International Journal of Engineering Business
and Social Science
Vol. 1 No. 06, July-Augusts 2023, pages: 553-566
e-ISSN: 2980-4108, p-ISSN: 2980-4272
https://ijebss.ph/index.php/ijebss
553
Ferroresonance Analysis Due to the Effect of External Faults on a
20 kV Voltage Transformer
Maulana Hidayatullah
1
Iwa Garniwa
2
1,2
Department of Electrical Engineering
Universitas Indonesia
Email: maulan[email protected] , [email protected]c.id
Keywords
Abstract
Ferroresonance, Inductive
Voltage Transformer,
Medium Voltage
Switchgear.
Medium Voltage (MV) Switchgear is an essential component in the electric power
distribution system with a working voltage of 20 kV. MW Switchgear consists of Circuit
Breaker (CB) and Voltage Transformer (VT). VT is one component that often fails in
MW Switchgear in the power distribution system, where conditions cause the VT iron
core to saturate. This saturation zone will make whatever capacitance reactance value
(X_C) that generated from the power system network will be the same value as the
inductive reactance value of the indsuctance VT (X_L), which causes the impedance
value to be zero. A very wide frequency range will be able to trigger a ferroresonance
which results in a large current flowing on the primary side of VT and has the potential
to cause failure in VT and MV switchgear, characterized by an explosion. This research
will focus on the main causes of ferroresonance emergence due to external disturbance,
20kV VT specification and MV Switchgear. Ferroresonance simulation is carried out by
ATPDraw Software, external disturbance variations, VT and MV Switchgear
specifications are given for simulation to observe the response of VT’s voltage and
current. The variables studied include disturbances of CB switching operations which
have an impact on the emergence of variations in network capacitance values and
produce subharmonic mode ferroresonance with voltage value reaches 150% of the
nominal voltage for Cg = 0,005 – 0,1 µF and 275,5% of the nominal voltage for Cs =
0,05 – 1 µF, then disturbances of lightning impulse currents which will cause
ferroresonance in networks with small capacitance values, this disturbance is very
dangerous because it creates ferroresonance with the amplitude of the primary voltage
VT can reach 14.391% of the rated voltage, and quasi-periodic mode’s ferroresonance
resulting from a single phase to ground fault which reaches 201.47 % of the rated voltage
value. The choice of a VT design with a voltage factor of 1.9Un/8h and an MV
Switchgear design which loads the VT burden with an inductance composition that is
greater than its resistance and approaches 80% of the VT burden specification can
mitigate the emergence of ferroresonance.
© 2023 by the authors. Submitted
for possible open access publication
under the terms and conditions of the Creative Commons Attribution (CC BY SA)
license (https://creativecommons.org/licenses/by-sa/4.0/).
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1. Introduction
Medium voltage switchgear is an electrical power system equipment that functions as a divider, connector,
controller, and protection. In Indonesia’s electric power system, managed by the State Electricity Company, medium
voltage switchgear is generally used in Substations. Medium voltage switchgear is one of the essential components in
the electric power distribution system with a 20 kV rated voltage. Several types of medium voltage switchgear have
main constituent components in the form of circuit breakers (CB). As its functions for protection and metering, medium
voltage switchgear is also equipped with instrument transformers in the form of Current Transformers (CT) and
Voltage Transformers (VT). There are various kinds of problems in medium voltage switchgear that cause failure of
the distribution system, one of the worst of these failures is an explosion which disrupts the distribution of electric
power to customers. Most of these failures are caused by explosions generated by the Voltage Transformer (VT). Based
on Research and Development Center of State Electricity Company shows that the ferroresonance phenomenon causes
the highest VT failure. The explosions of VT due to ferroresonance occur in certain types of VT, and production years.
Therefore, designing the right VT and medium voltage switchgear is essential to avoid this ferroresonance
phenomenon.
