Uninterrupted Switching based on VSG Control between Grid-connected and Stand-alone Operation of Single-Phase Grid-Tied Inverter

This paper proposes an uninterrupted switching method between a grid-connected operation and a stand-alone operation by the output-frequency command switching of a single-phase grid-tied inverter. The proposed switching is characterized by a virtual synchronous generator (VSG) control that switches from a grid-connected operation to a stand-alone operation. The VSG control achieves the uninterrupted switching. However, the VSG control makes a frequency fluctuation during the stand-alone operation. In addition, it is impossible to lock the voltage phase. This is the cause of inrush current at the switching to the grid-connected operation. The proposed switching method uses multiple control systems depending on the operations. A constant voltage constant frequency control is applied to suppress the frequency fluctuation during the stand-alone operation. Moreover, a phase-locked loop is applied to suppress the inrush current at the switching to the grid-connected operation. The proposed switching method is demonstrated by the experiments with a 5-kVA prototype. The experimental result shows that uninterrupted switching is achieved by the proposed switching method.