Selective frequency synchronization technique for a fast seamless mode transition in droop control

This paper proposes seamless frequency synchronization scheme for fast mode switching of parallel grid-connected inverters operated by droop control in stand-alone mode to grid-connected mode. The proposed scheme analyzes the phase difference between the grid and the inverter for fast frequency synchronization and predicts the synchronization time for frequency increase and decrease. By selecting and controlling the direction of high-speed synchronization using the predicted value, there is an advantage that the synchronization time can be reduced. Also, in order to maintain uniform load sharing and power quality, all inverters were made to participate in the mode switching process based on the secondary controller, and the frequency and the voltage were limited within allowable values. The effectiveness of the proposed method was verified by simulation with PSIM.