The gas insulated switchgear (GIS) has been widely used in the power plant and substation because of its advantages such as being independent of the atmospheric environment, small footprint, high reliability, low maintenance and so on [1]. However, the very fast transient overvoltage (VFTO) with steep front and high amplitude would occur during the switching operation in GIS [2]. The fundamental reason for VFTO is the breakdown of the contact gap, which leads to a steep rising voltage spread inside the GIS. Due to the different structure and parameters of GIS circuit components, the transient impulse voltage caused by constant refraction and reflection in GIS may cause damage to the insulation safety of the equipment [3–5]. A 220 kV GIS CB fault happened in the transmission line between Maoming power plant and Xie Ping Ling (XPL) station in China in May 2016, this fault led to the explosion of the CB and the burning of the potential transformer (PT). The CB was checked by X-ray immediately after the fault, where, the result showed that A-phase movable contact was not in the normal opening position, and it was not opened completely comparing with the position of other phase parts. Through analysing the voltage waveform obtained from the phasor measurement unit (PMU) recorder system before this fault, it is found that such waveforms are very rare in the current study. This failure may be due to the incompletely open of the CB, causes the system power repeated charged to the CB, and the VFTO would generate in the repeated strike process after the relevant analysis. To verify the cause of this fault, computer simulations and further analysis are necessary.

1. The Journal of Engineering
Research Article
Simulation and analysis of a gas insulated
switchgear explosion accident caused by a
failure of high-voltage circuit breaker in a
thermal power plant
eISSN 2051-3305
Received on 21st December 2018
Accepted on 21st January 2019
doi: 10.1049/joe.2018.5468
www.ietdl.org
Xun Zhang1, Kai Dai2 , Wei Niu1, Rongbin Xie3, Huarong Zeng1, Yi Wen1, Xiaohong Ma1
1
Electric Power Research Institute of Guizhou Power Grid Co. Ltd, Guiyang 550002, People's Republic of China
2
State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044,
People's Republic of China
3
Guiyang Power Supply Bureau, Guiyang550002, People's Republic of China
E-mail: 20161102121t@cqu.edu.cn
Abstract: This study analyses a gas insulated switchgear explosion accident caused by a failure of the circuit breaker (CB) in a
thermal power plant, which resulted in the explosion of the CB and the burning of the potential transformer (PT). Through X-ray
detection and disassemble examination of the damaged CB, it is found that the A-phase moveable contact was not opened
completely. After analysing the voltage waveform obtained from the phasor measurement unit recorder system, it is speculated
that this failure is caused by the process of repeated arc strike. For verification and further analysis, the fault process is
simulated in the EMTP; the arc is properly modelled by an extended KEMA's equation-based model which is proposed in this
study. The simulation results match well with the actual situation; the contact gap would be repeated breakdown because its
breakdown voltage is reduced, and the very fast transient overvoltage (VFTO) would be generated in these repeated strike
processes. Due to the steep front and high amplitude of VFTO, the insulation of the PT would be damaged and led to the
occurrence of PT short-circuit fault. The current would increase rapidly and causes the CB to explode subsequently.
1 Introduction
The gas insulated switchgear (GIS) has been widely used in the
power plant and substation because of its advantages such as being
independent of the atmospheric environment, small footprint, high
reliability, low maintenance and so on [1]. However, the very fast
transient overvoltage (VFTO) with steep front and high amplitude
would occur during the switching operation in GIS [2].
The fundamental reason for VFTO is the breakdown of the
contact gap, which leads to a steep rising voltage spread inside the
GIS. Due to the different structure and parameters of GIS circuit
components, the transient impulse voltage caused by constant
refraction and reflection in GIS may cause damage to the insulation
safety of the equipment [3–5].
A 220 kV GIS CB fault happened in the transmission line
between Maoming power plant and Xie Ping Ling (XPL) station in
China in May 2016, this fault led to the explosion of the CB and
the burning of the potential transformer (PT). The CB was checked
by X-ray immediately after the fault, where, the result showed that
A-phase movable contact was not in the normal opening position,
and it was not opened completely comparing with the position of
other phase parts.
