Computation of the Electric Field in Aged Underground Medium Voltage Cable Insulation

In this paper, the distribution of the electric field in the polyethylene insulation of a medium voltage power cable is computed in the absence and in the presence of water trees and space charge. The computations are made in variable regime, from the voltage switch-on up to one hour of application of the electrical stress. The water trees are considered of semi spherical shape and of known permittivity and conductivity. The conductivity and the characteristics of the water trees are determined experimentally. The influence of the dimensions of water trees and of the associated space charge layers (length and charge density) are analyzed in terms of local values of the electric field and of reduction of the cable breakdown voltage. It is shown that, for a water tree of given length, the probability of insulation breakdown reaches its maximum for a given ratio between space charge density and length of the space charge area.