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Switching Studies for Islington Substation Capacitor Banks with Surge Arresters
Vipashna Kaushik
Department of Electrical and Computer EngineeringUniversity of Auckland, Auckland, New Zealand
Abstract
The following report has been commissioned by
Transpower New Zealand Ltd. to make an assessment
on the existing and proposed surge arresters at Islington
Substation. Surge arresters are devices that protect
components from transient overvoltages caused during
capacitor switching. These arresters are installed on the
two existing 220kV capacitor banks and are proposed to
be installed on the two new 220kV capacitor banks in
order to protect them from switching transients.Switching studies were done with a circuit breaker
restrike occurring while isolating a capacitor bank. Two
surge arrester voltage ratings were analyzed; 198kV and
216kV in order to find which arrester would provide
more protection. Surge arresters were then placed
parallel to the capacitor banks in two different
configurations. The energy absorbed by the surge
arrester during a switching surge was calculated and
compared to its energy rating capability. The 216kV
surge arrester proved more suitable for the Islington
substation capacitor banks as the maximum energy
absorbed by this surge arrester was less than its energy
absorption capability of 1555.2kJ.
1. IntroductionThe Islington substation is a 220kV/66kV/33kV
substation located in the South Island. Transpower New
Zealand Ltd. is currently installing two capacitor banks
in addition to the existing capacitor banks to provide
voltage support to the surrounding areas at times of high
demand (refer to Appendix 1) [1]. These will be
switched on a daily basis to maintain control of the
system voltage levels during load changes. While such
switching has its advantages, it can also create high
voltage and current transients that can destroy
neighboring equipment in the substation. Surge arrestersare installed in such cases to protect the surrounding
equipment from such high surges.
A study had been carried out by the Power Systems
Consultants (PSC) in Wellington to report the effect of
capacitor bank switching on the proposed and existing
capacitor banks [1]. These studies however did not
include the existing or the proposed surge arresters. In
order to give Transpower an accurate picture on the
efficiency of the new surge arresters, the Islington
substation was modelled with surge arresters on
PSCAD.
2. ScopeThe intent of this report is to provide information on the
effect of capacitor bank switching on the selected surge
arresters. Two worst case scenarios were taken into
account: circuit breaker restrike and bus fault. The aim
of this report is to recommend whether the transientsgenerated during these scenarios exceed the energy
rating of the proposed surge arresters. The scenario ofrestrike has been discussed in detail in this report.
3. Islington SubstationA substation is a place where voltages are increased and
reduced for transmission and distribution purposes. They
also serve as connection points between different parts
of a transmission system. The Islington substation,
located in the South Island near Christchurch, is part of a
transmission and distribution system. As shown in
Figure 1, the southern generators at Tekapo, Ohao,
Benmore and Livingston are connected to Islingtonthrough 220kV lines. This is then reduced to 66kV and
33kV in order to distribute it near the Christchurch area.
Figure 1. Location of the Islington Substation
4. Transients in Power SystemsAn electrical transient is initiated whenever there is a
sudden change of circuit conditions. It is represented by
voltages and currents that are higher in magnitude and
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frequency than the system values. These transients
mostly occur when lightning strikes the transmission line
or when switching of a component is involved. The
period of a transient is normally very short; they can
however cause a lot of damage to the components in a
substation.
In this report the transients that are generated during theswitching of capacitor banks is focused on. The times
that transients occur during switching are while a) the
closing of a circuit breaker to energize the capacitor
bank or b) the opening of a circuit breaker to isolate or
de-energize the bank.
All components in a utility are made of capacitive and
inductive parameters. In an alternating current circuit,
energy is transferred cyclically between the inductances
and the capacitances of the circuit as the current and
voltage rise and fall at the frequency of the supply [2].
When a sudden circuit change takes place there is a
redistribution of energy in order to meet the new
conditions in the circuit [3]. However this redistributionof energy cannot take place instantaneously due to two
reasons: a) the voltage across a capacitor cannot increase
instantaneously without an infinite increase in current
and b) an instantaneous change in the current across an
inductor would require an infinite voltage to bring it
about [3].
5. Capacitor Bank SwitchingA capacitor bank is an assembly of capacitors and all
necessary equipment in one location [4]. They are
primarily installed in a transmission system for VAR
control and voltage control, while the secondary benefits
include an increase in the system capacity and a
reduction in power losses [4]. The components in a
substation are mostly inductive in nature and with the
addition of capacitor banks, the system losses are
reduced by improving the power factor of the system.