Through analysing the voltage waveform obtained from the
phasor measurement unit (PMU) recorder system before this fault,
it is found that such waveforms are very rare in the current study.
This failure may be due to the incompletely open of the CB, causes
the system power repeated charged to the CB, and the VFTO
would generate in the repeated strike process after the relevant
analysis. To verify the cause of this fault, computer simulations and
further analysis are necessary.
Arc generation and extinguishment are an integrated process
involving physical quantities such as heat, gas, and magnetism, and
that is very complex. At present, the arc model can be roughly
classified into two categories: physical model [6] and black box
model [7]. The black box model has been paid more and more
attention by researchers because of its simpler mathematical
equation and the dynamic study of the arc conductance [8].
The famous arc models include the early Mayr model and
Cassie model, which provide a qualitative description of the arc in
the high- and low-current regions, respectively. Then some models
based on these classic models are proposed, such as Habedank
model, Schwarz model, KEMA model and so on [9–12], which are
widely used and keeps updated in arc analysis [13, 14].
The KEMA model is a semi-empirical arc model based on the
Mayr–Cassie equation and a large number of experiments. Since
the mathematical description of the KEMA model is derived from
the Cassie and Mayr equations, which has a complete physical
meaning. In theory, it can simulate the arc of the various circuit
breakers (CBs) more accurately [15].
To make an accurate simulation of this fault, a simulation model
of the repeated breakdown of the CB contact gap is established,
and the repeated strike process is simulated in ATP-EMTP. The
proposed model includes the block of the KEMA arc model and the
CB breakdown control unit. Since most of the black box models
lack the capabilities to realise the analysis of the repeated
breakdown of CB and related research rarely exists, this system
will be of great significance to the study of arc modelling.
To analyse the VFTO accurately, accurate models for the GIS
components are established. Based on the equivalent model, a
simulation of the accident will be carried out. The simulation
results will be compared with the record data, and the process of
fault development will be analysed in detail. However, it is difficult
to simulate the reason of the PT damage, therefore, based on the
simulation and the recorded data, an inference of the reason of the
PT damage will be present at the last.
The rest of this paper is organised as follows. In Section 2, the
background of the CB failure is described, and a simple analysis of
this failure is carried out. In Section 3, the proposed CB model is
described in detail. In Section 4, based on the simulation results in
ATP-EMTP, the failure is analysed in detail. The discussion and
conclusion are drawn in Sections 5 and 6, respectively.
J. Eng.
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1

2. 2 Failure background
2.1 Failure description
On 10 May 2016, due to the regular maintenance schedule, the
second transmission line between Maoming power plant and XPL
station was planned to be changed from the working state to the
standby state. The #2970 SF6 CB (245 kV/50 kA) at the Maoming
power plant side was switched off at 9:00 a.m., with the relevant
equipment operating normally.
The CB at the XPL station side was switched off at 9:40 a.m.,
later the bus differential protection operated on the second bus of
Maoming power plant and the voltage was also getting lost then. It
was found that the GIS transformer room was smoking according
to the on-site inspection, while the #2970 SF6 CB broke down and
the PT burned down, as shown in Fig. 1. The simplified wiring
diagram between the Maoming power plant and XPL station is
illustrated in Fig. 2, and the fault sequence diagram is shown in
Fig. 3.
2.2 On-site examination
The CB was checked by X-ray immediately after the fault. The
result showed that the movable contact of phase A was not in the
normal opening position, and it was not opened completely
comparing with the position of B-phase movable contact, as shown
in Fig. 4.
This fault occurred after the CB at the XPL station side was
opened, and the protection record at the power plant side did not
start. However, voltage and current waveforms of the faulty
transmission line before the fault were obtained from the PMU
recorder system, as shown in Fig. 5. The ratio of the actual voltage
to the recorded voltage is 2200, and the ratio of the actual current
to the recorded current is 1600.
To make a detailed analysis of the fault, the recorded data are
divided into two stages in chronological sequence.
In stage I, which is from 9:40:00 to 9:42:46, the voltage
presents the shape of the flat-topped waveform while the current is
an impulse-like waveform.
