







































M. Rajabali Pour and M. Azimian /Future Energy                                                                       May 2022| Volume 01 | Issue 01 | Pages 16-23 

16 

 

 

 

Article 

Analysis of high voltage shunt capacitor bank 

over-voltage breakdown detection 
Mohsen Rajabali Pour1, Mohammad Azimian2* 

1Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Jalan Sultan Yahya Petra, 54100, 
Kuala Lumpur, Malaysia 
2Engineering Department, Razak Faculty of Technology and Informatics, Universiti Teknologi Malaysia, Jln Sultan Yahya 
Petra, 54100, Kuala Lumpur, Malaysia 

A R T I C L E   I N F O 
 

Article history: 
Received 10 March 2022  
Received in revised form 
11 April 2022 
Accepted 17 April 2022 
 
Keywords: 
Shunt capacitor banks, Component aging 
over-voltage breakdown, Detection method 
Mallet algorithm 
 
Corresponding author 
Email address: mazimian82@gmail.com 
 
 
DOI: 10.55670/fpll.fuen.1.1.11 

A B S T R A C T 
 

The traditional over-voltage breakdown detection method ignores the 
suppression of the inrush current of over-voltage breakdown, resulting in low 
over-voltage signal detection accuracy and a large detection deviation. As a 
result, a method is proposed for detecting and analyzing the ageing over-voltage 
breakdown of high voltage shunt capacitor banks. The breakdown model of 
aged components of the container group is built to determine the breakdown 
time of the container's overvoltage. Based on this, the annual load of the 
capacitor bank's aged component material is evaluated, and data on over-
voltage breakdown strength is collected. The Mallet algorithm is used to 
decompose and convert strength data into electrical signals. The high voltage 
shunt capacitor's over-voltage breakdown inrush current suppressor is built by 
combining the second-order under damping circuit and the voltage divider. 
Based on this, the parameters of HV shunt capacitor overvoltage breakdown are 
obtained, and the detection of HV shunt capacitor overvoltage breakdown of 
aged HV shunt capacitor banks is completed. The simulation results show that 
the proposed detection method's over-voltage breakdown output is in perfect 
agreement with the monitoring device's actual output and has ideal application 
performance. 

 
 

 
1. Introduction  

Capacitor faults in substations occur frequently, which 

pose a great threat to the safe operation of the power grid and 

cause great economic losses to power enterprises. The failure 

of shunt capacitors is related to the manufacturing level, 

operating conditions, and the reliability of the control 

protection device. The correct analysis of capacitor faults and 

the reduction of faults, and the improvement of grid reliability 

is of great importance to the improvement of economic 

benefits of power enterprises and the society [1-2]. 

Therefore, some good research results have been obtained for 

the breakdown detection of aged over-voltage of HV shunt 

capacitor banks in related fields. In literature [3], the over-

voltage dynamic control method of circuit breaker was 

proposed to detect the over-voltage breakdown of aged 

capacitor banks. Develop capacitor overvoltage breakdown 

strategy. The dynamic puncture characteristics of the contact 

gap of the shunt capacitor during the over-voltage phase are 

obtained by using the puncture characteristics measuring 

device. According to the dispersion of the over-voltage time 

between the capacitor and the shunt capacitor, the 

corresponding control strategy of the over-voltage 

breakdown of the capacitor is developed. This method is 

difficult to suppress the over-voltage inrush and easy to cause 

the damage to capacitor equipment in actual operation. 

Literature [4] is analyzed the capacitor group of internal 

components in the process of running through a string, the 

second string, and the breakdown of discharge voltage of fault 

phase capacitor transient variation. The peak value of 

discharge current is estimated. In the EMTP simulation 

software to establish the breakdown model of the capacitor, 

calculated and analyzed the breakdown of the element 

equivalent circuit parameters on the influence of the peak 

discharge current: The larger the resistance value is, the 

smaller the peak breakdown current will be. With the 

increase of resistance value, the decline of the peak 

breakdown current will slow down. 

