Partial Discharge Characteristics of Electrical Treeing in XLPE insulation utilizing Multi Level PWM Waveforms

Applying pulse width modulated (PWM) shaped voltages allows a flexible control of power and energy. 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 power frequency. Among various degradation mechanisms, electrical treeing is one important process and it can be linked to the presence of partial discharge (PD) activity. In this paper, the changes in PD characteristics at different stages of the degradation by electrical treeing were investigated for XLPE based insulation exposed to rapidly changing voltages of different inverter level. The test specimen in the investigation were subjected to two and three level wave form, each characterized by applying a short rise-time. Here the rise time was 0.75 \ub5s and all tests run applying 414 Hz carrier frequency. 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. As a rule, continuous PD appearance was observed between 23 and 37 kV pp , however the inception voltage was on average found higher for the three level inverter. The total number of PDs and their amplitude per cycle were monitored at the degradation process. Here the experimental results show that the trees had a significantly less branched appearance compared to longer voltage rise times and sinusoidal waveform in previous work. The conclusions from two and three level inverters suggest this method present new opportunities to study the PD characteristics during treeing processes