Obtaining fatigue-based frequency domain specifications for the design of controllers in wind turbines

Abstract. This work presents a methodology to (i) estimate fatigue using linear models and (ii) generate control specifications directly linked to the mechanical fatigue caused by driving loads for a wind turbine applications. The method is intended for frequency domain controller design techniques such as QFT or H_∞ and is based on Dirlik’s method for fatigue assessment. The main advantage of using frequency domain approach is that the need of computationally expensive processess such as the generation of turbulent wind fields or aeroelastic simulations is reduced. As a consequence, the controller design method becomes more agile. The method has been validated by designing controllers for the reference 15 MW wind turbine based on fatigue specifications, obtaining simulation results with OpenFAST and comparing the fatigue results from a rainflow algorithm with the linear estimation. Fatigue has been reduced by 22 % to 35 % at different wind speeds corresponding to above rated operation. The mean fatigue estimation error is 1.07 %, proving the method is suitable for a wind turbine control application.