Water Tree Simulation on Underground Polymeric Cable Using Finite Element Method

Abstract

Most insulation failures in polymeric underground cables have been caused by the formation of water tree in the polyethylene insulation that leads to electrical tree. Electric field intensity is fundamental to water tree growth, hence studying and modeling water tree in a cross-linked polyethylene (XLPE) insulation is vital as insulation failure is frequently triggered by water tree. The aim of this study is to determine the electric field intensity and to identify the electric potential distribution in XLPE insulation used in the underground medium voltage cable which are affected by water tree. Finite Element Method is used to perform the simulation works. The Electrostatic numerical models of 11kV single core XLPE cable affected by the variations of water tree models and size of water tree are analyzed. The two types of water tree, vented tree and bow-tie tree are modeled in the simulation and the properties of the models were set by the experimental value found in the literature. The simulation results revealed that regardless of water tree type, size, length, shape, dimension or location, water tree contributes to higher electric field at the affected region and thus reduces the dielectric strength of cable insulation. Nevertheless, the relative permittivity, shape, length and location of water tree induce a significant variation of electric field intensity in the insulation. The electric field is found to be more intensified at the region where water tree is closer to the conductor. Therefore, electrical tree is more likely generated from the vented water tree initiated from the outer surface of the insulation that grows towards the conductor rather than the other types of water tree.