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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.

The starting point of AC electrical trees in polyethylene and the surrounding microstructures were observed by a transmission electron microscope (TEM) for the first time. To minimize the size of tree paths, the authors developed a new apparatus for preparing specimens in which the tree growth was suppressed to less than 10 µm in length. The tree was started from a needle electrode made of carbon-filled polyethylene. The AC tree was a hollow pipe of about 100 nm diameter immediately after its generation. There were constrictions inside the tree, resulting in a corrugated surface. The starting point of the AC tree was a microscopic convex of several tens to one hundred nanometers, which was based on the cohesion of carbon particles of the electrode. The AC tree path was virtually parallel to the lamellae.

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