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A novel three-port cascaded multilevel solid-state transformer (CM-SST) is proposed in this article, which is based on modular multilevel converter submodules (SM) interconnected by high-frequency link (HFL), forming a decoupling channel to SMs and a low-voltage dc port. The common high-frequency bus in HFL helps to reduce the numbers of secondary full bridges in the form of multiplexing. The decoupling channel achieves the automatic balancing of capacitors voltage and natural elimination of SM ripple power, so the arms second-order circulating current can be eliminated from the root cause. Therefore, the CM-SST can simultaneously realize the switches decreasing, capacitance size reducing, arms circulating current elimination, and control simplification. In this article, the topology, equivalent model, HFL design, ripple-power decoupling, power loss, and control scheme are analyzed in detail. In addition, an evaluation of CM-SST compared with the traditional methods is provided, including sizing, components count, control, and efficiency. Finally, the correctness and effectiveness of the proposed scheme are verified by the simulation and experiment. [ABSTRACT FROM AUTHOR]

Two types of common mode voltage (CMV) exist in medium-voltage motor drive system based on modular multilevel converter (MMC) type topology, that are, arms CMV (ACMV) and system CMV (SCMV). The former will produce circulating current between phases, and the latter will do harm to the motor. This article proposes a suppression scheme of CMV with a dual dc bus MMC-based solid-state transformer (DBM2C-SST) based drive system. The horizontal submodules (SMs) of MMC are cascaded with a quadruple-active-bridge (QAB), then a low voltage is formed. Plus the original medium voltage bus, the system is characteristic as a dual dc bus. The primary triple-active-bridge of QAB which characterized by low-impedance and synchronous-switching helps to achieve the natural suppression of the fluctuating voltage of the MMC horizontal three SMs within the entire operating frequency range under the relevant modulation strategy, thus realizing the full suppression of ACMV and low-frequency components suppression of SCMV simultaneously, and with the additional minimum method of SCMV high-frequency components, the DBM2C-SST can achieve zero CMV. As the SMs fluctuating voltage and CMV are decreased, the values of SMs capacitance and arms inductance can be significantly reduced. [ABSTRACT FROM AUTHOR]