Ferroresonance is a non-linear resonance phenomenon that can affect the voltage and current conditions in
the electrical power network. This phenomenon can cause abnormal harmonic values, overvoltage, and overcurrent
transient or steady state, which are dangerous for electrical equipment [3]. Ferroresonance is described as a complex
resonant oscillation in a series RLC circuit. This phenomenon often occurs in electric power distribution systems due
to the saturated non-linear inductance of voltage transformers and capacitive effects from the distribution network
[2,4]. The capacitive effect is provided by some reason, such as protection equipment (switches), electric power
transmission components (conductor capacitance to earth, coupling between line circuits, capacitor banks), insulation
components (bushings capacitance), or measurement components (Capacitive Current Transformers). Ferroresonance
occurs when there is a transient fault (transient overvoltage, lightning overvoltage, or transient fault) or switching
operation (energize transformer or fault relief) (Hernanda et al., 2020)[5].
This research will discuss more specifically the triggering factors for the ferroresonance occurrence in the
form of changes in the magnitude of the VT load from electromechanical to digital meters and relays with small VA
power consumption. In addition, VT design errors and its use in medium voltage switchgear trigger a sudden saturation
of the VT iron core, which causes ferroresonance and failures and explosions, characterized by an increase in the value
of current and voltage on the primary side of VT, so that the primary side becomes burned. This research will be carried
out on the main cause of the saturation of the iron core VT which triggers the emergence of ferroresonance in the VT
installed in the medium voltage switchgear. Differences in the VT saturation point design will be combined with
variations in disturbances in the power distribution system, such as circuit breaker switching operations, single phase
to ground faults and the presence of impulse currents or lightning, and VT load value in medium voltage switchgear
design. After finding the cause of ferroresonance, an ideal composition of the VT and medium voltage switchgear will
be designed to mitigate the occurrence of ferroresonance (Kraszewski et al., 2022).
2. Materials and Methods
A. Inductive Voltage Transformer
Conventional inductive voltage transformers work with the principle of an open secondary winding, as shown
in Figure 1 (Minkner & Schmid, 2021).
Fig. 1 Inductive Voltage Transformer
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The secondary voltage
is proportional to the primary voltage
and the ratio of the number of turns of
the secondary winding
and the primary winding
. The secondary voltage can be calculated to

.
The accuracy of the voltage transformer depends on the size of the iron core, the resistance of the primary winding,
the leakage inductance, and the connected burden.
Figure 2 shows the equivalent circuit diagram of the inductive voltage transformer, reduced to the secondary
side. The primary winding is represented by the ohmic resistance Rp of the primary winding and the leakage inductance
LP, and the secondary winding by the ohmic resistance RS of the secondary winding and the secondary leakage
inductance LS. The behaviour of the iron core is represented by the main inductance Lm and the ohmic losses of the
core RFe. The impedance of the external burden is represented by the ohmic part RB and the inductive part LB. The
primary values are converted to the secondary side (Minkner & Schmid, 2021).
Fig. 2 Inductive Voltage Transformer Equivalent Circuit
There are 3 VT magnetization zones, as shown in Figure 3. The Linear Zone Section 0 – straight line, normal
VT operated. The turning point/saturation point is the risk point for VT to experience a saturation zone. In the saturation
zone, whatever the value of the capacitance reactance will match the value of the inductance reactance VT and
compensate each other so that the impedance becomes 0. The frequency range for the occurrence of ferroresonance
becomes very wide. So, it prevented VT not to operate in the saturation zone .
B. Ferroresonance
Ferroresonance is a non-linear resonance phenomenon caused by two main factors: non-linear inductance and
a particular capacitance value. Non-linear inductance is caused by the iron core's ferromagnetic nature, which can
saturate in Voltage Transformers (Pal & Roy, 2022).
Fig. 3 VT Magnetization Zone
While the value of capacitance is due to the operation of switching circuit breakers in medium voltage
networks, substations, the use of CVT (Capacitive Voltage Transformer), and distribution channels at specific lengths,
the emergence of ferroresonance is triggered by electrical power breakers operation, such as load shedding, energizing
transformers, de-energizing transformers, fault clearing, or disturbances that occur in the power network such as
overvoltage transient disturbances due to lightning strikes and short circuits. These triggering factors encourage
transformer saturation [5]. A simple circuit of ferroresonance is shown in Figure 4 [3].
