Journal of International Council on Electrical Engineering
ISSN: (Print) 2234-8972 (Online) Journal homepage: https://www.tandfonline.com/loi/tjee20
The Protection System Optimization of 154kV
Shunt Capacitor Bank in Korea
Myeong-Hee Lee & Cha-Soo Park
To cite this article: Myeong-Hee Lee & Cha-Soo Park (2013) The Protection System Optimization
of 154kV Shunt Capacitor Bank in Korea, Journal of International Council on Electrical Engineering,
3:2, 141-145, DOI: 10.5370/JICEE.2013.3.2.141
To link to this article: https://doi.org/10.5370/JICEE.2013.3.2.141
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Published online: 10 Sep 2014.
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Journal of International Council on Electrical Engineering Vol. 3, No. 2, pp.141~145, 2013
http://dx.doi.org/10.5370/JICEE.2013.3.2.141
The Protection System Optimization of 154kV Shunt Capacitor
Bank in Korea
Myeong-Hee Lee† and Cha-Soo Park*
Abstract - The reactive power is the most required element to maintain the voltage of electric power
system. KEPCO continuously increases the installation of 154kV Shunt Capacitor (Sh.C) Banks to
maximize the utilization of the substation and transmission facilities by the compensation of the reactive
power. In 2010, Kepco operated 256 Banks' Shunt Capacitor. With increasing the number of 154kV Sh.C,
the fault and the operation stop also increase. For the last five years, the number of the fault and the
operation stop has been 223 and 140 cases of them are the result of the activation of the protection relay
caused by the overvoltage in CB closing and unknown reasons. To solve the problem, we analyzed data in
the protection relay. The result is that the improvement about the structure of existing Sh.C bank and the
protection system is required. So I standardized the structure of Sh.C bank system and the protection
system and applied them to new installation of Sh.C. In conclusion, electric power system requires the
standard structure of the facility and matched protection system. The change of electric power system
without an advance review of the protection system spreads the fault of the system, increases the
maintenance cost and obstructs the accomplishment of the purpose of facilities.
Keywords: Protection, Relay, Capacitor, Shunt Capacitor Bank,
In this thesis, I wish to introduce the current status of
154kV Shunt Capacitor Bank fault, the result of fault
analysis, following system structure standardization and
optimized protection system.
1. Introduction
In electric power system, the reactive power is one of the
most important factors to maintain potential of it. KEPCO
is increasing the number of 154kV Sh.C bank to maximize
the efficiency of transmission-substation facility by
compensating reactive power and, in the end of 2010, there
were 256 banks of Sh.C in 113 substations are in operating.
Furthermore, because of the delay of electric power system
construction and the concentration of load, 10banks of Sh.C
construction is expected in every year.
The result of categorization of fault and unknown shutdown of 154kV Sh.C by the cause for five years is as
follow. The number of the protection relay activation by
overvoltage in closing and unknown reason is 140,
occupying 63%, other reasons are Capacitor Cell fault, Fuse
Link fault etc.
154kV Shunt Capacitor Bank is important facility to
compensate reactive power and relieve instability of electric
power system. The stable operation of this facility is to
relieve the limitation of electric power system and reduce
huge operation cost of electric power system.
2. Current Status of the 154kV Sh.C Bank
Operation and the Fault
2.1 Operation Status
Table 1. 154kV Sh.C bank operation status.
Voltage
Sh.C installed
Substation No.
Num. of Bank
Capacity
154kV
113 substations
256 Banks
12,620[MVar]
2.2 Fault Trend
† Corresponding Author: Dept. of Transmission & Substation Operation,
Korea Electric Power Corporation, Korea (eelmhee@kepco.co.kr)
* Dept. of Electrical Engineering, Dong-Eui Institute of Technology,
Korea (packcs@dit.ac.kr)
Received: March 13, 2013; Accepted: March 14, 2013
Fig. 1. 154kV Sh.C bank fault trend for last 10years.
In last 10years trend, the fault has increased rapidly for
141
The Protection System Optimization of 154kV Shunt Capacitor Bank in Korea
last five years. And it's because the number of Sh. C has
increased for the same time.
3.2 Harmonics Analysis of Fault Current Wave Pattern
[Fig. 3] is the analysis of harmonics in closing current. It
shows that 6th harmonics is 3.5times larger than the normal
current.
