Reduction of MMC Capacitances Through Parallelization of Symmetrical Half-Bridge Submodules

Abstract

Modular multilevel converters (MMCs) enjoy the benefits of modularity and scalability. In particular, MMCs with symmetrical half-bridge submodules enable bipolar operation yet with a simple structure and low conduction losses. However, bulky dc capacitors in MMC submodules act as one major obstacle that retards the further improvement of system size, weight, and cost performance. Additionally, tight regulation and balance of dc capacitor voltages necessitate expensive voltage sensors paired with dedicated voltage controllers. This article proposes an effective hardware-based strategy that achieves MMC capacitance reduction through parallelization of symmetrical half-bridge submodules. On top of capacitance saving, the proposed strategy balances capacitor voltages in a sensorless fashion, which translates into the removal of voltage sensors and great simplification of control efforts. In addition, the proposed strategy allows fault-tolerant operation of MMCs. Finally, simulation and experimental results validate the effectiveness of the proposed strategy in capacitance reduction and capacitor voltage balancing of MMCs as static compensators.