An Enhanced Control Method Based on Reduced-order Resonant PLL for Inverters Interfaced with Weak Grid

Phase-locked loop (PLL) is one of the most widely used synchronization solutions in the field of three-phase power generations. However, it will exert a negative impact on current control under weak grid conditions. In order to address the problem, a PLL based on the reduced-order resonant controller is proposed, which could improve the stability of the system. The output impedance model of the three-phase grid-connected inverter considering the proposed PLL is deduced to verify its effectiveness. Moreover, a practical impedance reshaping method is proposed, which can solve the frequency fluctuation problem while the initial control scheme cannot. The negative impedance region of the proposed method is much smaller, so PLL with higher bandwidth can be used in the weak grid, and the dynamics can be better on the premise of maintaining the system stability. The proposed method is verified by simulation and the prototype experiments.