As key equipment for the integration of renewable energy generation into the grid, converters are increasingly experiencing system oscillations when interconnected with the AC grid. The impedance model is the basis for analyzing the stability of the "converter-grid" coupled system. Based on the ?? coordinate system, this paper establishes the grid system impedance mathematical model of specific conditions. Then a unified impedance mathematical model under all working conditions is established. From the impedance model, the operational condition and controller parameters are the main factors influencing the system impedance characteristics. The Nyquist stability criterion is used to quantitatively analyze the influence of two kinds of influence factors on system stability. Then it proposes to increase control loop Kpi parameters to suppress system resonance under a small signal and enhance system stability. Finally, the simulation and experimental results under different operating conditions verifies the accuracy of the proposed mathematical model of impedance in all operating conditions in analyzing the stability, and also verifies the effectiveness of the stability improvement method.

This work is supported by the National Key Research and Development Program of China (No. 2018YFB0106300).