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Synchronization algorithms are of great importance in control of grid-connected inverters as an integral part of distributed power generation units such as photovoltaic systems. A new all-digital closed-loop phase-locked algorithm for the synchronization signals of three-phase grid-connected inverters is presented even considering seriously distorted and variable-frequency utility conditions. The proposed synchronization algorithm can suppress the negative sequence utility voltage at fundamental frequency and high-frequency harmonic components effectively, and lock the positive sequence phase at fundamental frequency accurately.Moreover, a frequency adaptation algorithm is also proposed for applications where large frequency variations are often expected. Finally, digital simulation and experimental results both prove the feasibility of the proposed phase-locked method. The introduced synchronization method provides higher degree of immunity and insensitivity to harmonics and other types of pollutions in the utility when used to synchronize inverters.

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