Space charge and breakdown strength of degassed XLPE insulation

Space charge formation and impulse electric breakdown strength of degassed cable grade XLPE insulation have been studied, using aluminium and semiconducting cable screen material electrodes. In both samples with aluminium and semiconducting cable screen material electrodes, the space charge accumulation was mainly controlled by electrode processes. The results from impulse breakdown tests of samples with aluminium and semiconducting cable screen electrodes pre-stressed with a de voltage showed that the breakdown strength was influenced by the space charge accumulation. For samples with aluminium electrodes, the breakdown strength was lower in case of impulse voltage of the same polarity as the applied stress than in the opposing case. For samples with semiconducting electrodes, there was no effect of the pre stressing on the impulse breakdown strength. This difference was explained by significantly higher magnitudes of accumulated charge in samples with aluminium electrodes.