Common High-Frequency Bus-Based Cascaded Multilevel Solid-State Transformer With Ripple and Unbalance Power Decoupling Channel

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.