These capacitor banks are normally switched on during
peak loading periods and switched off during light
loading periods. Such connecting of a capacitor bank to
the bus line results in a rise in the voltage level of the
system. The switching of the capacitor banks thus adds
flexibility over the control of voltage and losses. This
switching is carried out with the help of circuit breakers
and an automatic control device that senses a particular
condition and disconnects or connects the capacitor
banks from the bus based on that condition [4].
There is however a problem that is associated with the
switching of a capacitor bank. Transient overvoltages
are always created during this switching.
Figure 2. Energizing a Capacitor bank.
Figure 3. Overvoltage produced across capacitor
The circuit in Figure 2 was simulated using PSCAD in
order to investigate the occurrence of overvoltages while
switching a capacitor. The circuit breaker is initially
open and closes 0.53sec after the circuit is simulated. At
this instant the voltage across the capacitor reaches a
peak value of 1.6pu as shown in Figure 3. This occurs
due to the redistribution of energy in the circuit.
5.1.RestrikeRestrike is a phenomenon that occurs while de-
energizing or isolating a capacitor during the zero
current crossing [5]. At this point the voltage will be at
its peak value and the capacitor is isolated while being
charged to the peak line voltage [5]. This voltage gets
trapped at the capacitor end of the circuit breaker and
half a cycle later, the voltage on the source side of the
circuit breaker is of the opposite polarity. There will
then appear a rise in voltage across the contacts of the
circuit breaker (TRV) and this voltage will overshoot to
a value which is equal to the difference between the two
contact voltages [5]. Due to the oscillatory nature of the
circuit, this overshoot will then damp and return to the
system voltage. In extreme cases, if the overshoot
exceeds the dielectric strength of the insulation of the
circuit breaker, there is a breakdown in the insulationand arcing results [5]. This is called a restrike of the
circuit breaker. This has been illustrated in Figures 4 and
5.
0.40 0.50 0.60 0.70 0.80
-90
0
90
capacitor voltage
0.002 0.00053.6
1.
0
5000.0
CB1
IaCIa
Ea
Time(s)
Voltage
(kV)
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Figure 4. De-energizing a capacitor bank
Figure 5. Capacitor restrike
5.2.Back-to-Back SwitchingThe switching of a capacitor bank that is connected in
parallel with one or more than one capacitor bank is
known as back-to-back switching. Generally back to
back switching of a capacitor bank when a capacitor
bank is already energized creates a higher magnitude of
transients [5]. In the case of the Islington substation
there are four capacitor banks that are connected in
parallel and switched alternatively depending on theload conditions.
The zero voltage that occurs at the moment of contact
closure when the second capacitor is energized makes it
appear to the system as a short circuit [5]. This
temporary short circuit will cause any energized
capacitor nearby to discharge into the second capacitor
[5]. In addition, the two capacitors in parallel appear as
a larger equivalent capacitance rather than one capacitor
alone, making the inrush current magnitude much larger
than for a single capacitor. However, when switching a
bank in parallel with and in close proximity to another
energized bank, the transient current is limited only by
the impedance between the banks [5]. This impedance istypically very low by comparison and therefore results in
much higher current values.
5.3.Grounded/Ungrounded Capacitor banksThe capacitor banks at Islington are connected in two
different configurations. The old capacitor banks C22 &
C25 have an ungrounded star connection and the new
capacitor banks have a grounded star connection. These
two connections have been illustrated in Figure 6.
Grounded capacitor banks provide a low impedancepath to earth from lightning surges and provide some
voltage surge protection [5]. Hence they are preferred in
substations that are effectively grounded such as the
Islington substation.
Ungrounded Capacitor banks need to have surge
arresters connected across them to protect them. The
absence of a physical connection to ground means that
the transients have no path to travel [5].
Figure 6. Grounded vs. Ungrounded capacitor bank
6. Surge ArrestersA surge arrester is a non-linear resistor that provides a
low-impedance path around the component being
protected [6]. It protects equipment from overvoltages
and surges that can result from circuit breaker operation
and faults in the system.
The non linear resistor has a property of diminishing its
resistance sharply as the voltage at its terminal increases
[3]. They are connected across the apparatus to be
protected and so experience the system voltage under
normal operating conditions. Their resistance is very
high so the power dissipation is minimal [3]. On the
incidence of a surge of voltage, the resistance falls
rapidly as the voltage increases, thereby diverting much
of the current and energy of the surge into the arrester
[3]. In Figure 7 below, a surge arrester has been added
to the earlier example in Figure 6. The addition of this
surge arrester connected parallel to the capacitor has
resulted in a reduction in the peak voltage across thecapacitor from 1.60pu to 1.40pu.
