Independent pitch control using rotor position feedback for wind-shear and gravity fatigue reduction in a wind turbine

In large-scale horizontal-axis wind turbines, turbulence, gravity, and wind shear effects make the effective forces on turbine blades vary considerably. These varying conditions result in significant blade vibration fatigue loading. In this paper, two separate realtime control algorithms are studied; the goal of each algorithm is to reduce blade fatigue cycle amplitudes caused by wind shear and gravity effects. Each algorithm adds a modulation signal to a blade´s pitch angle based on the rotational position of the rotor; rotor position is used as the feedback signal as it relatively easy to measure. One algorithm focuses on reducing flatwise loading while the other focuses on edgewise loading. The algorithms are derived and analyzed based on a simplified 4th-order dynamic model of a blade. Evaluation of the flatwise controller shows it holds good potential for providing loading reduction; similar analysis shows the edgewise algorithm cannot provide realistic load reduction. The flatwise control strategy is then applied to a highly-detailed computer simulation model of a 1.5-MW machine using the ADAMS/WT finite-element simulation tool. The simulation results demonstrate that the control algorithm can significantly reduce flatwise fatigue loading for a large machine with very little affect on the machines power output and tower oscillations.