Energy Stress on Surge Arrester

8/12/2019 Energy Stress on Surge Arrester

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CALCULATION OF ENERGY STRESS ON SURGE ARRESTERS IN

275kV TRANSMISSION LINES

R. Bhattarai*, H. Griffiths, N. Harid and A. Haddad

High Voltage Energy Systems Group, School of Engineering, Cardiff University, UK

*Email: [email protected]

Abstract: Application of line surge arresters is found to be an efficient tool to improve the

lightning performance of transmission lines. Suitable selection of arrester rating and configurationalong the line are crucial for achieving improved reliability of the line. This paper presents a

systematic calculation of energy stress carried out on gapless Zinc-Oxide surge arresters applied to

a 275 kV double circuit shielded transmission line. An operational line was modelled in TFlash,

and simulations were carried out under conditions of lightning strokes hitting the phase conductor

and the shield wire. To determine arrester characteristics, a number of important parameters that

influence energy stress calculations were evaluated. The arrester failure performance analysis wascarried out to estimate the overall reliability of the transmission line. Stroke to phase conductor

was found to be the main source of arrester failure in the line.

1. INTRODUCTION

Faults caused by lightning are the main source of line

outages especially in areas with high ground flash

density, high earth resistivity and poor shielding.

Previous studies [1-3] have shown that lightning

performance can be guaranteed by careful selection and

location of zinc-oxide (ZnO) surge arresters. These linearresters are exposed to high-magnitude lightning

strikes and have to survive higher energy discharge duty

imposed by the lightning current.

In comparison to the substation arrester, the line arrester

may experience more energy stress. This is because the

incoming surge to a station is limited either by lineinsulator flashover or by the discharge to earth through

shield wire. Therefore, adequate selection of a line

arrester also depends upon assessing its energy

absorption capability so that it does not fail under

conditions of lightning striking either the phase

conductor or the shield wire.

In this study, a systematic calculation of energy stress

was carried out on gapless ZnO surge arresters installed

on a 275 kV double circuit shielded transmission line.The line was modelled in EPRIs TFLASH program

which is designed to examine all arrester options andtheir potential benefits to improving line performance

[4]. The simulation was carried out under conditions of

lightning stroke hitting the phase conductor (shielding

failure) and the shield wire (which may lead to

backflashover). In both cases, the energy stress in the

arrester depends on the line and lightning strokeparameters. A parametric analysis was carried out to

select the appropriate arrester, considering different

parameters that affect its energy calculation. Using these

studies, it is possible to determine an optimum

application of surge arresters, and make a more accurateselection of arrester rating in terms of protective level

and energy stress. Furthermore, arrester failure

performance analysis was carried out to assess the risk

of failure due to lightning strikes onto the phase

conductor and shield wire of surge arresters installed on

the line. A statistical simulation was performed to

calculate the risk of arrester failure on the line.

2. SIMULATED DATA

2.1. 275kV transmission line

An existing 35 km long, 275 kV double circuit shielded

line was considered in this study. The line with 300 m

span length was assumed to be located on flat terrain

with ground flat density of 0.5 flashes per kilometre

square per year (flashes/km2/year). Figure 1 show thetower structure and conductor geometry in each tower

of the line.

Twin 175 mm2 Lynx type ACSR conductors wereused in phase conductors and a single Lynx ACSR

conductor was used as shield wire. The diameter of each

conductor is 19.53 mm, and the bundle spacing of 30.48

cm was used for the twin phase conductors.

A 3.31 m long line insulator string composed of 16

individual glass insulator disc producing an overallcritical flashover voltage (CFO) of 1646 kV was used.

2.2. Surge arrester

The following specifications of zinc-oxide surge

arresters were used.

Nominal discharge current : 10 kAMax. continuous operating voltage (MCOV) : 220 kV

Energy capability : 7.8 kJ/kV of MCOV

Table 1 summarises the arrester V-I curve under an 8/20

impulse.

Table 1: Arrester discharge voltage for 8/20 impulsecurrent.

I (kA) 3 5 10 15 20 40

V(kV) 581 601 635 666 690 762

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Copyright c 2009 SAIEE, Innes House, Johannesburg

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3. TFLASH MODEL

Modelling transmission line components in TFLASH is

facilitated with its existing database of towers, earth

types, insulators, conductors and arresters [5]. Facilities

are provided within the program to allow further input

and modifications of the above components if required.