The fact that in every Sh.C fault, the same wave pattern
is shown and the protection relay is activated by the
excessive 6th harmonics is the basis of the judgment that
the improvement of bank system structure or protection
system of Sh.C bank is required.
Table 2. The fault classifications by causes.
Year
Overvoltage in
closing
Unknown
Cell
fault
Fuse
cut
etc.
Total
[cases]
2006
11
5
4
4
0
24
2007
19
15
4
11
3
52
2008
13
11
9
10
1
44
2009
21
25
14
10
5
75
2010.4
17
3
3
2
3
28
Total
81
59
34
37
12
223
3. Fault Analysis
3.1 Wave Pattern Analysis in Sh.C Closing
As shown in the table 2, Protection relay activation by
the overvoltage in closing Sh.C occupies 36%(81cases) of
154kV Sh.C Bank fault cases. And, it is the reason to focus
on the analysis of the Sh.C closing condition.
Fig. 2 is the wave form of the fault current in closing
Sh.C Bank, caught by the activated protection relay. All of
the results collected from many substations in KEPCO
show the same pattern. And. this wave pattern shows that
because 154kV Sh.C Banks in KEPCO resonates with 6th
Harmonics, 6th harmonics included in closing surge
produces huge current in early time of Sh.C closing.
Fig. 3. Harmonics analysis of inrush current in Sh.C closing.
4. Consideration of Bank System Structure
4.1 Current Bank System Structure
Table 3. Bank system structures (Overview)
A Type
B Type
C Type
206 Bank
48 Bank
2 Bank
Fig. 2. Current wave pattern of 154kV Sh.C bank closing.
6th harmonics current in early time of Sh.C closing is
1.8∼3.4times larger than the normal current and this
abnormally large current activates over current factor in the
protection relay. And the current at N phase rises to the
maximum 173A and this can be the cause of the Resistor
factor activation.
There are 3 types of 154kV Sh.C Bank system structure
in KEPCO.
142
Myeong-Hee Lee and Cha-Soo Park
All types have a reactor in series with capacitors and a
resistor at the neutral point for restricting the inrush current.
The difference of them is that A Type uses 6parallels of 5
cell series structure and B and C type uses 2parallels of
8cell series structure.
So, each type has a different cell specification.
4.3 System Structure Improvement and Standardization
For above study result, KEPCO improved 154kV Sh.C
Bank system structure by following table.
Table 5. 154kV Sh.C Bank structure technical standard
Table 4. Each type's technical spec. calculation (one phase)
Items
C (1phase)
XL
Xc
Z
Vp
I
VL
Vc
Vcell
QL
Qc
Q
Qt
Resonance
Frequency
Formula
(161kV)
A Type
B,C Type
6parallels *
5series
2parallels *
8series
[㎌]
2πfL
1/(2πfC)
√(XL-Xc)2
VL/√3
V/Z
I×XL
I×Xc
Vc/series
I2×XL
I2×Xc
Qc-QL
3×Q
1/2π√(LC)
5.00
5.25
13.19
530.52
517.32
92.95
179.68
2.37
95.32
15.89
0.43
17.13
16.70
50.11
380.45
13.19
505.25
492.06
92.95
188.91
2.49
95.45
11.93
0.47
18.03
17.56
52.68
371.28
frequency
6.3
6.2
Items
current status
Improvement
designed voltage
nothing
rated voltage 170kV
discharge
characteristic
nothing
discharge characteristic
calculation report required
5. Protection System Improvement
5.1 Consideration Terms of Protection System Structure
As shown in table 3, different system requires different
protection system.
But, there has not been a specific technical standard of
protection system and have used the protection system
provided by 154kV Sh.C Bank makers. As a result, there
has been a lot of fault cases by the overvoltage in Sh.C
closing and protection relay activation with unknown
reason. So, the optimization of the Sh.C system is required.
Table 4 is the result with considering continuous
operation voltage as 161kV. But, the actual survey of 5
substations for a month showed higher than 161kV
operation voltage in heavy load time and there was a case
approaching 170kV in a flash. If Sh.C is operating over
170kV, even it is very short time, it can be accumulated,
change the mechanical and electric characteristic of Sh.C
and be a cause of the fault as a result.
5.2 154kV Sh.C Bank Structure Standardization
Bank System
Double-Y Type
161kV(Cont.)
170kV(Max.)
60Hz, 50MVar
4.2 Capacitor Discharge Performance Analysis
Reactor
Currently, KEPCO standard of discharge performance
follows IEC60871 and the discharge time of the capacitor
in KEPCO is calculated by following formula.
