A survey of low-switching frequency modulation techniques for medium-voltage multilevel converters

Multilevel converters (MLC) are now a standard solution for medium-voltage (MV) high-power applications and several modulation techniques have been reported in literature. However, high-power applications require low device switching frequencies to reduce the dominating switching losses and thus improve the overall efficiency of drive system. On the other hand, total harmonic distortion (THD) of current waveforms increases at low device switching frequencies. Therefore, the challenge is to maintain the quality of current waveforms while operating MLCs at low device switching frequencies. The main goal of our study was to provide brief review of the various low-switching frequency modulation techniques proposed for MLCs in MV high-power applications and then, discuss in detail about one of the emerging control technique known as synchronous optimal pulsewidth modulation (SOP). The experimental results from seven and nine-level inverters fed induction motor drive demonstrated the effectiveness of SOP technique. Also, some of the future trends in the applications of MLCs are discussed to encourage further research into development of low-switching frequency modulation techniques.