 

 

Future Energy 

Open Access Journal 

https://doi.org/10.55670/fpll.fuen.1.1.11 

 

  

 

 

May 2022| Volume 01 | Issue 01 | Pages 16-23 

Journal homepage: https://fupubco.com/fuen 

 

ISSN 2832-0328 

mailto:mazimian82@gmail.com
https://doi.org/10.55670/fpll.fuen.1.1.11
https://fupubco.com/fuen


M. Rajabali Pour and M. Azimian /Future Energy                                                                       May 2022| Volume 01 | Issue 01 | Pages 16-23 

17 

 

In this paper, a breakdown model of aged components in 

the container group is constructed for the breakdown fault of 

aged capacitors. The Mallet algorithm is used to decompose 

the strength data and convert it into electrical signals. 

Suppress overvoltage breakdown magnetizing inrush current 

and complete the detection of overvoltage breakdown of high 

voltage shunt capacitor banks. The result of the simulation 

experiment verifies that the detection deviation of the 

proposed method is very small compared with the actual 

monitoring result and the simulation result, which indicates 

that the proposed method has certain practical value in 

engineering. 

2. Over-voltage breakdown detection of aged HV shunt 

capacitor banks 

2.1. The breakdown model of aging components in the 

container group was established 

When the aged element of the HV shunt capacitor bank 

operates under too high a voltage, the dielectric dissociation 

increases, and partial discharge may occur, which will 

increase the dielectric loss and lead to a dielectric breakdown 

in serious cases. The damage to the capacitor in operation is 

analyzed. The typical damage of capacitance aged element is 

the electric breakdown, and the fault feature is a short circuit. 

Assume that the running capacitor has a breakdown failure of 

an internal capacitance aged element. It is usually the 

individual components inside the capacitor that break down 

first, and the faulty component that breaks down becomes 

short-circuited, causing the faulty capacitor to lose a capacitor 

string. Suppose that the capacitance of a single capacitor is C. 

The total number of internal capacitance aging components in 

series is m series (4 strings in this paper). If the faulted 

capacitor has a series breakdown and the outer fuse is not 

fusing in time, the equivalent circuit of the faulted capacitor is 

shown in Figure 1. 

2.2. Breakdown sequence of capacitor overvoltage 

The over-voltage of high voltage shunt capacitors in 

different periods is realized according to a certain time 

sequence. Not in the same period, over-voltage said the timing 

of Figure 2. At the initial stage, the HV shunt capacitor is 

powered on, and the digital signal processor performs the 

reset operation. At this point, it is necessary to disconnect the 

contacts of power relays
1K ,

2K  connected to the HV shunt 

capacitor, and complete the initialization of different 

modules. In the second stage, the shunt capacitor enters the 

standby state. During this period, the SHUNT capacitor 

continuously detects the DC voltage of the HV shunt capacitor. 

When the DC voltage of the HV SHUNT capacitor circuit is 

greater than the high limit or less than the low limit, the 

operation starts from the initial stage again. Assuming that 

the DC voltage in the standby stage is always normal, the third 

stage will be entered. Relay contacts and 2K  are closed, the 

voltage and frequency of the HV shunt capacitor are detected, 

and the RMS value of the shunt capacitor voltage is calculated. 

After the delay time sT B, the effective value of capacitor 

output frequency and voltage is determined. Assuming that 

both items are in the normal range at this time, then the 

contact M is closed, and the HV shunt capacitor enters the 

power generation state after the completion of overvoltage 

over the same period. Assuming that the frequency and 

voltage of the network are abnormal, the HV shunt capacitor 

enters the initial stage. 

