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Effects of Simulation Models of Overhead

Transmission Line Basic Components on

Backflashover Surges Impinging on GIS SubstationsPantelis N. Mikropoulos1 , Thomas E. Tsovilis, Zacharias G. Datsios and Nikos C. Mavrikakis

High Voltage Laboratory, Schoolof Electrical & Computer Engineering,Faculty of Engineering,Aristotle University of Thessaloniki,Thessaloniki 541 24, Greece

1pnm@eng.auth.gr

Abstract- Overvoltages arising in 150 kV and 400 kV GISsubstations due to backflashover of the incoming overheadtransmission lines were computed with the aid of ATP-EMTPsimulations, by considering the effects of several simulationmodels of the basic transmission line components. The

protection offered against impinging surges by surge arrestersoperating at the substation entrance is evaluated with respect to

the basic insulation level of the GIS system. The computedovervoltages vary considerably among tower grounding systemmodels and among insulator string flashover models whereasrather insignificantly among tower simulation models. There isno systematic variation in computed overvoltages amonginsulator string flashover models. Single vertical lossless linemodels and a constant rather than a current dependent

resistance are considered, in terms of simulation simplicity andsafe design, as satisfactory for simulating transmission linetower and its grounding resistance, respectively, in insulationcoordination studies of substations.

Index TermsATP-EMTP, backflashover, fast-frontovervoltages, GIS substations, insulation coordination, lightning,overhead transmission lines.

I. INTRODUCTION

Lightning surges may impinge on substations due to either

backflashover in the connected overhead transmission lines,

that is, flashover of line insulation caused by lightning flash

to shield wire, or shielding failure in the incoming overhead

lines, that is, lightning flash to phase conductors. The

impinging surges may cause substation outages, thus,

consequently, interruptions in power supply, which result in

economic losses and affect reliability of power systems. Thus,

insulation coordination of substations necessitates the

estimation of the arising backflashover and shielding failure

overvoltages. This can be accomplished analytically [1] or

through computer simulations [2].

For modeling of the basic components of overhead

transmission lines, that is, the towers and their grounding

system and flashover of insulator strings, several simulation

models have been proposed in literature. Specifically, tower

segments are modeled either as single-phase vertical lossless

lines with the same [3]-[10] or different surge impedance

(multiconductor models) [10]-[13] or as a combination of the

latter and lumped circuit elements (multistory models) [14]-

[17]. The tower grounding system is represented by models of

either constant [6], [18] or current dependent grounding

resistance; the latter models consider the reduction in the

tower grounding resistance associated with soil ionization

[19]-[25]. Flashover of insulator strings is represented by

voltage-dependent switches, considering either the voltage-

time characteristic of the insulator strings [26], [27] or leader

progression models [28], [29].

In the present study the overvoltages arising at the entrance

of 150 kV and 400 kV GIS substations due to backflashoverof the incoming overhead transmission lines, normally

associated with higher amplitude and steepness than shielding

failure overvoltages, are evaluated with the aid of ATP-

EMTP simulations, by considering several simulation models

of the basic components of the lines. The computed

overvoltages vary significantly among tower grounding

system models and among insulator string flashover models;

the effect of tower simulation models is rather insignificant.

The present analysis is important for insulation coordination

of substations since the computed peak overvoltages are used

for the evaluation of the substation outage rate as well as for

the selection of the necessary protection measures.

II. SIMULATION MODELS OF THE BASIC COMPONENTS OFOVERHEAD TRANSMISSION LINES

A. Tower Models

Transmission line towers are divided in segments between

the crossarms. Each segment is represented by either a

vertical lossless single-phase frequency-independent

distributed parameter line or by a combination of the latter

and lumped circuit elements [3]-[17] (Fig. 1). Thus, tower

models can be classified into three categories.

Fig. 1. Tower models: (a) single vertical lossless line models [3]-[10], (b)

multiconductor models [11], [12] (c) Hara et al. multiconductor model [10],(d) multistory models [14]-[16], (e) Baba & Ishii multistory model [17].

