Material anisotropy effects on space charge distribution in high voltage cables

High voltage XLPE (Cross-Linked PolyEthylene) insulated cables can reveal some angular anisotropy of material properties due to extrusion and characterised by a higher level of residual stresses. These stressed regions are located by birefringence measurements and found at precise and reproducible location in the dielectric. This parameter has been taken into account since the beginning of this work. The cable under investigation was electrically aged in laboratory at a maximum AC field of 27 kV/mm for 3000 hours. The influence of both the level of residual stresses and high field electrical ageing on the physico-chemical and dielectric properties of the XLPE has been studied. The space charge distribution in the insulating wall, induced by application of a DC polarisation, has been measured using the Pressure Wave Propagation method (PWP) at different angular locations in the cable. The anisotropy of material properties is revealed by this method. This work also shows that the angular parameter must be integrated for any study on cable material ageing