Indirect Current Control Based Seamless Transfer of Three-phase Inverter in Distributed Generation

Abstract

The distributed generation (DG) should supply continuous power to the local critical load, even in the condition of utility outage. And many efforts have been made for the three-phase inverter in the DG to transfer between the islanding mode and the grid-tied mode seamlessly. This paper proposes a transfer strategy based on indirect current control for the three-phase inverter in the DG, which is composed mainly of two cascaded feedback loops, i.e., capacitor voltage loop and external grid current loop, in synchronous reference frame. In the grid-tied mode, the grid current is controlled by regulating the capacitor voltage. When islanding happens, the reference of the voltage loop is fixed by the limiter. As the local load is located in parallel with the filter capacitor in the proposed strategy, the load voltage is always controlled by the capacitor voltage loop directly, and the quality of the load voltage can be guaranteed. Besides, the performance of the capacitor voltage loop is further improved by introducing an inner capacitor current loop. Moreover, the steady state and transient state of the system, including the output of the limiter, are comprehensively analyzed, and then the compensators and limiters are designed. Finally, the proposed control strategy is verified by the simulation and experimental results.