An Improved Feedforward Control Method Considering PLL Dynamics to Improve Weak Grid Stability of Grid-Connected Inverters

Phase-locked loop (PLL) is commonly used for three-phase grid-connected inverters to obtain the information of grid synchronization, and PLL dynamics are the key factors for stable operation of the inverters. Under weak grid conditions, the coupling between PLL and grid impedance can result in harmonic resonance, or even instability in the system. In this paper, the effect of PLL dynamics and grid impedance on the stability of three-phase grid-connected inverters is studied with the d-q impedance model. Besides, the variable transfer relationship is used to analyze the influence of PLL perturbations on output current under weak grid conditions. To suppress low-order harmonics of the network current caused by PLL perturbations under weak grid conditions, a novel feedforward control method is proposed to compensate PLL perturbations and revise the output impedance, where the operation of the inverter and PLL dynamics have been taken into account in the design process. By analyzing the impedance characteristic of the system, it can be demonstrated that the proposed method can enhance the stability of grid-connected inverters under weak grid conditions and reduce the impact of PLL perturbations on grid-connecting current. The experimental results indicate that the low-order harmonics of the network current can be suppressed effectively, which verifies the analysis.