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

AC and impulse voltages were applied to needle electrode specimens modeling the interface of the semiconducting layer and insulation of XLPE (cross-linked polyethylene) cables. TEM (transmission electron microscope) photographs of minute trees (<10 μm) immediately after their initiation were examined in detail. It is shown that AC trees are hollow tubes of ≈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 to 100 nm. AC tree initiation is supposed 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 ≈100 nm in diameter, and the lamellae concentration around the tree starting point is not clear. The impulse tree is more parallel to the lamallae than the AC tree and contains no corrugated constrictions. Impulse tree initiation is concluded to be a filamentary breakdown over several micrometers