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When a power system experiences off-nominal conditions such as frequency deviation and power oscillation, the accuracy of phasor estimation may degrade. Moreover, the estimated value deviates farther from the theoretical value because of decaying DC components. New phasor estimators are proposed in this paper. They are able to handle the dynamic characteristics as well as the decaying DC components so as to improve the accuracy of phasor estimation. A new frequency estimator is proposed for this purpose. Two filters are employed to estimate the fundamental component and DC components, respectively. Then, least squares method (LSM) gives accurate estimation by reassigning the estimations obtained by short time Fourier transformation (STFT). The performance of the proposed algorithm is evaluated by computer-simulated signals, EMTDC-generated signals, and signals recorded by a commercial relay from real time digital simulator (RTDS). The evaluation results indicate that the proposed algorithms can achieve accurate phasor estimation, frequency estimation, and even power estimation in presence of frequency deviations and decaying DC components with a minimal increase of computational cost and time delay compared with the conventional STFT.