Putu Agus Aditya Pramana 2017 Inrush current investigation of capacitor bank switching for 150kV electrical system in Indonesia

2017 International Conference on High Voltage Engineering and Power System
October 2-5, 2017, Bali, Indonesia
Inrush Current Investigation of Capacitor Bank
Switching for 150kV Electrical System in Indonesia
Putu Agus Aditya Pramana, Aristo Adi Kusuma, Buyung Sofiarto Munir
Transmission and Distribution Department
PLN Research Institute
Jakarta, Indonesia
putu.pramana@pln.co.id
switching and the second one is for back to back switching.
According to [2], the maximum amplitude of inrush current
and inrush current frequency are 20kA peak and 4.25kHz,
respectively. These formulas are given in Table I.
Abstract—Circuit breaker (CB) installed for capacitor bank is
the CB which commonly experiences the switching process due to
the capacitor bank function as a voltage regulator. In closing
switching process, CB encounter transient condition due to
inrush current. In this paper, the simulation for studying the
inrush current characteristic of capacitor bank switching in
Indonesia has been performed. Simulation results show that the
inrush current for isolated switching complies with the IEC
standard. However, for the back to back switching, the inrush
current exceeds the standard’s allowable limit. By the
measurement data, the contact resistance value for the CB with
the back to back switching condition has a highly different value
for each phase, and it tend to broke the CB. To mitigate the
inrush current, damping reactor utilization has been studied and
the results showed that the damping reactor can minimize the
inrush current value by tuning the right value of damping
reactor inductance related to reactive power of capacitor bank.
Keywords—switching; circuit
damping reactor
I.
TABLE I. EQUATION FOR INRUSH CURRENT CALCULATION
breaker; inrush current;
Condition
Parameter
Isolated
capacitor
switching
Inrush
current (A)
Inrush
current
frequency
(Hz)
Back to back
capacitor
switching
INTRODUCTION
In the power system, capacitor bank is used to maintain the
operational voltage value always on the acceptable range.
Therefore, the circuit breaker for capacitor bank (CB) will
perform switching process frequently, which consists of
isolated switching and back to back switching. Isolated
switching is CB switching when there is only one capacitor
bank mounted on the busbar. Whereas, back to back switching
is CB switching when there are more than one capacitor
mounted on the same busbar[1].
When a closing switching process is performed, CB will
encounter transient condition which called as inrush current.
The inrush current has a higher order of frequency (commonly
on kHz order) which called as inrush current frequency. The
inrush current value is affected by the capacitance, inductance,
and resistance value of the electrical system [1][2]. In this
paper, the inrush current characteristic of 150kV system in
Indonesia has been evaluated, including the impact of inrush
current to the CB and the solution to mitigate the inrush
current.
Information
The standard reference for calculating the inrush current
that caused by the capacitor bank switching is given in [1]. The
inrush current calculation in this standard is divided into two
conditions. The first condition is calculation for isolated
Eq.
Number
‫ܫ‬௣௘௔௞ ൌ ξʹඥ‫ܫ‬௦௖ ‫ܫ‬ଵ
(1.a)
‫ܫ‬௦௖
݂ ൌ ݂௦ ඨ
‫ܫ‬ଵ
(1.b)
‫ܫ‬௣௘௔௞
Inrush
current (A)
ܷ௥ ݅ଵ ݅ଶ
ൌ ͳ͵ͷͲͲඨ
݂௦ ‫ܮ‬௘௤ ሺ݅ଵ ൅ ݅ଶ ሻ
(2.a)
Inrush
݂௦ ܷ௥ ሺ݅ଵ ൅ ݅ଶ ሻ
current
݂ ൌ ͻǤͷඨ
(2.b)
frequency
‫ܮ‬௘௤ ሺ݅ଵ ݅ଶ ሻ
(kHz)
݂௦ is system frequency (Hz)
‫ܮ‬௘௤ is equivalent inductance (uH)
݅ଵ ǡ ݅ଶ are current in capacitor 1 and capacitor 2
respectively (A)
‫ܫ‬௣௘௔௞ is the peak value for inrush current (A)
ܷ௥ is rated voltage (kV, rms)
‫ܫ‬௦௖ is short circuit current (A, rms)
The Eq. (1.a) and Eq. (1.b) are used to calculate the inrush
current and the inrush current frequency for isolated switching,
respectively. Eq. (2.a) and Eq. (2.b) are used to calculate the
inrush current and inrush current frequency for back to back
switching. The value of ݅ଵ and ݅ଶ are calculated by the Eq. (3)
where ‫ܥ‬௜ is the capacitance value of i-th capacitor bank in
Farad (F), ߱௦ ൌ ʹߨ݂௦ is the system angular frequency in rad/s,
and ‫ܮ‬௦ is source inductance in Henry (H).
