Development of Model Predictive Control for Three-Stage MMC-based Solid State Transformer to Interconnect High and Low Voltage AC Grids

Solid state transformer (SST) has gained remarkable attention in various high-voltage applications and modern power systems. This paper explores the application of modular multilevel converters (MMC) to realize a three-stage SST. This three-stage MMC-SST configuration is adopted to integrate two AC grids operating at different voltage levels. The typical three stages of the MMC-SST consist of an MMC-based rectifier, isolated dc/dc converter, and inverter. Model predictive control (MPC) is developed to control high-voltage and low-voltage side MMCs. A proportional-plus-integral controller-based phase shift control is used for the MMC-based isolated dc/dc converter to enable bidirectional power flow. This MMC-SST configuration provides a low-cost, modular, scalable, and flexible solution to interface high and low-voltage AC grids. The developed control provides a reliable solution for balancing voltage across the submodule capacitors, fulfils power requirements, and regulates DC-bus voltages. Exhaustive simulation studies are performed to validate the effective function of the MMC-SST and control method under different test scenarios.