A brief description of the line model used in this study

is given in the following sections.

3.1. Line model

Each span on the transmission line is represented as a

multiphase untransposed distributed parameter line

section. In order to avoid reflections in the line,matching impedances were used at both terminations of

the line. Each span section is further divided into

smaller sub sections to enable stroke simulation at anumber of points along the spans. In this model, 8

towers from the middle of the line were used in each

direction with lightning surge striking at a mid tower

position.

A high-voltage transmission tower can be considered as

a network of short transmission lines carrying transientcurrent from its top to the earth and its reflection back

towards the top [5]. Therefore, in this study, the tower

itself is modelled as a short vertical transmission line

section with constant surge impedance describing the

voltage produced on the tower, per unit current flowing

through it. The steel lattice tower has surge impedance

of 173.1 calculated using Equation (1) [6].

+=

21

avg1

Thh

rtan5.0cotln60Z (1)

where, h1 and h2 are the tower heights from base to

midsection and midsection to the tower top respectively

and ravg is the weighted average tower radius given byEquation (2).

( )

)hh(

hrhhrhrr

21

1321221avg

+

+++= (2)

where, r1, r2 and r3 are the radii at the top, midsection

and base of the tower respectively.

The tower footing resistance plays an important role in

calculating the arrester energy. A non-linear tower

footing resistance model as shown in Figure 2 was used.

The resistance is calculated using Equation (3) [7].

g

lci

I

I1

RR

+

= (3)

With;2lc

g

gR2

EI

= (4)

where, Rlcis the low-current tower footing resistance,

is the soil resistivity, I is the stroke current through the

tower footing and, Eg the soil ionisation criticalelectrical field (4 kV/cm).

In this study, a low current tower footing resistance of

10 with soil resistivity of 200 m was used in case of

a stroke hitting the phase conductor, and a resistance

value of 80 with soil resistivity of 1600 m was used

for the case of a stroke hitting the shield wire at the

tower top.

3.2. Arrester model

In TFLASH, arresters are modelled as nonlinear

voltage-controlled current sources, and the arrestercurrent is calculated based on the applied voltage across

the arrester [5]. Therefore, the resulting equivalent

circuit of an arrester installed between a tower structure

and a line conductor is shown in Figure 3.

The tower and line conductor node voltages are given

by Equations (5) and (6) respectively.

30.48 cm

19.24 m(12.19) m

25.33 m(18.26) m

31.42 m(24.37) m

36.88 m(30.22) m

4.57m

4.26m

4.03m

22.55 m

6.09 m

6.09 m

2.15 m

A1

A2

B1 B2

C1

C2

E

Figure 1: 275kV shielded double circuit transmission

line tower. Values in brackets are mid-span heights.

I

Rlc

Figure 2:Non-linear tower footing resistance model.

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TA

ST Z2

IIV

= (5)

LA

L Z2

IV

= (6)

3.3. Lightning stroke parameter model

A range of different lightning impulse shapes were used

in simulating lightning strikes to the transmission line.

In this study, a double exponential, 4/77.5 impulse

current wave as recommended by CIGRE [6] was used.

CIGRE also recommends specific peak values of

lightning current for the different simulations: a peak

current magnitude of less than 20 kA under shieldingfailure scenario and currents above 20 kA for

backflashover scenarios [6]. In order to evaluate the

maximum energy absorbed by surge arresters, unless

otherwise specified, a 20 kA lightning stroke hitting the

phase conductor A1 was used for the shielding failure

case and a 200 kA stroke hitting the tower-top was usedin the backflashover case.

4. ENERGY STRESS ON SURGE ARRESTERS

Based on the line lightning performance analysis carried

out in a previous study [1], the maximum energy

absorbed by a line surge arrester was calculated usingthe product of voltage and current traces computed by a

travelling wave simulation technique [5]. In this

investigation, the insulator flashover was neglected

since the voltage measured across the line insulator

which is protected by the surge arrester was found to be

much lower than the CFO of the insulator string evenunder high magnitude lightning strike. Therefore, it is

assumed that the insulator does not flashover when it is

protected by a surge arrester.

