Surface degradation of XLPE insulation at oil-water interfaces

Water ingress in high voltage oil-filled XLPE cable terminations has been found to be one of the main reasons for service breakdown causing a high risk of life for nearby personnel due to a possible explosion of the component. Some utilities perform regularly maintenance of such installations by measurements of water content of the oils. It was found that for some installations the water content was high, where even liquid water was present in the oil [1]. The breakdown mechanism for such situations is yet not clear. The main purpose of this work has therefore been to examine the effect of liquid water in the oil in such installations. The work has been performed on small scale model terminations. A small glass container was filled with oil having water at the bottom. The AC voltage was increased to 4.4 kV/mm in steps of 0.7 kV/mm with 30 minutes at each step. Subsequent measurements of partial discharge and video recording were done at each step. Long-term tests were also performed on six similar set-ups. The results show that partial discharges can only occur with the water layer grounded. It is also observed that a thin irregular water film is growing more than 10 mm along the cable surface in the oil up to about half the service stress (1.6 kV/mm) and stop growing further. The partial discharges, varying from 5 to 75 pC, are closely related to the release of water droplets from the irregular water film at higher stresses. Long-term tests resulted in water induced breakdown for one of the set-ups.