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Figure 7. Energizing in the presence of a surge arrester
Figure 8. Reduction in the initial overvoltage
Another important property of protective devices is their
ability to store and dissipate energy [6]. When current is
diverted into a surge arrester, and voltage is generated
across it, it is at the same time absorbing energy.
When metal-oxide arresters are energized, valve
elements of the arrester will absorb energy that results in
a temperature increase of the valve elements [6]. Under
normal operating conditions there is a balance between
the heat generated by the valve elements and the heat
dissipated by the arrester [6]. Overvoltage events disturbthis stable condition by causing the valve elements to
absorb increased levels of energy. If the temperature rise
of the valve elements due to energy absorption is too
high, the arrester can be driven into a state of thermal
runaway, resulting in further increase in valve element
temperature [6]. If the temperature of a valve element
reaches a high enough level, damage to the valve
elements can occur, leading to an electrical breakdown
and failure of the arrester [6].
The amount of energy an arrester absorbs depends on
the magnitude and duration of the surge. The surge
arrester should be capable of handling this energy by
either storing or dissipating it without damaging itself.
The surge arresters used at Islington are metal oxide
arresters. The surge arresters in this project were
modelled with two different voltage ratings: 198kV and
216kV. This was done to find the best possible rating for
the surge arresters.
7. Case StudiesThe Islington substation circuit is a complicated circuit
and it needs a power systems software in order to
analyze it. Power Systems Computer Aided Design
(PSCAD) was used for this purpose as it is an
Electromagnetic Transients Program (EMTP) software.
In order to study the complete effect of switching
overvoltages on surge arresters, a worst case scenario
was modeled on PSCAD. These cases involved restrike
occurring on the circuit breaker during the isolation of
capacitor banks C22 and C21. These cases have been
described in detail below:
7.1.Case 1A
Figure 9. Steps taken to complete Case 1A
Simulations were carried out on PSCAD with the
existing capacitor banks C22 & C25. A restrike occurred
on the circuit breaker connecting C22 to the 220kV bus
while isolating the capacitor bank (refer to Figure 10).
The duration of the restrike was 20mS and it occurred
half a cycle after the capacitor current is interrupted, on
phases A and C, while phase B remained unaffected
CBC22Ia
CBC22a
CBC22Ib
CBC22b
CBC22Ic
CBC22c
0.00050.002
0.0023.6 0.0005
0.0020.0005
3.6
3.6
10000000.0
Figure 10. Connection of Capacitor Bank C22
The device illustrated in Figure 11 controls the opening
of the circuit breaker on capacitor bank C22. The first
input slider labeled Open ABC is set to open the
circuit breaker at 0.5355sec. This is when the system
current is at the zero crossing. The next slider labeled
Restrike Start is set to start restrike at 0.1s. This is half
a cycle after the circuit breaker opens.
CASE 1A
Restrike
With Surge
Arresters
Without Surge
Arresters0.40 0.50 0.60 0.70 0.80
-90
0
90
capacitor voltage
0.002 0.00053.6
1.
0
5000.0
CB1
IaCIa
Ea
Time(s)
Voltage
(kV)
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TIME
A
B
Compar-ator
TIME
A
B
Compar-ator
Open ABC
D
+
F
+
Restrike Duration
A
B
Compar-ator
TIME
C+
D+
F
+
Open CB Control
CBC22a
CBC22c
CBC22b
Restrike start
Figure 11. Restrike Control for Case 1A
The last slider labeled Restrike Duration governs how
long restrike lasts and is set to 0.02s. As a result of this
control device, restrike only occurs on phases A and C,
while the circuit breaker for phase B opens with no suchoccurrence. The peak voltages affecting the capacitor
banks and other components were noted. This simulation
was then repeated in the presence of surge arresters
connected phase-to-ground. The results of this
simulation can are compared in Table 1.
7.2. Case 1B
Figure 12. Steps taken to complete Case 1B
Simulations were carried out with the two proposed 5th
harmonic filter banks (C21 & C26) and the existing
capacitor banks (C22 & C25). A restrike occurred on
circuit breaker connecting C21 to the 220kV bus while
isolating the capacitor bank (refer to Figure 13). In this
case restrike occurred on phase A half a cycle later and
was 20mS in duration. The same simulation was then
repeated in the presence of surge arresters. The
placement of these surge arresters was altered in the
following ways:
(i) Connected from the capacitor side of thecircuit breaker to ground (CapGnd).
