Modular Multilevel Converters With Three-Active-Switch Symmetrical-Half-Bridge Submodules and Parallel Connectivity

Modular multilevel converters (MMCs) enjoy growing popularity primarily due to modularity and scalability, but their applications are still constrained by the complexity of hardware and control. MMCs with symmetrical-half-bridge (SHB) submodules can achieve bipolar operation via the simplest structure and reduced cost. However, the imbalance of submodule capacitor voltages remains as a challenge. As a solution, this letter proposes a novel MMC with three-active-switch SHB (3AS-SHB) submodules, which benefits from simple structure and parallel connectivity. Through parallel connection of capacitors among submodules, the proposed MMC allows sensorless voltage balancing, and hence the removal of many voltage sensors, thereby reducing both hardware and control complexity as well as lowering the system cost. Moreover, inherent voltage balancing avoids overmodulation and output waveform distortion. To achieve parallel connectivity, we propose a novel modulation strategy to generate complementary driving signals with adjustable double-edge deadbands. Finally, simulations and experiments validate the effectiveness of the proposed MMC and voltage balance scheme.