Electric Field in Polymeric Cable due to Space Charge Accumulation under DC and Temperature Gradient

When power cables are loaded under high voltage direct current (HVDC), an accumulation of space charge and a radial distribution of temperature gradient are developed across the insulation material. Such existence and accumulation of space charge within the insulating material poses a threat to the reliability of the operation of dc power cables. The electric field of a practical dc power cable is affected by the conductivity of the material, which is a function of both temperature and electric field. This causes difficulties in identifying the electric field distribution. In this paper, a method of determining the electric field distribution in dc power cables was proposed by considering the influence of space charge on the conductivity of the insulating material under different temperatures. Commercial 11 kV ac cross-linked polyethylene (XLPE) power cables were used and the space charge in these cables under dc conditions was measured using a modified pulsed electroacoustic (PEA) system with an attached current transformer. Therefore, a replica of a power cable under load conditions is obtained, which allows an investigation of the formation, migration and accumulation of space charge in a power cable with and without temperature gradients across the insulating material. COMSOL Multiphysics software package was used to accurately determine the electric field distribution in the dc power cable with consideration of the influence of electric field on the conductivity of the insulating material. The numerical modelling is based on the hopping conduction mechanism and its parameters were obtained from experiments carried out on the XLPE insulation material.