Figure 4 shows energy stress distribution in surge

arresters installed in all phases in the case when a

lightning strike hits a phase conductor or a shield wire.When a low current lightning strike hits a phase

conductor, the energy absorbed by arresters at any tower

is different. As expected, the arrester installed on a

stricken phase absorbs excessively high energy

compared with arresters on other phases. However,when high current lightning hits the shield wire, it was

shown that any two arresters installed at the same height

absorb equal energy. The arresters installed on the

bottom phases absorb more energy than the arresters

installed on the other four phases above.

In a previous study [1], it was shown that the top phase

conductors are more vulnerable to shielding failure

while those at the bottom phases are more vulnerable to

backflashover. Considering this case and the results

shown in Figure 4, arrester energy requirements can be

such that the top arresters are more likely to experience

direct strikes of lower magnitudes while the lower ones

can be subjected to stresses equivalent to those causing

backflashover.

4.1. Parametric analysis

Appropriate selection of an arrester as a function of its

energy stress depends upon different parameters. These

parameters can be classified as line parameters and

lightning stroke parameters. Parameters such as

arresters in adjacent towers, tower footing resistance

and angle of power frequency voltage are considered as

line parameters whereas stroke peak current magnitude,

front time and tail time are considered as lightningstroke parameters. Influence of each of these parameters

on arresters energy stress was analysed in this

investigation.

Influence of line parametersIn practice, the energy shared by arresters at a tower is

highly influenced by the presence of arresters at

neighbouring towers. The nature of influence of these

adjacent arresters on the struck arresters energy

stress depends upon the position of the lightning stroke

hitting the line. Figure 5 shows the energy discharged

by an arrester as a function of number of arresters in

neighbouring towers. It is clearly understood that, whenstroke hits the phase conductor, the neighbouring

arresters help in sharing some of the duty and, hence,

this reduces the energy stress on the arrester at the

struck tower. However, the case is different when high

current lightning stroke hits the shield wire or the towertop. In this case, the energy absorbed by the arresters atthe tower increases with increasing number of arresters

at the adjacent towers. This is explained by the current

passing through the adjacent arresters being of opposite

IA ZLZL

ZTZT

IS

IA/2IA/2

IA/2IA/2

VT

VL

Surge

Arrester

Figure 3: Equivalent circuit of an arrester installed

between a line conductor and a tower structure. ZT =

Tower surge impedance, ZL= Line surge impedance, IS

= Stroke current, and IA= Arrester current

0

10

20

30

40

50

60

70

80

90

A1-C2 B1-B2 C1-A2 A1-C2 B1-B2 C1-A2

Arrester Position in Phase

Arre

sterEnergy[kJ] 20 kA

10

Stroke to shield wireStroke to phase conductor

200 kA

80

Figure 4:Distribution of energy stress in surge arresters

installed at a stricken tower.

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polarity, and hence flows back to the striking point

resulting into the increase of energy absorbed by the

arrester at the struck tower [8].

Figure 6 shows the percentage of energy shared by

arresters at adjacent towers, when lightning strikes aphase conductor and shield wire. Therefore, it can be

said that when lightning hits the phase conductor, the

nearest arresters to the strike absorb almost 40% of

energy, and the arresters on immediate adjacent towers

absorb 28%. This value decreases to only 19% for the

arresters at far end on the line section considered in this

study. However, when lightning hits the shield wire, the

nearest arresters absorb only 10%, while the arresters at

the far end does not absorb any energy.

The energy discharge duty of surge arresters depends on

tower footing resistance. Figure 7a shows this effectwhen lightning strikes of different current magnitude hit

the phase conductor. Figure 7b shows the case when

200 kA lightning strike hits the shield wire. The tower

footing resistance was varied from 10 to 80 . For

stroke to a phase, the energy absorbed by the arrester on

the phase decreases with increasing footing resistance.

For a stroke to shield wire, however, high value of

footing resistance increases the arrester energy

discharge. In this case, arresters installed on the top

phases are more stressed with low tower footing

resistance value whereas bottom phase arresters are

more stressed in the case of high footing resistance

value.

Figure 8 shows the effect of power frequency voltage

angle on arrester energy stress. Significant influence of

this angle is seen on both the case of lightning hitting

the phase conductor and shield wire. When stroke hits

the phase conductor, the maximum energy in struck

arrester is obtained at voltage angle of 0

o

. In case ofstroke hitting the shield wire, the maximum energy is

found at voltage angle of 180o. With change in voltage

angle, the energy stress in arrester increases when stroke

hits the shield wire, but the energy discharge in this case

is rather low and is unlikely to exceed the maximum

energy absorption capability of the arrester.

