Synchronous Optimal Pulsewidth Modulation for Low-Switching-Frequency Control of Medium-Voltage Multilevel Inverters

This paper presents the mechanism and details of synchronous optimal pulsewidth modulation (PWM) generation for control of medium-voltage induction motor drives using multilevel inverters at low switching frequency. Multilevel inverters allow operation at multiple of dc-link voltage and reduce the total harmonic distortion (THD). Synchronous optimal PWM control permits setting the maximum switching frequency to a low value without compromising THD. Low switching frequency reduces the switching losses of the power semiconductor devices. An optimal control procedure is explained in detail. The performances of three- and five-level inverter topologies are compared. The experimental results of a five-level inverter drive using optimal PWM control are presented.

I. Introduction

Medium-Voltage ac drives based on voltage source inverters are in increasing demand for various industrial applications. To achieve better efficiency at higher power, the voltage rating, rather than the current, of the inverter is increased. Multilevel inverters are a preferred choice for medium-voltage drive applications [1]–[4]. They allow operation at multiple of dc-link voltage, which reduces the total harmonic distortion (THD) [1]–[4] while offering several other benefits discussed in [1]–[3]. Several possible configurations to realize multilevel inverters are discussed in [1]. Space vector modulation (SVM) for multilevel inverters for medium-voltage applications has been discussed in [2]–[4].