Refinements and Tests of an Advanced Controller to Mitigate Fatigue Loads in the Controls Advanced Research Turbine

Abstract

Wind turbines are complex, nonlinear, dynamic systems forced by aerodynamic, gravitational, centrifugal, and gyroscopic loads. The aerodynamics of wind turbines are nonlinear, unsteady, and complex. Turbine rotors are subjected to a complicated 3-D turbulent wind inflow field, with imbedded coherent vortices that drive fatigue loads and reduce lifetime. Design of control algorithms for wind turbines must account for multiple control objectives. Future large multi-megawatt turbines must be designed with lighter weight structures, using active controls to mitigate fatigue loads, while maximizing energy capture. Active damping should be added to these dynamic structures to maintain stability for operation in a complex environment. At the National Renewable Energy Laboratory (NREL), we have designed, implemented, and tested advanced controls to maximize energy extraction and reduce structural dynamic loads. These control designs are based on linear models of the turbine that are generated by specialized modeling software. In this paper, we present field test results of an advanced control algorithm to mitigate blade, tower, and drivetrain loads in Region 3. The advanced state-space controller uses independent blade pitch to mitigate the effects of shear across the rotor disk, and a collective pitch component to add active damping to the tower’s first fore-aft bending mode, and, to regulate turbine speed. In addition, a separate generator torque control loop adds active damping to the tower’s first side-side mode and the first drivetrain-torsion mode. In this paper we show a refinement to the generator torque control loop to account for actuator delay. We discovered a delay in actuation between commanded generator torque and the torque actually applied to the highspeed shaft. If this delay is not properly accounted for in the plant model used for control design, the generator torque control loop tends to destabilize the first drivetrain torsion mode. We show modifications to the torque control loop to account for this delay, and, to prevent unnecessary control actuation at certain harmonics in the rotorspeed. We present field tests of this controller and make comparisons with a simple PID baseline controller for the 2-bladed Controls Advanced Research Turbine (CART2) located at NREL’s National Wind Technology Center.