Flexible multi-state switch (FMSS) is a controllable power electronic device that provides accurate active and reactive power flow control. When the feeder fails, the flexible multi-state switch can quickly detect and isolate faults, and quickly restore the power loss load. This paper establishes the mathematical model of the normal grid-connected side and the fault-off-grid side of the three-port FMSS, and analyzes the working principle and control strategy of the three-port FMSS in normal operation mode and fault off-grid operation mode. The controller state tracking control method is used to realize the smooth switching of the control mode of the three-port FMSS, and compared with the hard switching mode, the advantages of the method for smooth adjustment and stable output are illustrated. In order to verify the practicability and effectiveness of the selection of the switching method, a three-port FMSS simulation model was built in the MATLAB/Simulink environment to obtain the dynamic response characteristics of the three-port FMSS in different operating modes and during the switching process.