In stage II, which is from 9:42:46 to 9:43:00, a sharp increase in
the current happened.
Generally, the line current should always be zero as well as the
line voltage should decay to zero soon from the operating voltage
when the CB at the station side was opened after the power plant
jumping out to the hot standby state. However, in this case, the
three-phase voltage constantly exhibited a flat-topped voltage
waveform which periodically changes polarity, and the impulse
current was monitored in A-phase at that time in stage I, as shown
in Fig. 5a. The comparison of the station side voltage of the faulty
CB and the power plant voltage is shown in Fig. 5c. The following
points can be seen from stage I.
Fig. 1 Failure equipment: the breakdown of the SF6 CB arc-extinguish
chamber
Fig. 2 Simplified wiring diagram between the Maoming power plant and XPL station
Fig. 3 Diagram of the fault sequence
Fig. 4 Result of X-ray inspection
(a) Moving contact of phase A, (b) Joint shaft of phase A, (c) Moving contact of phase B, (d) Joint shaft of phase B
2 J. Eng.
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3. i. The variation of the faulty transmission line voltage polarity
was consistent with the change of system voltage polarity.
When the voltage between the CB contact gaps reaches a limit
value, the transmission line voltage jumps to the system
voltage at that moment. The high-frequency component of
shock and rapid attenuation appeared at the moment that the
flat changed the oscillation transients may occur at the voltages
higher than the system operating voltage. As the sampling rate
of the PMU recorder system is 4.8 kHz, the details of the high-
frequency shock cannot be clearly recorded. However, the
record shows that the high-frequency shock was there and
maybe VFTO.
ii. The current appeared as a discontinuous impulse current
pattern, and the maximum value is 475 A. The pulse
occurrence time corresponds to the time of voltage polarity
conversion that means the high-frequency shock explained in
(i) causes these impulse current.
iii. The voltages also existed in the B-phase and C-phase, but the
amplitude is significantly lower than that of A-phase. The
comparison of the A-phase and B-phase voltage is shown in
Fig. 5d, which demonstrated a good phase congruency, it
means that the observed B-phase and C-phase voltage are
induced from the A-phase.
However, the voltage and current in stage II presented a different
pattern from stage I. The voltage rose to the system voltage after
the breakdown but then decreases and reverses, and the current is
very small at this time. Then the current increased rapidly to 4.7
kA, and the voltage fluctuated within small amplitude, as shown in
Fig. 5b. The following points can be seen from stage II.
i. Instead of maintaining the magnitude of the system voltage at
the time of breakdown, the line voltage decayed and became
negative after the breakdown. This indicated that the energy
was oscillating in the system inductance and capacitance
element, so the voltage had changed.
ii. The current rose sharply to 4.7 kA at 9:42:48, indicating a
short circuit happened in the system. Since the current
measured by the current transformer (CT) was mainly the
current flowing through the GIS busbar and the current in the
PT, and the melting hole on the GIS is the position where the
PT is mounted, it can be seen that a short circuit had occurred
at the PT.
It is possible to speculate about the cause of the fault from the
PMU record. The #2970 CB at the Maoming power plant side was
first opened and malfunction in this process, leading to that the
breakdown voltage of A-phase was significantly reduced to a value
below the operating voltage of the system. However, the
transmission line at the XPL station side was still connected to the
system, and the voltage across the 2970 CB is synchronised so the
fault did not appear from 9:00 to 9:40.
The line voltage no longer changes with the system voltage
when the CB at the XPL side opened at 9:40, so the voltage across
the 2970 CB shows periodic changes. As the breakdown voltage of
the 2970 CB dropped below the operating voltage, the CB contact
gap would be broken down when the voltage across it reaches a
critical value, and then the transmission line charged by the power
source at the plant side through the arc generated in the CB. Then,
the voltage of the transmission line would rise rapidly to the supply
voltage, and the impulse current appeared. The voltage of the
transmission line would remain unchanged until the next
breakdown after the arc extinguished.
