New grid impedance estimation technique for grid-connected power converters

Grid impedance estimation is crucial 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 due to harmonics caused by the injected disturbances. 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. The unique feature is that the proposed new grid impedance can be easily estimated, even under distorted and unbalanced grid voltage conditions with no need of the intentionally injected disturbance signals any more. First of all, the system equivalent impedance circuit is analyzed to clarify the proposed method, along with a theoretical comparison with the conventional one. Aside from that, an interesting estimation of voltage and current switching components are presented for determining the grid impedance with two well-defined equations. And then, the performance tests are carried out and the results verify the effectiveness of the proposed method. Finally, a typical application of the proposed method is presented and discussed.