Switched Midpoint Modular Multilevel Converter With Third-Order Harmonic Injection, Extended Natural Balancing, and Fault-Blocking Capability

This article considers the switched midpoint modular multilevel converter (SMPC) topology, which has been recently proposed for the integration of renewable sources and interconnections in ac-dc grids. The SMPC employs a shared flying stack of half-bridge submodules (HBSMs) per phase, which enables zero-voltage switching (ZVS) of its director switches (DSs). It reduces the number of required submodules and lowers energy storage requirements by 17.7% and 40% , respectively, compared to the conventional modular multilevel converter (MMC). This article extends the SMPC to operate with significantly higher modulation indexes by considering the third-order harmonic injection and fault-blocking capability, making it particularly attractive for reliable high-voltage direct current (HVdc) applications requiring a wide voltage variation. Other benefits of the extended SMPC include simplified harmonic current elimination required for stack voltage balancing and reduced semiconductor stress, resulting in significant loss reduction, surpassing even the most efficient competitor, i.e., HBSM-MMC. In addition, its full-bridge submodule (FBSM)-based outer stacks offer inherent dc fault protection through its capacitors' back EMF generation and dc fault ride-through. The principles of operation and energy-balancing concepts are presented and compared with previous well-known counterparts. Finally, the simulation results and experimental evaluations using a lab-scale prototype based on OPAL-RT MMC test bench are given to validate the superiority of the converter's performance.