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i. Single vertical lossless line tower models

By following a simplified approach, towers can be

represented by a simple geometric shape, such as a cylinder

or a cone, with surge impedance calculated according to

expressions derived theoretically or through scale model

experiments (Table I). Thus, all tower segments between the

crossarms are represented by using an equal surge impedance,

ZT() [Fig. 1(a)]. The surge propagation velocity is assumedequal to 85% of the speed of light [6]. The surge impedance

of the 150 kV and 400 kV line towers under study (Fig. 2)

according to different tower models is given in Table I.

Fig. 2. Typical towers of (a) 150 kV and (b) 400 kV double-circuit lines ofthe Hellenic transmission system; the length of insulator strings is 1.86 m and

3.62 m for the 150 kV and 400 kV line, respectively.

ii. Multiconductor tower modelsBy representing tower segments by multiconductor vertical

lossless lines [11] or, for simplicity, by single lossless lines

[11], [12], the surge impedance of each segment can be

calculated through expressions derived from electromagnetic

field analysis. Thus, according to multiconductor models the

tower segments are simulated as single vertical lossless lines

with different surge impedance, ZTi [Fig. 1(b)]. Hara et al.

[10] based on scale model experimental results proposed a

simulation model, which takes also into account the effect of

bracings and crossarms by representing the former by lossless

lines, ZLi, in parallel to the lines of the tower segments, ZTi,

and the latter by lossless horizontal lines, ZAi[Fig. 1(c)]. The

surge propagation velocity is assumed equal to the speed of

light. The surge impedance of the segments of the towers of

the 150 kV and 400 kV lines (Fig. 2) calculated according to

models [10]-[12] is given in Table II.

iii. Multistory tower models

Each tower segment is modeled as a lossless line in series

with an R-L parallel circuit [Figs. 1(d) and 1(e)]. The values

of the surge impedance of tower segments, ZTi, and the

attenuation coefficient (Table III) are determined by a trial-

and-error process so as to fit measured [14]-[16] or accurately

TABLE ITOWER SURGE IMPEDANCE FOR SINGLE VERTICAL LOSSLESS LINE MODELS

TABLE IISURGE IMPEDANCE ACCORDING TO MULTICONDUCTOR TOWER MODELS

computed [17] voltage waveforms at several points along

Japanese large-scale towers. he damping resistances and

inductances, Ri and L

i respectively, can be estimated for

models [14]-[16] [Fig. 1(d)] as

1 2 3

2 ln, 1 3Tii i

ZR h i

h h h

= =

+ +

and 4 42 lnTR Z = (1)

, 1 4ii iT

k hL R i

v= = (2)

where his the tower height, h1, h2, h3 (m) are defined in Fig.

1, ki= 2, and vTis the surge propagation velocity equal to the

speed of light. Baba and Ishii model [17] incorporates fixed

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TABLE IIISURGE IMPEDANCE AND ATTENUATION COEFFICIENT ACCORDING TO

MULTISTORY TOWER MODELS

TABLE IVDAMPING RESISTANCE AND kiACCORDING TO BABA &ISHII [17]

TABLE VDAMPING RESISTANCE AND INDUCTANCE FOR MULTISTORY TOWER MODELS

values of Ri (Table IV) and inductances, Li, calculated

according to (2) by using the values of kigiven in Table IV.The values of the damping resistances and inductances of the

towers of the 150 kV and 400 kV lines (Fig. 2), calculated

according to models [14]-[17], are listed in Table V.

B. Tower Grounding System Models

Following a simplified approach, a concentrated tower

grounding system can be represented as a constant resistance

equal to the low current and low frequency grounding

resistance, R0 [6], [18]. Alternatively, according to models

[19]-[25], a concentrated tower grounding system can bemodeled by a current dependent grounding resistance,R(I), so

as to consider the reduction due to soil ionization in the

grounding resistance from the initial value of R0. Soil

ionization occurs when the lightning current, I, flowing

through the grounding system, results in the electric field

strength at the surface of the grounding electrodes to attain a

value equal to soil ionization gradient, E0. The tower

grounding system can be modeled in ATP-EMTP on the basis

of the expressions shown in Table VI with the aid of

MODELS.

