Simultaneous measurement of microscopic image and discharge pulses at the moment of electrical tree initiation

This paper reports some experimental results concerning the tree initiation from a semiconductive needle electrode in polyethylene under a 50 Hz AC voltage. With a newly developed measuring system, the correspondence between partial discharge pulses and tree initiation and growth up to 10-20 µm in length was clarified for the first time. Tree observation and pulse measurement were made simultaneously. At the moment of tree initiation, positive pulses (current from the needle towards the grounding electrode) of 0.05-0.1 pC were first observed. A very small shade of the initial tree was observed when continuous positive pulses appeared; it grew rapidly to about 10 µm in length, along with the positive and negative pulses. After the tree branched, the growth rate dropped for a while. In many cases, the negative pulses disappeared during this stage.