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Fig. 4 Simple Circuit of Ferroresonance
Key
󰇛󰇜 = AC Voltage Source
󰇛󰇜 = Voltage at series capacitance

󰇛󰇜 = Voltage at the main inductance of the voltage transformer (VT)
󰇛󰇜 = Circuit current
= Series Capacitance

󰇍
󰇛
󰇛
󰇜
󰇜= Flux density as a function of the current i(t)
󰇛
󰇍
󰇜= Non-linear ain inductance of voltage transformer
The schematic shown in Figure 4 is for series resonance. Resonance occurs when
If written in
complex numbers, then 

. Inductance and capacitance have different polarities, so a short circuit will
occur when the voltage applied to the impedance is equal to 0. It seems to result in a short circuit in the primary winding
due to ferroresonance. This occurs at a particular frequency of a value
and
[1,5]. Where
is the primary winding
inductance VT and
equal to capacitance from the network or due to CB switching operations (Heidary et al., 2020).
Based on IEC 61869-102, soft excitation is a ferroresonance oscillation with a slow increase, while hard
excitation is an oscillation with a sudden increase rapidly [3]. This hard excitation often occurs in electric power
systems, especially distribution systems, as discussed in this study. A sudden iron saturation in the transformer iron
core is caused by a switching process in the network or a ground disturbance [3]. Then from these two groupings, there
are steady-state and non-steady-state excitations.
EXPERIMENT AND SIMULATION MODEL
In this research, disturbance modeling that can potentially cause ferroresonance will be carried out using
ATPDraw software. In addition, variations on the characteristics and burden loading of VT were carried out. To carry
out this simulation, initial parameter values are given for VT specification as shown in table I and II.
Table I VT Parameters
Value
Unit
20
kV
50
Hz
998.67
Ω
0.0003536
mH
0.046
Ω
0.4356
mH
95
MΩ
200
-
100
Ω
5
mH
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Table II Non-Linear Inductance Parameters
I(A)
Flux linked (Wb-T)
0.0075
123.79
0.018562
129.59
0.0745246
136.45
0.222739
137.82
A. Ferroresonance due to Circuit Breaker Switching Operations
In this simulation, the disturbance of the Circuit Breaker (CB) switching operation, which causes a value of
grading capacitance (Cg), together with the ground capacitance (Cs) that represent channel capacitance to ground, will
change the value of capacitance in a distribution network. In this simulation, the variations in capacitance value and
its response to ferroresonance phenomenon will be observed. The simulation circuit is shown in Figure 5.
Fig. 5 CB Switching Operation Disturbance Simulation Circuit
Additional and disturbance parameters are varied to carry out the simulation, as shown in Table III.
Table III CB Operation Disturbance Simulation Parameters
Parameters
Value
Unit
Channel Resistance (Rline)
0.0020161
Ω/m
Channel Inductance (Lline)
0.001285
mH/m
Grading Capacitance (Cg)
0.001 - 100
µF
Shunt Capacitance (Cs)
0.001 - 100
µF
CB Switching time
0.25
s
Channel Length
1000
m
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B. Ferroresonance due to Ground Fault
Fig. 6 Ground Fault Disturbance Simulation Circuit
Table IV Ground Fault Disturbance Parameters
Parameters
Value
Unit
Channel Capacitance
0.005 - 5000
pF/m
Channel Length
1000
m
Lightning strike time
0.25
s
Short Circuit Resistance (Rgnd)
1
Ω
In this simulation, the disturbance of ground fault will occur after 0.25 seconds from the simulation starting
point. The ground fault simulation circuit is shown in Figure 6, and the parameter variations were the capacitance of
the distribution line, as shown in Table IV. The primary voltage value will be observed after a ground fault occurs.