TABLE VICURRENT DEPENDENT TOWER GROUNDING RESISTANCE MODELS

C.

Insulator String Flashover Models

According to [26], [27], flashover of line insulation occurs

when the voltage across the insulator strings becomes equal

to flashover strength VFO(kV) determined by the voltage-time

characteristic of the insulator strings. VFO(kV) is given as [27]

( )0.75400 710FOV t D= + (3)

whereD(m) is the insulator string length and t(s) the elapsed

time after lightning stroke.

According to leader progression models [28], [29] line

insulation flashover occurs when the insulator string length is

bridged by a leader. The leader progresses when the average

electric field strength in the unabridged gap becomes equal to

or higher than a critical value E0. Leader progression models

employ differential equations (Table VII) to compute the

leader length,L, at each time instant; flashover occurs whenL

becomes equal to the gap length, D, that is, the insulator

string length. Thus, the 150 kV and 400 kV line insulator

strings were represented by voltage-dependent flashover

switches with the aid of MODELS by incorporating (3)

according to [26], [27] and the differential equations shown inTable VII according to leader progression models [28], [29].

TABLE VIILEADER PROGRESSION MODELS

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III. OVERVOLTAGES ARISING IN GISSUBSTATIONS

ATP-EMTP simulations were performed for two 150 kV

and 400 kV GIS substation configurations (Fig. 3). For both

substations, the last section of 1.75 km of the incoming

overhead transmission lines was represented by a sequence of

J.Marti frequency dependent models, considering the line

span (350 m) and the tower geometries shown in Fig. 2. The

underground XLPE power cables were represented by theBergeron model. Surge arresters were modeled as nonlinear

current dependent resistors taking into account their

characteristics (Table VIII). GIS bays were represented as

lossless stub lines with a surge impedance of 75 [2]. The

step-up transformers were represented by a capacitance pi-

circuit together with BCTRAN model. Cable connections and

the surge arrester lead lengths shorter than 3 m were modeled

by a lumped parameter inductance of 1H/m [2]. The earth

resistivity was assumed 200 m.

Fig. 3. Schematic diagrams of the evaluated systems (a) 150 kV and

(b) 400 kV GIS substations.

TABLE VIIISURGE ARRESTER CHARACTERISTICS

The effects of simulation models of towers and their

grounding system and of insulator string flashover on the

arising overvoltages in the GIS substations were investigated

for the following worst case scenario: lightning is assumed to

strike to the first tower close to the substation, at the time

instant of positive power-frequency voltage peak of the upper

phase of the overhead transmission line. Lightning stroke was

represented by a current source of negative polarity producing

a current waveshape 7/77.5 s with front upwardly concave

[21], [30], [31]. The lightning current had an amplitude of200 kA and a maximum steepness calculated according to

[31]. Finally, simulations were performed with and without

surge arresters operating at the substation entrance so as to

evaluate the protection offered against impinging surges with

respect to the basic insulation level, BIL, of the GIS system;

the latter is 750 kV and 1425 kV for the 150 kV and 400 kV

substations, respectively.

A. Effect of Tower Models

Fig. 4 shows the computed peak overvoltages arising at the

entrance of the 150 kV and 400 kV GIS substations by

Fig. 4. Computed peak overvoltages at the entrance of the (a), (c) 150 kV

and (b), (d) 400 kV GIS substations; models are numbered according toTables I-III, vertical bars indicate the variation of peak overvoltages among

tower models, red line corresponds to safety margin of BIL/1.15.

