Accurate Impedance Model of a Grid-Connected Inverter for Small-Signal Stability Assessment in High-Impedance Grids

抄録

Power quality problems caused by grid-connected renewable energy inverters have been reported increasingly in recent literature. Excessive harmonics and interharmonics 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, in presented dq-domain models the grid voltage feedforward loop employed by the inverter is not included. To fill this gap, this paper presents a new impedance model, which includes the effect of feedforward, to analyze impedance-based stability in the presence of large grid impedance. The model was verified by impedance measurements from a laboratory prototype. The model gave accurate predictions of small-signal stability when the Nyquist stability-criterion was applied. Thus, the model can be used to re-shape the inverter impedance to avoid stability problems. The developed impedance model also provides a useful tool to monitor stability margins online, which necessitates adaptive impedance-shaping of grid-connected inverters.