A technique for voltage-source inverter seamless transitions between grid-connected and standalone modes

A technique for the seamless transition of voltage-source inverters switched between grid-tied and standalone modes of operation is presented in this paper. In this technique, only the inverter current and voltage sensors are utilized and no control over the grid-side static transfer switch is needed. The presented technique contains two strategies for grid-tied-to-standalone and standalone-to-grid-tied transitions. In the standalone-to-grid-tied transition strategy, a novel algorithm is presented for estimating the grid angle nearly instantaneously, which allows the three-phase inverter to respond very quickly if the grid and point of common coupling voltages are out of phase. This fast response allows the inverter to effectively eliminate the transient overcurrent that would normally occur if it were connected to the grid without first being synchronized. The fast response also allows the inverter to return to normal operation very quickly after such an event. The strategy for the seamless transition from grid-tied to standalone mode is also presented. These strategies have been verified through experiments, and the results are presented in this paper.