The fault can be summarised as the repeated process that the
transmission line charged by the power source at the plant side
through the arc generated in the CB. Every time the CB contact gap
was broken down, a steep rising voltage would spread inside the
GIS. It can be inferred that the constant refraction and reflection of
Fig. 5 Data obtained by the PMU recorder system
(a) Three-phase voltage and Three-phase current in stage I, (b) Three-phase voltage and three-phase current in Stage II, (c) Comparison of station side voltage of the faulty CB and
the power plant voltage in Stage I, (d) Comparison of A-phase voltage and B-phase voltage in stage I
J. Eng.
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3

4. the steep rising voltage in GIS would initiate VFTO, and the
insulation of the PT would be destroyed by the VFTO at last. The
current becomes large because of the short circuit of the PT, and
then the CB exploded. This process will be validated by
simulations in Section 4.
3 Arc model for the repeated breakdowns
3.1 Black-box arc model
The significant characteristic of the black-box arc model is the
dynamic study of the arc conductance. The widely-used arc models
include the early Mayr model and Cassie model, which provide a
qualitative description of the arc in the high- and low-current
regions, respectively. The expression of the Mayr is illustrated in
(1) and Cassie model is illustrated in (2)
1
g
dg
dt
=
dln g
dt
=
1
τ
ui
P
− 1 , (1)
1
g
dg
dt
=
dln g
dt
=
1
τ
u2
Uc
2 − 1 , (2)
where g is the conductance of the arc, u is the voltage across the
arc, i is the current through the arc, τ is the arc time constant, P is
the cooling power, and Uc is the constant arc voltage.
Cassie model and Mayr model are under a different assumption,
and only one kind of heat dissipation is considered in these two
models. The main application of Cassie model is the large current
period before the current zero crossing, but the Mayr model is
suitable for a small current period with the current zero crossing. In
fact, the arc extinguishing process is carried out by the combination
of these two assumptions.
The KEMA model, which combined Cassie model and Mayr
model, can calculate the arc voltage itself, it is not entirely
dependent on the input parameters. This advantage implies that the
determination of arc voltage is more realistic, and the results will
be in good agreement with the laboratory measurements. The
differential equations of the KEMA model are described by three
series of modified Mary equation, which is used to represent the
three regions of the arc. The expression of the KEMA model is
shown in (1)
dgi
dt
=
1
Πiτi
gi
λi
ui
2
−
1
τi
gi, i = 1, 2, 3, (3)
where gi is the conductance of the ith arc; ui is the voltage across
the ith arc; τi is the time constant of the ith arc; Πi is the power
loss; λi is the Cassie–Mayr control of the ith arc. λ is 1 for Cassie
model, and λ is 2 for Mayr model.
The arc voltage U, arc current I and arc conductance g are
described as follows:
1
g
= ∑
i = 1
3
1
gi
, (4)
U = ∑
i = 1
3
ui, (5)
I = U ⋅ g . (6)
This model contains nine parameters with six of them being
empirically derived constants, and satisfies the following numerical
quantities (5) or relation (6)
λ1 = 1.4, λ2 = 1.9, λ3 = 2, (7)
τ2 =
τ1
K1
, τ3 =
τ2
K2
, Π2 =
Π3
K3
. (8)
K1, K2, and K3 are constants decided by the design of the breaker,
Π1, Π2 and τ1 are considered as free parameters, describing the
state of the CB.
3.2 Proposed model
To verify the aforementioned inference and make an accurate
analysis of the cause of this accident, an extended KEMA's
equation-based model is proposed in this study. The schematic
diagram of the extended KEMA's equation-based model for
simulations is depicted in Fig. 6. This proposed model is composed
of two parts. One is the control unit of CB breakdown, which
controls the breakdown of the CB contact gap; the other part is the
block of KEMA arc model, which describes the dynamic change of
the arc resistance during the extinguished process.
The control unit of CB breakdown includes the EMTP TACS
control, where the voltage across the CB and the breakdown
voltage as input parameters, the trip will be assigned then as an
output parameter to decide the TACSSWIT state. The function of
the CB breakdown control unit is implemented by the MODEL
module in ATP-EMTP, using Fortran programming language. By
comparing the breakdown voltage and the voltage across the CB,
this unit will send out a control signal at each time step.
