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CALCULATION OF LIGHTNING FLASHOVERS AND BACKFLASH LEVEL

ON 230kV TRANSMISSION LINES

Bander J. Al-Qahtani * M. H. Shwehdi

SAOO-NGPD-TSU Electrical Engineering Department

Saudi Aramco King Fahd University of Petroleum & Minerals

Abqaiq, Saudi Arabia Dhahran, Saudi Arabia

[email protected] [email protected]

ABSTRACT

Lightning has been one of the important problems for

insulation design of power systems and it is still the

main cause of outages of transmission and distribution

lines. Lightning caused outages can be reduced by

lightning protection devices such as ground wires and

lightning arresters.

This paper presents a comparative studies used to

determine the lightning backflashovers level on 230kV

transmission lines utilized by Saudi Electric Company

(SEC) in Saudi Arabia, using two well known

approaches CIGRE, and the simplified method. The

studies include lightning flashovers, backflash

analysis, as dependent on the tower design parameters

which is considered the main parameters that reduce

the rate of lightning bachflashovers in the

transmission lines. The study results can be applied to

reduce the number lightning flashovers and therefore

reduce the transmission lines outages.

KEY WORDSLightning flashovers, backflashovers, simulation and

ground wires

1. Introduction

A complete awareness of the parameters of lightning

strokes is essential for the prediction of the severity of the

transient voltages generated across power apparatus either

by a direct stroke to the power line/apparatus, or by an

indirect stroke. However, no two lightning strokes are thesame. Therefore, the statistical variations of the lightning-

stroke parameters must be taken into account in assessing

the severity of lightning strokes on the specific design of a

power line or apparatus.

The lightning return-stroke current and the charge

delivered by the stroke are the most important parameters

to assess the severity of lightning strokes to power linesand apparatus. The return-stroke current is characterized

by a rapid rise to the peak, I p, within a few microseconds

and then a relatively slow decay, reaching half of the peak

value in tens of microseconds. The return-stroke current is

specified by its peak value and its waveshape. The

waveshape, in turn, is specified by the time from zero to

the peak value (tf , front time) and by the time to its

subsequent decay to its half value (th, tail time). The tailtime being several orders of magnitude longer than the

front time, its statistical variation is of lesser importance

in the computation of the generated voltage. Thegenerated voltage is a function of the peak current for

both the direct and indirect strokes.

For backflashes in direct strokes and for indirect strokes

the generated voltage is higher the shorter the front time

of the return-stroke current [1]. The front time (and the

tail time, to a lesser extent), influence the withstandcapability (volt-time characteristics) of the power

P.O.Box 66467, Dammam 31576, Kingdom of Saudi Arabia

mailto:[email protected]





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apparatus. The charge in a stroke signifies the energy

transferred to the struck object. The ancillary equipment

(e.g., surge protectors) connected near the struck point

will be damaged if the charge content of the stroke

exceeds the withstand capability of the equipment. Thereturn-stroke velocity will affect the component of the

voltage which is generated by the induction field of thelightning stroke [1]. Field tests have shown that the

parameters of the first stroke are different from that of the

subsequent strokes.

2. Lightning Flashes

Lightning damages a power apparatus in two ways: (i) it

raises the voltage across an apparatus such that theterminals across the struck apparatus spark over causing a

short circuit of the system or the voltage puncturesthrough the apparatus electrical insulation, causing

permanent damage. (ii) The energy of the lightning stroke

may exceed the energy handling capability of the

apparatus, causing meltdown or fracture.

A lightning flash generally consists of several strokes

which lower charges, negative or positive, from the cloud

to the ground. The first stroke is most often more severethan the subsequent strokes. Low current continues to

flow between two strokes, thus increasing the total energy

injected to the struck object. The transient voltage from

the lightning strike is generated by: (i) direct stroke and

(ii) indirect stroke. For direct strike, it can strike an

apparatus. In that case, the apparatus will be permanently

damaged. Most often, lightning strikes the phase

conductor of the power line. In that case, a travelingvoltage wave is generated on the line; it travels along the

line and is impressed across the terminals of an apparatus

or most often the insulator between the phase conductor

and the cross-arm of the tower at the end of the span. If

the voltage is high enough, the insulator flashes over

causing a short circuit of the system.

