Modeling and control of the modular multilevel converter (MMC) based solid state transformer (SST) with magnetic integration

Solid state transformer (SST) can provide more advanced functionalities compared with conventional transformer, and has great potential in smart grid application. Recently, the SST with medium frequency (MF) isolation link and magnetic integration feature has been proposed, which can reduce the system volume and thus increase the power density. However, the magnetic integration also introduces strong coupling between the line frequency (LF) and MF variables, which poses a great challenge on modeling and control issues. This paper proposes a modeling and control method for an SST with magnetic integration and mixed-frequency modulation. A mathematical model based on dual d-q references is deduced, and then a cascaded control system is designed according to the model. Parameters of the controller for the variables at one frequency are properly designed to avoid disturbance from the variables at the other frequency. The simulation and experimental results show good decoupling effect and satisfactory dynamics performance of the proposed control system.