Nucleation of Electrical Tress in Polyethylene

The inception voltage Vi for 1 pC partial discharges has been measured for vacuum molded needle-plane samples of low density polyethylene. The principal parameters studied are needle radius R (3¿R¿100 ¿m), ambient gas (air, N2, Ar, SF6), and frequency of the applied ac Voltage (60 Hz, 1 kHz). Experimental results during this "nucleation phase" indicate that electrons, field-emitted from the needle tip, transit through microvoids in the polymer before being stopped by collision with polymer chains. During transit they interact with molecules of the particular ambient gas which permeate the polymer\´s "free volume". In spite of the extremely small size of microvoids, Townsend avalanche-like behavior is inferred.