Parameters Influences Analysis and Optimization Design Method for Synchronous Transformer of Switched-Capacitor MMC-SST

In the switched-capacitor-based modular multilevel converter-solid state transformer scheme, the switched-capacitor circuit plays a key role in power transfer, electric isolation, voltage clamping, and low-frequency voltage ripple decoupling. However, the operational performances of switched-capacitor circuits are depended on the distribution parameters of synchronous transformers. To explore the parameter constraints, the mathematical model is established, the relationship between the active power transmission capacity and the transformer time constant is studied, the dynamic inrush current stress is also given. The analysis results show that the distribution resistance should be minimally designed to reduce the voltage clamping error, however, a large distribution resistance is good for suppressing the inrush current. Moreover, if the distribution resistance is large and the leakage inductance is small, the time constant is small, which benefits the power transfer. So, there are some contradictions in the transformer design, which needs to make a tradeoff. For coordinating the parameter optimization, the design criteria and influence factors of distribution parameters for the synchronous transformer are analyzed. Then the transformer optimization design flow considering distribution parameter constraints is proposed. Finally, the toroidal transformer prototype and the switched-capacitor experimental platform are developed to verify the correctness of the proposed scheme.