Seamless mode transfer control for master-slave microgrid

This study proposes a simple mixed droop-v/f control strategy for the master inverter of a microgrid to achieve seamless mode transfer between grid-connected and autonomous islanding modes. The proposed control utilises (i) a modified 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 (ii) v/f control in islanding mode, which is realised by simply reducing the droop coefficient to zero gradually after islanding, to provide voltage and frequency references to the slave inverters. Pre-synchronisation on the voltage and frequency between microgrid and utility grid is used for the reconnection of them in a seamless manner. The rationale and detailed design of the system with the proposed control strategy are elaborated, after which its effectiveness is proved by simulations and experiments.