The effect of DC bias on electrical tree growth characteristics in epoxy resin samples is investigated, using three waveforms types: AC, AC with positive DC bias, and AC with negative DC bias. Point-to-plane samples were used. AC tests resulted in 62% and 48% longer average time to breakdown than positive and negative DC biased tests respectively. The negative DC bias test had 14% longer average time to breakdown than positive DC bias test. It is suggested that this is due to space charge injection modifying the field at the tree tip. 4 stages of distinct tree growth are identified in AC tests compared to 3 stages in DC bias tests. In particular, the phenomena of trees growing in the ‘reverse direction’ (from the planar to the point electrode) observed in the latter stages of AC tests, is not seen in the DC tests reported here. This is thought to be due to the peak field magnitudes involved in each case.
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