Cf

R1

Break f

R2

Cron

Vrf

i

 

Figure 1. Fuse fusing capacitor inside not outside string 

element breakdown equivalent figure 

 

Initial stage The third stageThe second stage

Check the output 

voltage and frequency 

of capacitor

Inrush suppressor 

access

Continuous detection 

of Capacitor DC 

voltage

3K1 2K K、  are closed
1 2 3K K K、 、 are break off are break off

 

Figure 2. The over-voltage sequence of high voltage shunt 

capacitor in different periods 

2.3. Breakdown strength data collection 

Prior to data acquisition, the annual load status of the 

capacitor bank material needs to be evaluated. Unit vector 

iterative search for each subregion of aging element material, 

and the iterative search objective function is: 

2

, , ,

1 1

= + exp( )
T N

M i i i t i i t i i i t

t i

E P P P    
= =

 + +                 (1)      

Where, 
i , 

i , 
i ,

i , and 
i  are the material loss 

Angle coefficients of conventional unit i  capacitor bank 

elements; T  for collecting time, divided into 24 hours a day, 

each time for one hour; ,i tP  refers to the micronized 

impurities of a unit i  in the t  period. The material fuel cost 

and annual load condition compensation of the aged 

components in the region were taken as the total cost of the 

breakdown strength data [5]: 

2

, ,

1 1

ch

, t d ,

1

( )

1 1
+ ( )

2

T N

C i i i t i i t

t i

T
D

Dch t Dch t

t D

F a b P c P

P K t W P t


= =

=

= + +

 + 




                         (2) 



M. Rajabali Pour and M. Azimian /Future Energy                                                                       May 2022| Volume 01 | Issue 01 | Pages 16-23 

18 

 

Where, 
ia , 

ib  and 
ic  are the environmental cost 

coefficients of the conventional unit i ; 
D  is the discharge 

efficiency of capacitor bank elements; ,Dch tP  is the 

discharging power of the elements of the capacitor bank at the 

time; 
tK  is the electricity price at time t ; t  is the discharge 

period; 
dW  is the discharge loss coefficient of capacitor bank 

elements; chD  is the discharge mechanical damage 

coefficient of capacitor bank components. Taking the daily 
load variance as the safety index of capacitor bank component 
materials, the average load of 24 time periods in a day is 
obtained as follows: 

,t , , ,

1

1
( )

T

D Ch t W t Dch t

t

P P P P P
T =

= + − −                                    (3) 

By obtaining the average load of aging component 

materials, the data signal acquisition function of the material 

breakdown strength of capacitor bank components is 

obtained: 

, 2

,t

1 ,

( )
T

W t C

V D

t M Dch t

P F
L P P

E P=

= − +                                           (4) 

Where, ,tDP  is the current load demand of aging 

element materials at a time t ; ,W tP  is the breaking elongation 

of capacitor bank elements at t  a time; ,Ch tP  is the 

depolarizing power of the material of the capacitor bank 
element at t  a time.  

Through the capacitor set of components material 
breakdown strength data acquisition function, can be a 
variety of biological damage and pollution degree to detect 
index classification data such as data collection[6].On the 
basis of obtaining the breakdown strength data of capacitor 
bank components, the signal conversion of the breakdown 
strength data is conducted. 

2.4. Breakdown of strength data signal conversion 

On the basis of data collection of material breakdown 

strength of capacitor bank components, signal conversion is 

carried out on the collected data. Mallet algorithm is used to 

decompose the data and reconstruct the decomposed data 

[6]. In order to eliminate the component material breakdown 

strength data of the capacitor bank will cause the deviation of 

the test result. The data on the material breakdown strength 

of capacitor bank components are processed. Firstly, the data 

of material breakdown strength of capacitor bank 

components are decomposed by wavelet, and the data of 

breakdown strength is converted by signal, and the wavelet is 

selected. The decomposition level N is calculated, and the 

breakdown strength data of capacitor bank elements are 

decomposed into N layers. The wavelet function needs to 

select an orthogonal wavelet, and the signal conversion 

formula of the breakdown strength data is as follows: 

( ) ( )j j n
V

n

X e x n e L 


−

=−

=                                           (5) 

Where, ( )x n  is the breakdown intensity data signal; 

( )jX e 
 represents a complex number, and the negative 

number varies with the angular frequency, and the negative 

number is used to represent the frequency domain signal of 

the breakdown intensity data;   is distributed between 

( , )− +  [6-7]. 

