Impact mechanism of frequency response on wind turbine fatigue load

Wind turbines' participation in frequency response is known to improve the frequency stability of power systems, but it can also have a negative impact on the fatigue load of wind turbines. The objective of this paper is to investigate the effect of frequency response on the fatigue loads experienced by various components of a wind turbine, including the low-speed shaft, tower, and blade. To achieve this goal, the authors develop a model of a variable speed horizontal-axis wind turbine based on a doubly fed induction generator. They derive explicit analytical equations of low-speed shaft torque, tower bending moment, and blade bending moment to describe the fluctuations of torque and moment related to the operating states of wind turbines, such as generator torque, rotor speed, and pitch angle, under frequency response. These equations allow for the evaluation of the impact of frequency response on torque and moment changes and fatigue load. Spectral density analysis and modal analysis are used to further analyze the analytical equations, examining the influence of frequency response on different operating conditions of wind turbines and determining the mechanism by which frequency response affects fatigue load qualitatively and quantitatively. The authors use the FAST V8 Code based on the NREL offshore 5-MW baseline wind turbine to demonstrate the effectiveness of the proposed analysis method in evaluating fatigue loads affected by frequency response. The results show that the fatigue load on the low-speed shaft and the lateral side of the tower will significantly increase due to wind turbine participation in frequency response.