The distributed power generation networks and smart grids are characterised by a high penetration of renewable energies. For purposes of improving the power quality of the distributed generation and achieving a reliable, efficient and safe operation of the grid, on the basis of the synchronisation techniques, this study proposes a robust frequency-locked loop (FLL) algorithm for grid synchronisation of single-phase applications under distorted grid conditions. The algorithm mainly includes a frequency-adjustable quadrature signal generator (QSG) and a cascaded adaptive complex filter (ACF), as well as a robust frequency adaptive controller. The suggested QSG can generate two-phase orthogonal voltages with the same attenuation ratio. In the proposed scheme, the normalised blocks and the cascaded ACF are introduced, for purposes of eliminating the influence of voltage fluctuation and the harmonics on the frequency adaptive adjustment effectively. Theoretical analysis indicates that the proposed FLL algorithm has good performance in aspects of rapidity and robustness even in the hostile envelopments caused by frequency variation, voltage fluctuation and harmonic distortion. Simulations and experimental results are provided at the end of this study to validate the good performance of the proposed algorithm.