Secondly, the high-frequency coefficients in the 

breakdown strength data of capacitor bank components are 

detected to reduce the deviation: The breakdown strength 

data in the N layer are processed with a high-frequency 

coefficient [8-9]. 

Finally, one-dimensional wavelet reconstruction is 

carried out for the breakdown strength data: The low-

frequency coefficients in the N layer breakdown strength 

data and the processed high-frequency coefficients are taken 

as the basis. The breakdown strength data is reconstructed by 

a one-dimensional wavelet transform [9]. 

1 , , ,

1
( )j

t t c Ch t Dch t Trip t

D

S S P t P t S X e 


−= +  −  −         (6) 

,Trip tS S L=                                                                              (7) 

min maxtS S S                                                                           (8) 

Where, 
c  is the material detection efficiency of aging 

components, and
tS  is the mechanical damage coefficient in 

the period t ; ,Trip tS  is the material detection deviation of 

capacitor bank elements in the period t ; S  is the average 

mechanical damage coefficient per unit distance; L  is the 

breakdown strength; 
minS  and 

maxS  are the upper and lower 

limits of detection indexes respectively. 

Through the above operation, the parts of the material 

breakdown strength data of the capacitor bank elements that 

will cause the deviation of the test results are eliminated [10]. 

The online test result of material breakdown strength of 

capacitor bank components is more accurate. 

2.5. Overvoltageltage breakdown inrush current 

suppression 

The over-voltage breakdown inrush current suppressor 

of high voltage shunt capacitor is constructed by combining 

the second-order under damping circuit and the voltage 

divider. It is connected to the third winding of the capacitor to 

ensure that the voltage amplitude on different windings of the 

capacitor is different, but the phase is the same in order to 

eliminate the inrush current that occurs in different over-

voltage stages of the HV shunt capacitor. The specific process 

is described as follows: 

In the third winding of high voltage parallel capacitor and 

power connection between the inrush current limiter. 

Disconnect the main winding of the shunt capacitor on the 

low-voltage side of the transformer and energize the third 

winding of the capacitor with an inrush current suppressor. 

Steady-state alternating flux is generated in the transformer 

core in parallel with the capacitor: 

2 cos( )m t  = − +                                                                 (9) 

Where,  is over-voltage Angle, m is excitation inductance, 

 is capacitor reactance, and t is capacitor running time. 



M. Rajabali Pour and M. Azimian /Future Energy                                                                       May 2022| Volume 01 | Issue 01 | Pages 16-23 

19 

 

On the basis of the formula (1), implementation of 

capacitor main winding over-voltage electricity. Without 

considering the active power loss of the HV shunt capacitor, 

the relationship between excitation voltage u  and core flux 

  in the equivalent circuit of the capacitor is as follows: 

 2 sin( ) /mu U t d dt  = + =                                          (10) 

Where, u  represents the capacitor's power supply 

side voltage, 
mU  represents the electrified signal of the gate 

coil of the HV shunt capacitor, d  represents the iron core loss 

in the instant of the capacitor's over-voltage breakdown, dt  

represents the primary side leakage inductance, and the iron 

core flux chain   can be expressed as follows: 

cos( )m t C  = − + +                                                        (11) 

Where, C  represents the capacitance value. 

