Proposal of Method for Water Tree Propagation in XLPE Power Cables under Continuous Heating and Heat Cycle Conditions

In order to investigate the effective temperature condition for artificially accelerating propagation of harmful water trees in insulation of 3.3 to 77 kV cross-linked polyethylene (XLPE) cables, an AC voltage of 13 kV and 1200 Hz was applied to a 22 kV XLPE cable removed from service with its outer semiconducting layer only partially immersed in tap water and no water immersion in its high voltage conductor. The temperature conditions were constant at room temperature (RT), 50 °C, 60 °C and 70 °C and under the heat cycle (HC) conditions between RT 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 constant and under the HC condition between RT and 70 °C, and shapes of the bow-tie water trees were narrow and long like those of harmful water trees. Factors that were effective for accelerating water tree propagation were discussed with the results of thermal, mechanical, and X-ray analyses and an evaluation of oxidation of the XLPE insulation.