Characteristics of Partial Discharge Pulse Count Rate and Surge Voltage at Motor Terminal under Inverter Drives

Abstract

In inverter-fed motor systems, output voltage of the inverter has a rectangular waveform with rise time of a few tens of nanoseconds, and consequently a motor suffers from repetitive surge pulses which may have adverse effect on motor insulation. Therefore it is very important to know partial discharge (PD) characteristics during inverter operation especially if there were insulation defects. We have investigated the PD characteristics under inverter surge during actual motor operation, especially about the dependence of PD pulse count rate on inverter DC bus voltage when PD defect exists at phase-phase or phase-ground at motor terminal. As a result, we found that the characteristics of PD pulse count rate depend on DC bus voltage considerably and differs whether the PD defect is at phase-phase or phase-ground. When PD defect is at phase-phase, the PD pulse count rate reaches immediately twice of the power frequency above the PD inception voltage and become constant even if the DC bus voltage is increase, and when the DC bus voltage is further increased, PD pulse count rate starts to increase again. On the other hand, when PD defect is at phase-ground, the PD pulse count rate has increased monotonously with the DC bus voltage until PD pulse count rate has reached twice of carrier frequency. This difference is caused by the difference of the voltage waveform between phase-phase voltage and phase-ground voltage under inverter drives. Therefore, we concluded that it is possible to distinguish whether the PD source is at phase-phase or phase-ground by the dependence of PD pulse count rate on the DC bus voltage.