Modeling of inverter output impedance for stability analysis in combination with measured grid impedances

Due to the changing structure of the electric power system, stability analysis in the distribution grid are increasingly important. In this paper an impedance based method to determine the stability and the robustness of a grid connected converter under various grid conditions is presented. Therefore, the inverter output impedance is modeled in a modular way based on state space systems. Additionally, grid impedance measurements in a public low voltage grid are carried out. Based on these impedances the stability of the system is evaluated by applying the Nyquist stability criterion. Further, the paper illustrates under which circumstances it is necessary to consider grid resonances to achieve stable operation and it points out how emerging grid impedance resonances and the inverter parameters influence the system stability. The output impedance model is validated by simulation.