A Modular Multilevel Converter Based Solid State Transformer (MMC-SST) for High Power Wind Generators

Solid-state transformer (SST) technology has emerged as a promising alternative in distribution networks providing transformation between voltage levels through the employment of power electronic converters and a high-frequency isolation transformer. SST operation is employed in either medium or high-frequency ranges, leading to a considerable reduction in overall weight and volume. This paper proposes a modified single shift modulation for Modular Multilevel Converter Based Solid State Transformer (MMC-SST) operation for high power wind-turbine integration. The power controller is designed such that the power requirement of the medium-voltage DC (MVDC) grid is achieved using a proportional-integral (PI) controller along with traditional MMC voltage balancing which further enhances the overall system performance. A MATLAB/SIMULINK model is constructed for a 4 MW DC microgrid to validate the proposed control and modulation scheme. Different scenarios for normal and fault conditions are investigated where the efficiency of the proposed system under balanced power distribution reaches 96%.