Advances in Load Frequency Control Topologies for Frequency Stability in Power Grids: A Review

The increased penetration of renewable energy sources and Inverter-Based Resources (IBRs) has resulted in power systems operating with significantly reduced inertia, thus increasing their susceptibility to frequency instability and posing challenges to the stability and security of modern power grids. Consequently, extensive research efforts have focused on enhancing frequency stability through improved Load Frequency Control (LFC), also referred to as Automatic Generation Control (AGC). Existing studies have proposed a wide range of control strategies that differ in controller structure, optimization techniques, and performance evaluation criteria. Conventional integral-order controllers remain among the most widely investigated approaches. Fractional-order controllers have also attracted considerable attention owing to their enhanced flexibility and dynamic performance. In addition, two-degrees-of-freedom and multi-degrees-of-freedom controllers have been explored to improve frequency regulation through the utilization of multiple control inputs. Fuzzy logic controllers have also been extensively investigated, with approaches varying from sole integration in systems to hybridizing with other control approaches. An emerging body of work has investigated other control approaches, such as sliding mode control, active disturbance rejection, model predictive control, and reinforcement learning. As more Energy Storage Systems (ESSs) are being adopted for ancillary services, recent studies have increasingly focused on integrating ESSs into frequency regulation schemes through both LFC-based control and independent control architectures. This review provides a comprehensive classification and critical analysis of existing LFC topologies, highlighting their characteristics, limitations, and emerging research directions for low-inertia power systems.