Submodule Voltage Balancing Methods in Modular Multilevel Converters – A Review

With its inherent modular structure, modular multilevel converters (MMCs) have outstanding scalability and application flexibility, but it also introduces the submodule (SM) voltage balancing issue. Numerous SM voltage balancing methods have been proposed for both MMCs in grid-frequency operation and modular multilevel dc-dc converters (MMDCs) in high-frequency operation. This paper provides a comprehensive review on the SM balancing methods including closed-loop control methods and open-loop self-balancing strategies with their advantages and disadvantages highlighted, which helps to inspire new methods for further optimization of the SM balancing in modular converters.