The effect of ageing on charge traps in XLPE cable insulation

The effect of ageing upon charge trap density and distribution is determined by measuring the decay dynamics of space charge introduced into peelings taken from XLPE insulated cable pieces that had previously experienced different types of stress. Control peelings from unstressed cables showed only negative space charge residing in two separate trap distributions covering the trap depth ranges 0.8 - 0.9 eV and 1 - about 1.5 eV. Peelings that were suspended from endurance testing (55 kV/mm & 363 K) after 6088 hours exhibited both positive and negative space charge, with the positive charge comprising about (about 45-65%) of the total charge residing in traps lying at depths of about 0.75 eV - 0.96 eV. The negative charge was present in deeper traps at 1.05 eV to about 1.5 eV. Peelings from thermally stressed cable had an increased trap density per eV for negative charges (deep traps) but very little change for the positive charge traps. These changes correlate with a significant reduction of endurance life observed in other peelings from the same thermally stressed cable piece. Similar measurements for peelings from the electrically stressed cable piece showed a reduction of endurance life only at low failure probabilities. The corresponding trap analysis however shows an increase in trap density for the positive charge traps. No change in the total negative charge was observed in the deeper traps. However it was found that the highest occupied trap levels of the negative charge were shifted to deeper energies (about 1.13 eV) commensurate with longer de-trapping times.