The absolute value of breakdown voltage at each moment get
from the PMU recorder system is shown in Fig. 7, the breakdown
voltage at each moment is different due to the instability of the arc.
To make the simulation results more realistic, the breakdown
voltage in the simulation takes the actual value obtained from the
PMU recorder system.
The switch is expressed by TACSSWIT (Type 13) in the
simulation, which will get the signal from trip. The switch will
close when the voltage across the CB is higher than the breakdown
voltage as the output value of trip is 1, which represents the
breakdown of CB. The current starts flowing through it as the arc
occurs and the resistance is a very small value, which is set at 2 Ω.
After the arc continues to develop for 1 × 10−5 s [16], the output
value changes to 0 and the switch is opened.
Then the arc starts to extinguish, and the block of MODEL
works. Whenever there is the voltage across the CB, the resistance
value is calculated by the KEMA arc equation at each time step,
Fig. 6 Schematic diagram of the proposed model of CB
Fig. 7 Breakdown voltage at each moment
4 J. Eng.
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5. and the value of the output resistance is presented on the
TACSRES (Type 91). The arc will extinguish when the resistance
goes to infinite, then the thermodynamic control is complete. With
the change of the power phase, the voltage across the CB may
exceed the breakdown voltage again, and the thermodynamic
control will start due to a new arc occurring.
4 Failure analysis
4.1 Simulation on stage I
To analyse the fault accurately, the components in GIS must be
modelled properly. Since the VFTO would occur in the repeated
strike process and mainly contains high-frequency components,
most of the components have their capacitances that dominate the
other parameters.
Then the busbar and overhead line are modelled as a distributed
transmission line with a surge impedance of 63 and 300 Ω,
respectively. The power transformer is considered as the
combination of lumped capacitance and inductance. The SF6
bushing, disconnecting switch, PT, CT, and surge arrester are
modelled as a lump capacitance to ground. The equivalent model
and the specific parameter of the GIS component is shown in
Table 1 [17].
As the maximum operating voltage of the system is 252 kV, so
the amplitude of the voltage is set as follows:
U = 252 kV ×
2
3
= 205.757 kV . (9)
The PT model considers saturation effects; the excitation branch is
composed of the parallel resistance and the non-linear inductor.
The resistance represents the voltage transformer loss, which is
taken as 10 MΩ. The magnetising curve of voltage transformer is
shown in Fig. 8.
Based on the model established above, the failure process is
analysed using ATP-EMTP. The open time of the CB at the XPL
station is set to 0.1 s and the time step is set to 1 × 10−9 s. The
voltage waveforms of the faulty CB on the station side and the
power plant voltage from the simulation are shown in Fig. 9a for
comparison.
The results show that the line voltage is normal before the CB
at the XPL station side switching to open, and it is consistent with
the voltage on the power plant side. As the CB of the XPL station
side is open the line voltage changes. With the change of the
voltage at the power supply side, the gap of the CB contact will
continue to break down. During the repeated strike process of the
CB contact gap, the breakdown occurred near the peak of the
voltage. The line-side voltage remains constant between the two
breakdowns, forming a stepped voltage waveform similar to a
square wave, and the waveform frequency is the same as the
frequency on the power supply side. The variation characteristics
of the line-side voltage are the same as the voltage obtained from
the PMU recorder system. Every time the CB contact gap was
broken down, a steep rising voltage would generate in the line. Due
to the different structure and parameters of GIS circuit components,
the VFTO with high amplitude and steep front would appear. The
waveform diagram and spectrum diagram of the VFTO are shown
in Figs. 9b and d, respectively.
The current obtained from the simulation is shown in Fig. 9c,
the impulse current flows through the GIS busbar at the moments
of the sudden changing of voltage. The variation characteristics of
the current get from the simulation are the same as the current
obtained from the PMU record. However, due to the low sampling
frequency (about 4.8 kHz) of the PMU recorder system, some
pulse may not be noticeable in the recorded waveform.