Influence of lightning stroke parameters

Lightning stroke parameters have significant influence

on energy stress in line surge arresters. This can be

considered as one of the key factor for selection of the

arrester. The oscillographic simulation was carried out

to understand the effect of these parameters for both thecase of stroke hitting a phase conductor and the shield

wire.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Tower Number

ArresterEnergy[%]

20 kA

10

200 kA

80

Figure 6:Percentage (with respect to energy absorbed

by arrester at tower hit by lightning) of energy shared by

adjacent arresters at towers along the line.

Figure 5: Arrester energy as a function of adjacent

arresters.

0

100

200

300

400

500

600

700

800

900

1000

1 3 5 7 9 11 1 3 1 5 1 7

Number of Towers with Arresters

ArresterEnergy[kJ]

0

5

10

15

20

25

30

35

40

45

50

ArresterEnergy[kJ]

200 kA

10

10 kA20 kA

10

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60 70 80

Tower Footing Resistance [ ]

ArresterEnergy[kJ]

Arr A1-C2

Arr B1-B2

Arr C1-A2

200 kA

80

200 kA

80

200 kA

80

b:Stroke to shield wire

Figure 7:Arrester energy dependence on tower footing

resistance

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70 80

Tower Footing Resistance [ ] ] ] ]

Ar

resterenergy[kJ]

10

20 kA

10 kA

a:Stroke to phase conductor

Figure 8: Arrester energy dependence on power

frequency voltage angle at lightning strike.

0

20

40

60

80

100

120

140

160

180

200

0 60 120 180 240 300 360

Power Frequency Voltage Angle in A1 []]]]

ArresterEnergy[kJ]

200 kA

80

20 kA

10

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Figure 9 shows the effect of stroke peak current

magnitude on arrester energy. This effect was examined

for different tower footing resistances. The energy

absorbed by the arrester increases with increasing peak

current magnitude, and this is obtained for all cases of

impact point and tower footing resistance.

The effect of impulse front time for different stroke

peak current magnitudes is shown in Figure 10. In the

case of lightning hitting the phase conductor, the change

in front time does not have any influence on the arrester

energy. However, the arresters are less stressed with

high front time when lightning hits the shield wire. Onthe other hand, the stroke current tail time has

significant influence on the energy absorbed by line

arresters (Figure 11). The arrester energy increases with

increasing tail time of the lightning impulse.

5. ARRESTER FAILURE PERFORMANCE

The objective of this study is to estimate the failure rate

of arresters due to excessive energy absorption and to

guarantee that the arresters installed on the line have

sufficient energy capability to withstand lightningstrikes to the phase conductors or to the shield wire. To

determine the arrester failure probability, the integrated

energy for each arrester is used with the failure

probability curve from EPRI report 1000461 [9].

The statistical simulation method was used. To integrate

the energy through the arresters over most of the strokeduration, the method used in TFLASH software adopts

different simulation time limits for strokes to phase

conductor and shield wire. These time limits are much

longer than the flashover statistics time limit (500 s for

a stroke to phase conductor and 100 s for a stroke to

shield wire). Figure 12 shows an example of a typicalwaveform used for energy calculation for a 20 kA

stroke current. In this case, the insulation flashovers

were disabled.

-5

0

5

10

15

20

25

30

35

40

45

0 25 50 75 100 125 150 175 200 225

Stroke Peak Current Ma gnitude [kA]

ArresterEnergy[kJ]

50

20

80

Figure 9:Effect of stroke peak current magnitude



-20

0

20

40

60

80

100

120

140

160

180

0 2 4 6 8 10 12 14 16 18 20

Stroke Peak Current Magnitude [kA]

ArresterEnergy[kJ]

80

10

40

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6

Front Time [s]

ArresterEnergy[kJ]

150 kA

200 kA

250 kA

80

Figure 10:Effect of front time (Tail time = 77.5s)



0

20

40

60

80

100

120

140

160

180

0 1 2 3 4 5 6

Front Time [s]]]]

Arreste

rEnergy[kJ]

5 kA10

20 kA

10 kA

0

50

100

150

200

250

0 25 50 75 100 125 150 175

Tail Time [s]

A

rresterEnergy[kJ]

10

5 kA

10 kA

20 kA

0

10

20

30

40

50

60

70

80

0 25 50 75 100 125 150 175

Tail Time [s]

