The influence of square wave voltage duty cycle on PD behavior

Partial discharge inception voltage (PDIV)and /or repetitive PDIV (RPDIV) tests must be performed in order to design and qualify Type I insulation system of electrical rotating machines driven by voltage converters. RPDIV is measured under voltage surges. Compared to PD detections at sinusoidal voltages (PDIV), these measurements are more complex, since the spectrum of the disturbance produced by the impulse generators might overlap with that of PD pulses. Therefore, the PD detection system design, the selection of impulsive voltage parameters and PD detection procedure should be carefully reviewed to obtain consistent RPDIV values. We have reported that rise time and frequency of repetitive impulsive voltage can influence PD features significantly, both in the time and frequency domain. Thus, the voltage impulse parameters should be carefully selected to improve RPDIV measurements. In this study, the influence of square wave voltage duty cycle on PD statistical features is studied resorting to crossed enameled wires. Based on a large number of PD tests performed under bipolar voltages of different duty cycles, results show that short duty cycles can induce asymmetry in the PD patterns and reduce the probability of incepting PD activities. Moreover, impulse voltage with low duty cycle gives rise to more than one PD at the voltage falling flanks of each cycle, very close to the peak of the disturbance impulses. This phenomenon could hide acquisition of PD pulses, causing not accurate RPDIV estimates when performing qualification and type types on inverter-fed rotating machines according to IEC standards. Therefore, impulsive voltage waveforms having duration time larger than 100 μs should be selected to increase the probability of incepting and detecting PD when performing RPDIV measurements.