The coupling effect between the control system of grid-connected inverters and grid impedance influences the stability of grid-connected inverter system under the weak grid condition due to grid impedance. The control system of grid-connected inverters usually uses the phase-locked loop(PLL)to acquire the synchronization information of grid. The dynamic characteristic of PLL is the key factor that affects the stability of inverter system. The effect of PLL on system stability under weak grid condition is analyzed. Besides

a method to improve the stability of inverters is proposed based on the analysis above. The output impedance modeling of a three-phase grid-connected inverter including the current loop

PLL and filter is derived under the synchronization reference frame. The internal relationship between the stability of inverter system with different grid impedance and the bandwidth of PLL is analyzed. Considering the influences caused by the system voltage perturbation on the current loop through PLL in the impedance model

an improved feed-forward control method to reduce the output effect of PLL is proposed. The feed-forward control strategy takes the system voltage

the dynamic of PLL and filter into account. The analysis result shows that the proposed control method can effectively improve the stability of grid-connected inverter under weak grid condition. The experimental results verify the correctness of the proposed control method.