Electric field computation in water treed polyethylene with space charge accumulation

Electrical degradation of the insulating systems of energy cables is close related to the water treeing and space charge development. In this paper the results of the calculations of the electric field repartition in the insulation of a medium voltage cable affected by water trees and their related space charge are presented. Continuous and individual water trees, growing from outer and/or inner semiconductor have been considered. The space charge density has been determined by measurements performed on plane samples of polyethylene by using the thermal step method. The repartition of the space charge in the water treed regions has been estimated from FTIR analysis on the repartition of the ions associated to water trees. The results show that the presence of water trees and of space charge determines an important increase of the electric field strength near to the semiconductor layers, which facilitate the inception of electrical trees in insulations and consequently the breakdown at lower voltage