C. Ferroresonance due to Lightning Strike
In this simulation, an impulse current with 10 kA amplitudes will be given to the simulation circuit at 0.25
seconds, as shown in Figure 7.
Fig. 7 Lightning Strike Disturbance Simulation Circuit
Distribution line characteristic data is shown in Table V, and the impulse variations characteristic to determine
primary voltage response after disturbance is shown in Table VI.
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Table V Lightning Strike Disturbance Parameters
Parameters
Value
Unit
Channel Capacitance
0.005
pF/m
Channel Length
1000
m
Lightning strike time
0.25
s
Impuls Current Front Time
0.5 - 2
µs
Impuls Current Tail Time
5 - 200
µs
Impuls Current
10
kA
Table VI Lightning Strike Disturbance Variations
Impuls Variations
Simulation
Number
front time (µs)
tail time (µs)
Constant tail
time
Decreasing Front
time
1
1.2
50
2
1
50
3
0.8
50
4
0.5
50
Increasing Front
Time
5
1.5
50
6
2
50
Constant
Front Time
Increasing Tail
Time
7
1.2
75
8
1.2
100
9
1.2
150
10
1.2
200
Decreasing Tail
Time
11
1.2
25
12
1.2
10
13
1.2
5
D. Voltage Transformer Non-Linear Inductance
After conducting experiments in the laboratory with the VT Analyzer, it was found that the specification of
the Voltage Factor, which is different from each VT, will affect the magnetization curve or saturation point, therefore
in this simulation, the response of the primary voltage to changes of non-linear inductance magnetization curve will
be observed. The variation of saturation points of VT is shown in Table VII.
Table VII Non-Linear Inductance Saturation Point 1.2x Initial
I(A)
Flux linked (Wb-T)
1.2x Initial
Flux linked (Wb-T)
0.8x Initial
0.0075
148.55
99.03
0.018562
129.59
103.67
0.0745246
136.45
109.16
0.222739
137.82
110.26
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E. Voltage Transformer Burden
In addition to the internal VT design factors that influence the emergence of ferroresonance, this simulation
will observe the response of the primary voltage value to the variation of the VT burden installed on the medium
voltage switchgear. This variation of burden loading is carried out by changing the resistance and inductance values,
as shown in Table VIII.
Table VIII VT Burden loading variations
No.
Simulasi
Rb
(Ω)
Lb
(mH)
(Ω)
Z (Ω)
Burden
(VA)
%Burden
Load
1
10
0.1
31.4
32.95
8.90
30%
2
10
0.2
62.8
63.59
17.17
57%
3
10
0.3
94.2
94.73
25.58
85%
4
10
0.4
125.6
126.00
34.02
113%
5
10
0.5
157
157.32
42.48
142%
6
25
0.1
31.4
40.14
10.84
36%
7
50
0.1
31.4
59.04
15.94
53%
8
75
0.1
31.4
81.31
21.95
73%
9
100
0.1
31.4
104.81
28.30
94%
10
125
0.1
31.4
128.88
34.80
116%
3. Results and Discussions
A. Normal Condition Simulation
Under normal conditions, the primary side voltage waveform VT can be observed, as shown in figure 8.
Fig. 8 Primary Voltage under Normal Condition
The rated voltage of the distribution system is alternating (AC) line-to-line RMS (

󰇜of 20 kV with 50
Hz Frequency.
Therefore, the line to neutral RMS voltage value RMS (

󰇜 is equal to this calculation:


(1)



So, the line to neutral peak voltage (

󰇜 can be calculated as follows:


(2)



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B. Ferroresonance due to Circuit Breaker Switching Operations Simulation, Variation of Cg Value, Cs Constant =
0.01 µF
After simulating the disturbance of CB switching operation on the distribution network, it can be seen in
figure 9 that after 0.25 seconds there are different responses to the variations Grading capacitance value, where
resonance with the subharmonic mode occurs at Cg = 0.01 µF, where the primary voltage peak increases up to 150%
of the nominal voltage and resonates with a long time.
