Topology and control of an arm multiplexing MMC with full-range voltage regulation

The compact design of the modular multilevel converter (MMC) has drawn great attention due to the miniaturization requirement of the distant offshore wind power integration system. Only 50% submodules (SMs) in the phase leg are inserted at any instant, which results in lots of redundant SMs in the conventional MMC. Combining modular structure with device series switches (DSs) can effectively reduce the number of modules by improving their utilization. However, a fixed modulation ratio is necessary for some compact topologies to balance arm energy, and additional complicated control algorithms are required to obtain a limited voltage regulation range. This paper develops an arm multiplexing MMC (AM-MMC) with multiple arms, in which the SMs in the middle arms are assigned to operate in time-division multiplexing (TDM) modes. A multiplexed nearest level modulation (MNLM) method applied to the AM-MMC is presented to achieve full-range voltage regulation without additional energy balance control. To ensure reliable operation of TDM modes, the structure design and zero-voltage switching scheme of arm selection switches are given in detail. The feasibility and performance of the developed AM-MMC has been demonstrated on a small-scale 150V/2.75kW experimental prototype and an 85MVA/ ±35kV AM-MMC HVDC simulation system based on Matlab/Simulink.