ArresterEnergy[kJ]

80

250 kA

200 kA

150 kA

Figure 11:Effect of tail time (Front time = 4s)



0

5

10

15

20

25

0 50 100 150 200 250 300 350 400 450 500

Time [s]

Current[kA]

For 20kA stroke current

Front time = 3.4 s

Tail time= 56.2 s

Figure 12:Equal probability waveform (20kA stroke

current)

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The arrester failure performance was analysed for the

whole line section (35 km) when arresters are installed

at each phase and at every tower. Table 2 gives a

summary of the arrester failures for the 35 km line. It

can be seen that the phase conductors are expected to be

hit by 3.704 strokes per year resulting in an arrester

failure rate of 0.128 failures per year, i.e. one arrestermay fail every 7 to 8 years. Since there are hundreds of

arresters installed on the line, the chances of the same

arrester failing again is very low. The table also shows

the failure rate on each phase. It can be seen that there is

no risk of arrester failure for stroke terminating on theshield wire or tower top but there is risk associated to

the arresters at the top phase (installed on phases A1and

C2) with direct strokes terminating on the phase.

Figure 13 shows the arrester failure rate at towers along

the section of the line. The maximum failure rate is very

low; e.g. an arrester at the same tower may fail every

815 years. This of course depends on the appropriateselection of the arresters.

Table 2: Arrester failure performance of 35km line.

(Values in per year basis)

Direct strikes per year = 3.704

Arrester failure per year = 0.128

Arrester Failure by Phase

Failure From

PhaseDirect

StrikesShield

Strikes

Phase

Strikes

All

Strikes

A1 0.269 0.000 0.065 0.065

B1 0.000 0.000 0.000 0.000

C1 0.000 0.000 0.000 0.000

C2 0.269 0.000 0.065 0.065

B2 0.000 0.000 0.000 0.000

A2 0.000 0.000 0.000 0.000

6. CONCLUSIONS

Energy stress analysis of ZnO surge arresters installed

on a 275 kV double circuit transmission line was

investigated. It was found that the energy requirements

on the line arresters were moderate. The energy

absorption studies were carried out for the line andstroke parameters which are essential in the selection

process of line surge arresters.

Lightning strikes hitting the phase conductors (shielding

failure) were found to be the main source of the risk of

failure. In this case, the surge arresters installed on other

phases on the same tower did not help to share the totalsurge energy. The risk of failure when lightning hits the

shield wire or tower was found to be zero.

7. ACKNOWLEDGMENT

The authors wish to thank the Engineering and PhysicalSciences Research Council (EPSRC) for financial

support.

8. REFERENCES

[1] R. Bhattarai, R. Rashedin, S. Venkatesan, A.Haddad, H. Griffiths and N. Harid, Lightning

performance of 275kV transmission lines, in Proc.

of 43rd

International Universities PowerEngineering Conference, Padova, Italy, Sept. 2008.

[2] J. A. Tarchini and W. Gimenez, Line surge arresterselection to improve lightning performance of

transmission lines, in Proc. of IEEE PowerTechConference, vol. 2, Bologna, Italy, June 2003.

[3] J. A. Martinez and F. Castro-Aranda, Lightningflashover rate of an overhead transmission line

protected by surge arresters, IEEE Power

Engineering Society General Meeting, Tampa,

Florida, USA, June 2007.

[4] EPRI, Lightning performance analysis of pacificpower company cascade craft substation, Draft

Report, EPRI, Palo Alto, Exhibit No: WWB-4,

Nov. 2006.

[5] EPRI, Handbook for improving overheadtransmission line lightning performance, EPRI,

Palo Alto, CA:2004. 1002019, Dec. 2004.[6] CIGRE WG 33-01, Guide to procedures for

estimating the lightning performance of

transmission lines, CIGRE Brochure 63, Oct.

1991.

[7] British Std. BSEN 60071-2, Insulationcoordination, part 2: application guide, 1997.

[8] A. R. Hileman, Insulation coordination for powersystems, Marcel Dekker, ISBN 0-8247-9957-7,

1999.

[9] EPRI, Transmission line surge arrester impulseenergy testing, EPRI, Palo Alto, vol. 1000461.

Figure 13: Arrester failure rate at each tower along a

section of the line.

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Tower Number

ArresterFailure/Year

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Energy Stress on Surge Arrester