As the penetration rate of renewable energy represented by photovoltaic (PV) power in power systems continues to increase

the synchronisation stability of grid-connected inverters under weak-grid conditions faces severe challenges. The double second-order generalised integrator–based phase-locked loop (DSOGI-PLL)

a key technology for grid-connected control in weak grids

exhibits excellent harmonic suppression capability. However

when significant fluctuations occur in grid frequency or voltage

its dynamic response and phase-tracking accuracy are significantly affected

even threatening the stable operation of the system. Analysis indicates that one of the key issues leading to the performance degradation of the DSOGI-PLL in weak grids is the lack of a fast and accurate frequency estimation. To address this

after comparing three frequency estimation methods—namely

the output frequency of the PLL itself

the grid-voltage zero-crossing detection frequency and the adaptive notch filter frequency—this paper proposes a multisource frequency fusion and adaptive damping DSOGI-PLL (MSFFAD-DSOGI-PLL). By analysing the harmonic content and frequency deviation of the power grid

the proposed method adaptively weight-fuses the results of the above three frequency estimation approaches and corrects the resonant centre frequency of the DSOGI in real time. On this basis

to balance the dynamic response and steady-state accuracy of the system under different operating conditions

an adaptive damping adjustment mechanism based on fuzzy logic linearisation is designed to achieve stable operation over a wide voltage/frequency range. Finally

experimental results demonstrate that compared with the traditional DSOGI-PLL

the proposed method exhibits stronger robustness and higher tracking accuracy under complex grid conditions such as large-range fluctuations in grid frequency and voltage.