Analysis and implementation of virtual impedance for fixed-frequency control strategy in microgrid

Classic droop control ensures the synchronization of distributed generation (DG) units inside a microgrid without requiring any deployment of communication links, however it causes unwanted frequency fluctuations and degrades system dynamics. Angle droop control and V-I control have been developed as two major global positioning system (GPS)-based control methods, both of which realize a fixed-frequency operation of the microgrid through synchronizing DG units with GPS timing technology and brings improvement to the dynamic response of the overall system. This paper reveals the similarities and the correlations between these two methods that they can both be regarded as different forms of virtual impedance control. A novel adaptive virtual impedance control method is proposed accordingly to generalize GPS-based control strategies into a unified control frame. An impedance and inner controller design approach considering both stability constraints and power quality requirements based on the small-signal model of GPS-based microgrids is presented for practical implementation. An adaptive transient resistance concept is adopted to enhance the system stability during large disturbances and grid faults. Case studies are presented to validate the system performance and fault ride-through abilities of the proposed control scheme.