Partial Discharge Characteristics of Electrical Treeing in XLPE Insulation Exposed to Voltages of Different Rise Times

The use of pulse width modulated (PWM) waveforms allows a more flexible energy applications and management The downside is that under such operating conditions the voltage stress imposed on the insulation systems increases, particularly as the frequency content is considerably higher than at the conventional 50 Hz sinusoidal waveform. Among various degradation mechanisms, electrical treeing is one important processes and it can be linked with a presence of partial discharge (PD) activity. In this paper, the changes in PD characteristics at different stages of degradation by electrical treeing were investigated for XLPE based insulation exposed to rapidly changing voltages of varying magnitude. The tested material samples were subjected to two square voltage shapes, each characterized by their rise-time. Here the shorter was 1 mu s and the longer 0.5 ms, all tests run at 414 Hz. To compare the voltage endurance, the voltage level was gradually increased until detectable PD activity and tree initiation could be observed, both electrically and optically. Typically continuous PD appearance was observed between 23 and 32 kVpp. The total number of PDs and their characteristics were monitored as the trees were gradually growing. Here the experimental results shows that the PDs appears at lower voltage magnitudes for the shorter rise times and that the trees had a significantly less branched appearance as compared to the longer voltage rise time. These results resemble the observations that recently been presented with tests on DC voltage pre-stressed material samples exposed to impulses of reversed polarity. Additionally the measured PD characteristics were non-symmetric, having a clear polarity dependence. This finding suggests a possibility to use the presented approach for a more detailed investigating of degradation processes in solid insulation systems suitable for both HVAC and HVDC applications.