Seamless Transfer of Single-Phase Utility Interactive Inverters with a Synchronized Output Regulation Strategy

This study presents a synchronized output regulation (SOR) strategy for controlling inverters operating in grid-connected and stand-alone modes. As a concept of networked dynamic systems, SOR involves nodes with outputs that are synchronized but display a desirable wave shape. Under the SOR strategy, inverter and utility are regarded as two nodes that comprise the simplest network. These two nodes work independently under stand-alone mode. An intermediate synchronization mode is highlighted because it is a standard SOR problem in the transition from stand-alone mode to grid-connected mode. In grid-connected mode, the inverter operates in a relatively independent way, with only the reference voltage changing for a general synchronization, in which the current between the inverter node and the utility node satisfies the required power injection. Such a relatively independent design leads to seamless transfer between operation modes. The closed loop system is analyzed in the state space on the basis of output regulation theory, which improves the robustness of the design. Simulations and experiments are performed to verify the proposed control strategy.