ఠ ஼௨
݅௜ ൌ ଵିఠೞ మ೔௅
ೞ
II. STANDARD CALCULATION FOR INRUSH CURRENT
Equation
ೞ ஼೔
(3)
Leq in Table I is the total summation value of the capacitor
1’s inductance (‫ܮ‬௖ଵ ሻ, capacitor 2’s inductance (‫ܮ‬௖ଶ ), line
inductance between capacitor 1 and CB capacitor 1 (‫ܮ‬ଵ ), line
inductance between capacitor 2 and CB capacitor 2 (‫ܮ‬ଶ ), and
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line inductance between CB capacitor 1 and CB capacitor 2
(‫ܮ‬௕௨௦ ). The typical values for Leq are given in Table II.
TABLE II.
TYPICAL VALUE FOR Leq
Rated Voltage
(kV)
Typical inductance value
between capacitor bank, Leq
(uH)
ч 17.5
36
52
72.5
123
145
170
245
10 s/d 20
15 s/d 30
20 s/d 40
25 s/d 50
35 s/d 70
40 s/d 80
60 s/d 120
85 s/d 170
Fig. 2. Source impedance distribution
III. MODEL AND SIMULATION
A. Source Impedance
From the Table I, to calculate the inrush current and inrush
current frequency, it needs to know the source impedance value
of the capacitor bank. The source impedance represents the
equivalent impedance of the system in front of the CB
capacitor’s busbar and its value can be derived from the short
circuit study on that busbar. Fig. 1 shows the illustration to find
the source impedance.
The vertical axis on Fig. 2 represents the number of CB
busbar which has source impedance value given in horizontal
axis. Based on Fig. 2, it can be seen that the source impedance
ranges from 2ȍ to 50ȍ. Most of the source impedance values
are distributed from 2ȍ to 22ȍ. Then, this source impedance
will be used to calculate the inrush current.
B. Inrush Current for Isolated Switching
Based on Eq. (1.a) and Eq. (1.b) in Table I, the inrush
current and inrush current frequency for isolated switching are
given in Fig. 3 and Fig. 4, respectively.
Fig. 1. Illustration in finding source impedance value
Fig. 1 (a) depicts a complex electrical system which
consists of Bus A that is connected to four bay. Each of bay is
connected to System A, System B, System C, and Capacitor
Bank. To get the simplification of this complex electrical
system, we perform short circuit study in Bus A. Once the
short circuit study is done, we can get the source impedance
and equivalent source as depicted in Fig. 1(b). In the next
calculation of inrush current value, we will use the simplified
model of the electrical system as given in Fig. 1(b). By this
short circuit study we can get the distribution of source
impedance of 150kV electrical system in Indonesia as given in
Fig. 2.
Fig. 3. Inrush current for isolated switching
Fig. 4. Inrush current frequency for isolated switching
Fig. 3 and Fig. 4 show the inrush current and inrush current
frequency for three different source impedance values which
are Z=4.6ȍ, Z=25ȍ, and Z=50ȍ. According to the Fig. 2,
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these source impedance values are chosen to represent the
source impedance values of 150kV electrical system in
Indonesia.
