A novel digital modulation scheme for multilevel cascaded H-bridge inverters

The multilevel cascaded H-bridge (CHB) inverters are widely used in megawatt variable speed drives, where the voltage level normally varies from seven to thirteen and the number of active switching devices used in the inverter is in the range of 36 to 72. The design of a simple modulation scheme for such inverters with superior harmonic performance is a challenging task. This paper presents a novel digital multilevel modulation (DMM) scheme for multilevel CHB inverters. This scheme is very simple and easy to implement. DMM scheme switching and conduction losses are evenly distributed among individual H-bridges. To generate gate signals for all the active switches in the inverter, the scheme needs only to calculate a three-phase sine function once in each sampling period followed by distributing gating pulses to all the switches in the inverter. In this paper, the principle of the proposed modulation scheme is elaborated and its harmonic performance is analyzed. Comparisons are carried out between the DMM scheme and carrier based modulation schemes including phase-shifted and level-shifted modulations. It is demonstrated that the harmonic performance of the DMM scheme is on par with the best of carrier based modulation techniques.