Nonlinear model predictive pitch control of aero-elastic wind turbine blades

Abstract

This paper proposes a Nonlinear Model Predictive Controller (NMPC) for pitch control of Horizontal-Axis Wind Turbines (HAWTs) in Region 3 to avoid flutter aero-elastic instability. First, an aero-elastic HAWT rotor model was derived based on extended Hamilton’s principle using the coupled flap-wise and torsional motions of each blade. As for the aerodynamic loading, expressions for lift and pitching moment are obtained based on modifications of Theodorsen’s fixed wing strip theory for a rotating HAWT blade. The model is spatially discretized using the Assumed Modes Method with the first three flap-wise and two torsional mode shapes for the fixed cantilevered blade under free loading. This was applied to the 5-MW NREL (National Renewable Energy Laboratory) reference HAWT. The time-domain response under aerodynamic loading of the developed model was compared to FAST (Fatigue, Aerodynamics, Structure and Turbulence) aero-servo-elastic HAWT simulator. Then, an NMPC pitch controller was designed using the developed model for prediction. This was compared to another NMPC pitch controller which used a lumped-mass drive-train model as a prediction model and to the baseline gain-scheduled PI pitch controller.