Based on Fig. 3 and Fig. 4, it can be seen that the effect of
source impedance differences to inrush current will be more
visible if the reactive power of capacitor bank is larger. The
maximum difference of inrush current value for source
impedance Z=4.6ȍ and Z=25ȍ is about 12% for the
100MVAR capacitor bank. In the case of isolated switching, it
can be seen that the maximum inrush current is 4.5kA for
capacitor bank with capacity from 10MVAR to 100MVAR.
This value is still complies with the maximum value set by
reference [2], which is 20 kA. Likewise, the maximum value of
inrush current frequency is 1.1 kHz and this value is still lower
than the maximum value set by reference [2], which is 4.25
kHz.
C. Inrush Current for Back To Back Switching
In the back to back switching condition, the capacitor bank
is switched close on a bus when there are already one or more
capacitor banks that have been connected to the bus. Equation
used to calculate inrush current and inrush current frequency
for back to back switching are given by Eq. (2.a) and Eq. (2.b)
in Table I. The relationship between inrush current and the
ratio of capacitors bank reactive power (which is switched back
to back) is given in Fig.5.
axis is "2" and blue line is being reviewed, so the magnitude of
capacitor bank reactive power is 20MVAR and 10MVAR.
Thus, the magnitude of inrush current frequency will be
22kHz. It can be observed that the magnitude of the inrush
current frequency for back to back switching will decrease with
an increase in the value of the capacitor bank ratio. In addition,
the inrush current frequency is also reduced for back to back
switching of capacitor bank with larger reactive power value.
Fig. 6. Inrush current frequency for back to back switching
From Fig. 5 and Fig. 6, it can be seen that the inrush current
magnitude for back to back switching of capacitors bank with
reactive power higher than 25MVAR will produce inrush
current higher than 20kA (maximum inrush current limit
according to [2] is 20kA). On the other hand, back to back
switching of capacitor bank with reactive power higher than
10MVAR will produce inrush current frequency higher than
4.25kHz (maximum inrush current frequency limit according to
[2] is 4.25kHz).
D. Effect of Inrush Current
Inrush currents magnitude which higher than the standard
value may cause degradation in the CB [3][4][5]. CB
degradation can be indicated by the significant difference of the
contact resistance at each phase of CB, as given in Table III.
TABLE III.
Fig. 5. Inrush current for back to back switching
CONTACT RESISTANCE VALUE OF BACK TO BACK CB
Fig. 5 shows the magnitude of inrush current due to back to
back switching. The horizontal axis represents the reactive
power magnitude ratio of the capacitor bank being switched to
the capacitor listed in the legend. For example, if the horizontal
axis is "2" and blue line is being reviewed, so the magnitude of
capacitor bank reactive power is 20MVAR and 10MVAR.
Thus, the magnitude of inrush current will be 15kA. It can be
observed that the inrush current for back to back switching
increases with an increase of the capacitor bank ratio. In
addition, the inrush current is also increased for back to back
switching with an increased the capacitor bank reactive power.
Fig. 6 shows the inrush current frequency due to back to
back switching. The horizontal axis represents the reactive
power magnitude ratio of the capacitor bank being switched to
the capacitor listed in the legend. For example, if the horizontal
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Table III shows the contact resistance of the CB that
encounter back to back switching during operation. These
values are collected from the CB routine maintenance data, and
showed an indication of the non uniformity of the contact
resistance (on R, S, and T phase of CBs). For comparison, the
contact resistance of CB for isolated switching is given in
Table IV.
TABLE IV. CONTACT RESISTANCE VALUE OF ISOLATED CB
The CB contact resistance for each phase in Table IV tends
to be more uniform when compared to the contact resistance in
Table III. The non uniformity of the contact resistance in Table
III can be used as an early indication that inrush current for
back to back switching will potentially degrade the CBs
[3][4][5]. Therefore, a tool for reducing inrush current is
required.
