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Partial discharge characteristics in an artificially-simulated tree channel

In attempt to clarify the electrical treeing degradation, partial discharges in an artificially-simulated tree channel in a low density polyethylene (LDPE) sample were investigated. The typical PD pulse sequences from the tree channel in our experimental conditions showed that at most one PD pulse took place in a half cycle and that PD appeared in several consecutive cycles and then ceased for several cycles. In other words, there were PD pulse trains with discharge free intervals in between. There existed a strong correlation between positive and negative PD pulses occurring in two consecutive half cycles. PD pulses in a train had almost the same magnitude, though the average magnitude changed with time. In order to understand the condition for PD to cease, we investigated the polarity of the last pulse in each train. The last PD pulse tended to have the positive polarity. After a few hours of voltage application, a new tree channel was formed at the tip of the artificial tree channel. The last pulses in PD pulse trains after the formation of a new channel tended to have the negative polarity.