Electrical Tree Initiation in LDPE under Positive DC Stresses Superimposed with AC Ripples

Electrical tree initiation in LDPE under positive DC stresses with controlled AC ripple is investigated. Needle-plane geometries were used and partial discharge (PD) signals recorded. Tree initiation occurred around +45 kV, and the initial growth within the first second was long compared to the more gradual processes seen with AC excitation. In each case, when the first tree image was captured, they were of the branch type and at least one order of magnitude longer than those seen under pure AC stresses. 5 kV peak AC superimposed onto +45 kV led to rapid sample failure. Unlike in pure AC cases, PD detection did not correlate well with the progress of electrical treeing (initiation, growth or breakdown) under pure DC stresses. Phase resolved PD analysis did however provide useful information in the presence of an AC ripple.