The resonance that occurs with other variations of grading capacitance shows a damped resonance. From the
variations of grading capacitance with magnitudes from 0.001 µF to 10 µF as shown in Table IX, the ferroresonance
phenomenon only occurs at capacitance quantities of 0.005 µF and 0.01 µF.
(i)
(ii)
Fig. 9 Primary Voltage under CB switching operation response of Cg variations (i)0.005 µF, (ii)0.01 µF
Table IX Voltage and Current value of VT under CB switching operation simulations response of Cg variations.
Cg (µF)
Peak Value
Ferro-
resonance?
Vp (kV)
% of rated
Vp
Ip (mA)
Vs (V)
Is(A)
0.001
10.264
62.9%
2.66
51.29
0.512
No
0.005
-19.73
120.8%
-5.12
-98.6
-1
Yes
0.01
-24.48
150.0%
-6.36
-122.3
-1.22
Yes
0.05
15.78
96.6%
4.1
78.86
0.788
No
0.1
16.106
98.6%
4.18
80.49
0.804
No
0.5
16.289
99.7%
4.2
81.45
0.813
No
1
16.162
99.0%
4.21
80.997
0.811
No
5
16.27
99.6%
4.22
81.3
0.813
No
10
16.295
99.8%
4.23
81.443
0.814
No
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C. Ferroresonance due to Circuit Breaker Switching Operations Simulation, Variation of Cs Value, Cg Constant =
0.01 µF
In the variation of the capacitance of the line to ground, the influence of Cs variation does not significantly
impact the magnitude of the VT primary voltage. The resulting ferroresonance mode is also the subharmonic mode, as
shown in figure 10.
(i)
(ii)
(iii)
Fig. 10 Primary Voltage under CB switching operation response of Cs variations (i)0.05µF, (ii)0.1µF, (iii)0.5 µF.
Table X Voltage and Current value of VT under CB switching operation response of Cs variations.
Cs (µF)
Peak Value
Ferro-
resonance?
Vp (kV)
% of
Nominal
VP
Ip
(mA)
Vs (V)
Is(A)
0.001
16.282
99.7%
4.24
81.649
0.816
No
0.005
16.252
99.5%
4.22
81.09
0.812
No
0.01
16.255
99.5%
4.23
81.451
0.814
No
0.05
44.99
275.5%
11.69
224.84
2.24
Yes
0.1
43.4
265.8%
11.28
217.43
2.17
Yes
0.5
27.32
167.3%
7.1
136.57
1.36
Yes
1
16.963
103.9%
4.4
84.77
0.847
Yes
5
7.313
44.8%
1.9
36.55
0.365
No
10
5.033
30.8%
1.3
25.15
0.25
No
Details of the simulation results on the variations of line to ground can be seen in Table X. It can also be
observed that the ferroresonance phenomenon occurs at small capacitance values or high impedance values.
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D. Ferroresonance due to Ground Fault
Fig. 11 Primary Voltage under ground fault disturbance with different sampling time.
As can be seen in Figure 11, the voltage waveform generated from the primary voltage VT due to a phase-to-
ground fault is a ferroresonance with quasi-periodic mode. However, this ferroresonance in a single-phase-to-ground
fault can only occur at very small network capacitance values or very high impedance values. With the fulfillment of
this low capacitance value, the magnitude of the voltage waveform soars very high and can even reach 201.47% of the
nominal voltage.
In Table XI, it can be seen in detail the response of the primary voltage VT to the variation of the network
capacitance value due to phase-to-ground fault. It can be observed that ferroresonance occurs over a wide range of
capacitance values.
Table XI Primary Voltage of VT under ground fault disturbance with Line capacitance variations
Line Capacitance
(pF/m)
Vp Peak (kV)
% of Nominal Vp
Ferroresonance?
