Space charge and impulse breakdown strength in XLPE

The impulse breakdown strength of XLPE samples made of cable-grade material poled at 35 kV/mm DC has been found to be reduced by approximately 35 kV/mm for the aiding case compared to the opposing polarity. The internal space charge distribution is, however, found to be reasonably homogeneous and consisting of negative net charge, seemingly contradicting the commonly assumed explanation of the polarity dependence effect, based on the occurrence of heterocharge accumulation in the material during the pre-stress period. The present results indicate that the local field strength near the anode controls the breakdown, but no hypothesis supporting such an effect is available