(ii) Across the capacitor section (CapSec)
48 kV surge arresters were also placed across the tuning
reactors.
CBC21Ia
CBC21a
CBC21Ib
CBC21b
CBC21Ic
CBC21c
0.41
0.1125
3.6
0.41
0.1125
0.41
0.1125
3.6
3.6
Figure 13. Connection of Capacitor Bank C21
The sliders in the control device in Figure 14 are the
same as those for Case 1A. The difference is that for
Case 1B, restrike only occurs on phase A of circuit
breaker CBC21.
TIME
A
B
Compar-ator
TIME
A
B
Compar-ator
Open ABC
D +
F
+
Restrike Duration
A
B
Compar-ator
TIMEC
+D
+
F
+
Open CB Control
CBC21a
CBC21c
CBC21b
Restrike Start
Figure 14. Restrike Control for Case 1B
7.3.ResultsThe following are the results from the case studies Case
1A and Case 1B.
Without
SA(PU)
With
SA(PU)
220Kv Bus 1.59 1.48
66Kv Bus 1.23 1.19
33Kv Bus 1.52 1.44
C25 voltage 1.96 1.51
C25 Current 45.84 45.02
C22 Voltage 2.17 1.71
C22 Current 48.77 45.63
T3 Tertiary Line Voltage 1.12 1.1
SVC current 1.94 1.8
SVC 5th filter current 1.80 1.64
SVC 7th filter current 3.51 3.14
Table 1. Case 1A (existing capacitor banks)
CASE 1B
Restrike
With surge
arrester (SA)
Without surge
arrester (SA)
SA-capacitor to
ground
SA-capacitor
section
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Table 2. Case 1B (new and existing capacitor banks)
Figure 15. Circuit breaker voltage waveforms
Figure 16. Circuit breaker voltage waveform with SA
In Figures 15 and 16, the peak voltage across the circuit
breaker reduces from -657.38 kV to -429.56 kV. This is
the TRV voltage that builds up across the circuit
breakers during a restrike.
8. DiscussionAs can be noted from Table 1, surge voltages reduced
proportionally after the addition of surge arresters.Because a surge arrester is a non-linear resistor, a high
reduction in voltage is made only when the overvoltage
is extremely high. This is evident from the results in
Table 1 and 2.
It was observed that the currents in C22 & C25 were
very high while C22 was being de-energized. This
occurred due to the back-to-back switching of capacitor
banks C22 & C25. The factor limiting these inrush
currents is the inductance between these two banks [5].
However this impedance was very low (0.002 + 0.0005j)
and hence there was a very high frequency current of the
order 45.84pu and 48.77pu rushing in the existing
capacitor banks. In order to investigate this further, the
impedance was increased to 0.02 + 0.005j and the
magnitude of inrush current reduced to 22.68pu.
Changing the ungrounded connection of the old
capacitor banks to a grounded connection did not create
favorable results. The inrush currents remained the same
as the previous values.
It is not practically possible to change the connections of
these components at Islington without a substantial
investment being made in altering the connections.
These observations should however be taken into
account when installing new capacitor banks.
From Table 2, it can be noted that the inrush currents
entering the capacitor banks while de-energizing
capacitor bank C21 have reduced. This is due to the fact
that the new capacitor banks are 5th
filter banks and the
inductance of the tuning reactor is 0.41 + 0.1125j. This
high impedance limits the currents and protects the
capacitor banks. However, there is a rise in current in
the existing capacitor banks C25 and C22 after the
addition of surge arresters. The value for the tuning
reactor was increased to 0.41 + 0.1125j for the old
capacitor banks and this inrush currents reduced from
approximately 12.55pu to 1.93pu for both the capacitor
banks C22 & C25. This finding also supports thefindings in Case 1A mentioned earlier.
There was not any marked difference in the voltage
peaks after altering the placement of surge arresters
(CapGnd-CapSec). Both configurations provided the
same amount of protection for the circuit components.
Without
SA(P.U.)
With
SA(P.U.)
With
SA(P.U.)
