Mitigation of high du/dt-originated motor overvoltages in multilevel inverter drives

An overvoltage at the motor terminals of a pulse-width-modulated voltage-fed drive can cause premature insulation failure in the windings of the motor. Pulsed inverter voltage and the impedance mismatch between the cable and the motor cause an oscillating overvoltage in the motor terminals. Several methods to reduce these overvoltages have been introduced, most of them being based on filtering with lossy passive components. Despite the recent interest in multilevel inverters, the overvoltage phenomenon in these applications has received only little attention. It is often assumed that in multilevel converters, oscillating overvoltages are relatively small, because a single voltage level constitutes only a part of the fundamental voltage. However, the voltage stress caused by high du/dt-rating may induce partial discharges also in a medium-voltage machine. In this paper, overvoltages are shown to exist in multilevel inverter drives. The paper proposes a method to mitigate these overvoltages; the method uses two properly timed consecutive switchings to almost completely cancel out the voltage oscillations at the motor end of the cable. This cancellation is possible by using oscillations in opposite phases. The method is analytically derived, and its feasibility is tested with a 3 kV inverter prototype and two different motor cable lengths.