Abstract: The control strategy of flexible multi-state switch（FMS）is a key technical challenge to ensure its high

efficiency，reliability and flexibility. In order to solve the problems that the parameter design of proportional resonant （PR）controller is complicated and is greatly affected by grid frequency disturbance in stationary reference frame，an improved control method of FMS based on proportional complex resonant（PCR）controller was proposed. The mathematical model of FMS was established in stationary reference frame，and the equivalent mechanism of control strategy in stationary reference frame and synchronization reference frame was deduced. In order to ensure the multimode stable operation of FMS，flexible switching control strategy of the rectifier side and inverter side converter were proposed and analyzed in detail. Finally，the simulation model of FMS was built by Matlab/Simulink simulation software，and the feasibility and effectiveness of the proposed control strategy are verified by the simulation results.

      Key words: flexible multi-state switch（FMS）；operation mode；stationary reference frame；coordinated

control；proportional complex resonan（t PCR）controller