Study on control scheme for smooth transition of micro-grid operation modes

To meet the requirements of both islanded and grid-connected operation of microgrid and to realize the smooth transition between the two modes,a reference adjusting control loop is added to the basic droop controller.By switching the loop,PQ control and droop control are implemented in grid-connected and island mode respectively.A pre-synchronizing controller is designed to synchronize the inverter to the grid when the islanded microgrid needs to reconnect to the grid.This control unit effectively restrains the impact current in the parallel connection.A mode transition process is proposed and validated by simulation.The microgrid simulation experiment also verifies several other operational conditions both in islanded mode and grid-connected mode.