DC Voltage Ripple Optimization of a Single-Stage Solid-State Transformer Based on the Modular Multilevel Matrix Converter

The modular multilevel matrix converter (M-3 C) is introduced and applied to a single-stage ac/dc solid-state transformer (SST), which uses only one concentrated medium-frequency transformer (MFT) for isolation. As a result, the transformer insulation design is simplified significantly and higher power density could be achieved. Meanwhile, a novel decoupling transformation based on two internal three-phase circulating systems is proposed to improve the previous method. Through this transformation, the unbalanced capacitor voltage components can be divided into four independent dimensions, which correspond to four kinds of capacitor voltage ripple frequency. And a dc-side vector model is then established to calculate the required circulating current reference for capacitor balancing. Besides, under the application of SST, the capacitor voltage ripple suppression strategy via injection of extra circulating current is proposed. The amplitude and frequency index is optimized to obtain the best effect. Therefore, the submodule capacitor volume can be reduced to achieve higher power density. Simulation and experimental results are also provided to verify the theoretical analysis.