Temperature dependence of inception of electrical tree from water-tree degradation

In order to clarify the mechanism of electrical-tree inception from water-tree degradation 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 higher temperatures the extent of water-tree degradation was much higher than that of water-tree degradation formed at lower temperatures. We, therefore, investigated the temperature dependence of water tree degradation and its effects on electrical tree inception. At 363 K, the water tree growth is much faster than low temperatures and the degradation is severer. When the water tree is wet, electrical tree inception voltage becomes higher, which is explained in terms of electrical-field moderation and suppression of electron acceleration due to the existence of water and so on. In the sample with water tree formed at 363 K, however, the increase in tree inception voltage is small. When the water tree is dry, water tree degradation tends to lower electrical tree inception voltage and to promote the degradation under prolonged ac voltage application. These are explained by structural change due to water tree degradation, such as scission of polymer chains and micro void formation.