit
calculations is now available in PSS®E Version 33.1. This MOV modeling will allow PSS®E users to
evaluate the short circuit capability of breakers used in the switching of series compensated lines and
series capacitor banks for symmetrical and asymmetrical fault currents. PSS®E fault current calculation
activities ASCC, IECS and SCMU model MOV protection.
The use of series capacitors as a solution to increase the power transfer capability of existing extra high
voltage (EHV) transmission lines has been tried extensively in recent years. The general idea is to reduce
the inductive reactance of the line, thereby increasing the electrical length and surge impedance loading
of a series compensated transmission line.
An important design issue of series capacitors in transmission lines is their overvoltage protection. A
widely used protection scheme based on metal (zinc) oxide varistors (MOV) has proven to be very
effective and reliable. MOVs present a nonlinear resistance characteristic that is approximated by
fundamental frequency nonlinear impedance during short circuit calculations. The series capacitors and
MOV protection are modeled together as equivalent branch impedance, Zeq= RCeq –j XCeq, controlled by
the branch current I flowing into the MOV-protected series capacitor branch1.
A typical MOV-protected series capacitor arrangement is shown in Figure 1. The MOV protection is
connected directly in parallel with the series capacitor and performs its function by holding the maximum
capacitor voltage within the designed protective level. MOV conduction normally occurs only during faults
because the capacitor protective level is usually specified above the maximum voltages expected during
overload or swing conditions. In normal operation, all current flows through the series capacitor and none
through the MOV protection.
1
Daniel L. Goldsworthy, "A Linearized Model for MOV-protected Series Capacitors," IEEE Transactions
on Power Systems, Vol. PWRS-2, No. 4, pp 953-958, November 1987. For additional details refer to
PSS®E Program Application Guide Volume I.
Power Technology
March 2012
C
Va
Ia
V’a
Va
MOV
C
Vb
Ib
V’b
Zeqc
MOV
C
Ic
Zeq
b
Vb
Vc
Vc
Zeq
a
Ia
V’a
Ib
V’b
Ic
V’c
V’c
MOV
Figure 1 – Three-phase, MOV-protected Series Capacitor Bank
An MOV is a nonlinear variable resistance that has very high resistance for low voltages across the
varistor, and then above the threshold voltage its resistance decreases rapidly. Figure 2 illustrates a
typical VI characteristic of a 120 kV rms rated MOV. Notice that the unit conducts little current up to a
voltage of about √3 x 120 = 208 kV and then, as the voltage across it increases further, the current
increases very rapidly.
700.0
600.0
Typical Arrester Voltage‐Current (VI) Characteristics Arrester Voltage (kVp) 500.0
400.0
300.0
200.0
100.0
0.0
1E‐05
0.0001
0.001
0.01
0.1
1
10
100
1000
Arrester Current (A)
Figure 2 - Typical VI Characteristic for MOV
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10000 100000 1E+06
Power Technology
March 2012
Solving a network containing a nonlinear impedance Zeq requires an iterative scheme. An initial value of
current entering this equivalent impedance is estimated, based on the system condition, and then the
network with the nonlinear impedance is solved using the values corresponding to this estimated current.
This procedure is repeated until the estimated line current change between two consecutive iterations is
within a desired tolerance. This procedure is described by the equation below where I is the MOVprotected series capacitor branch current, α is an accelerating factor, and k is the iteration number.


I k 1  I k   α  I k 1  I k  for α  1.0
The desired tolerance, accelerating factor and maximum number of MOV loop iterations values are
provided as PSS®E options. Their default values are as follows: acceleration factor equals 0.3, tolerance
equals 0.01 and maximum number of MOV loop iterations equals 20. These default values have been
tested to assure convergence when working with many real world cases.
PSS®E Data Requirements
The series capacitor installation must be explicitly modeled in the PSS®E working case as a branch with
an impedance equal to 0.0 – j XC, i.e., the branch resistance must be zero. Also, branch reactance in
positive and zero sequence must be the same.
The MOV data is entered into the working case zero sequence non-transformer branch data records
found in the Sequence Data File. Each zero sequence branch data record has the following format:
I, J, ICKT, RLINZ, XLINZ, BCHZ, GI, BI, GJ, BJ, IPR, SCTYP
where
IPR
MOV rated current for a series capacitor branch; entered in kA. IPR must be
positive. The default for IPR is 0.0.
SCTYP
MOV protection mode
The three protection modes allowed are:
0 for a normal branch (i.e., not an MOV-protected branch)
1 for MOV Protection enabled
2 for MOV Protection disabled
The default value for SCTYP is 0.
(For other elements in this data record, please refer to the PSS®E Program Operation Manual.)
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Power Technology
March 2012
Figure 3 shows PSS®E branch data editor dialog that can be used to edit the MOV protection mode and
MOV-rated current data.
