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A new digital phase-locked-loop (DPLL) method is proposed for single-phase grid-connected power conversion systems in this paper. The analysis of a phase detector using trigonometric function transformation is presented in detail. A sliding-window-based data sampling method is utilized to obtain the orthogonal signal related to the original input signal according to the periodic characteristic of the steady-state sine fundamental component, and the fictitious active and reactive power components are constructed. On this basis, the DPLL method is developed. It consists of an even harmonics elimination network, a grid voltage feedforward network, and a fictitious instantaneous power feedback phase correction network. The fundamental signal of the grid voltage is extracted by an even harmonics elimination network and a digital Butterworth filter and a unit value controller to generate a unity sine reference signal for the feedforward network, whereas only the signs of the fictitious instantaneous active and reactive powers are used to revise the phase shift of the unity sine reference signal by means of a real-time variable step-size control in the feedback phase correction network. The method is validated to have good dynamic and static performances through MATLAB simulations, simulator investigations, and experiments with a grid-connected photovoltaic system under different voltage conditions.