Partial Discharge Pattern Changes due to Electrical Tree Growth in Polyethylene

Abstract

Partial discharges (PD) pulses from electrical treeing in low-density polyethylene with a needleplane electrode system were observed by using a computer-aided partial discharge measurement system. Simultaneously the tree shape was observed optically, and the correlation between the tree shape and the PD pattern was investigated. Unlike the magnitude of PD pulses originating from a void, the magnitude of PD pulses originating from tree path increased with the phase angle of applied voltage (the wing-like pattern) and the PD pulse number in every half cycle was less than the number of tree trunks and much less than the branch number. These facts and the one-cycle pulse-sequence analysis suggested the following: (1) each PD pulse in every half cycle originates from a different discharge path which is connected to the needle electrode; (2) at most one discharge takes place in each discharge path in every half cycle. By applying a pattern recognition technique to the wing-like pattern, the tree shape was successfully assessed.