Research on Triple-Port SST Scheme Based on the Natural Elimination of MMC Submodule Voltage Fluctuation and Imbalance

A triple-port solid state transformer (TP-SST) topology based on the combination of modular multilevel converter (MMC) and resonant push-pull converter (RP2C) is proposed in this paper. For an MMC-based SST, large size capacitor caused by fluctuating power and voltage imbalance of MMC sub-modules (SMs) are two important problems. Based on a simple open-loop control scheme, the RP2C based high frequency links (HFL) are characterized by a voltage source with fixed conversion ratio and low impedance, and then can form a power free-coupling channel between SMs and realize the automatic balance of capacitors voltage. Moreover, the fluctuating power can cancel each other based on their three-phase symmetry, which obviously reduces the capacitance size, and the 2nd-order harmonic circulating current of the arms could be eliminated at the same time, greatly simplifying the control of system. In this paper, the working principle of TP-SST are described, and the methods of HFL realizing SM voltage ripple elimination and self-balancing are analyzed in detail. In addition, a comparison with traditional SST in efficiency and cost is presented. Finally, simulation and experimental results verify the correctness and effectiveness of the proposed topology scheme.