In order to realize the smooth transition of the flexible multi-state switch（FMSS） state in case of feeder fault

we proposed a DC voltage coordination strategy and control port selection method suitable for FMSS based on the master-slave control and distribution network information. Moreover

we proposed a port mode smooth switching strategy based on the improved control structure in which PI regulator and steady state inverse model are conbined. A phase smoothing control structure was designed by adding off-grid locking and grid-connected pre-synchronization links on phase-locked loop. The simulation results show that the DC voltage fluctuation of FMSS does not exceed 0.2% of rated voltage and the stabilization time is about 10 ms during the switching process under different conditions. Besides

the AC voltage and current of each port transit stably

and the active power overshoot of Udc–Q port does not exceed 3.5% of its rated value. Hence the strategy proposed in this paper can be adopted to realize smooth switching of FMSS under various conditions.