The simulation results are in good agreement with the actual
situation in stage I, confirming the inference above towards the
failure. The stage I can be summarised as the power supply
repeatedly charges the capacitance of ground from the line. The
line-side voltage remains constant between the two breakdowns,
forming a stepped voltage waveform similar to a square wave. The
impulse current occurs at the time of gap breakdown, and the
VFTO with steep front and high amplitude would appear in the
repeated strike process.
4.2 Analysis of stage II
The recorded data showed that the current rose to 4.7 kA before the
fault occurred, indicating that the fault could be caused by the
overcurrent. As the current measured by the CT was mainly the
current flowing through the GIS busbar and the current in the PT,
and the melting hole on the GIS was the position where the PT is
mounted, showing that a short circuit had occurred at the PT. Based
on the simulation of stage I and the record data of stage II, the
damage of the PT may be caused by the VFTO.
Due to the steep front and high amplitude of VFTO, the voltage
distribution in the PT windings would be very uneven, especially at
the head end of the winding. Therefore, a high voltage would drop
at the head end of the winding, which would cause the insulation of
the winding inter-turn to be damaged.
If an inter-turn short circuit fault occurred, an additional closed
loop would be added in the winding, as shown in Fig. 10a. The
equivalent circuit for the shorted turns occurred in the winding is
shown in Fig. 10b, the induced electromotive force in shorted turns
is the source of current in shorted turns. As the impedance of the
shorted turns was very low, the current would be as high as several
times of the rated current [18, 19].
As the inter-turn short circuit caused the increase of the current,
the temperature of the shorted turns would increase rapidly. The
insulation of the surrounding turns would be damaged under high
temperature, resulting in more and more shorted turns. As the
current would also increase with the increase of the number of
shorted turns, the fault would expand rapidly and eventually lead to
the short-circuit fault of the PT.
In addition, when the voltage at the terminal of the PT goes
beyond line-to-ground voltage, the core of PT would carry out deep
Table 1 Component models and their parameters
Component Equivalent model Equivalent parameter
transformer inductance and
capacitance
L = 24 mH, C = 2500 pF
GIS busbar distributed
transmission line
Z = 83 Ω, V = 2.9 × 108 m/s
overhead line distributed
transmission line
Z = 300 Ω, V = 3 × 108 m/s
SF6/oil bushing capacitance to
ground
C = 300 pF
disconnecting
switch (closed)
capacitance to
ground
C = 80 pF
PT capacitance to
ground
C = 300 pF
CT capacitance to
ground
C = 20 pF
surge arrester capacitance to
ground
C = 19 pF
Fig. 8 Excitation characteristic curve of the voltage transformer
J. Eng.
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5

6. saturation, resulting in a sharp increase of the PT magnetising
current.
As the heat was increased with the square of the current, the
increased current led to a continuous increase in the heat of the PT.
Due to the high amplitude of the current, the heat of the PT would
increase rapidly.
However, the magnetisation of the core would be reduced with
the increase of the heat of the PT [20]. Then the magnetising
inductance would decrease with the reduction of the magnetisation
of the core, which would lead to an increase of the current.
Then the heat of the PT continued to accumulate, excitation
current continued to rise and this was a positive feedback
acceleration process. With the continuous increase of heat, the
thermal stability of PT was damaged, eventually led to the
occurrence of PT short-circuit fault.
The DC resistance of three-phase primary winding of damaged
PT was measured after this fault, and the results are shown in
Table 2. The results show that the DC resistance of A-phase is far
greater than the normal value, and the B-phase and C-phase are
normal, this indicates there is a breakdown in the primary winding
of A-phase.
The damaged PT was disintegrated then, it is found that the
winding was burnt black and some coils were broken, as shown in
Fig. 11. This proves the inferences mentioned above, indicating
that the accident was caused by VFTO.
5 Discussion
As GIS plays an important role in the power system, it is necessary
to study the fault cause of GIS. The results shown in this study
confirm that the cause of this certain fault is the incompletely open
of CB through theoretical analysis and related simulation analysis.