0.005
21.17
129.64%
Yes
0.05
21.26
130.19%
Yes
0.5
21.175
129.67%
Yes
5
19.85
121.56%
Yes
50
16.81
102.94%
Yes
500
-32.9
201.47%
Yes
5000
-29.4
180.04%
Yes
E. Ferroresonance due to Lightning Strike
This simulation gives a disturbance as a lightning impulse current at 0.25 seconds. The magnitude of the given
lightning impulse current is 10 kA. Based on the simulation results, It can be observed that the primary voltage
increases with a very high voltage magnitude which can cause VT to explode with a voltage order of thousands of
kilovolts, as shown in Figure 13. It also can be seen that the generated ferroresonance mode is quasi-periodic with a
very high frequency. This resonance fault continues up to 0.14 seconds after the lightning strike disturbance.
Fig. 12 Primary Voltage under lightning strike disturbance
Table XII shows a more detailed value of the primary voltage response to lightning strike disturbances with
front time and tail time variations based on Table VI. It can be observed that the reduction and addition of the front
time do not have a significant impact on the resulting impulse waveform and the response of the primary voltage VT.
Still, a long tail time will cause a greater magnitude of the primary voltage, whereas vice versa, if the tail time is
shorter, the magnitude of the impulse waveform also gets smaller. The disturbance resonance time of the front time
and tail time variations of the lightning impulse current show the same number, 0.14 seconds.
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Table XII Voltage and Current value of VT under lightning strike disturbance variations.
Simulation
Number
Vp
(kV)
% of rated
Vp
Vs (kV)
Ip (A)
Is (A)
Ferroresonance?
1
2,350
14,391%
11.64
70.2
9.25
Yes
2
2,342
14,342%
11.612
70.05
9.22
Yes
3
2,335
14,299%
11.57
69.8
9.19
Yes
4
2,324
14,232%
11.52
69.5
9.15
Yes
5
2,361
14,458%
11.7
70.5
9.29
Yes
6
2,381
14,581%
11.8
71.1
9.37
Yes
7
2,391
14,642%
11.85
73.4
9.67
Yes
8
2,411
14,764%
11.9
75.03
9.89
Yes
9
2,432
14,893%
12.05
76.6
10.11
Yes
10
2,444
14,966%
12.11
77.48
10.22
Yes
11
2,208
13,521%
10.9
61.2
8.03
Yes
12
1,707
10,453%
8.46
37.8
4.87
Yes
13
758
4,642%
3.75
-13.5
-1.63
Yes
F. Voltage Transformer Non-Linear Inductance Characteristics Variation
In this simulation, variations are made on the characteristics of the non-linear inductance VT, where an initial
non-linear inductance magnetization curve has been given and shown in figure 14, from this value a variation is carried
out by increasing and decreasing the saturation point of the non-linear inductance. Then a disturbance of the switching
CB operation is given with Cg = 0.01 µF and Cs = 0.01 µF, this disturbance chosen because it occurs most often in the
distribution system.
1) Normal Saturation Point
Fig. 13 Initial VT’s Magnetization Curve
Variations are made by multiplying each initial fluxlinked quantity by 1.2 to increase the saturation point of
the magnetization curve and decreasing the saturation point of the magnetization curve by multiplying the initial
fluxlinked by 0.8.
2) 1.2 Normal Saturation Point
Fig. 14 1.2x initial VT’s Magnetization Curve
3) 0.8 Normal Saturation Point
Fig. 15 0.8x initial VT’s Magnetization Curve
565 e-ISSN: 2980-4108 p-ISSN: 2980-4272 IJEBSS
IJEBSS Vol. 1 No. 06, July-Augusts 2023, pages: 553-566
Table XIII Primary Voltage Value under CB Switching Operation disturbance with VT’s magnetization curve
variations.
Saturation Point
Variations
Vp (kV)
% of rated Vp
Ferro-
resonance?
Normal
18,207
111,5%
Yes
1,2 x
12,365
75,7%
No
0,8 x
16,971
103,9%
Yes
It can be seen in Table XIII that with a higher saturation point, VT will be more resistant to ferroresonance.
