Breakdown mechanisms of XLPE cable with insulation defects

Defects in Cross-Link Polyethylene (XLPE) insulation accelerates the insulation ageing. This is mainly caused by the high amount of discharges caused by the defects. Voids enclosed in the XLPE insulation are one such type of defects. This study compares the possible breakdown mechanisms of two XLPE samples with protrusions created by a needle electrode. One of the samples prepared with a defined void to analyse the effect of it on the breakdown mechanism. Both samples were prepared by inserting a needle electrode to a slice of XLPE cable. The breakdown voltage of these samples at 100 Hz was measured. Finite Element Method (FEM) simulations were conducted on the samples to investigate the distribution of electric fields during the sample breakdown. As per the FEM results, it can be seen that for the case with a void, the breakdown can be caused by the ionization of air in the void, which results in discharges between the needle and the conductor and ultimately leading to a breakdown of the XLPE. This was further verified by the visual electroluminescence discharges observed just before the breakdown of the sample during the laboratory experiment. In comparison, for the sample with no void, such visual discharges were not observed and the electric field at the breakdown is in the range of the intrinsic breakdown electrical stress of XLPE.