When polymeric insulation systems are subjected to high electrical stresses, they are susceptible to localised degradation around stress enhancements caused by voids or contaminants within the insulation or due to protrusions at the electrode/insulation interface. Electrical tree formation is one of the main causes of the polymeric insulation degradation. The growth of electrical trees within solid dielectric is a result of partial discharge (PD) activity within the tree channels. When treeing occurs, the material around the stressed area decomposes, building hollow carbonised channels. These channels have a hair-like structure with a diameter of a few micrometers [1]. The partial discharge behaviour of electrical trees has been investigated [2-6]. In particular, combined partial discharge detection and video monitoring of tree growth have shown a strong correlation between the partial discharge activity and the spatial and temporal development of the electrical tree [6].
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