Four-Consecutive-Samples based Frequency Estimation for Three-Phase Grids with DC-offsets

In power grids, DC offsets can result in the frequency oscillations. So it is a challenging problem which can't be ignored in frequency estimation. To resolve this problem, this paper proposes a method called TD-FLL (time delay frequency-locked-loop) based on four-consecutive-samples (4CSs), which estimates frequency and the positive-and negative-sequence components (PNSC) of the grid voltage with DC offsets in a fast and accurate way. Compared with the conventional methods, the proposed method not only has fast dynamic response but also can completely reject the DC offsets. By using the basic trigonometry and algebra, the relationship of the fundamental grid voltage, DC offsets and consecutive sample signals has been derived. According to the relationship, a model based on basic trigonometry and algebra has been established. Based on the model, the frequency has been estimated with DC offsets suppression. Finally, the simulation results indicate that the 4CSs-model estimates the frequency accurately and has good capability of rejecting DC offset.