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An algorithm based on the concept of an adaptive notch filter is proposed for the frequency estimation of three-phase power signals. The algorithm permits direct estimation of frequency and its rate of change for power system signals. The proposed method uses the information of all three-phase signals together. The structure, mathematical formulation, and theoretical stability analysis of the proposed technique are presented, and its performance is studied in a variety of scenarios where three-phase power signal attributes such as frequency and amplitude undergo variations over time. This study confirms the desirable transient and steady-state performances of the proposed algorithm. In contrast to the conventional three-phase PLL-based frequency estimator, the proposed method is less sensitive to distortions such as harmonics and interharmonics and is not sensitive to unbalanced system conditions.