The flux linkage in the iron core of the HV shunt 

capacitor at different over-voltage periods is conserved, then: 

0
cos( ) cos( )m t tt C t   =− + + = +                             (12) 

Where, 
0t t=  represents the initial running time of the 

capacitor, then the magnetic bias in the iron core of the HV 

shunt capacitor at the moment of over-voltage breakdown is: 

0r C = =                                                                                      (13) 

The transient process of the HV shunt capacitor can be 

avoided by the above process. The third winding is 

disconnected at any time after the overvoltage breakdown of 

the HV shunt capacitor. At this point, there is no effect on the 

main magnetic circuit of the shunt capacitor, and the steady-

state operation can be entered. The inrush current 

suppressor is realized by a second-order under a damped 

system. The main reason for using this system is that the 

characteristic roots of the transfer function of the second-

order under a damped system are a pair of conjugate complex 

roots. Only the damping ratio of the second-order under a 

damped system needs to be set effectively. It can track the 

input voltage frequency and phase of each circuit of the HV 

shunt capacitor. The purpose of restraining the inrush 

current in each loop is realized. The input voltage source of 

the HV shunt capacitor is set as follows: 

2 sin( )i mu U t = +                                                              (14) 

The corresponding expected output voltage of the 

second-order under a damped system can be expressed by 

the following formula: 

0 2 [1 exp( / )]sin( )mu U t T t = − − +                                 (15) 

Where, T  is the time constant. The input signals ( )iU s  

and output signals 0 ( )U s  of each circuit of the HV parallel 

capacitor are Laplace transform [11]. There are: 

2 2

0 2 2 2

2
( ) 2

2 1
( ) 2 ( )

( 1/ )

i m

m

U s U
s

U s U
s s T




 


= +


 = −
 + + +

                        (16) 

Where, s  stands for sinusoidal current. The transfer 

function of the second-order under a damped system can be 

obtained from the above equation: 

0( ) ( ) / ( )iH s U s U s=                                                                   (17) 

By combining formula (8) and formula (9), it can be 

known that the transmission function of the second-order 

under a damped system is a function of   andT , and the 

variation interval of   is given by using the following 

formula: 

min max

min

max

0

2

  



 

 


=
 =

                                                                         (18) 

Where, 
min  and 

max respectively represent the 

maximum and minimum over-voltage angles. 

Based on the above considerations, the value T  should 

be appropriately increased: (1) When the value is large, the 

influence   can be minimized; When the value is small, the 

above second-order under a damped system can be regarded 

as an equivalent second-order system, and the equivalent 

second-order system has a lower design cost. (2) By 

increasing the value T , the steady rise of voltage amplitude 

on the third winding of the HV shunt capacitor can be 

guaranteed. The constant variation of the voltage amplitude 

at the output end of the capacitor leads to the change in the 

magnetic flux of the iron core, which reduces the possibility 

of causing the inrush current of the capacitor. The transfer 

function given in formula (9) is simplified, then: 

2 2

2 2
( ) 1

( )

s
H s

s a





+
 = −

+ +
                                                         (19) 

According to the design scheme of capacitor in surge 

suppressor described in the above formula, appropriate HV 

shunt capacitors, inductors and resistors are selected by the 

following formula to control the control characteristics 

required by different over-voltage of HV shunt capacitors. 

22

3

1 2

2

2

2

1 1
( )

N

N

UR

L T U

R R

L T

LC T



=


 +

=



= +


                                                                        (20) 

Where, 2NU  and 3NU  respectively represent the 

corresponding rated capacitance values of the capacitor two-



M. Rajabali Pour and M. Azimian /Future Energy                                                                       May 2022| Volume 01 | Issue 01 | Pages 16-23 

20 

 

phase winding and three-phase winding, L  representing the 

inductance and 
1 2R R R= =  represents the resistance. 

2.6. Parameter condition and detection of the over-

voltage breakdown of high voltage shunt capacitor 

Suppose that 
TU  the effective value of the primary 

voltage of the isolation transformer during the grid-

connected power generation stage of the high-voltage parallel 

capacitor 
LI  is the effective value of the current when the 

grid-connected power generation is at the maximum power 

and m  is the capacitor circuit regulation. The vector triangle 

relationship of the high-voltage shunt capacitor is given by 

the following formula [12]: 

2 2

1 2[ ( ) ]D T f m m LmAU U L L I= + +                                     (21) 

Where, f  is the speed, 
1mL  and 

2mL  are the 

inductance values of the shunt capacitor circuit, and 
2

TU  is 

the open-circuit voltage of the shunt capacitor 

2 2

1 22 [ ( ) ]T f m m L

D

U L L I
U

mA

+ +
=                                    (22) 

It can be seen from the above formula that when the 

high-voltage shunt capacitor outputs the equivalent 

maximum power, the smaller the m  value is, the higher the 

corresponding open-circuit voltage of the capacitor is, and the 

higher the voltage borne by the high-voltage shunt capacitor. 

