Partial discharge characteristics of electrical trees in polymeric cable insulation

When polymeric insulation systems are subjected to high electrical stresses, they are susceptible to localized degradation at stress enhancements caused by contaminants within the insulation or protrusions at electrode/insulation interfaces. The degradation is referred to as electrical treeing, tree-like structures of gas-filled channels growing from the stress concentration. There are three phases of electrical treeing, tree initiation which is degradation leading to formation of the first channel, tree growth which is the extension of the channels and the final breakdown when one or more of the channels bridge the electrodes to produce failure. The growth phase is caused by partial discharges (PD) within the channels. Although the PD characteristics have received some attention in the past these studies have used conventional narrow band PD detection equipment which gives information on the apparent discharge magnitudes and repetition rates of the pulses. However, valuable information about the discharge processes, which is available from the pulse shape, is lost. This paper describes the initial results of a study of the amplitudes, repetition rate and shapes of the discharge pulses observed during the growth of electrical trees in XLPE cable insulation using a wide band PD detector