Low-Frequency Voltage Ripple Suppression for MMCs With Split-Capacitor Submodules

This article proposes a split-capacitor submodule (SC-SM) based active power decoupling scheme to suppress the fundamental and double-frequency voltage ripples in modular multilevel converter (MMC) submodules (SMs) and reduce the capacitance required by the system. The operating principle is delicately analyzed to reveal the constraints for the SC-SM and establish the theoretical model showing the nonlinearity of the SC-SM. An asymmetric split-capacitor power decoupling scheme is proposed and the corresponding parameter is designed according to the constraints and operating principles. An easy-to-implement double closed-loop control strategy is proposed based on the decoupled and simplified SC-SM model developed. The feasibility and effectiveness of the proposed scheme are experimentally verified. The experimental results show that the SC-SM does not affect the steady-state performance and transients during load changes of the MMC system. The low-frequency voltage fluctuation of the SC-SM is significantly reduced by 88% compared with the conventional MMC with half-bridge SMs. The voltage ripple content with respect to the dc voltage can be reduced to 2.02%.