CapGnd CapSec
220Kv Bus 1.17 1.07 1.06
66Kv Bus 1.10 1.06 1.06
33Kv Bus 1.07 1.03 1.03
C25 voltage 1.08 1.04 1.04
C25 Current 2.25 12.55 12.64
C21 Voltage 2.40 1.24 1.24
C21 Current 8.20 2.27 2.29
C26 Voltage 2.31 1.23 1.23
C26 Current 7.26 2.31 2.41
C22 Voltage 1.09 1.03 1.02
C22 Current 2.26 12.41 12.45
T3 Tertiary Line
Voltage 1.14 1.04 1.05
SVC current 1.64 1.33 1.34
SVC 5th filter
current 1.78 1.36 1.36
SVC 7th filter
current 1.59 1.39 1.39
0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700
-600
-300
0
300
600CB C21 Voltage
0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700
-600
-300
0
300
600
CBC21 Voltage
Voltage
(kV)
Voltage
(kV)
Time(s)
Time(s)
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9. Energy CalculationsThe energy being absorbed by the surge arrester (Table
3 & 4) was calculated by placing a voltage and current
probe at the arrester and measuring the Vpk and Ipk
entering this arrester. These two values were then
multiplied by the time it took the surge to settle down to
the system voltage. In the case of restrike, this was 20
ms.
Energy = Vpk* Ipk* t (1)
Measured
Parameters
Energy Absorption (kJ) for 216kV
and 198kV rated SA
Restrike(216kV) Restrike(198kV
C22 17.118 26.127C25 17.119 26.127C21 31.714 42.896C26 17.119 26.132
Table 3. Maximum energy absorbed during Restrike
Measured
Parameters Energy Absorption (kJ)
Bus Fault(216kV) Bus Fault(198kV)
C22 1240.204 5609.126C25 1240.204 5609.126C21 1246.613 6303.676C26 1246.562 6304.666
Table 4. Maximum energy absorbed during Bus Fault
The energy absorption capabilities of both the surge
arresters have been mentioned below:
SA Energy Rating (216kV) = 1555.2kJ
SA Energy Rating (198kV) = 1425.6kJ
The energy absorbed by the surge arrester during
restrike was less than that during bus fault. This was
mainly due to the difference in duration of these two
scenarios. The bus fault lasted 300ms while the restrike
only lasted 20ms. Also the peak currents produced
during bus fault were higher than the peak currents
produced during restrike.
The 216kV rated surge arrester would be a better option
to use, as the maximum energy absorbed by the surge
arrester in a worst case scenario was 1246.613kJ, which
is well below its energy rating of 1555.2kJ.
10.ConclusionsTransients that were generated during restrike have been
successfully controlled with the addition surge arresters.
The 216kV surge arrester performed better than the
198kV surge arrester.
Altering the placement of surge arresters did not supply
any marked differences. Either of the two connections
can be used for the new capacitor banks.
The peak voltage (TRV) across the circuit breaker
during a restrike reduced from -657.38 kV to -429.56
kV.
11.RecommendationsIt is recommended that in order to reduce the generation
of transients, inductances between capacitor banks
should be increased. Also controlled switching should be
incorporated in order to ensure that switching does not
occur at zero current crossings.
Future capacitor banks should be connected in a
grounded star connection.
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Acknowledgements
I would like to thank the following people:
Dr. Nirmal Nair: for his guidance and support during the
course of this project.
Mr. Nihal Kularatna: for his advice on how to start a
project effectively.
Deeksha Srivastava: who has been a pleasure to work
with these past few months.
12.References[1] Deller, R. (2005) Proposed B.E. Final year project
on 220kV Islington Capacitor bank switching studies,
Transpower New Zealand Ltd.
[2] Shwehdi, M. H. and Sultan, M. R. (2000) Power
Factor Correction Capacitors; Essentials and Cautions
Power Engineering Society Summer Meeting, 2000.
IEEE Volume 3, 16-20 July 2000 Page(s):1317 - 1322
[3] Greenwood, A. (1971) Electrical Transients in
Power Systems John Wiley & Sons, New York.
[4] IEEE guide for Application of Shunt Power
Capacitors (1992). IEEE std. 1036-1992. Transmission
and distribution committee of the IEEE Power
Engineering Society.
[5] IEEE Guide for the Protection of Shunt Capacitor
Banks (2000). IEEE std. std C37.99-2000. Power
System Relaying Committee of the IEEE Power
Engineering Society.
[6] IEEE Guide for the Application of Metal-OxideSurge Arresters for Alternating-Current Systems
(1997). IEEE std. C62.22-1997. Surge protectivedevices committee of the IEEE Power Engineering
Society.
Appendix 1
The single phase circuit in the column below illustrates
the connections of the capacitor banks at Islington.
They are all connected to each other through the 220kV
bus line. The existing capacitor banks are C22 and C25
and the proposed capacitor banks are C21 and C26.