Non-linear pitch control of wind turbines for tower load reduction

As the modern wind turbines (WTs) become larger in size, the structural loads experienced by the WTs increase dramatically, which would shorten the working life of the WTs and increase the maintenance cost. To mitigate such problem, this study proposes a non-linear pitch control strategy including tower load reducing objective for the WTs operating in the above-rated wind speed region. The proposed non-linear controller consists of a rotor speed regulator, a tower oscillation damper and a lead compensator. The rotor speed regulator and tower oscillation damper are designed separately based on inverse system method, which is a powerful tool in solving the control problem of non-affine non-linear system. The lead compensator is utilised to compensate for the phase lag of the pitch angle command caused by the pitch actuator. Furthermore, light detection and ranging (LIDAR) preview measurements are introduced to provide wind information for the proposed controller in order to improve its control performance. The simulation results with the aero-elastic model of fatigue, aerodynamics, structures and turbulence (FAST) show that, the proposed non-linear controller outperforms the baseline gain-scheduled proportional-integral controller. The proposed non-linear controller can work well in the whole above-rated wind speed region, and is easy to implement for application.