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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.