A seamless transfer strategy of islanded and grid-connected mode switching for microgrid based on droop control

In this paper, the structure and control method of microgrid in both islanded mode and grid-connected mode are presented. And a seamless transfer strategy for droop controlled distributed generations from islanded mode to grid-connected mode is proposed. This transfer strategy is based on droop method and consisted of frequency synchronization and phase synchronization. It could avoid a frequency and phase oscillation, and do not have an effect on the power sharing of droop control. After synchronizations the bypass switch closes at a zero-crossing of voltage, and at the same time the control mode charges from the voltage sources based on droop control to current sources. Simulation results are presented to show the performance and feasibility of this proposed strategy.