The influence of impulsive voltage frequency on PD features in turn insulation of inverter-fed motors

The repetitive impulsive surges from inverters has been found undoubtedly to cause insulation reliability flaws in inverter-fed motors. To mitigate this problem, PD detection and life testing under sinusoidal and/or repetitive square voltages should be performed on new types of inverter-fed motor insulation, according to IEC standard and Technical specification(TS), (i.e. IEC 60034-18-41 and IEC TS 60034-18-42). The obtained partial discharge inception voltage (PDIV) and lifetime of insulation in the tests are used to evaluate the insulation performance and to improve the insulation designs. Under repetitive square voltages with short rise times, the influence of the square voltage frequency on PD activity should be investigated (a) to understand the influence of frequency and duty cycle of the repetitive square voltage frequency on PDIV and life, and (b) to increase the signal-to-noise ratio when extracting PD pulses from the strong noise generated by the operating switch of impulse generators. Therefore, the main purpose of this study is to investigate the effects of repetitive square voltage frequency on PD events. Insulation models of Type II (high-voltage inverter-fed motor) were used to obtain PD statistical features under square wave voltages with frequency from 1Hz to 5 kHz. Experimental results indicate that higher frequencies give rise to less PD events with lower PD magnitudes and shorter delay times in one square voltage cycle. This phenomenon could be attributed to the surface charge decay times and the initial electron emission probabilities resulting from different square voltage frequencies.