A novel digital modulation scheme for multilevel cascaded H-bridge inverters in high power AC drives

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 project presents a novel digital multilevel modulation (DMM) scheme for multilevel CHB inverters. This scheme is very simple, flexible and easy to implement. To generate gate signals for all the active switches in the inverter, the scheme needs only to calculate a three-phase sine function once followed by a number of subtractions in each sampling period. In this project, 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. On the inverter side, line-to-line voltage THD of inverter output for the proposed modulation scheme is as low as IPSD modulation scheme. On the line side, line current THD is as low as phase-shifted modulation scheme because of balanced poser consumption in each power cell. Under the condition of device switching frequency unchanged, a new algorithm is developed to combine the advantages from both phase-shifted and IPD modulation schemes and conquer the drawback of both schemes. The simulation result shows that new modulation scheme has almost same line-to-line voltage harmonic spectrum from light to rated load with balanced power cells.