Evolution of AC breakdown strength of XLPE HV cable after long term test and correlation with physical properties

A dry cured XLPE (cross-linked polyethylene) cable with 15 mm insulation thickness was aged for several years at 135 kV (corresponding to 13 kV/mm maximum stress) on site with load cycles. Electrical tests were carried out on unaged and aged cables. These cables were investigated in their radial direction by cutting thin films on a lathe. Some characteristics of aged insulation such as crystallinity, interfacial diffusion, bow-tie tree distribution, and AC breakdown strength were studied and compared with those of the unaged insulation. Radial breakdown strength obtained from thin films was compared with test results from cables. It was shown that ageing can have a very different influence on electrical characteristics along the radial axis of insulation due to competing factors such as crystallinity, mechanical stresses, and diffusion at semiconductive-shield insulation interfaces