Water treeing in underground power cables: modelling of the trees and calculation of the electric field perturbation

Abstract

In order to explain the development of different types of water trees and the related dielectric breakdowns in extruded power cables, it is necessary to analyse the dielectric properties of the corresponding treed regions and their influence on the distribution of electric field. The study presented in this paper is both experimental and theoretical. Experimentally, we performed the laboratory ageing of a power cable for accelerated conditions of applied voltage and frequency: we inspected the different shapes, orientations and dimensions of vented, bow-tie, single-bow-tie and global water trees produced in the polymeric insulation. From a theoretical point of view we proposed adequate models for obtaining the field distribution analytically and using a suitable finite element method. A local electric field amplification is calculated according to the different characteristics of the water trees: length, shape, permittivity and law of variation for permittivity. From our results, a compared analysis of the risk of electric breakdown is performed for the different types of water trees according to their configurations.