Modulation Error Control for Medium Voltage Drives with LC-Filters and Synchronous Optimal Pulse Width Modulation

Synchronous optimal pulse width modulation (SO- PWM) techniques provide an appropriate solution to the increasing demand on very low switching frequencies of power semiconductors, in order to reduce switching losses. The shape of their pulse patterns is offline computed, and the modulation techniques assume a steady state operation of the drive system. Medium voltage drives when applied to the retrofit of existing fixed speed induction motors often include an output LC-filter which introduces a resonant circuit. Thus, the combination of an LC- filter with offline optimized pulse patterns deliver a satisfactory performance of the drive system only in steady state operations. Weakly damped oscillations in the filter occur when operating conditions change, resulting in high overcurrents in the motor. In this contribution, two control methods which were recently proposed by the authors are compared. The control methods allow a dynamic operation of drives with LC-filters when SO-PWM is used. Both methods actively damp the LC-filter resonance and compensate the dynamic modulation errors without increasing the switching frequency. The comparison of the modulation error controllers is done by simulations of a 2.4 kV induction motor drive.