Abstract
Contrary to the phase-locked loop (PLL), which has almost reached a mature stage of development in power and energy applications (particularly in three-phase systems), the frequency-locked loop (FLL) is not a mature technique yet. This is probably because of the implementation of FLLs in the stationary reference frame, which makes their modeling, tuning, and performance enhancement more complicated than PLLs. The aim of this paper is conducting a research on three-phase FLLs. Providing a review of recent advances, introducing the concept of inloop filters for designing more advanced FLLs, demonstrating the FLL modeling and tuning in the presence of an inloop filter, analyzing the advantages and disadvantages of using an inloop filter in the FLL structure, and establishing a connection between FLLs and PLLs are the main parts of this research.