Optimization Scheme Based on High-Frequency Link Interconnection of Submodules

Submodule (SM) capacitor voltage ripple and imbalance are inherent problems with conventional modular multilevel converter (MMC). In addition, the large size capacitance in SM reduces the power density of MMC, and the life defect of electrolytic capacitor also decreases reliability of the converter. This article proposes a modified switched-capacitors based MMC (SC-MMC) topology with ripple-power decoupling channels. Without relying on any closed-loop control, the natural elimination of SMs ripple-power and automatic balance of SMs voltage can be realized simultaneously, which greatly increase the power density and simplify the control design of system. In this article, the ripple characteristic of the conventional MMC and the constraint on capacitance are studied, and an SMs ripple-power decoupling method is analyzed. The power characteristics and impedance constraint analysis of the switched-capacitors are described in detail and the configuration assessments including volume, cost, and efficiency of the SC-MMC are also analyzed. Finally, a down-scaled experimental platform is built, and the experimental results verify the correctness and effectiveness of the proposed scheme.