considering in simulations the tower models presented in

Section II.A; insulator string flashover was modeledaccording to [27]. From Figs. 4(a) and 4(b) it is obvious that

the overvoltages, obtained by using a fixed tower grounding

resistance of 10 , do not vary significantly among tower

models. Actually, as can be deduced for Figs. 4(c) and 4(d),

the overvoltages, increasing with tower grounding resistance,

vary among models within 10% over the range of tower

grounding resistance of 5-20 ; generally, the differences in

overvoltages among tower models are less pronounced for

higher tower grounding resistance. It is noteworthy that

increasing tower surge impedance within the range given in

Table I results in a small increase in overvoltage peak,

especially for lower tower grounding resistance; however,

this is insignificant when surge arresters are operating at the

substation entrance. The increase of overvoltage peak due to

an increase in either tower surge impedance or grounding

resistance can be explained by the corresponding increase in

the instantaneous flashover voltage of line insulation. It is

important that when surge arresters are operating at the

substation entrance the peak overvoltages are limited to

values lower than the safety margin of BIL/1.15 [32] (Fig. 4).

B. Effect of Tower Grounding System Models

Fig. 5 shows the computed peak overvoltages at the

entrance of the substations under study for constant and

current dependent tower grounding resistance by assuming alow current low frequency grounding resistance,R0, of 10 ;

towers and insulator string flashover were modeled according

to [6] and [27], respectively. From Fig. 5 it can be deduced

that the computed overvoltages vary a little among tower

grounding system models. However, when a higher value of

R0 is employed in simulations (Fig. 6), differences in

overvoltages among simulation models increase up to 19%.

The overvoltages, increasing with R0, are highest when the

tower grounding system is modeled by a constant grounding

resistance equal to R0, as suggested by [6] and [18], or

according to model [25]; this is more pronounced for

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Fig. 5. Computed peak overvoltages at the entrance of the (a) 150 kV and(b) 400 kV GIS substations; low current and low frequency groundingresistance 10 , red line corresponds to safety margin of BIL/1.15.

Fig. 6. Computed peak overvoltages at the entrance of the (a) 150 kV and(b) 400 kV GIS substations as a function of low current and low frequency

grounding resistance; red line corresponds to safety margin of BIL/1.15.

relatively higher R0. However, when surge arresters are

operating at the substation entrance the peak overvoltages are

limited to values lower than the safety margin of BIL/1.15[32] (Figs. 5 and 6) and do not vary significantly, less than

6%, among tower grounding system models. Summarizing,

the use in simulations of a constant tower grounding

resistance, yielding the highest computed overvoltages in the

substations, may result in a safer design.

C. Effect of Insulator String Flashover Models

Fig. 7 shows the computed peak overvoltages at the

entrance of the 150 kV and 400 kV GIS substations by

considering in simulations the insulator string flashover

models presented in Section II.C; towers were represented

Fig. 7. Computed peak overvoltages at the entrance of the 150 kV and

400 kV GIS substations; tower grounding resistance 10 ; red linecorresponds to safety margin of BIL/1.15.

Fig. 8. Computed peak overvoltages at the entrance of the (a) 150 kV and(b) 400 kV GIS substations as a function of tower grounding resistance; red

line corresponds to safety margin of BIL/1.15.

according to [6] and were terminated by a constant grounding

resistance of 10 . The computed overvoltages vary notably

among models, up to 18%, especially for the 400 kV system.

They increase with increasing tower grounding resistance

(Fig. 8), especially when surge arresters are not operating atthe substation entrance. It is important to note that for the

400 kV system when leader progression models are employed

in simulations the overvoltages, even when surge arresters are

installed, are higher than the safety margin of BIL/1.15 for

tower grounding resistance higher than about 15 . This,

contrary to common practice [33], indicates the necessity of

using a higher insulation level for the GIS equipment, and is

considered as unrealistic since care is normally taken to

reduce the last tower grounding resistance to values lower

than 10 . As a general result, due to the non-systematic

variation in overvoltages among insulation string flashover

models conscious selection of the latter is needed forinsulation coordination of substations.

IV. CONCLUSIONS

The effect of the simulation models of the basic

components of overhead transmission lines on the arising

overvoltages at the entrance of 150 kV and 400 kV GIS

substations due to backflashover of the incoming lines has

been investigated with the aid of ATP-EMTP simulations.