It is assumed that the DC voltage of the high voltage 

shunt capacitor is low at the current stage, and m  reaches the 

maximum, and has no output power. At this time, the voltage 

can only maintain the operation of the shunt capacitor itself. 

At this time, formula (22) can be converted into [13]: 

min 2D TmU U=                                                                          (23) 

minDU  is used to describe the minimum open-circuit 

voltage of the shunt capacitor. 

According to relevant certification standards [14], the 

normal operation output frequency range of a high-voltage 

shunt capacitor is 48.5-50.5hz, and the range of single AC 

voltage is 186-240v. These two factors can constitute the AC 

side parameter conditions of the over-voltage breakdown of 

a high-voltage shunt capacitor. The shadow area given in Fig. 

3 is used to characterize the over-voltage breakdown area of 

the shunt capacitor. 

When the relay contacts 
1K  and 

2K  are closed, the 

high-voltage shunt capacitor should detect the effective 

value and frequency of the grid voltage in different periods. 

When the voltage frequency and effective value of different 

periods are in the shaded area in Fig. 3, it indicates that the 

power grid is in normal operation at this time, and the shunt 

capacitor is allowed to close the power relay contact 3K , 

and the high-voltage shunt capacitor will turn into the 

power output state after full over-voltage, otherwise, the 

contact will be disconnected Point 1K , 2K  capacitor into 

standby mode[15]. The output voltage of the high-voltage 

capacitor is detected after closing contacts 
1K  and 

2K . In 

order to make the output voltage stable enough time, 
dT  is 

required to be long enough. After many times of debugging, 

80dT ms=  is selected. After 
dT  a period, the average 

output voltage and whether the power grid is normal are 

determined. If the current period is normal, 
3K  is closed, 

and contacts 
1K  and 

2K  are disconnected. 

Voltage/V

Frequency/Hz

 

Figure 3. Over-voltage breakdown area of high voltage 

shunt capacitor 

2.7 Construction of online detection platform 

According to the data processing results, an online 

detection platform for the breakdown strength of capacitor 

bank components is constructed. The platform is mainly 

composed of a signal processing unit, signal receiving unit, 

and signal transmitting unit. The narrow pulse signal is 

transmitted from the signal transmitting unit to the 

capacitor bank component material. In the capacitor bank 

component material, the narrow pulse signal will 

continuously propagate and be affected by the breakdown 

strength of the capacitor bank component material, thus 

generating the reflection pulse corresponding to the 

breakdown strength. The signal receiving unit is set at the 

other end of the capacitor to receive the data and transmit 

the signal. The reflected pulse and transmitted pulse emitted 

by the unit are stored in the buffer of FPGA, and the data is 

transmitted to the upper computer by the serial port of 

FPGA. The reflected pulse is analyzed by the detection 

algorithm in the upper computer, and the breakdown 

strength of the capacitor bank component material 

corresponding to the reflected pulse is obtained, so as to 

realize the online detection of the breakdown strength of the 

capacitor bank component material. The specific detection 

process is shown in Figure 4. 

