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Small-signal stability problems often occur when the inverter for renewable energy generation is connected to weak grid. A small-signal transfer function integrated model reflecting the interaction of grid impedance, phase locked-loop (PLL), and current control loop is established in this paper. Based on the established model, the oscillation mechanism of the grid-connected inverter system is revealed: the inductance current flowing through the grid impedance can produce a voltage disturbance, which will eventually affect the inductance current through PLL and current control loop. Therefore, the loop composed of the grid impedance and PLL can easily lead to the oscillation of the grid-connected inverter system under weak grid condition. To suppress the oscillation, a control parameters design method of the grid-connected inverter is proposed. Without changing the control method, the proposed control parameters design method can ensure the stable operation of the grid-connected inverter system under the very weak grid condition when the short-circuit ratio (SCR) is 2. Finally, the experimental results verify the correctness of the stability analysis and the effectiveness of the control parameters design method proposed in this paper.

Modeling and Control Parameters Design for Grid-Connected Inverter System Considering the Effect of PLL and Grid Impedance