Dielectric relaxation and DC conductivity of transformer pressboard and XLPE cable insulation

The authors present results from preliminary laboratory measurements of return voltage made on samples of oil-impregnated transformer pressboard and XLPE (cross-linked polyethylene) cable insulation. The effects of temperature and water content have been examined. Particular attention has been paid to the dielectric response of water-tree-aged cable insulation. The qualitatively different effects of water on XLPE and impregnated paper observed in these preliminary investigations may indicate that, while interfacial polarization is a dominant effect in the paper-oil system, electron (or hole) trapping in sites in the tree regions is an effective charge storage mechanism in XLPE. The fact that the presence of water apparently reduces the trapped charge mobility in XLPE objects containing trees is difficult to explain, but again it must be related to the trapping mechanism(s) being modified by water