Figure 3 – PSS®E Branch Data Editor Dialog
Numerical Example
A ten-bus test system with two series compensated lines (4010-4011 and 4020-4021) is used in this
numerical example. The bus fault currents calculated using activity ASCC for symmetrical, three-phase,
and asymmetrical, LG, LLG and LL faults are shown in Table 1. The test system used in this example is
effectively grounded (X0/X1<3 and R0/X1<1) and thus asymmetrical short circuit currents involving
ground will tend to be bigger than three-phase symmetrical fault currents.
The first column in each block set shows the momentary short circuit current at the faulted bus with MOV
protection disabled; the second column presents the short circuit results with MOV protection enabled.
Note that the magnitude of the momentary short circuit currents is smaller when the MOV protection is
enabled, as the effective impedance presented by the MOV-protected series capacitor is larger than the
effective impedance when the series capacitor installation has its MOV protection disabled.
Faulted
Bus
3 Phase Fault
(Amps)
LG Fault
(Amps)
LLG Fault
(Amps)
Disabled
Enabled
Disabled
Enabled
Disabled
Enabled
Disabled
Enabled
4000
11660.3
11587.1
14435.5
14353.9
18943.5
18881.2
10098.3
10034
4001
6112.7
5966.9
7953.8
7767.4
11405.8
11202.4
5289.1
5163.2
4002
6041.9
6035.6
6343.6
6335
6677.3
6669.5
5232.4
5224.6
4010
1434.9
772.8
1487.4
796.5
1544.2
807.6
1242.6
692
4011
1486.1
738.1
1515.9
742.6
1546.9
751.3
1287
638.3
4020
1434.9
774.6
1487.4
796.5
1544.2
807.6
1242.6
692
4021
1486.1
738.1
1515.9
742.6
1546.9
751.3
1287
638.3
Table 1 - PSS®E Short Circuit Results with Activity ASCC
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LL Fault
(Amps)
Power Technology
March 2012
The short circuit results can be presented graphically on a slider diagram. The slider shows the series
capacitor installations specified with MOV protection by means of a symbol that shows a series capacitor
in parallel with its MOV protection. Figures 4 and 5 below display the faulted bus, the momentary threephase short circuit current at bus 4021, as well as the post-fault branch currents and bus voltages
resulting from the application of the bolted three-phase fault at bus 4021. Figure 4 shows results with the
MOV protection disabled; Figure 5 shows results with the MOV protection enabled.
4002
BUS2
4300
GEN3LV
1
14778.4
12.6
167.1
12731.6
-172.0
334.2
8.0
27487.6
1
4001
BUS1
167.1
-172.0
8.0
167.1
4200
GEN2LV
723.8
27574.7
-132.4
47.6
167.1
41271.7
1
-172.0
10.5
8.0
1
-6.3
238.0, 24.1
6.4, 27.0
33473.4
-47.9
6.1, -1.4
4021
SC2B2
4011
SC1B2
207.1
28.8
207.1
389.0
389.0
93.6
-86.4
207.1
-151.2
1201.3
28.8
-37.5
205.0, 32.1
0.0, 0.0
1486.1, -48.9 (3PH)
225.0, 0.2
4010
SC1B1
4000
GEN1HV
207.1
-151.2
207.1
28.8
4020
SC2B1
207.1
1201.3
1201.3
1201.3
-151.2
142.5
-37.5
142.5
4100
GEN1LV
216.5, 9.7
1298.5
-29.1
25969.9
480.5, -127.5
150.9
25969.9
1
TEST SYSTEM #3 -SSR
217.1, 41.8
WED, FEB 15 2012 12:01
ASCC : Three Phase Fault: Bus 4021, IA (AMPS), VA (VOLTS), (POLAR)
-29.1
11.4, 42.7
Figure 4 – Fault Currents for Three-phase Fault at Bus 4021 and MOV Protection Disabled
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Power Technology
March 2012
4002
BUS2
4300
GEN3LV
1
14778.3
12.6
169.2
12892.3
-171.8
338.4
8.2
27650.3
4200
GEN2LV
4001
BUS1
169.2
-171.8
8.2
169.2
695.5
26493.5
-131.7
48.3
41343.7
169.2
1
1
6.1, -1.6
238.0, 24.1
6.4, 27.0
1
-6.5
8.2
-171.8
10.6
33882.3
-46.2
4021
SC2B2
4011
SC1B2
170.4
28.5
170.4
389.9
389.9
93.2
-86.8
170.4
-151.5
410.3
28.5
-41.3
203.9, 25.8
0.0, 0.0
738.1, -63.5 (3PH)
225.6, -0.1
4010
SC1B1
4000
GEN1HV
170.4
-151.5
170.4
28.5
170.4
-151.5
4020
SC2B1
410.3
410.3
138.7
-41.3
410.3
138.7
4100
GEN1LV
225.0, 18.1
495.4
-22.5
9908.8
10.7, -62.9
157.5
9908.8
1
TEST SYSTEM #3 -SSR
234.2, 42.8
WED, FEB 15 2012 11:31
ASCC : Three Phase Fault: Bus 4021, IA (AMPS), VA (VOLTS), (POLAR)
-22.5
11.9, 43.2
Figure 5 – Fault Currents for Three-phase Fault at Bus 4021 and MOV Protection Enabled
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