Hybrid Modular Multilevel Converter With Self-Balancing Structure

In the hybrid modular multilevel converter (MMC), the voltage imbalance between the capacitors of half-bridge submodule (HBSM) and full-bridge submodule (FBSM) would occur when the over-modulation index exceeds certain range, which is caused by the different charging/discharging properties of the HBSM and FBSM. An improved hybrid MMC with self-balancing branch (SBB) is proposed in this article to avoid the voltage imbalance under over-modulation situations. The SBBs could offer additional energy transmitting channel for each bypassed HBSM at the interval of negative arm voltage. Thus, the HBSMs could possess the same energy charging/discharging properties as the FBSMs, and then the voltage imbalance between the HBSM and FBSM capacitors could be avoided under any over-modulation indexes. The operation mechanism of the proposed SBB is revealed in detail, according to which the component parameter in the SBB could be selected. The effectiveness of the proposed hybrid MMC with SBBs is verified by the simulation and experimental results. Also, it shows that the proposed hybrid MMC could realize the voltage balancing under over-modulation situations with lower energy losses according to the comparisons with other MMC topologies.