An Enhanced Adaptive Frequency-Locked Loop Based on Fixed-Length Transfer Delay

The fixed-length transfer delay-based adaptive frequency-locked loop (TD-AFLL) has drawn much attention due to its concise structure and faster response speed than conventional second-order generalized integrator-based FLL. However, the application of TD-AFLL is still limited owing to the poor performance in terms of phase-angle/amplitude detection under distorted grid conditions. This paper explores an inherent input-output relationship of TD-AFLL, and a notch filter is employed to represent its filtering feature. Then a high-order filter is designed based on this notch filter, and an enhanced TD-AFLL (ETD-AFLL) with improved phase-angle/amplitude detection is further derived. The ETD-AFLL highly improves the phase-angle/amplitude detection accuracy under distorted grid conditions yet keeps the same fast response speed as TD-AFLL. In addition, the ETD-AFLL only requires a slightly additional computational cost. Experimental results are given to support the proposed ETD-AFLL.