Many overhead power lines are equipped with shield

wires to shield the phase conductors. Even then, shielding

failures occur when lightning bypasses the shield wiresand strikes a phase conductor. When lightning strikes a

tower, a traveling voltage is generated which travels back

and forth along the tower, being reflected at the tower footing and at the tower top, thus raising the voltages at

the cross-arms and stressing the insulators. The insulator

will flash over if this transient voltage exceeds its

withstand level (backflash). Even if lightning strikes a

shield wire, the generated traveling voltage wave willtravel to the nearest tower, produce multiple reflections

along the tower, causing backflash across an insulator.

When lightning hits the ground several hundred meters

away from the line (indirect stroke), the electric and

magnetic fields of the lightning channel can induce high

voltage on the line for the insulators of the low-voltage

distribution lines to spark over causing a short circuit of the system. Thus, assuming the lightning channel to be a

current source, the transient voltages across the insulator of a phase conductor are generated in three ways: (i)

lightning striking the phase conductor (shielding failure),

(ii) lightning striking the tower or the shield wire

(backflash), and (iii) lightning striking the nearby ground

(indirect stroke). The severity of these three types of

transient voltages is influenced by different lightning parameters [2, 3].

The significance of lightning parameters on power

systems is gauged by the severity of the transient

overvoltages they create and the consequent damages to

the power system. As mentioned before, these

overvoltages are generated by three different ways.

3. Computation of Insulator Voltage

The lightning return-stroke current is the most significant

parameter in the estimation of the response of electricalapparatus and systems to lightning strikes. The return

stroke current rises to its peak in a few microseconds and

then decays to the half value in a few tens of

microseconds [4]. The return-stroke current is identified

by three parameters: peak value I p, front time tf and time

to half value th. The difficulty with the exponential

function representing a return-stroke current is that it is

not easy to select the parameters of these analyticalexpressions to fit the three parameters ( I p, tf and th ).

However, this problem does not arise if the return-stroke

current is represented as linearly rising and linearly falling

functions [4]:

)()()()(21 f f

t t ut t t tut I −−−= α α (1)

Where α1 = I p/tf , and α2 = (2th –If )I p/2tf (th –If ). For short tf in

the order of a few microseconds, eqn. 1 seems to work very well. With eqn. 1, the three parameters of the return-

stroke current can be varied very easily. Starting with the

return-stroke current, the various voltage components

across the insulator were computed.

3.1 First and Second Voltage Components

To compute the first voltage component, i.e. the cross-

arm voltage V ca, the tower was assumed to be a vertical

transmission line of a fixed surge impedance Z t. The

voltage and current waves were assumed to travel along

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the tower with a constant p.u. velocity of β t. The first

reflections from the adjacent towers for the shield-wire

voltages were also included in the computation. The

tower footing resistance was assumed to be constant, Rtf .

The tower-top was terminated by shield wire(s) and thelightning channel of constant surge impedance Z ch. The

cross-arm voltage due to the multiply reflected voltagewaves along the struck tower was computed by following

a previous method as shown in [4]. Although Z t, βt, R tf and Zch were assumed as constant, they were used as input

variables which could be changed for parametric analysis.

The second voltage component is the voltage induced onthe phase conductor due to electromagnetic coupling with

the shield wire. This voltage is equal to k cf Vt, where Vt is

the tower-top/shield-wire voltage and the coupling factor

k cf is equal to Z ps/Zsh. Z ps is the mutual surge impedance

between the phase conductor and the shield wire; Zsh is

the shield-wire surge impedance [4]. The tower-top

voltage was computed following the same procedure as

for Vca. The insulator-string voltage due to the first andsecond voltage components is:

t cf cainsV k V V −= (2)

3.2 Third Voltage Component

The third voltage component is the voltage induced on the

phase conductor due to the electromagnetic fields of thelightning channel. The computation of the phase-

conductor voltage followed previous analysis [4], with the

difference that, in the present case, the stroke hits the

tower top instead of the ground. This difference is

manifested in the inducing voltage Vi , which is the

voltage in space (in the absence of the phase conductor)

caused by the residual charge in the upper part and the

return-stroke current in the lower part of the lightningchannel. Vi is:

∫ ∂

∂+∇=

ph

i dz t

AV

0

)( φ (3)

Where Φ is the scalar potential due to the residual charge

in the upper part of the lightning channel, and A is the

vector potential due to the return-stroke current in the

lower part of the channel. For stroke to ground, Φ and A

are

r d r r

c

r r t r q

t r

ch

z

′′−

′−−′

= ∫ ′

),(

4

1),(

0

0πε φ

(4)

r d r r

c

r r t r I

t r A

z

′′−

′−−′

= ∫ ′

0

0

),(

4),(

π

µ (5)

where r and r are field and source points, respectively: I׳

is the return-stroke current: q0 is the constant linear charge

density of the leader stroke: hc is the cloud height: and isthe instantaneous height of the upward-moving head of

the return stroke above ground. For a stroke to ground, z ׳

increases as a function of time and the return-stroke

velocity, with its lower and upper limits 0 and h c. For a

stroke to tower of height hc , the lower and the upper limitsof z ׳ are ht and hc. Thus, for a stroke to tower, the

voltages induced on the phase conductor were computedfor two different cloud heights (hc and ht), and then the

second induced voltage (for ht ) was subtracted from the

first induced voltage (for hc ).

