Droop-controlled inverters with seamless transition between islanding and grid-connected operations

This paper presents a seamless transition method for droop-controlled inverters to operate in both islanding and grid-connected modes. A local PLL and a virtual inductance are designed to ride through transient when the inverter switches between two modes with no synchronization. The proposed method can cooperatively work with well-developed droop controls so that the inverters are able to share load among them as well as subsist under transient events of the utility. Theoretical analysis and experimental results validate effectiveness of the proposed method.