As this accident is very strange and the waveforms recorded in
the PMU recorder system are relatively rare, it is very meaningful
to study the mechanism of the generated waveform. Since the
recorded data presented a different pattern in two stages, it is
necessary to analyse the two stages separately.
To verify the cause of this fault, computer simulations and
further analysis are necessary. However, most of the black box
models are used to simulate the interruption process of CB and lack
the capabilities to realise the analysis of the repeated breakdown of
CB. The breakdown control unit is added in this study to extend the
KEMA arc model, and then the arc model is established to simulate
the repeated strike process of the CB contact gap. The simulation
results are in good agreement with the values in the actual case,
which verifies the soundness and applicability of the arc model.
The simulation reproduces the process in stage I. Since the
breakdown voltage between the two contacts of the CB is less than
the peak value of the system voltage, the contact gap will break
down twice in one cycle. The line-side voltage will remain constant
Fig. 9 Voltage and current obtained through the simulation
(a) Comparison of station side voltage of the faulty CB and the power plant voltage, (b) Voltage at the moment of breakdown, (c) Current through the CB from the simulation, (d)
Spectrum diagram of the voltage at the moment of breakdown
Fig. 10 Inter-turn short-circuit fault of winding
(a) An inter-turn short-circuit fault occurred in the winding, (b) Equivalent circuit for a
shorted turns occurred in the winding
Table 2 Primary winding dc resistances of the PT
Phase Measure value Normal value, kΩ Difference, %
A 50.80 MΩ 34 ± 0.5 49,410
B 33.58 kΩ 34 ± 0.5 1.24
C 33.58 kΩ 34 ± 0.5 1.24
6 J. Eng.
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7. between the two breakdowns, forming a stepped voltage waveform
which is similar to a square wave, and the high-frequency shock
happens in time of the voltage polarity conversion cause the
impulse current. The simulation results show that the repeated
strike process happen in stage I and the high-frequency shock was
VFTO,
Owing to the difficulty of simulation on stage II, the analysis of
stage II is carried out in the simulation on stage I and the recorded
data. Due to the steep front and high amplitude of VFTO, the
current will increase rapidly in the PT. The insulation of the PT
would be damaged and led to the occurrence of PT short-circuit
fault.
The reason for the fault is very clear now; the results indicate
the cause of this accident is the continuous discharge (VFTO) of
the contact gap in A-phase. Due to the rareness of this fault, this
study will have a great reference value to the engineering practice
and application.
In this study, the accident is analysed by the voltage and current
waveform before the accident occurs. To learn more about this
fault, the change process of voltage and current at the time of the
accident should be analysed. However, the relevant data cannot be
obtained due to the equipment has been burned, so it cannot further
reveal the fault process, and the conclusions in this study may have
some limitations. It is suggested that some research work can focus
on the development and application of the non-contact voltage and
current sensors, to improve the efficiency of accident analysis, and
to take the response measures.
6 Conclusions
This study analyses a 220 kV GIS explosion accident in the
Maoming power plant through simulation and practical record
information analysis. Through X-ray detection and disassemble
examination of the damaged CB, it is found that the A-phase
moveable contact was not opened completely. Combining the
relevant test and the PMU recorded waveform analysis, the result
indicates the cause of this accident is the continuous discharge of
the contact gap in A-phase.
Also, the arc is modelled by an extended KEMA's equation-
based model; the fault is reproduced by simulation in the ATP-
EMTP properly. The simulation results show that this fault is due
to the incompletely open of CB, which leads to the repeated
breakdown between the contact gaps. Every time the CB contact
gap was broken down, the VFTO with steep front and high
amplitude would occur in GIS and rushes into PT winding. The
repeated action of the VFTO for 3 min destroys the insulation of
the PT winding, resulting in the damage of PT. The amplitude of
the current becomes large because of the short circuit of the PT,
and then the CB exploded.
7 Acknowledgment
This work was supported by the Guizhou Power Grid Co., Ltd
Science and Technology Project (GZ2015-2-0034).
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Fig. 11 Damage of the PT winding insulation
J. Eng.
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