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A new simple approach to the design of digital algorithm for simultaneous reactive-power and frequency estimations of local system is presented. The algorithm is derived using the weighted-least-square method. During the algorithm derivation, a pure sinusoidal voltage model was assumed. Cascade finite-impulse-response (FIR) comb digital filters are used to minimize the noise effect and to eliminate the presence of harmonics effect. The most important point of this paper is the mathematical model that transforms the problem of estimation into an overdetermined set of linear equations. The investigation was simplified because the total similarity to the state of the problem of the active-power and frequency estimations was noticed. The only difference is the adaptive phase shifter applied to the voltage signal. In addition, coefficient-sensitivity problems of the large-order FIR comb cascade structure are overridden by using a multirate (decimation) digital signal processing technique. Even more, by using antialiasing filters, the parameter estimation accuracy is improved. The effectiveness of the proposed techniques is demonstrated by both simulation and experimental results. The algorithm shows a very high level of robustness, as well as high measurement accuracy over a wide range of frequency changes.