Frequency-Freezing FLL for Enhanced Synchronization Stability of Grid-Following Converters during Grid Faults

Transient instability is an issue for grid-following converters operating under grid faults when complying with low-voltage ride-through requirements. This has initiated much research with the aim to understand, model, and prevent loss of synchronization for synchronous-reference frame phase-locked loop (SRF-PLL)-synchronized systems. However, as the majority of grid faults are asymmetrical, a more complex synchronization unit is needed for the extraction of voltage sequences and phase tracking. This paper proposes a method for enhanced transient stability during severe grid faults for more complex synchronization structures designed to deal with asymmetrical fault conditions. This is done by freezing the frequency of a stationary-reference frame frequency-locked loop. The global asymptotic stability of the method is mathematically proven, and its performance is experimentally verified. Based on the mathematical equivalence between frequency-locked loops and phase-locked loops, it is shown that the presented method can be generalized to both stationary-reference and synchronous-reference frame structures and can, therefore, be a suitable solution in a wide range of applications.