Low-Frequency Voltage Ripples Decoupling With Switched-Capacitor Conversion for an MMC-Based SST

The modular multilevel converter based solid state transformer (MMC-SST) is characterized by a large number of switches and large-size submodule (SM) capacitors, complex system control schemes. In this article, a novel high-frequency link (HFL) interconnection scheme based on MMC-SST (MHFL-SST) is proposed. All the SM capacitors are connected to the common bus (CB) through the full bridges triggered by the same driver signals, operating as a switched-capacitor conversion. The low frequency currents of SMs are decoupled on the CB; thus, the SM capacitance value can be reduced significantly and the circulating currents are also suppressed substantially. Furthermore, the voltages of all SM capacitors are always clamped with each other to be the same due to the switched-capacitor circuits. The bidirectional power conversion of the low voltage dc bus is implemented by phase shift control between a multiplexed H-bridge cell and the CB. The parameter constraint of the switched-capacitor loop circuit and the closed-loop control scheme for MHFL-SST are also carried out. The correctness and effectiveness of the proposed scheme are verified by simulations and experiments.