The effects of asymmetry repetitive square wave voltages on PD statistics and endurance

Repetitive square wave voltages should be used to perform partial discharge inception voltage and endurance (lifetime) tests on insulation systems of rotating machines fed by power electronics in qualification and type tests according to new published International Electrotechnical Commission (IEC). To obtain objective test results, the influence of waveform parameters on partial discharge (PD) and endurance results should be carefully considered and thus accordingly, the appropriate waveform selected. In our previous research, we have concluded that the rise time of the repetitive square wave voltages can influence the PD statistics in both time/frequency domain and the lifetime of magnet wires. However, all the results were obtained under symmetrical repetitive square wave voltages having same rise times and fall times. In fact, many researchers and manufacturers have been using impulsive voltages having asymmetric waveforms with fast rise times (ns level) while much slower fall times (μs level). This study aims to investigate the effects of this type of asymmetric waveform on PD statistics and endurance tests results. A large number of PD and endurance tests were performed on single-contact crossed wires made up of enameled wires used in inverter-fed motors. Experimental results show that repetitive square wave voltages with fast rise time and slow fall time give rise to asymmetrical phase resolved PD patterns, i.e. they give rise to PD events with high magnitude when the voltage slew rate is large, while PDs with much lower magnitude can be observed in correspondence of the flanks with low slew rate. Moreover, endurance results indicate the asymmetric repetitive square wave voltages will, probability, lead to much longer endurance compare with that obtained at symmetrical voltages.