Partial Discharge Measurement in a Motor Winding Fed by a SiC Inverter – How Critical is High dV/dt Really?

Abstract:
This paper investigates partial discharge in a sin-gle-tooth motor winding fed by a SiC inverter with high dV/dt and compares the results with state of the art PD measurement. In order to achieve this, a very noise immune PD sensor with a Rogowski coil and a fine tuned high-pass filter is developed and calibrated in a 50 Hz PD test setup. Here, par-tial discharge events can be seen as soon as the peak-to-peak value of the sinusoidal voltage ex-ceeds VPP ≥ 2200 V. In a next step, a pulse width modulated voltage with a high voltage gradient of up to dVDS/dt = 88 kV/mus is applied at the winding. It turns out that as long as the peak-to-peak value of the PWM voltage is below VPP < 2200 V no partial discharge occurs, despite the very high dV/dt. Only a further increase of the PWM peak-to-peak volt-age to VPP > 2200 V causes PD at the transition of positive and negative PWM voltage blocks. First of all, this means, even in PWM operation with a high dV/dt partial discharge only occurs if the peak-to-peak voltage exceeds the critical value found in a state of the art PD test. No additional effect of the high dV/dt could be observed. Secondly, the num-ber of PD events that degrade the motor insulation depend on the electrical frequency of the convert-er output current. An increased switching frequen-cy doesn’t accelerate ageing. These highly unex-pected outcomes are traced back to the electrical properties of the single-tooth motor winding. To this end, these investigations show that not all motor windings are in danger of high dV/dt. This releas-es new potentials to further improve the efficiency of power electronics and electric drives.