4. Computation of Backflash Rate

The overhead ground wires or shield wires have been

located so as to minimize the number of lightning strokes

that terminate on the phase conductor. The remaining and

vast majority of strokes and flashes now terminate on theoverhead ground wires. A stroke that so terminate forces

current to flow down the tower and out on the ground

wires. Thus voltage are built up across the line insulation.

If these voltages equal or exceed the line CFO, flashover

occurs. This event is called a backflash. By referring to

figure 1, equations for the crest voltage, the voltage at the

tower top prior to any reflections from the footing

resistance, and the final voltage can be derived as follows

Figure 1. Surge voltages at the tower and across the insulation [5]

( ) I RV

I K K V

I K K V

e F

TA spTA

TT spTT

=

=

=

(4.1)

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And the current through the footing resistance is

I R

R I

i

e R = (4.2)

Where

( )

+

−+

−−−=

+=

+=

412111 f

S

T R

f

S

T RSP

f

AT T eTA

f

T T T eTT

t

T

t

T K

t

T Z R K

t T Z R K

α α α α

α

α

(4.3)

For these equations:

i g

i g

e R Z

R Z R

2+=

i g

i g

iT

iT

T R Z

R Z

R Z

R Z

2

2

+

−≈

+

−=α

i g

g

R R Z

Z

2+=α (4.4)

Also, the tail of the voltages can be conservatively

approximated by a time constant τ:

S

i

g T

R

Z =τ (4.5)

That is, the equation for the tail of the surge isτ /)( f t t

F TT eV e−−= (4.6)

To be complete the definition of the variables are:

tf = time to crest of the stroke current, μsC = coupling factor ZT = surge impedance of the tower, ohms

Zg = surge impedance of the ground wires, ohms

TT = tower travel time, μs

TA = tower travel time to any location on the tower A,

μs

TS = travel time of a span, μs

I = stroke current, KA

IR = current through footing of struck tower, KAR o = measured or low-current footing resistance, ohms

R i = impulse or high-current footing resistance, ohms

τ = time constant of tail, μs

Now, to provide first estmate of the backflash rate, theBFR, examine figure 6. The surge voltage on the ground

wires produces a surge voltage on the phase conductor

equal to the coupling factor C times the voltage on the

ground wires, or CVTT. Also note that the voltage VTA is

located on the tower opposite the phase conductor.

Therefore, the crest voltage across the insulation V1 is

[ ]SP TT TA

K CK K I V −=1 (4.7)

Also, note that the crest voltage VIF across the insulation

caused by the footing resistance is

( ) I RC V e IF −= 1 (4.8)

For a flashover to occur, the voltage across the insulator V1, must be equal to or greater than the CFO of the

insulation. Replacing V1 of Eq. (7) with CFO, the current

obtained is the critical current IC at and above whichflashover occur, i.e.,

( ) SP TT TA

C K CK K

CFO I

−= (4.9)

Since K TT is in many cases approximately equal to K TA,

then approximately,

( )SP TT

C K K C

CFO I

−=

1(4.10)

The probability of a flashover is the probability that the

stroke current I equals or exceeds the critical current IC, or

( ) ( ) ( )dI I f I P I I ob

C I

C C

∫

∞

==≥Pr (4.11)

The backflash rate BFR is this probability times the

number of strokes, NL, that terminate on the ground wires,

or

BFR= L N ( )C I P (4.12)

Where

( )10

28 6.0

g

g L

S h N N

+= (4.13)

Where h is the tower height (meters), Sg is the horizontal

distance between the ground wires (meters), and Ng is the

ground flash density (flashes/km2-year), thus the BFR is

in terms of flashovers per 100 km-years.

