Study on Artificial Water Tree Degradation Method for XLPE Power Cables (Part 2)

－Proposal of Method for Water Tree Propagation under Continuous Heating and Heat Cycle Conditions－

Abstract

In order to determine the effective temperature condition for artificially accelerating propagation of harmful water trees in insulation of 3.3 to 77 kV XLPE cables, an AC voltage of 13 kV and 1200 Hz was applied to a 22 kV XLPE cable removed from service at room temperature, 50°C, 60°C and 70°C, and under the heat cycle conditions between room temperature and 50°C, 60°C or 70°C. As a result, it was found that bow-tie water trees propagated up to a few mm at 70°C and under the heat cycle condition between room temperature and 70°C, and shapes of bow-tie water trees were narrow and long like those of harmful water trees. Factors that were effective for accelerating water tree growth were discussed with the results of thermal, mechanical, and X-ray analyses and an evaluation of oxidation of the XLPE insulation. In addition, AC withstand voltage tests were performed for the cable samples after 300 days of water tree degradation at 70°C and under the heat cycle condition between room temperature and 70°C.