Inception and propagation of electrical tree from water tree degradation [polyethylene cable insulation]

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 (PD) characteristics, electrical-tree inception and propagation and associated phenomena. The ac electrical-tree inception voltage increased with increasing water-tree degradation, which was attributed to field moderation due to space charge formed in the water-tree region. The relations among ac tree propagation, gas formation and PD as well as water tree growth from electrical-tree tips were observed and discussed. Impulse voltage application, which usually does not lead to significant space charge formation, showed more increase in the tree inception voltage than ac voltage application, which suggests some other mechanisms to increase breakdown strength in and/or near the water tree-region. The effect of the ion concentration in the water was also studied.