Mechanism analysis of ultra-low-frequency oscillations in high-penetration hydropower systems and suppression strategy based on PID-QPR control

Ultra-low-frequency oscillations (ULFO), increasingly observed in high-penetration hydropower systems and asynchronously interconnected power grids, have emerged as a significant threat to grid stability, with the potential to cause cascading failures and large-scale blackouts. To enhance small-signal stability during active frequency regulation, a hybrid control strategy combining proportional–integral–derivative (PID) and quasi-proportional–resonant (QPR) controllers is developed. The stability characteristics are first analyzed using a single-machine system model derived from the primary frequency regulation dynamics of hydropower units. Subsequently, a multi-machine frequency response model incorporating both hydropower and thermal power units is constructed. The damping torque method is then employed to reveal the generation mechanism of ULFO in power grids with varying hydropower penetration levels, as well as their impact on grid stability. Inspired by the advantages of QPR controllers in frequency selectivity, precise control of specific target frequencies, and superior dynamic performance, a combined PID-QPR control strategy is proposed. Additional damping is integrated into the governor side, and the parameters of the PID-QPR controller are optimized using an improved Beluga Whale Optimization algorithm. Finally, simulation models of single-machine, multi-machine, and four-machine two-area systems are constructed on the MATLAB/Simulink platform for validation. Through a comparative analysis of three optimization strategies (PSO, GA-PSO, and MISER3.2), the proposed method demonstrates superior robustness and adaptability. It reduces overshoot by around 4.25 % and shortens settling time by approximately 20 %, confirming its effectiveness in suppressing ULFO and enhancing system stability.