Combined Feed-forward/Feedback Control of Wind Turbines to Reduce Blade Flap Bending Moments

In above rated conditions, wind turbines are often subjected to undesirable high structural loading. We investigate two feedback control techniques in combination with a feedforward control method for reducing blade flap bending loads in above rated wind conditions. The feedback controls studied include both disturbance accommodating/tracking and integral augmented/repetitive types that incorporate models of persistent disturbances at DC (step changes in wind) and at the once per revolution frequency. Each method is combined with a feed-forward method utilizing the wind speed as measured at the blade tips and also based on the blade tip average wind speed. Performance is assessed by simulating the combined feed-forward/feedback systems on a three bladed turbine model with the National Renewable Energy Lab’s FAST wind turbine code. It is found that feedforward of blade tip average wind speed measurements can provide significant reduction of blade root loads and improved speed regulation in time varying wind that is uniform across the rotor plane. However, the improvements are not as great in non-uniform and turbulent conditions in which case the blade tip average wind speed measurements provide incomplete information for conditions that can be unique at each blade. We use an extension of the linearized turbine model to design feed-forward compensation that uses individual measurements of the wind at each blade tip. Results suggest that using blade local measurements provides substantially greater reduction in blade loads, but assessing the full potential of using this more detailed information requires more accurate modeling of the way perturbations local to each blade couple into the turbine.