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.
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