Reduced Switching Frequency (RSF) Voltage Balancing Technique for 21-Level Hybrid MMC

The conventional half-bridge sub-module (HB-SM) based modular multilevel converter (MMC) fails to provide the DC fault handling capability. Nevertheless, this drawback can be overcome by replacing the HB-SMs with the full-bridge (FB) SMs since the FB SMs can generate bipolar voltages thus handling the DC fault condition. But the drawback of using FB-SMs arises as it employs double the number of switches used that in case of HB-SM-based MMC, thus increasing the switching losses due to an increase in device count. Then, the Hybrid MMC, which employs HB SMs in addition to FB SMs, came into the picture and mitigated the drawbacks mentioned earlier significantly. This paper presents two cases for 21-level Hybrid MMC, wherein the first case implements the hybridization of 50% conventional SMs and 50% FB-SMs, and case two implements the same with 33.33% conventional SMs and 66.66% FB-SMs. The reduced switching frequency (RSF) capacitor voltage balancing approach is employed to balance the capacitor voltages of HB-SMs and FB-SMs for the first combination (50% HB and 50% FB-SMs); whereas the modified voltage balancing approach has been applied to the second case (33.33% HB and 66.66% FB-SMs). To evaluate their performances, a comparative analysis of 21-level conventional HB-SM-based MMC, FB-SM-based MMC, and Hybrid MMC with both mentioned combinations, is carried out in MATLAB simulation for the corresponding voltage balancing techniques used. The impact of voltage balancing approach on switching transitions is also presented.