Multilevel modulation method for mitigation of high du/dt-originated oscillating overvoltages at motor terminals

Abstract

Multilevel inverters have drawn wide interest in the field of high-power applications. Regardless of the attention paid to inverters, the overvoltage phenomenon, which is well known in two-level inverters, has not been investigated thoroughly in multilevel applications. The oscillating overvoltages occur after high du/dt voltage edges if the propagation delay of the motor cable is long compared to the voltage rise time. These oscillating voltages at the motor terminals cause, for example, partial discharges in the stator windings and deteriorate them. A multilevel modulation method that suppresses the oscillations is introduced in this study. The method is based on the timing of two consecutive voltage edges, so that the oscillations caused by the edges counteract each other. This counteraction mitigates the resulting oscillating voltage. The principles of the method are given in detail for a wide scale of multilevel inverter levels. The method is implementable to almost any multilevel inverter topology. Measurements with a 100 kVA, 3 kV series-connected H-Bridge inverter confirm the method.