Adaptive fast frequency support control for VSC-HVDC interconnected offshore wind turbines with exponential trajectory recovery

Integrating large-scale offshore wind farms could pose considerable challenges to the frequency stability of the onshore AC grid. To mitigate this issue, variable-speed wind turbines, equipped with auxiliary controllers to harness kinetic energy stored in the rotating mass of turbine rotors, can provide valuable fast frequency support during grid frequency events. This paper proposes an adaptive fast frequency support control scheme that enables voltage source converter-high voltage direct current interconnected offshore wind turbines to provide fast frequency support in a communication-free manner, effectively exploiting their available kinetic energy while preventing secondary frequency drop. Specifically, based on dynamic rotor speeds and grid frequency characteristics, the fast frequency support control gains are adaptively adjusted via hybrid fuzzy logic-linear tuning strategies. Further, considering differences in the inertia constant of individual wind turbines, an inertia discrepancy factor is embedded in the adaptive control gain tuning strategies to ensure a proper allocation of fast frequency support burdens among turbines according to their available kinetic energy. Once the grid reaches the frequency nadir, each turbine seamlessly transitions to the rotor speed recovery stage, employing an exponential trajectory recovery scheme to smoothly restore to its pre-disturbance state while preventing secondary frequency drop. Simulations conducted in MATLAB/Simulink show that the proposed adaptive control scheme outperforms other control frameworks in enabling fast frequency supports of wind turbines to improve grid frequency nadir and prevent secondary frequency drop across various wind turbine operating conditions and wind speeds.