Inception of electrical tree from water-tree degradation

In order to clarify the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we carried out simultaneous observation of partial discharge characteristics, electrical-tree inception and propagation and associated light emission on both samples with a wet water tree and a dried water tree. The ac electrical-tree inception voltage increased with increasing water-tree degradation in a sample with a wet water tree, which was attributed to field moderation. On the other hand, under impulse voltage application the increase in tree-inception voltage is much, higher than ac-tree-inception voltage, suggesting increase in the intrinsic breakdown strength in and/or near water-tree region. On the basis of these results, effects of structural change in water tree region on electrical tree inception were discussed by investigating effects of drying of water tree and ac prestressing.