The effect of gas impregnation on the time to electrical tree inception in XLPE

Long-term aging tests were performed on semiconductor protrusions embedded in cross-linked polyethylene (XLPE) cables impregnated with different gases. During aging, electroluminescence emitted at the protrusion was monitored continuously until tree initiation. The results showed that oxygen in the free volume of the polymer plays an important role in the deterioration of XLPE insulation subjected to high electrical stress. Decreasing the concentration of oxygen by degassing or impregnating with N/sub 2/ or SF/sub 6/ increased the time to electrical treeing. While the results for N/sub 2/- and SF/sub 6/-impregnated XLPE were comparable, degassed XLPE had the longest time to tree inception. It is shown that ultraviolet light emitted during electroluminescence photodegrades the polymer and can account for the differences in the times to electrical tree inception in XLPE impregnated with different gases.<
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