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MODELLING OF LONG GROUND ELECTRODES FOR LIGHTNING STUDIES |
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M.E. Ahmdda |
M.T. Correia de Barros |
1ST - Universidade Teeniea de Lisboa / Instituto da Energia - fNTERG Av. Rovisco Pais, 1049¡¤0 01 Lisboa, Portugal | |
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Abstract - For evaluating the lightning perfonnance of transmission lines by cOIl1puter simulation, the accurate modelling of tower footing is very important. Considering the high frequencies and magnitudes characteristics of lightning currents, an adequate representation of earth electrodes must take into account the distributed nature of their parameters, as well as its non-linear behaviour, due to soil ionisation pheoomena. The purpose of this paper is to study the non-linear behaviour of long ground electrodes, based on a fmite -differences transmission line mO.del able to take non-linearities¡¤ into account [1],_ Results obtained show the influence of soil ionisation process at the voltage and current distributions along the electrode and the important role played by the lightning current rise time. |
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1. Introduction |
Computer simulation is an important tool for evaluating the lightning performance of transmission lines and the adequate modelling rechniques for the different systems components have to be established. In particular, it has been emphasised by different authors that, the predicted lightning backflashover rates are very sensitive to the tower footing behaviour, thus being essential an adequate groWld electrode model.
Considering the high frequencies and magnitudes characteristic of lightning currents, the influence of these parameters have to be taken mto account in tower footing modelling. In order to reproduce accurately . the high frequency behaviour, a distributed model may be needed, depending on the length of the ground electrode.
In high resistivity soils it is often required to install long horizontally buried conductors, counterpoises, in the tower footing. The accurate modelling of coWlterpoises fo r lightning currents requires their representation by a distribuled parameters model [2- 5].
If high magnirude currents flow from the tower footing into the soil, the critical field strength of the soil can be exceeded, and Its partial breakdown occurs. Then, the conductor is surrounded by a corona-type discharge pattern. TIle ionised area occupies a confined space in which the conductivity becomes much greater than in the rest of the son. In this situation, the ground electrodes display a nonlinear transient behaviour and present a lower resistance to groWld.
Different models have been, developed to describe the non-linear behaviour, due to soil ionisation process, of concentrated ground e lectrodes. These models can be classified in three categories: empirical models [6-7], variable geometry models [2-5][8] and variable resistivity models [9-11]. The same approaches can also be used for long ground electrodes.
In this paper the counterpoise distributed nature and non-linear behaviour is taken into account using a transntission line model, based on a finite-differences algoritlun [I]. |
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2. Non-Linear Counterpoise Distributed Model |
![](http://www.ground.co.kr/board/images/pds_tec/110_img_01.gif)
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3. Simulation Results |
![](http://www.ground.co.kr/board/images/pds_tec/110_img_05.gif)
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4. Conclusirms |
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![](http://www.ground.co.kr/board/images/pds_tec/110_img_10.gif) |
![](http://www.ground.co.kr/board/images/pds_tec/110_img_11.gif) |
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5. Bibliography |
[1] Correia de Barros, M.T.; Almeida, M.E.; DubIS, L,; Stein B" "A general-purpose transmission line model and its interface with an electromagnetic transients programme", Electrical Power & Energy Systems, Vol. 19, No. 14, pp. 249-254, May 1997.
[2] Mazzetti, c.; Veca, Q,M., "Impulse behaviour of ground electrodes", IEEE Traits. 011 Power Apparatus and Systems, Vol. 102, No, 9, pp.3148-3156, September 1983.
[3] Velazquez, R.; Mukhedkar, D" "Analytical modelling of grounding electrodes transient behaviour", IEEE Trans. 011 Power Apparatus and Systems, Vol. 103, No.6, pp. 1314-1322, June 1984.
[4] Papalexopoulos, A.D.; Meliopoulos, A., "Frequency dependent characteristics of ground 5ystems", IEEE TrailS. 011 Power Delivay, Vol. 2, No.4, pp. 1073-1081, October 1987.
[5] Filho, S.V.; Portela, CM" "Modelling of earthing systems for lightning protection application, including propagation effects", Proc. ICLP-92, pp. 129-132 Berlim, September 1992,
[6] IEEE Working Group on Estimating the Lighming Perfonnance of Transmission Lines, "Estimating lightning performance of transmission lines I]- updates to analytical models", IEEE TrallsaCliolls 0/1 power Delivery, Vol. 8, No.3,pp.1254-1267,JulyI993.
[7] CIGRE Workins Group 0 I (Lightning), Study Committee 33 (Overvoltages and Insulation Coordination), "Guide to procedures for estimating the Jighming perfonnance of transmission lines", Monograph No. 63, October 91.
[8] Bel1ashi, P.L.; Annington, R.E.; Snowden, A., "Impulse and 60-cycle c!J.aracteristics of driven grounds - II", AlEE Trans., Vol. G 1, pp. 349-363, 1942.
[9] Liew, kC.; Darveniza, M., "Dynamic model of impulse characteristics of concentrated e8ths", Proc. lEE, Vol. 121, No.2, pp, 123-135, February 1974.
[10J Almeida, M.E.; Correia de Barros, M.T., ;Modelling the hysteresis behaviour of the transmission tower focting", 9th ]llIerna!ional Symposium 011 High Vollage EngineerIng-ISH 95, VoL S6, pp. 6799 1-4, Graz, 1995.
[11] Almeida, M.E.; Correia de Barros, M.T., "Accurate modelling of rod driven tower footing", IEEE Trolls. on power Delivery, Vol. II, No.3, pp. 1606-1609, July 1996.
[12] Dwight, H.B. , "Calculation of I sistances to grOlllld", AlEE Trans., Vol. 55, pp. 1319¡¤ 1328, December 1936. | | |
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