Accurate impedance model of grid-connected inverter for small-signal stability assessment in high-impedance grids

Power quality problems caused by grid-connected renewable energy inverters have been reported in recent literature. Excessive harmonics and interharmonics may arise when the inverter starts to interact with the grid impedance. Small-signal impedance models have been proven to be useful tools to analyze the stability margins. However, most often the grid voltage feedforward loop employed by the inverter is not included in impedance-based analysis. To fill this gap, this paper presents an impedance model, which includes the effect of feedforward, to analyze impedance-based stability in the presence of large grid impedance. The model is verified by impedance measurements from a laboratory prototype. The model is shown to give accurate prediction of small-signal stability when the Nyquist stability-criterion is applied. Thus, the model can be used to re-shape the inverter impedance to avoid stability problems. The developed impedance model will also provide a useful tool to monitor stability margins online, which necessitates adaptive impedance-shaping of grid-connected inverters.