Unified Control Strategy for Microgrid Solid-State Transformers

Solid-state transformers (SST) are particularly useful in distributed generation systems (DG) and microgrids in that they provide several functions besides voltage and current conversion and electrical isolation, such as better controllability and power factor correction. When designing an SST, many aspects need to be considered, such as the choice and design of the actual high-frequency transformer, the SST topology, and its control method. In AC systems, the SST is connected to the grid through an inverter which needs to be controlled to ensure the proper operation of the DG in grid-tied mode and independently, i.e., in islanded mode and the switching between the two modes. This paper proposes a unified and an efficient control scheme for the inverter which works in both modes of operation and provides smooth transfer between them. It also controls the isolation stage and a battery energy storage system (BESS), which serves several functions, as will be discussed herein. The proposed scheme gives the opportunity for incorporating many other functions in it without having to add new systems or components. To demonstrate this point and for further contribution, this paper also proposes two such functions, i) a method to mitigate the current harmonics caused by the connection of the SST to the grid and ii) an efficient low-voltage ride-through (LVRT) scheme. Both come at no extra cost using the proposed unified control scheme. The SST model was constructed in Simulink, and the algorithm was written as a MATLAB function that outputs all the necessary control parameters. The proposal's validity is verified through the simulation of the presented case studies. The main advantages of the proposed control method are its versatility and efficiency.