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To quickly and accurately estimate the parameters of the fundamental positive- and negative-sequence under the unbalanced and distorted grid voltage, a synchronization method is presented in this paper. The proposed method is based on both a harmonic decoupling network consisting of multiple dual second-order generalized integrators (MDSOGIs) and an improved frequency locked loop (IFLL), so it is called the MDSOGI-IFLL. Due to the IFLL, the system has the feature that the dynamic performance of estimating the fundamental frequency is independent of the variation of both the fundamental positive- and negative-sequence voltage. In this paper, a first-order linear frequency adaption model is established for the design of the IFLL. Finally, the good performance of the proposed MDSOGI-IFLL is validated by the simulation and experiment. [ABSTRACT FROM AUTHOR]