Two types of electrical trees have been observed to grow in a glassy epoxy resin of needle-plane geometry under 50 Hz AC voltage. They differ in their constituent channel radii and the associated partial discharge activity. The so-called ‘filamentary tree’ has finer channels and much smaller dischargemagnitudes than the ‘PD tree’. This work describes an electrical treeing process containing a clear transition from filamentary tree growth to PD tree growth with associated changes to partial discharge measurements. Apart from emphasizing the importance of recognizing tree channel thickness, it provides experimental evidence for an electro-mechanical treeing process in which filamentary channel propagation is followed by enlargement of the channel radius by partial discharges. Moreover, the PD characteristics during the transient stage areassessed. The magnitude of partial discharges during the transition stage was found to be polarity dependent.
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