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Real-time grid impedance estimation technique for grid-connected power converters

Grid impedance is one of the most important parameters for operating and controlling of grid-connected converters, especially with the high penetration of renewable energy sources into microgrid and smart grid. One of the technical challenges is how to fast and accurately estimate the grid impedance, even under the distorted and unbalanced conditions. Many impedance estimation methods have been presented in the past decades. Most of them intentionally inject one or more disturbance signals, low frequency or high frequency, to excite the grid response. And then the grid impedance can be estimated by the voltage/current information of two steady-state operating points before and after the disturbance. However, these injected disturbances will deteriorate the power quality. In order to mitigate the problem, a new impedance estimation method is presented in this paper. Its basic idea lies in the inherent switching feature of the grid-connected power converter. Therefore, the grid impedance can be easily estimated with no need of the intentionally injected disturbance signals anymore. The theoretical analysis and performance evaluation results verify the effectiveness of the proposed method.