One-Leg Phase-Shifted-Carrier PWM Control of a Full Bridge Cell for Hybrid Modular Multilevel Converters operating in buck and boost modes

This paper proposes a new phase-shifted-carrier (PSC)PWM-based control approach for Hybrid modular multilevel converters (HyMMC) operating in both buck and boost modes. When the conventional PSCPWM for Half Bridge (HB)MMC and Full Bridge (FB)MMC is (respectively) applied directly to HB and FB cells in HyMMC, some mismatched pulses occur in the output voltage, which cause lower effective switching frequency and reduced efficiency. The effective solution (based on PSCPWM) to remove these pulses is already demonstrated in the literature for the buck operating mode of HyMMC. However, the solution proposed in this work is effective for both buck and boost modes of HyMMC, and the effective switching frequency is equal to the HBMMC. In this approach, only one leg of FB cells is operated in PWM mode, while the other leg is switched at fundamental frequency in the boost mode, and no switching occurs in the buck mode. This creates further opportunity to improving efficiency and cost of FB cells in HyMMC by optimally selecting devices in the fundamental frequency switching leg. Simulation and experimental results validate the proposed approach.