Cable 

status

Signal processing 

unit

Signal 

transmitting unit

Data 

signal 

collection

State judgment

Trend analysis

On line 

detection of 

breakdown 

strength data

 

Figure 4. Specific detection process 



M. Rajabali Pour and M. Azimian /Future Energy                                                                       May 2022| Volume 01 | Issue 01 | Pages 16-23 

21 

 

Among them, the hardware of the signal processing unit 

mainly includes a controller, analogue-to-digital conversion 

chip, and upper computer. The model of the controller is 

HDL Verilog, and the controller can work with various 

simulation tools; the specific model of analogue-to-digital 

conversion chip is AD7810, which is a low-power A/D 

conversion acquisition chip, which is powered by a single 

power supply, has the highest sampling frequency of 

100kHz and serial data interface, and can be connected with 

the upper computer to realize the hardware function of the 

signal processing unit. The software of the signal processing 

unit is the breakdown strength detection unit, which mainly 

uses the detection algorithm to analyze the reflected pulse 

so as to realize the detection of the breakdown strength of 

capacitor bank components. 

The signal receiving unit mainly includes a step delay 

chip and a programmable digital delay pulse receiver. The 

specific model of step delay chip is AD9501, which can 

support both CMOS level and TTL level and can achieve the 

minimum delay time of 10ps and the maximum trigger 

frequency of 50MHz; the programmable digital delay pulse 

receiver is mainly responsible for the reflection pulse 

corresponding to the breakdown strength Receive. 

The signal transmitting unit mainly includes a delay 

pulse trigger circuit and programmable digital delay pulse 

transmitter. The delay pulse trigger circuit mainly uses an 

A/D conversion chip to trigger the delay pulse, which is 

mainly composed of a voltage comparator, step wave 

generator, and oblique wave generator; the programmable 

digital delay pulse transmitter is mainly responsible for 

transmitting narrow pulse signal. Through the signal 

processing unit, the signal receiving unit, and the signal 

transmitting unit, the online detection of the breakdown 

strength of the capacitor bank components is realized. 

3. Experimental results and analysis 

3.1 Output results of aging over-voltage breakdown 

detection of high voltage shunt capacitor bank 

components 

Simulation calculation of capacitor breakdown in a series 

of time, the solid dielectric inside the capacitor is only a few 

tens of microns film, the breakdown of capacitor aging 

components belongs to voltage breakdown. The breakdown 

of a c-phase capacitor is simulated by the simulation model. 

The breakdown of capacitor aging components occurs at the 

time when the c-phase bears the peak voltage. The phase of 

the sinusoidal current on the capacitor is 90° ahead of the 

voltage phase at both ends of the capacitor. The current 

breakdown phase is approximately 0. In the simulation 

calculation. When the C phase is set at 9.8ms, the voltage at 

both ends of the capacitor is close to the peak value of phase 

voltage. The breakdown process is simulated as the control 

switch is closed. The breakdown point of a series of 

components of the capacitor is equivalent to the series branch 

of small resistance and small inductance. The main observed 

quantities are the current of the branch where the capacitor 

is broken down and the voltage and current of the three-

phase capacitor circuit. Figure 5 shows the current waveform 

on the breakdown capacitor branch during the breakdown 

process. Figure 6 shows the voltage waveform on the three-

phase capacitor. The positive peak voltage waveforms in the 

Figure are B, C, and A-phase capacitors respectively. The 

voltage on the c-phase capacitor drops rapidly and the voltage 

drop is 392V. The breakdown current waveform and voltage 

waveform of the three-phase capacitor in the process of over-

voltage breakdown are shown in Fig. 5 and Fig. 6. 

9 9.5 10 11.51110.5 12

200

400

600

800

1000

1200

1400

t/ms

I/
A

(a) Breakdown current waveform

9 9.5 10 11.51110.5
t/ms

200

400

600

800

1000

1200

1400

I/
A

(b) Breakdown current enlargement

 

Figure 5. The current waveform on the breakdown capacitor 

branch 

0 5 10 15 20 25 30
-10

-5

0

5

10

t/ms

U
/k

V

 

