The influence of morphology on electrical tree initiation in the polyethylene under AC and impulse voltages

AC and impulse voltages were applied to needle electrode specimens modeling the interface of the semiconducting layer of the XLPE (cross-linked polyethylene) cable. TEM (transmission electron microscope) photographs of minute trees (not longer than 10 μm) immediately after their initiation were examined in detail. AC trees are found to be hollow tubes of about 100 nm in diameter in the initial stage. The tree starts from a minute projection of the electrode due to cohesion of carbon particles. Lamellae are radially concentrated at the tree starting point. The tree extends along lamellae and has corrugated constrictions at a pitch of about 50-100 nm. AC tree initiation is suggested to be a local breakdown due to concentration of electric current at a minute region on the order of 100 nm. Minute impulse trees are found to be about 100 nm in diameter. The lamella concentration around the tree starting point is not clear. The impulse tree is more parallel to the lamellae compared with the AC tree, and has no corrugated constrictions in it. Impulse tree initiation is suggested to be a filamentary breakdown over several micrometers