A Self-Adaptive Controller for Inverter With Seamless Transfer and Automatic Pre-Synchronization Capability

A self-adaptive controller is proposed to operate inverter in both grid-connected (GC) state and stand-alone (SA) state and ensure seamless transfer between them without reconfiguring the control structure. The controller is mainly composed of phase-locked-loop (PLL) and two cascaded control loops. The role of PLL is not only to estimate the frequency but also to generate the reference of voltage loop. When the grid is normal, the reference and the feedback value of voltage loop are always equal and the proposed controller accurately regulates the grid current of inverter. When islanding occurs, the inverter can automatically convert from current control mode to voltage control mode without critical islanding detection. The self-adaptive controller has several advantages. First, compared with direct current control, it can operate in both strong and weak grid conditions. Second, its dynamic performance in the GC state is much better than that of indirect current control. Third, when the grid returns to normal, the pre-synchronization of inverter's output voltage and grid voltage can be achieved automatically without any extra control algorithm. Finally, simulation and experimental results are provided to verify the effectiveness of proposed self-adaptive controller.