Nonlinear pitch angle control of an onshore wind turbine by considering the aerodynamic nonlinearities and deriving an aeroelastic model

Abstract

In this paper, the control problem of a wind turbine in region 3 (where the wind velocity is between the rated wind velocity and cut out wind velocity) has been investigated by considering the aerodynamic nonlinear behavior of the wind-structure interaction. The model has been developed by using the blade element momentum (BEM) theory to obtain the aerodynamic torque and aerodynamic loads in edgewise and flapwise directions. For validation, the aerodynamic behavior of the onshore NREL 5 MW turbine has been compared with the Fatigue, Aerodynamics, Structures, and Turbulence (FAST) aeroelastic code in terms of the power coefficient. Wind speed is modelled as a three-dimensional profile with Kaimal spectrum distribution. The wind profile data is modeled in the frequency domain by the Kaimal spectrum distribution function. Then, an inverse Fourier transformation is needed to convert the data into the time domain. In the next, the sliding mode approach is applied to the three DOFs model of the drivetrain system which includes the rotor speed, low-speed shaft angle, and pitch angle. Finally, to consider the complete behavior of the onshore wind turbine, an eleven DOFs model is obtained. This model considers three DOFs for the flapwise vibrations of the blades, another three DOFs for the edgewise vibrations of the wind turbine, two DOFs for the side-side and fore-aft vibrations of the tower, and three DOFs for the drive-train system of the wind turbine. The performance of the proposed sliding mode control methods has been compared with the conventional PID controller for the above-rated wind velocity region. Results demonstrate that the wind turbine performance has been improved.