The equations for K TT and K I show that the voltage across

the insulation increases as the time to crest of the stroke

current decreases. This is caused by the tower component

of voltage. Thus the critical current increases as the timeto crest increases. Therefore, theoretically, all fronts

should be considered. To do this, the equation for BFR

should be changed to the following:

BFR=0.6 NL P(IC) (4.14)

5. Simulation & Results

The 230 kV HV line of figure 2 whose characteristics are

given in table I, are used to calculate the backflash rate

using different methods. Also, this case study will include

the following

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1. The effect of decrease of resistance from R o versus R i2. One versus two shield wires

3. The effect of underbuilt shield or ground wire

As shown in the figure 3 & 4 the backflash rate for the

above mentioned high voltage lines with span length of

300 meters and CFO of 1200kV has been calculated byusing CIGRE method software and simplified method.The comparison appears acceptable for the line with

tower height of 35 meters, but for tower height of 70

meters the simplified method is inadequate. So, the

CIGRE method is always the proper tool.

Using the CIGRE method, the BFR of the single circuit

230 kV is shown in Fig. 5 as a function of R O with the

ratio ρ/R O as a parameter. To illustrate the effect of thedecrease of resistance with current, a curve labeled R i=R Ofor which the footing resistance is not decreased is also

presented.

Figure 2. 230 kV Tower Dimensions

For some applications, where the cost of two shield wires

is not economically and technically justified, or wherethere is low ground flash density, a single shield wire can

be used. The single wire increases the value of R e,

decreases the coupling factor, and thus increase the BFR.

To illustrate, the curves of Fig. 6 have been constructed to

compare one and two shield wires for a 230 kV double-circuit line and two shield wires for a single-circuit 230

kV line. Using one shield wire on the double-circuit line

essentially doubles the BFR as compared to the two-shield-wire case.

A ground wire located below the phase conductors cannot

truthfully be called a shield wire, since it has no shielding

function. Rather, its function is to increase the coupling

factor to the lower phases, those phases that are most

likely to flashover. For example, for the 230-kV double-

circuit, two-ground-wire line with a shield wire height of

35 meters and coupling factor to the top, middle, and

bottom phase of 0.350, 0.248, and 0.183, respectively,installing a ground wire at 12 meters above ground at the

center of the tower increases these coupling factors to0.441, 0.347, and 0.307, respectively. Thus all coupling

factors are increased and are more uniform. Figure 7

shows the dramatic decrease in BFR for this case.

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6

X10Ro, ohms

B F R , F l a s h o v e r s /

CIGRE

Method

Simplified

Method

Figure 3. Comparasion of BFRs for CIGRE method and simplifiedmethod, 230kV double circuit towers with two ground wires and height

of 35 meters

0

2

4

6

8

10

12

1 2 3 4 5 6

X10Ro, ohms

B F R , F l a s h o v e r s

Simplified

Method

CIGRE

Method

Figure 4. Comparasion of BFRs for CIGRE method and simplified

method, 230kV double circuit towers with two ground wires and height

of 70 meters

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8

X10Ro, ohms

B F R , F l a s h o v e r s / 1

p/Ro=40

p/Ro=20

p/Ro=10

Figure 5. Effect of decrease to high-current footing resistance

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0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6

X10, ohms

B F R , F l a s h o v e r

2GrdWire

1GrdWire

Figure 6. Tow shield wires for the 230kV double circuit line with height

of 35 m decrease the BFR, p/Ro=20

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8

X10Ro, ohms

B F R , F l a s h o v e r s

2GrdWires

2Grd

Wires+under

built grdwire

Figure 7. An underbuilt ground wire decreases the BFR, 230kV double

circuit line with height of 35 m, p/Ro=20.

6. Conclusion

The most significant parameters of the lightning return

stroke to estimate the severity on the power system are: (i)

peak current, (ii) current front time, (iii) velocity and (iv)

total charge of the flash.

The electromagnetic fields of the lightning channel and

the magnetic fields of the traveling current waves alongthe power-line tower will significantly affect the

insulator-string voltage, and hence the outage rate due to

backflash. Analytical methods to estimate the backflash

outage rate have been proposed, which should result in

better prediction of the lightning performance of overhead

power lines.

In this report, equations were developed to estimate theBFR that include the tower component of voltage; their

use is called CIGRE method. This method is suffiently

complex so that the use of the computer program is

suggested. The effect of decrease of the concentrated

grounds value on the BFR was addressed. Also, the effectof the number of shield wires as well as adding underbuilt

shield or ground wire were highlighted.

The 230 kV line design from SEC is considered very

highly engineered, using two ground shield wires with 7.3

meter span at each side made almost a full cover for both

circuits. This tower can be considered as lightning proof.