Tower simulation model does not affect significantly the

computed overvoltages, especially with increasing tower

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grounding resistance. Thus, single vertical lossless line

models are considered as satisfactory for simulating

transmission line towers, due to their simplicity, in

insulation coordination studies of substations.

Tower grounding system simulation model affects

considerably the computed overvoltages, which increase

with tower grounding resistance; however this is

insignificant when surge arresters are operating at thesubstation entrance. A constant rather than current

dependent resistance, resulting in higher computed

overvoltages, is considered as satisfactory in terms of safer

design for simulating tower grounding resistance in

insulation coordination studies of substations.

The computed overvoltages vary significantly however

non-systematically among insulator string flashover

models. Thus, conscious selection of the overhead line

insulator string flashover model is needed for insulation

coordination of substations.

ACKNOWLEDGEMENTS T. E. Tsovilis wishes to thank the Research Committee of

Aristotle University of Thessaloniki for the support provided

by a merit scholarship.

REFERENCES

[1] A. R. Hileman, The incoming surge and open breaker protection, in

Insulation Coordination for Power Systems. Boca Raton, FL: CRC

Press, Taylor & Francis Group, New York, 1999.

[2] IEEE Task Force, Modeling guidelines for fast front transients,IEEETrans. Power Delivery, vol. 11, no. 1, pp. 493506, Jan. 1996.

[3] C. A. Jordan, Lightning computations for transmission lines withoverhead ground wires, Gen. Elec. Rev., vol. 37, 1934.

[4] C. F. Wagner and A. R. Hileman, A new approach to calculation of

lightning performance of transmission lines III A simplified method:

Stroke to tower,AIEE Trans. Power App. Syst., vol. 79, pp. 589603,Oct. 1960.

[5] M. A. Sargent and M. Darveniza, Tower surge impedance, IEEETrans. Power App. Syst., vol. PAS88, no. 5, pp. 680687, May 1969.

[6] IEEE Working Group, A Simplified method for estimating lightningperformance of transmission lines, IEEE Trans. Power App. Syst.,vol. PAS-104, no. 4, pp. 919-932, Apr. 1985.

[7] C. Menemenlis and Z. T. Chun, Wave propagation on nonuniform

lines, IEEE Trans. Power App. Syst., vol. PAS101, no. 4, pp 833839, Apr. 1982.

[8] W. A. Chisholm, Y. L. Chow, and K. D. Srivastava Travel time of

transmission towers, IEEE Trans. Power App. Syst., vol. PAS104,no. 10, pp. 29222928, Oct. 1985.

[9] IEEE Working Group on Estimating Lightning Performance of

Transmission Lines, IEEE guide for improving the lightningperformance of transmission lines, Proposed IEEE P1243 draft, 1996.

[10]

T. Hara and O. Yamamoto, Modelling of a transmission tower forlightningsurge analysis,IEE Gener. Transm. Distrib., vol. 143, no. 3,pp. 283289, May 1996.

[11] A. Ametani, Y. Kasai, J. Sawada, . Mochizuki, and . Yamada,

Frequencydependent impedance of vertical conductors and amulticonductor tower model, IEE Gener. Transm. Distrib., vol. 141,

no. 4, pp 339345, Jul. 1994.

[12] D. Rondon, M. Vargas, J. Herrera, J. Montana, H. Torres, M. Camargo,

D. Jimenez, and A. Delgadillo, Influence of grounding systemmodelling on transient analysis of transmission lines due to direct

lightning strike,inProc. Power Tech, St. Petersburg, Russia, 2005, pp.

1-6.[13] J. A. Gutierrez, P. Moreno, L. Guardado, J. L. Naredo, Comparison of

transmission tower models for evaluating lightning performance, in

Proc. Power Tech, Bologna, Italy, 2003, pp. 1-6.[14] M. Ishii, T. Kawamura, T. Kouno, E. Ohsaki, K. Shiokawa, K.

