Reduction in Quantity of Submodules in UC-FB-MMC by Injecting Zero-Sequence Voltage

Unidirectional-current full-bridge submodulebased modular multilevel converter (UC-FB-MMC) can reduce the submodule capacitance by increasing the modulation index. However, compared with the most-common half-bridge submodule-based MMC (HB-MMC), the UC-FB-MMC utilizes many more submodules. Moreover, because the UC-FB-MMC commonly implements the active circulating current injection, the arm voltage are not standard sine waves, and thus, the existing zero-sequence voltage injection (ZSVI) method cannot maximize its potential. This paper proposes a ZSVI method for the UC-FBMMC. The basic idea is to dynamically adjust the injection index and the phase, thereby symmetrically suppressing the peak of the arm voltages. Theoretical analysis results indicate that the injection of the proposed zero-sequency voltage can reduce the peak of the arm voltage by 9.3 %, indicating that the corresponding portion of submodules can be eliminated. Simulation results verify the feasibility and effectiveness of the proposed zero-sequence voltage injection method.