Partial Discharge Measurement in a Motor Winding fed by a SiC Inverter - How critical is high dV/dt really?

摘要

This paper investigates partial discharge in a single-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, partial discharge events can be seen as soon as the peak-to-peak value of the sinusoidal voltage exceeds V_(PP) ≥ 2200 V. In a next step, a pulse width modulated voltage with a high voltage gradient of up to dV_(DS)/dt = 88 kV/μs is applied at the winding. It turns out that as long as the peak-to-peak value of the PWM voltage is below V_(PP) < 2200 V no partial discharge occurs, despite the very high dV/dt. Only a further increase of the PWM peak-to-peak voltage to V_(PP)>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 number of PD events that degrade the motor insulation depend on the electrical frequency of the converter output current. An increased switching frequency doesn't accelerate ageing. These highly unexpected 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 releases new potentials to further improve the efficiency of power electronics and electric drives.