New grid impedance estimation technique for grid-connected power converters
Abstract
Grid impedance estimation is crucial for operating and controlling of grid-connectedconverters, especially with the high penetration of renewable energy sources into microgridand smart grid. One of the technical challenges is how to fast and accurately estimate the gridimpedance, even under the distorted and unbalanced conditions. Many impedance estimationmethods have been presented in the past decades. Most of them intentionally inject one or moredisturbance signals, low frequency or high frequency, to excite the grid response. And thenthe grid impedance can be estimated by the voltage/current information of two steady-stateoperating points before and after the disturbance. However, these injected disturbances willdeteriorate the power quality due to harmonics caused by the injected disturbances. In order tomitigate the problem, a new impedance estimation method is presented in this paper. Its basicidea lies in the inherent switching feature of the grid-connected power converter. The uniquefeature is that the proposed new grid impedance can be easily estimated, even under distortedand unbalanced grid voltage conditions with no need of the intentionally injected disturbancesignals any more. First of all, the system equivalent impedance circuit is analyzed to clarify theproposed method, along with a theoretical comparison with the conventional one. Aside fromthat, an interesting estimation of voltage and current switching components are presented fordetermining the grid impedance with two well-defined equations. And then, the performancetests are carried out and the results verify the effectiveness of the proposed method. Finally, atypical application of the proposed method is presented and discussed.References
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