Variable structure robust voltage regulator design for microgrid master-slave control

Solid state transformers (SSTs) can be interconnected with each other through a distribution feeder to form a smart grid. When the main grid is disconnected, SSTs form an islanded microgrid. With available communications, master-slave microgrid control structure can be adopted. The system-level stability analysis can be simplified by considering only the grid interface stage of the SST. The loop feeder impedance uncertainty effects on system stability is more complex compared to that of conventional grid connection. By using mu synthesis method, grid impedance uncertainty, slave SST load uncertainty and master SST DC link uncertainty can be considered at the stage of controller design. Mu synthesis considers the structured parameter uncertainties. However, when all SSTs are disconnected from grid, the system stability cannot be guaranteed due to large structure variation. This paper proposes a variable structure robust control design method achieving both robust performance and stability considering grid parameter and structure variation. The method used in this paper can be extended to systems with a large number of SSTs. Without losing generality, a 7-SST system is considered as an example. The designed controller is validated through detailed PLECS simulations and controller hardware in the loop (CHIL) testing.