Influence of mechanical strain and stress on the electrical performance of XLPE cable insulation

XLPE insulation used in high-voltage cables is not only subjected to electrical stress but also to mechanical strains, either from internal residual strains created during manufacturing or externally applied when the cable is bent sharply. Due to thermally activated viscoelastic motion in the polymeric material, the original mechanical stress responsible for the strain may be completely or partly relaxed, with the permanent strain remaining unchanged. It is generally accepted that mechanical strain has a strong influence on the structural integrity and electrical performance of polymeric insulating materials. However, the influence of mechanical strain on the life and performance of insulating materials has still to be fully ascertained, since it depends on the nature (compression or tensile) and the direction of the strain and the magnitude of the remaining (unrelaxed) mechanical stress. In this paper, long-term and short-term breakdown tests were conducted on XLPE ribbon samples. To isolate the effect of the strain, a series of samples was drawn at different drawing ratios; the drawing rate and temperature allowed a situation of near-total stress relaxation. Results are presented for samples subjected to simultaneous tensile mechanical and electrical stress