Inverter-embedded Partial Discharge Testing for Reliability Enhancement of Stator Winding Insulation in Low Voltage Machines

Partial discharge (PD) due to fast risetime, pulse-width modulation (PWM) has been identified as one of the leading causes of insulation failure in low voltage (LV) motors with random wound stator windings fed by variable frequency drives (VFD). With the recent increase in electrical stress with faster risetime wide bandgap power devices and higher voltage for improved efficiency and power density of the system, PD-induced insulation failures are expected to increase. Since LV motor insulation is not resistant to PD, an increasing number of VFD motors are being qualified for PD-free operation. However, the PD inception voltage (PDIV) of qualified motors can decrease with insulation aging and the operating environment making LV motors susceptible to failure. In this paper, a new concept for automated inverter-embedded PD testing under impulse excitation is proposed. The main concept is to perform impulse PD testing using the inverter at increased voltage levels whenever the motor is at standstill for providing advanced warning of PD activity for preventing forced outage of VFD motors. Two options for applying increased impulse test voltage for implementing the proposed concept are presented. Test results on an 800 V VFD driven motor shows that the proposed methods can provide automated PD testing whenever the motor is stopped for improving the reliability of the VFD motor stator insulation.