Figure 6. The voltage waveform of the three-phase capacitor 

during the breakdown 

3.2 Comparative analysis of simulation results and fault 

data recorded by a monitoring device 

In order to verify the effectiveness of the proposed 

method, the proposed method is used to detect the over-

voltage breakdown process of aging components of a real 

shunt capacitor bank, and the results are compared with the 

real monitoring results of the monitor. In order to monitor the 

failure process of capacitor bank, a power supply bureau in 

Guangdong Province. A high voltage shunt capacitor online 

monitoring device is installed on several capacitor banks with 

frequent faults. It can record a variety of fault data of the 

capacitor bank in time, including current and phase voltage 

data of each capacitor branch. The data sampling frequency of 

the monitoring device is 10 kHz. It can record various 

waveforms of the breakdown process quickly. Two 

breakdowns occurred in the capacitor. The effective value of 

capacitor C8 current increased from 55.2A to 83.4A. The 

online monitoring device of the shunt capacitor shows its 

fault and gives an alarm. The current of other capacitors has 

no obvious change. The effective value of the C8 current of the 

capacitor bank increased to 134.4A again after the capacitor 

bank was maintained in this working state for about 75 min. 

The current of other capacitors is basically unchanged. After 

the capacitor bank is in this state for about 200 ms, the 

capacitor bank is removed from the system. After the failure 

of the capacitor bank, it is detected. Faulty capacitor C. The 

variation of capacitance is 53.6 μF. According to the fault 

current records and post fault detection results of each 



M. Rajabali Pour and M. Azimian /Future Energy                                                                       May 2022| Volume 01 | Issue 01 | Pages 16-23 

22 

 

capacitor branch, it can be inferred that the first increase of 

C8 current is the performance of a series of capacitor aging 

components in its internal four strings, and the second 

current increase is the performance of a series of capacitor 

aging components in the remaining three strings. At this time, 

an unbalanced current is detected by neutral line unbalance 

protection. After 220 ms of protection setting value. The relay 

protection of capacitor bank acts. Make the vacuum circuit 

breaker of the capacitor bank open the capacitor bank at 

740ms. Fig. 7 is the fault recording of the breakdown of the 

internal capacitance aging element of a capacitor in phase C 

recorded by the monitoring device and the detection results 

of the method proposed in Fig. 8. 

96 100 104 108 112 116 120 124 128
-400

0

400

800

1200

t/ms

I/
A

 

Figure 7. Waveform recording of monitoring device 

 

100 104 108 112 116 120 124 128
t/ms

-400

0

400

800

1200

I/
A

 

Figure 8. Test results of the proposed method 

It can be seen from the fault waveform in figure 8. 

When a series of capacitor aging components are broken 

down, the current of the capacitor branch is about 0 before 

the breakdown, and then there is a high-frequency discharge 

oscillation for about 2 ms. The discharge front is very steep, 

and the first peak value is about 1050A. In terms of 

breakdown current peak value, oscillation frequency, and 

transition time, the above theoretical and simulation analysis 

are in good agreement with fault waveform records. 

4. Conclusions 

As the main reactive power compensation device of the 

substation, the shunt capacitor bank is in a high temperature 

and high voltage environment for a long time. Due to 

insulation aging and other reasons, the dielectric strength of 

the capacitor will gradually decrease. In addition, due to the 

instantaneous over-voltage between the electrodes of the 

capacitor, the weak components inside the capacitor may 

break down, resulting in capacitor damage. This paper 

presents an analysis method of the aging over-voltage 

breakdown of high voltage shunt capacitor banks. The 

simulation results show that the detection results of the 

proposed method are identical to the actual detection results, 

and the application performance is good. 

Ethical issue 
The authors are aware of and comply with best practices 

in publication ethics, specifically with regard to authorship 
(avoidance of guest authorship), dual submission, 
manipulation of figures, competing interests, and compliance 
with policies on research ethics. The authors adhere to 
publication requirements that the submitted work is original 
and has not been published elsewhere in any language. 

Data availability statement 
Data sharing is not applicable to this article as no datasets 

were generated or analyzed during the current study. 

Conflict of interest 

The authors declare no potential conflict of interest. 

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