Temperature dependence of electrical tree inception from water tree degradation

In order to clarify the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we have investigated temperature dependence of water tree and electrical tree inception and propagation from water-tree degradation. In the samples with water tree of the area above ca. 5000 mum2, the ac electrical-tree inception voltage increased with increasing water-tree degradation in high temperature as well as room temperature. This is explained by the moderation of electrical field in the region of water tree. In the samples with water tree of the area less than 5000 mum2 and in the samples without water tree, however, the ac electrical-tree inception voltage is higher at 70degC than at room temperature and ca. 15 kVrms regardless of water-tree area. On the other hand, the samples with dried water tree degradation show reduction in impulse tree inception voltage by ac prestressing at 70degC. Moreover, at 90degC the rate of water-tree growth and the extent of degradation is much higher than at 30degC. This suggests possible danger of water tree degradation at high temperatures.