Polarization and charge trapping in cross-linked polyethylene (XLPE) cables AC aged in humid environment

The authors report the results of TSDC (thermally stimulated discharge current) and TTC measurements of XLPE (cross-linked polyethylene) cables AC aged in water for a period up to 4000 h. Absorption and resorption current measurements were also performed during the aging to get more information about the penetration of water and ions into the cable. The results suggest that initially the polar moieties of cross-linked by-products together with the antioxidant play a dominant role in the internal polarization of the polymer. The diffusion of ions with water under the AC stress also is enhanced with time. The dipole density gradually reduces with aging time due to diffusion. With increasing AC aging there is also the enhancement of the density of the localized energy states and the oxidation level, which in turn increases the injection efficiency of charge carriers which are subsequently trapped. The observed TSDC spectra of the AC aged and DC poled samples may thus have a complex origin