00_electrc_watertre_g - 860 IEEE Transactions on...

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860 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7 No. 6, December 2000 The Effect of Water Treeing on the Electric Field Distribution of XLPE Consequences for the Dielectric Strength I. Radu; M. Acedo? J. C. Filippini Laboratoire d’Elcctrostatique et Mat6riaux Di6lectriques de Grenoble Grenoblc, Frailcc P. Notingher Politehnica University of Bucharest Bucharest, Romania and F. Frutos Departamento de Fisica Aplicada Facultad de Informatica Universidad de Sevilla, Spain ABSTRACT This paper describes the influence of water trees on dielectric properties of flat samples cut from a HV crosslinked polyethylene (XLPE) cable insulation. In order to obtain experimental results in a short period of time, we used accelerated aging conditions of voltage and frequency as well as differeat techniques for producing a multitude of artificial water tree inception points. The average value of the water tree permittivity and a law for the time-dependence of permittivity inside the treed degraded areas were deduced from measurements of capaci- tance and water tree lengths. On the basis of the experimental knowledge permittivity and water tree length, we determined with analytical/numerical methods, the distribution of the electric field for different shapes of treed regions, Because of the increase in length and per- mittivity of the water tree, the electric field in front of the degraded area is amplified, which is an important factor of risk leading to the electric breakdown. 1 INTRODUCTION N medium voltage (MV) and HV polymeric power cables, operating I in a wet environment, a degradation process known as water treeing may occur. Water treeing was first detected in 1969 by Miyashita [l]. Presently, water treeing is considered responsible for the majority of failures in underground distribution cables. Whereas the causes of initiation and propagation of water trees have been studied thoroughly and are more or less understood, the mecha- nisms responsible for breakdown, once water trees are present in the insulation, are far from being clarified. Several mechanisms have been proposed [2-41 but no definite conclusion can be drawn, in particular because the properties of the water trees have not been thoroughly char- acterized. Among the proposed mechanisms, the mechanism based on a local amplification of the electric field (LAP) due to the growth of water trees [4] is particularly attractive because of its simplicity and because of the fact that the amplification of electric field can be calculated from geometric and dielectric data. The mechanism can be summarized as follows: due to the higher permittivity of the water tree relative to the surrounding medium, the electric field is amplified in the undamaged region facing the tree, and if a defect, e.g.
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This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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00_electrc_watertre_g - 860 IEEE Transactions on...

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