No abstract

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 light emission. 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. To investigate the intrinsic tree inception process without the effect of field moderation, tree inception and propagation under impulse-voltage application and under dry condition was studied. The increase in tree-inception voltage under impulse voltage was much higher than ac-treeinception voltage, suggesting increase in the intrinsic breakdown strength in and/or near water-tree region. The drying of water-tree region led to lower tree inception voltage and this is consistent with the above presumption.

In order to clarify the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we have investigated effects of water-tree degradation condition and impulsevoltage application on electrical-tree propagation from water tree, As for the condition of water-tree degradation, we have investigated electrical-tree inception voltage by using NaCl solution with different concentrations. Regardless of concentration, the inception voltage becomes higher with the watertree size. This is explained by electrical-field moderation. On the other hand, under impulse-voltage application where less field moderation is expected, the increase in tree inception voltage is much higher. This suggests increase in intrinsic breakdown strength in andor near water-tree region.

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.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 andor near water tree-region. The effect of ion concentration in water was also studied.