Failure mechanism of variable frequency drives in ungrounded distribution systems

This work investigates an actual violent failure of a number of variable frequency drives that were supplied by an ungrounded power source. It was noted that when a phase-to-ground fault was manifested in one of the motors, all the VFDs were destroyed except the one that is connected to the motor where the fault occurs. Circuit analysis and simulations using PSpice was carried out in an effort to explain the cause of such destruction. It is found that the inverters used in this facility were not designed for ungrounded power supplies, and the cause of such failure is the presence of an EMI capacitor filter that is placed between the DC link midpoint and ground.