Voltage and Power Balance Control Strategy for Three-phase Modular Cascaded Solid Stated Transformer

摘要

The three-phase modular cascaded solid stated transformer (SST) is an important element in the micro-grid systems as its outstanding adventures compared with conventional power transformer. It consists of three stages, the three-phase cascaded modular rectifier stage, the dual active bridge (DAB) converter stage and the three-phase inverter stage. Three output-paralleled DAB converters offer three dc-buses, which forming the interfaces of renewable energy. However, unbalanced modular voltage or transferred power causes overvoltage or overcurrent issues, which increase the stress of the semiconductor switches and even the instability of SST system. Focusing on the imbalance issues, this paper proposes a systematic control strategy for the three-phase modular cascaded SST. A d-q vector-based common-duty-ratio controller is applied to the rectifier stage aiming at balancing the modular current in each phase. Voltage controller for DAB stage achieves the balanced dc-link and dc-bus voltage, and proportional-resonant controller is applied to inverter stage. The effect of this proposed control method has been verified theoretically. In addition, a scaled-down experimental prototype is built to prove the performance.