7. Acknowledgment

The authors express appreciation to Saudi Electric

Company engineers for thier time and support also their

gratitude to KFUPM for educational, studies facilities and

support.

References:

[1] P. Chowdhuri, J. G. Anderson, W. A. Chisholm,

T. E. Field, M. Ishii, J. A. Martinez, M. B. Marz, J.

McDaniel, T. R. McDermott, A. M. Mousa,T. Narita,

D. K. Nichols, & T. A. Short, Parameters of

Lightning Strokes: A Review, IEEE Transactions

and Power delivery, March 28, 2003.

[2] P. Chowdhuri, A.K. Mishra & B.W. McConnell,

Volt-time characteristics of short air gaps under nonstandard lightning voltage waves, ibid., Vol. 12,

No. 1, pp. 470-476, 1997.

[3] P. Chowdhuri, A.K., Parameters of Lighting

Strokes and Their effect on Power Systems, Vol. 12,

No. 1, pp. 1047-1051, 2001

[4] P. Chowdhuri, A.K., S. Li & P. Yan Rigorous

analysis of back-flashover outages caused by direct

lightning strokes to overhead power lines, IEEE

Proceedings, 2002

[5] Andrew R. Hileman, Insulation Coordination for

Power Systems, (Eastern Hemisphere Distribution,

New York, 1999)

[6] R. Thottappillil & M. A. Uman, Comparison of

lightning return stroke models, J. Geophys. Res., vol.

98, pp. 22 903–22 914, 1993.

[7] V. Cooray & R. E. Orville, The effect of the

variation of current amplitude, current rise time and

return stroke velocity along the return stroke channel

on the electromagnetic fields generated by the return

stroke, J. Geophys. Res., vol. 95, pp. 18 617–18 630,1990.

[8] Dennis W. Lenk, F. Richard Stockum & David

E. Grimes,A new approach to distribution arrester design, IEEE Transactions on power delivery, vol. 3,

No. 2, April 1988.

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[9] P. Pinceti & M. Giannettoni, A simplified model

for zinc oxide surge arrester, IEEE Transactions on power delivery, Vol. 14, No. 2, April 1999.

Biographies

Bander J. Qahtani; Born in Al-Khobar 1979. He obtained hisB.Sc. degree in electrical engineering with honors from KingFahd University of Petroleum & Minerals (KFUPM) in 2002. In

the year 2000 he was selected as distinguished student for SaudiAramco Scholarship program. During his studies at KFUPM hehas conducted several term projects and studies dealing withIndustrial power systems. Upon graduation, Bandar was

employed by Aramco as instrument engineer with SouthernArea Producing Engineering Department (SAPED) in Abqaiq.He is enrolled in the Msc. Program at KFUPM. Bandar has

published and presented many technical papers and reports to

region, and international conferences.

M. H. Shwehdi (S'74, M'85, SM 90) received the B. SC. degree

from University of Tripoli, Libya in 1972. He obtained the M.

Sc. Degree from the University of Southern California and Ph.D.

degree from Mississippi State University in 1975 and 1985respectively all in electrical engineering. He was a consultant to

A.B. Chance Company, and Flood Engineering. Dr. Shwehdiheld teaching positions with the University of Missouri-

Columbia, Texas A & I University, University of Florida andPenn. State University from 1991-1993. At present he is

associate professor with the King Fahd University of Petroleum& Minerals (KFUPM), Saudi Arabia. His research interest

includes, power system analysis, Power Quality & Harmonics,overvoltages analysis on Power Systems, Transmission andDistribution Systems. Dr. Shwehdi is active in IEEE activities.He is listed as a distinguished lecturer with the DLP of the

IEEE/PES DLP upon the Board selection, was named andawarded the 2001 IEEE/PES outstanding chapter engineer,. Hewas named and awarded the 1999 IEEE WG for standard award,the GCC-CIGRE 1998 best applied research award, IEEE/IAS

Outstanding Supervisor for Student Research 1989, 1990, andthe IEEE outstanding student advisor in 1990.

Table I Characteristics of Lines, Distances in meters

System

Voltageh yA yB yC Sg Sa S b Zg ZT CA CB CC

230 35 29 24 18 5 8 11 379 190 .35 .25 .18a230 35 29 24 18 0 8 11 600 190 .22 .16 .12

230 70 64 59 53 5 8 11 421 210 .42 .34 .28 b230 35 29 24 18 5 8 11 239 190 .44 .35 .31

a Single ground wire. b Underbuilt ground wire at h=12 m at center of tower

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