35[mH/Phase]
2.81kV
1,757A(2sec)
100kV
(RMS,
0.5 sec)
Discharge Time =
Capacitor
Internal Fused
3Φ 50MVar
Impulse : 750kV
KEPCO operation standard defines that Sh.C bank can be
turned on again after 5minutes from turning off. But,
calculating result with considering the resistance of
discharge coil in the capacitor is that the residual voltage of
capacitor drops to below 75V after 12 minutes(718sec) rest
time.
* Sh.C can be turned on again when residual voltage is under 75V
Resistor
20A, 50[Ω]
1,700A(Max.)
Fig. 4. 154kV Sh.C bank standard structure.
143
The Protection System Optimization of 154kV Shunt Capacitor Bank in Korea
Considering advantages and disadvantages of each type
in Table 3 and other requirements that each capacitor's fault
can be found easily, number of capacitor is small and
maintenance is easy, Double Y protection system is adopted
to the standard.
The reason to construct system 2parallels-8series is for
searching each capacitor cell fault more precisely. Existing
method compared each phase voltage of main bus to decide
capacitor fault. But improved method uses differential
current among each capacitor group.
6. Conclusion
Till now I introduced the 154kV Sh.C Bank system
improvement adopted to KEPCO and following protection
system optimization.
154kV Shunt Capacitor Bank has the role to relieve
voltage instability and transmission limitation by
compensating reactive power of electric power system.
Because these devices are not categorized to the direct
power supply facility, engineers do not have a strong
attention to them. But, they have a very importance in
complicated and organic electric power system.
The attention to them in electric power system and the
optimization of the system structure improves the stability
of whole electric power system and raise the efficiency of
power supply facilities. So, the importance of them is rising
with increasing complexity of the electric power system.
Thinking about the importance of this device, it is irrational
to apply the method decided by maker's simple review
without considering site-conditions.
In this thesis, I simply analyzed the causes of occurred
faults in the meantime and tried to seek a piecemeal
solution to prevent recurrence.
For the fundamental solution, the study of Sh.C bank
structure optimization to KEPCO electric power system and
following suitable protection system is required.
5.3 Protection System Optimization of Standard System.
The protection system suitable to this Double-Y structure
was decided by the basis of IEEE Std. C37.99 (2000)
recommendation and existing fault and protection
activation cases.
2 individual protection systems can guarantee the successful
protection even if one of them is unhealthy.
2 individual protection systems can be used as a primary
and secondary protection system and each protection has
different protection functions. Primary protection relay
(Relay 1) is for System Protection and secondary (Relay 2)
is for Bank Protection.
Table 6. System protection (Relay 1)
Activation factor
Protection function
Device No.
Phase OCR
Short circuit fault
50/51
Ground OCR
Inner bank ground
50/51N
Negative phase OCR
Mid-rack Flash Over
46
Harmonics OCR
Overload protection
50/51R
References
[1] KEPCO Transmission Operation Dept., 154kV Sh.C
Bank Fault Reviewing Report (2010.7)
[2] IEEE Std C37.99(2000), IEEE Guide for the Protection
of Shunt Capacitor Banks (R2006)
[3] Bothell, Washington, J. Lewis Blackburn, [Protective
Relaying Theory and Applications (Second Edition)]
(1997)
Table 7. Bank protection (Relay 2)
Activation factor
Protection function
OVR
System overvoltage
Device No.
59
UVR
Related system fault
27
Unbalance(N)
Unbalance (Neutral)
47
Unbalance(Ph)
Arcing Fault
60
Unbalance(L)
Inner fuse cut
60
OCR(R)
Resistor over-current
Myeong-Hee, Lee received M.S in
Electrical Engineering from Han-yang
University, and he works at Korea
Electric Power Corporation as a
General Manager of Transmission &
Substation Operation Dept.
50/51N
The difference of the protection function compared to
existing protection system is that each phase's current
differential factor is reinforced and each phase's parallel
condenser group's inter mid-rack flash over detection
function is added. Protection system reliability is improved
by adding precise protection function.
144
Myeong-Hee Lee and Cha-Soo Park
Cha-Soo, Park received M.S and Ph.D
in Electrical Engineering from Pusan
National University, and he is currently
a professor at Dong-Eui Institute of
Technology. His research interests
include plasma processing and Power
Systems
145