Murotani, and T. Higuchi, Multistory transmission tower model for

lightning surge analysis, IEEE Trans. Power Delivery, vol. 6, no. 3,pp. 13271335, Jul. 1991.

[15] T. Yamada, A. Mochizuki, J. Sawada, E. Zaima, T. Kawamura, A.

Ametani, M. Ishii, and S. Kato, Experimental evaluation of a UHV

tower model for lightning surge analysis, IEEE Trans. Power

Delivery, vol. 10, no. 1, pp 393402, Jan. 1995.

[16] H. Motoyama, K. Shinjo, Y. Matsumoto, and N. Itamoto, Observationand analysis of multiphase back flashover on the Okushishiku test

transmission line caused by winter lightning, IEEE Trans. Power

Delivery, vol. 13, no. 4, pp. 13911398, Oct. 1998.[17] Y. Baba and M. Ishii, Numerical electromagnetic field analysis on

lightning surge response of tower with shield wire, IEEE Trans.

Power Delivery, vol. 15, no. 3, pp 10101015, Jul. 2000.[18] A. Ametani and T. Kawamura, A method of a lightning surge analysis

recommended in Japan using EMTP, IEEE Trans. Power Delivery,

vol. 20, no. 2, pp. 867875, Apr. 2005.

[19]

E. E. Oettle, A new general estimation curve for predicting theimpulse impedance of concentrated earth electrodes, IEEE Trans.Power Delivery, vol. 3, no. 4, pp 20202029, Oct. 1988.

[20] W. A. Chisholm and W. Janischewskyj, Lightning surge response of

ground electrodes, IEEE Trans. Power Delivery, vol. 4, no. 2, pp.13291337, Apr. 1989.

[21] CIGRE Working Group 33.01, Guide to procedures for estimating thelightning performance of transmission lines, Technical Bulletin 63,Oct. 1991.

[22] K. H. Weck, The current dependence of tower footing resistance,CIGRE 33-88 (WG01), 14 IWD, 1988.

[23] P. Chowdhuri, Grounding for protection against lightning, in

Electromagnetic transients in power systems. Research Studies PressLtd., John Wiley & sons inc., New York, 1996, pp. 104-113.

[24] A. V. Korsuntchev, Application of the theory of similitude to the

calculation of concentrated earth electrodes, Electrichestvo, no.5, pp.

31-35, May 1958.[25] Y. Yasuda, Y. Hirakawa, K. Shiraishi, and T. Hara, Sensitivity

analysis on grounding models for 500kV transmission lines, Trans.IEE Japan B, vol. 121, no. 10, pp. 13861393, 2001.

[26] M. Darveniza, F. Popolansky, and E. R. Whitehead Lightning

protection of UHV lines,Electra, no. 41, pp. 3969, Jul. 1975.[27] IEEE Working Group, Estimating lightning performance of

transmission lines II Updates to analytical models, IEEE Trans.

Power Delivery, vol. 8, no. 3, pp. 1254-1267, Jul. 1993.[28] K. H. Weck, Lightning performance of substations, CIGRE SC 33,

Rio de Janeiro, Brazil, 1981.

[29] A. Pigini, G. Rizzi, E. Garbagnati, A. Porrino, G. Baldo, and G.Pesavento, Performance of large air gaps under lightning overvoltages:

Experimental study and analysis of accuracy of predeterminationmethods, IEEE Trans. Power Delivery, vol. 4, no. 2, pp 13791392,

Apr. 1989.

[30]

Lightning and Insulator Subcommittee of the T&D Committee,Parameters of lightning strokes: A review, IEEE Trans. Power

Delivery, vol. 20, no. 1, pp. 346-358, Jan. 2005.

[31] R. B. Anderson and A. J. Eriksson, Lightning parameters forengineering application,Electra, no. 69, pp. 65102, 1980.

[32] IEC 60071-2, Insulation Coordination, 1996.

[33] Joint Working Group 33/23.12, Insulation co-ordination of GIS:Return of experience, on site tests and diagnostic techniques, Electra,

no. 176, pp. 67-97, Feb. 1998.