Effect of electrical and thermal stressing on charge traps in XLPE cable insulation

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Insulation peelings were taken from a single 90 kV ac transmission cable whose segments had been stressed for between ~8000 and 10 000 h either at elevated temperature (363 K) or field 19.5 kV/mm (rms) (T= 293 K). Control peelings were taken from a cable segment that had experienced no stressing. The various peelings were subjected to an endurance test at 70 kV/mm (rms) and T = 363 K, which demonstrated that only those peelings that had experienced thermal stressing had a reduced lifetime compared to the set of control peelings. Space charge measurements made prior to the endurance test showed only negative space charge in all types of peelings and an analysis of the decay dynamics in the control set showed that it resided in two trap distributions at 0.8 to 0.9 eV and 1 to ~1.5 eV. Peelings from a second endurance test (55 kV/mm (rms), T = 363 K) were suspended when the test was terminated at 6088 h. Space charge measurements showed that these samples exhibited positive as well as negative space charge with the positive charge being ~60% of the total. The space charge decay dynamics also showed two trap distributions, ~0.65eV to 0.85 eV and 1eV to ~1.3 eV, with both positive and negative charge in each energy range. Detailed differences in the trap energy range and density in both distributions were found between peelings with different cable histories, and those differences that correlate with a reduction of endurance life are identified. A tentative explanation in terms of anti-oxidant consumption and trap generation by chemical reaction is proffered.