Seamless Mode Transfer Control for a Master-Slave Microgrid

A simple mixed droop-v/f control strategy is proposed for the master inverter in a microgrid to achieve seamless mode transfer between grid-connected and autonomous islanding modes. The proposed control utilizes 1) droop control in grid-connected mode to achieve seamless transfer from grid-connected mode to islanding mode in case of intentional and unintentional islanding events, and 2) v/f control in islanding mode, which is realized by simply reducing the droop coefficient to zero gradually after islanding, to provide voltage and frequency references to the slave inverters. Pre-synchronization on the voltage and frequency between microgrid and utility grid is used for the reconnection of them in a seamless manner. The validity of the proposed method is verified by experiments performed by a 15kVA microgrid.