Switching Device Number Reduction for Three-Phase Cascade-Modular Solid-State Transformer System with Employment of Three-Phase T-Type Converter

This paper proposes a three-phase cascade-modular solid-state-transformer (SST) system employed with a three-phase T-type converter to reduce the switching device number. A conventional three-phase SST system requires three single-phase SST subsystems, whereas the proposed three-phase SST system requires only two single-phase SST subsystems due to the application of the three-phase T-type converter. The operation of the proposed three-phase SST system is confirmed by both simulation and experiment. In the simulations, the three-phase SST system with the input voltage of 6.3 kV and the output voltage of 400V is evaluated. The unity power factor operation and the low grid current total distortion harmonic (THD) of 0.96% is achieved, where the output current with low current ripple is also achieved, satisfying the condition of the grid quality. In the experiments, the grid currents with high current THD of about 13.0% is obtained.