Self-Balanced Clamp-Double Submodule for Modular Multilevel Converter

The modular multilevel converter (MMC) has been widely used in the field of high-voltage direct current (HVDC) transmission and motor drivers due to its many advantages. As voltage sensors and sorting algorithm are often used to depress the voltage fluctuation of submodules (SMs), it suffers from the algorithm complexity and hardware cost, especially for a large number of output levels. In this article, a new clamp-double-SM (CDSM) composed of two capacitors and five transistors is proposed. The symmetrical operation of the two capacitors enables the CDSM to output three levels and their voltages are self-balanced. Moreover, the capacitor voltages among different SMs of a CDSM-based MMC are also self-balanced by adding two auxiliary balance branches between adjacent CDSMs. It means that the new MMC can operate normally without using complex balancing algorithms based on capacitor voltage sorting. Compared with existing self-balanced MMCs, the new MMC uses fewer components in both the main power topology and auxiliary balancing circuits. In addition to topology, modulation, parameter design, and power loss are analyzed in detail. Finally, the feasibility of the CDSM-based MMC is verified by simulation and experimental results.