Correlation between the AC breakdown strength and space charge distribution of water tree aged XLPE

To understand the electrical characteristics of water tree aged XLPE, artificially aged XLPE specimens of which the water tree with uniform length grew on the large area have been fabricated. The AC breakdown strength of the specimens has been reduced as the growth length increased and the trend of the reduction has been similar to that of water tree growth rate. Space charge distributions in the virgin and aged specimens have been measured by the PEA method and, as a result, it has been shown that the homo-charge is heavily concentrated at the tip of the water tree path while a little hetero-charge is gathered in front of electrode contacted to the water tree. Through the electric field calculation using the charge distribution, the local field enhancement at the tip of water tree path was also verified. Based on the experimental results and analysis, it can be said that the water tree reduces the breakdown strength of XLPE because of the homo-charge concentration at the tip of water tree path, which results in the reduction of effective insulating thickness.