E. Mitigation of Inrush Current Using Damping Reactor
Currently, there are several methods that can be used to
reduce inrush current value such as pre insertion resistor
addition, controlled switching, or damping reactor addition
[6][7][8]. This study focused on inrush current mitigation using
damping reactor due to the other methods limitation.
Controlled switching requires good coordination between the
equipment that control the switching process, therefore the
success of reducing inrush current is depends on the condition
of the switching controller which commonly is an electronic
device [8]. On the other hand, utilization of pre insertion
resistor only reduce the magnitude of inrush current without
reducing the magnitude of inrush current frequency due to back
to back switching [7].
It is important to use a proper reactor specification to
reduce the magnitude of inrush current. Selection of reactor
specification is based on the result of transient simulation. The
effects of damping reactor addition to the inrush current of CBs
are given Fig. 7, Fig.8, Fig.9, and Fig.10.
Fig. 7. Inrush current with damping reactor for 25MVAR capacitor bank
based
Fig. 8. Inrush current frequency with damping reactor for 25MVAR capacitor
bank based
Fig. 7 and Fig. 8 show the magnitude of inrush current and
inrush current frequency for back to back switching with
damping reactor addition. The horizontal axis represents the
reactive power magnitude ratio of the capacitor bank being
switched to 25MVAR capacitor bank. For example, if we
perform a back to back switching with a ratio of “2” (meaning
back to back between 25MVAR and 50MVAR capacitors
bank), then when 330uH of damping reactor is used on each
capacitor (graph with blue line), it will be obtained the inrush
current value is about 9kA and inrush current frequency about
5kHz. Similarly, when a reactor with an inductance value of
660uH is used, it will be obtained an inrush current and inrush
current frequency about 7kA and 4kHz, respectively. Then, if
880uH damping reactor is used, it will be obtained an inrush
current and inrush current frequency about 6kA and 3kHz,
respectively. On the other hand, for capacitor bank based value
of 50MVAR, the effects of adding the damping reactor to the
inrush current on the CBs are given Fig.9 and Fig.10.
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back switching occurs for capacitors with reactive power
higher than 25MVAR, the inrush current will exceed the limits
permitted by [2]. Thus, the effect of inrush current may be
indicated by the significant difference of contact resistance in
all CBs phases. Therefore, mitigation can be performed by
installing damping reactor in series on each capacitor bank with
the specifications given in Table V.
TABLE V. DAMPING REACTOR SPECIFICATION FOR BACK TO BACK
SWITCHING
Fig. 9. Inrush current with damping reactor for 50MVAR capacitor bank
based
V. CONCLUSION
An investigation of inrush current characteristic for 150kV
electrical system in Indonesia has been performed. The results
show that the inrush values comply with the IEC Standard for
isolated switching. However, for back to back switching, the
inrush values may exceed the limit of CBs withstand capability
which designed using IEC Standard. Inrush current and inrush
current frequency for back to back switching can be reduced by
using damping reactor on each capacitor bank.
REFERENCES
Fig. 10. Inrush current frequency with damping reactor for 50MVAR
capacitor bank based
[1]
Fig. 9 and Fig. 10 show the value of the inrush current and
inrush frequency in the back to back switching using damping
reactor. The horizontal axis represents the reactive power ratio
of the capacitor bank being switched to 50MVAR capacitor
bank. For example, if we perform a back to back switching
with a ratio of “2” (meaning back to back between 50MVAR
and 100MVAR capacitors bank), then when 330uH of damping
reactor is used on each capacitor (graph with blue line), it will
be obtained the inrush current value is about 14kA and inrush
current frequency about 3.5kHz. Similarly, when a reactor with
an inductance value of 660uH is used, it will be obtained an
inrush current and inrush current frequency about 10kA and
2.5kHz, respectively. Then, if 880uH damping reactor is used,
it will be obtained an inrush current and inrush current
frequency about 6.5kA and 2kHz, respectively.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
IV. DISCUSSION
The simulation result shows that the characteristic of
isolated switching for 150kV electrical system in Indonesia
complies with the standard value. However, when the back-to-
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