Using the UHF technique to investigate PD produced by defects in solid insulation

The partial discharges produced by internal defects in solid insulation have been investigated using the ultra high frequency (UHF) technique. The work has shown that the discharges produced by a virgin cavity generate a UHF signal that may be detected using this UHF technique. The discharges are observed as a series of UHF pulses that occur less frequently as time progresses. The reduction in the number of UHF pulses has been attributed to a progressive change from the streamer discharge mechanism, that is observed with a virgin cavity, to the Townsend mechanism, which occurs when the cavity has been exposed to prolonged discharge activity. It also has been shown that the discharges associated with tree formation generate a detectable UHF signal. The paper describes a method of distinguishing between the streamer discharges and the discharges that cause treeing