In contrast, a lower saturation point will make it more susceptible to ferroresonance. It can also be seen in figure 15
that multiplying the saturation point by 1.2x of the initial value will dampen the occurrence of ferroresonance. On the
other hand, the lower the saturation curve point of the non-linear inductance, the ferroresonance continues to oscillate
and is not damped, as shown in Figures 14 and 16.
G. Voltage Transformer Burden Variations
Fig. 16 Primary Voltage Waveform due to burden variations,
R = 125 Ω L = 0.1 mH
This simulation is carried out by varying the value of the burden load, which consists of resistance and
inductance. A disturbance of the switching CB operation is also given with Cg = 0.01 µF and Cs = 0.01 µF. With an
increasing inductance value with constant resistance, as shown in the data in Table XIV, the voltage value has a lower
peak waveform. Still, the ferroresonance disturbance continues to resonate long with the fundamental mode.
Meanwhile, increasing the resistance value and keeping the inductance at a lower and constant value will produce a
higher and damped primary voltage ferroresonance peak waveform magnitude, as shown in Figure 20.
Table XIV Primary Voltage Value under CB Switching Operation disturbance with VT’s burden loading variations.
Simulation
Number
Vp (kV)
%rated VP
Ferroresonance ?
1
8,85
54,2%
No
2
8,85
54,2%
No
3
8,86
54,3%
No
4
8,87
54,3%
No
5
8,88
54,4%
No
6
10,86
66,5%
No
7
12,46
76,3%
No
8
15,43
94,5%
No
9
-24,7
151,3%
Yes
10
-49,95
305,9%
Yes
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IJEBSS Vol. 1 No. 06, July-Augusts 2023, pages: 553-566
4. Conclusion
From the various disturbance simulations given to the VT installed in the medium voltage switchgear, several
conclusions can be drawn as follows: CB switching operation is a disturbance that often occurs in the distribution
system, and this impacts the appearance of network capacitance values which results in saturation of the VT iron core
and causes ferroresonance. The type of ferroresonance generated is the subharmonic mode, with voltage value reaches
150% of the nominal voltage for Cg = 0,005 – 0,1 µF and 275,5% of the nominal voltage for Cs = 0,05 – 1 µF.
Ferroresonance resulting from a single-phase to ground fault occurs at high line impedance values with a high-
frequency quasi-periodic mode where the magnitude of the primary voltage can reach 201.47 % of the rated voltage.
For lightning impulse current disturbances, the variation of the long tail time will significantly impact the magnitude
of the primary voltage. This lightning impulse disturbance is the most dangerous, so it is necessary to have special
protection equipment, especially a fuse to protect this VT because the resulting ferroresonance has a huge magnitude
which can reach 14.391% of the rated voltage.
Based on the results of the simulations and analyses that have been carried out in this study, it can be
concluded that ferroresonation can occur due to various kinds of disturbances in the distribution system, especially in
the case of this study, the emergence of voltage and current spikes which make the primary side of the VT explode or
burn. It is also necessary to pay attention to the VT loading installed to the medium voltage switchgear because it can
trigger ferroresonance. MV Switchgear design which loads the VT burden with an inductance composition that is
greater than its resistance and approaches 80% of the VT burden specification can mitigate the emergence of
ferroresonance. Besides that, the design factor of the magnetization curve of the non-linear inductance VT is also
critical because it can prevent VT from ferroresonance due to capacitance variations that arise in the distribution
network, The choice of a VT design with a voltage factor of 1.9Un/8h mitigate the emergence of ferroresonance.
5. References
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Study of ferroresonance in 150 kV high voltage inductive voltage transformer. 2020 International Seminar on
Intelligent Technology and Its Applications (ISITIA), 386–391.
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Minkner, R., & Schmid, J. (2021). The Technology of Instrument Transformers: Current and Voltage Measurement
and Insulation Systems. Springer Nature.
Pal, R. S., & Roy, M. (2022). Investigation on the Occurrence of Ferroresonance with the Variation of Degree of
Transformer Core Saturation. In Sustainable Technology and Advanced Computing in Electrical Engineering:
Proceedings of ICSTACE 2021 (pp. 747–756). Springer.