Inception of electrical tree from water-tree degradation at high temperature

In order to clarify the mechanism of electrical-tree inception which often determines the remaining life of power cables, we have been investigating electrical tree inception and propagation from water-tree degradation in polyethylene. Some of the results suggested that at 363 K, the continuous permissible temperature of XLPE cables, the extent of water-tree degradation was much higher than that of water-tree degradation formed at lower temperatures and water-tree degradation at 363 K virtually determines the remaining life. We, therefore, investigated the effects of structural change induced by water-tree degradation at 363 K on AC and impulse tree inception. The experimental results for the samples with water tree dried after it was formed at 363 K showed that the AC and impulse electrical-tree inception voltages were lowered, even though they were slightly higher than those without water tree degradation in the samples with wet water tree. This suggests that the structural change due to severer water-tree degradation such as formation of micro voids leads to lower breakdown strength.