Hybrid Modular Multilevel Converter with Self-Balancing Structure

A hybrid modular multilevel converter (MMC) topology with self-balancing structure is proposed in this paper. The self-balancing circuit is established for the half-bridge submodules (HBSM) to offer additional energy transferring channels. As a result, the HBSM capacitors can be discharged with positive arm current and be charged with negative arm current by the self-balancing circuit. Thus, the capacitor imbalance between the HBSMs and FBSMs will be avoided under DC voltage reduction without any extra circulating current injections. The control approaches of the self-balancing switches are introduced, and the parameter design of the switches in self-balancing circuits are given. With the self-balancing circuits, the hybrid MMC can be equalized as an FBSM based MMC during over-modulation operations caused by DC voltage reduction. The effectiveness of the proposed hybrid MMC with self-balancing circuit is verified by simulations.