Frequency-Locked Loop Techniques in Power System Synchronization

Frequency-locked loops (FLLs) are indispensable for modern power system synchronization, providing direct estimation of grid frequency and phase in grid-connected converters. Unlike phase-locked loops, which infer frequency from phase error, FLLs employ orthogonal signal generation and adaptive observers to track rapid frequency variations, harmonic distortion and dc offsets. Key implementations utilise second-order generalized integrators (SOGIs) and their modified forms (mSOGIs) to generate orthogonal components of single-phase signals, enabling parallel harmonic estimation and frequency adaptation. Recent advances integrate frequency-fixed SOGI structures to enhance stability in weak or low-inertia grids, while repetitive controllers and harmonic-decoupling networks have been applied to suppress grid interferences. These techniques support high-penetration renewable energy sources by ensuring robust synchronization under unbalanced, distorted and dynamically changing grid conditions, with practical implementations demonstrating fast settling times, minimal phase error and immunity to dc offsets.

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Recent work on frequency-fixed SOGI-based loops has introduced arbitrarily delayed signal cancellation to mitigate the detrimental effects of dc offset on single-phase synchronization. By adopting a frequency-fixed SOGI-PLL structure with systematic gain design, this method achieves fast transient response and accurate frequency estimation even under significant offset injection, outperforming conventional adaptive loops in settling time and dynamic stability.

A comprehensive review of SOGI-based phase-locked loops under complex grid conditions highlights the challenges posed by non-ideal scenarios such as variable frequency, voltage sags and inverter-induced harmonics. This body of research identifies nonlinear loop coupling and negative-resistance behaviour as primary sources of instability, proposing enhanced filter designs and optimised bandwidth allocation to preserve robustness in weak-grid environments.

Innovations in repetitive-controller-based frequency-locked loops demonstrate a simplified architecture that simultaneously attenuates harmonics and tracks frequency variations. By embedding a repetitive controller within the FLL feedback path, the proposed structures reduce computational burden and extend resilience against common grid disturbances, delivering rapid frequency convergence and enhanced harmonic rejection in grid-synchronization experiments.

Frequency-Locked Loop Techniques in Power System Synchronization publication trend

The graph below shows the total number of articles in frequency-locked loop techniques in power system synchronization across all publications each year (not limited to Nature Index journals).

Technical terms

Frequency-Locked Loop (FLL): A control system that directly estimates the frequency of an AC signal by processing orthogonal components and minimising frequency error in a feedback loop.

Second-Order Generalized Integrator (SOGI): A signal processing block used to generate quadrature components of a single-phase signal, enabling decoupled estimation of amplitude, phase and frequency.

Modified SOGI (mSOGI): An enhanced SOGI structure incorporating gain normalisation, anti-windup and rate-limitation to improve convergence speed and robustness under varying grid conditions.

Repetitive Controller: A controller that embeds an internal model of periodic disturbances to achieve precise attenuation of harmonics in tracking loops.

DC Offset: A non-zero average component in a periodic signal that can introduce estimation errors and instability in synchronization loops if not compensated.

References

1. Modified Second-Order Generalized Integrators With Modified Frequency Locked Loop for Fast Harmonics Estimation of Distorted Single-Phase Signals. IEEE Transactions on Power Electronics (2019).
2. Overview of SOGI-Based Single-Phase Phase-Locked Loops for Grid Synchronization Under Complex Grid Conditions. IEEE Access (2021).
3. DC offset rejection in a frequency-fixed second-order generalized integrator-based phase-locked loop for single-phase grid-connected applications. Protection and Control of Modern Power Systems (2022).
4. Frequency-Locked Loop Based on a Repetitive Controller for Grid Synchronization Systems. IEEE Access (2020).