A Partial Discharge Study of Medium-Voltage Motor Winding Insulation Under Two-Level Voltage Pulses With High Dv/Dt

Abstract

Medium-voltage (e.g. 10 kV rated) silicon carbide (SiC) devices have great potentials in medium-voltage variable speed drives. But their high switching dv/dt can increase the voltage stress on motor windings and cause partial discharges. This paper presents a partial discharge study of a medium-voltage form-wound winding under two-level square-wave voltage pulses. A 10 kV SiC device-based test platform is built to generate voltage pulses with high dv/dt. A three-step test approach is proposed and employed to systematically investigate the effects of various voltage parameters on partial discharges. These include voltage rise/fall time, voltage pulse width, pulse repetitive rate, duty ratio, voltage polarity, fundamental frequency, and modulation index. Partial discharge inception voltages (PDIVs) and repetitive partial discharge inception voltages (RPDIVs) of the sample are measured with varied voltage parameters. Test results show that voltage rise/fall time is a major affecting factor which reduces PDIVs of the winding sample by 6.5% when it decreases from 800 ns to 100 ns. Based on test results, a hypothetical partial discharge mechanism is presented to explain the effects of fast voltage rise/fall edges. An empirical equation is also derived to estimate PDIVs of a winding sample under various voltage rise/fall time and pulse width conditions.

Motor Winding Insulation Degradation Under Repetitive Voltage Pulses

“…Moreover, because PD activities depend on the peak-topeak value of excitation voltage, the PDIV of the unipolar waveform should be at least double that of the bipolar waveform. The experimental results in [35] are consistent with this finding. Compared with unipolar waveform, bipolar waveform excitation is the worst case for PD detection, and this also corresponds to the instruction mentioned in standard IEC/TS 61934:2011 [58].…”

“…It can be concluded that regardless of the test objects or insulation models, both the PDIV and PDEV for shorter rise time PWM waveform (unipolar and bipolar) are lower than those of longer rise time waveform, and the PDIV & PDEV values for DC excitation are always larger than both unipolar and bipolar PWM waveform. [35] tests the RPDIV of Type II machine insulation. It finds when the peak-to-peak value is fixed (ignoring overshoot), the RPDIV of motor insulation is slightly higher under bipolar voltage excitation.…”

“…Furthermore, it suggests that the higher frequency of impulse voltage of 50% duty cycle will result in significantly reduced PD magnitude [66]. If the polarity of the applied voltage changes too fast to provide sufficient time for electrons to accumulate, the PD number and magnitude will be less [34,35]. Therefore, faster switching speeds will result in fewer and smaller PD events per impulse.…”

Voltage rise rate‐related generalised probabilistic lifetime model of turn‐to‐turn insulation in inverter‐fed motors

“…Many approaches have been proposed to investigate the effects of various parameters of repetitive impulse voltage stress, including voltage duty cycle, voltage frequency, voltage rise/fall time, voltage amplitude, and voltage polarity. Hu et al [33] investigated the effects of various voltage parameters on partial discharges of medium-voltage form-wound winding. They obtained an empirical equation to estimate partial discharge inception voltages (PDIV) at various voltage rise/fall times and pulse width conditions.…”

“…Hu et al. [33] investigated the effects of various voltage parameters on partial discharges of medium‐voltage form‐wound winding. They obtained an empirical equation to estimate partial discharge inception voltages (PDIV) at various voltage rise/fall times and pulse width conditions.…”