Lee J. Fingersh

Control of variable-speed wind turbines: standard and adaptive techniques for maximizing energy capture

This article considers an adaptive control scheme previously developed for region 2 control of a variable speed wind turbine. In this paper, the question of theoretical stability of the torque controller is addressed, showing that the rotor speed is asymptotically stable under the torque control law in the constant wind speed input case and L/sub 2/ stable with respect to time-varying wind input. Further, a method is derived for selecting /spl gamma//sub /spl Delta/M/ in the gain adaptation law to guarantee convergence of the adaptive gain M to its optimal value M*.

Methods for Increasing Region 2 Power Capture on a Variable-Speed Wind Turbine

The standard region 2 control scheme for a variable-speed wind turbine,t c5Kv 2 , has several shortcomings that can result in significant power loss. The first of these is that there is no accurate way to determine the gain K; modeling programs are not accurate enough to represent all of the complex aerodynamics, and these aerodynamics change over time. Furthermore, it is not certain whether the value of K used in the standard control even provides for the maximum energy capture under real-world turbulent conditions. We introduce new control methods to address these issues. First, we show in simulation that using smaller values of K than the standard can result in increased energy capture. Second, we give simulation results showing that an optimally tracking rotor control scheme can improve upon the standard scheme by assisting the rotor speed in tracking wind-speed fluctuations more rapidly. Finally, we propose an adaptive control scheme that allows for maximum power capture despite parameter uncertainty. @DOI: 10.1115/1.1792653#

Electrical Collection and Transmission Systems for Offshore Wind Power

The electrical systems needed for offshore wind farms to collect power from wind turbines--and transmit it to shore--will be a significant cost element of these systems. This paper describes the development of a simplified model of the cost and performance of such systems.

Field Testing LIDAR Based Feed-Forward Controls on the NREL Controls Advanced Research Turbine

Wind turbines are complex, nonlinear, dynamic systems driven by aerodynamic, gravitational, centrifugal, and gyroscopic forces. The aerodynamics of wind turbines are nonlinear, unsteady, and complex. Turbine rotors are subjected to a chaotic three-dimensional (3-D) turbulent wind inflow field with imbedded coherent vortices that drive fatigue loads and reduce lifetime. In order to reduce cost of energy, future large multimegawatt turbines must be designed with lighter weight structures, using active controls to mitigate fatigue loads, maximize energy capture, and add active damping to maintain stability for these dynamically active structures operating in a complex environment. Researchers at the National Renewable Energy Laboratory (NREL) and University of Stuttgart are designing, implementing, and testing advanced feed-back and feed-forward controls in order to reduce the cost of energy for wind turbines.

METHODS FOR INCREASING REGION 2 POWER CAPTURE ON A VARIABLE SPEED HAWT

β blade pitch (degrees) ψ yaw error (degrees) M adaptive gain (m 5 /rad 3 ) The standard region 2 control scheme for a variable speed wind turbine, τc = kω 2 , has several shortcomings that can result in significant power loss. The first of these is that there is no accurate way to determine the gain k; modeling programs are not accurate enough to represent all of the complex aerodynamics, and these aerodynamics change over time. Furthermore, it is not certain whether the value of k used in the standard control even provides for the maximum energy capture under real-world turbulent conditions. New control ideas are introduced to address these issues. First, it is shown in simulation that using smaller values of k than the standard can result in increased energy capture. Second, an optimally tracking rotor control scheme improves upon the standard scheme by assisting the rotor speed in tracking wind speed fluctuations more rapidly. Finally, an adpative control scheme is proposed that allows for maximum power capture despite parameter uncertainty. n number of steps in adaptation period K∆M positive gain on adaptation law (rad 3 /m 5

Controls Advanced Research Turbine (CART) Commissioning and Baseline Data Collection

During FY2002, the CART turbine and controller were developed and commissioned. This included developing and checking out the protection and operational control systems. More than 50 hours of data were collected in constant and variable-speed modes. A new strategy, which underwent limited testing on the machine, was created for avoiding tower resonance. All the data from the checkout through the operational periods were organized, archived, and backed up.

Stability analysis of an adaptive torque controller for variable speed wind turbines

Variable speed wind turbines are designed to follow wind speed variations in low winds in order to maximize aerodynamic efficiency. Unfortunately, uncertainty in the aerodynamic parameters may lead to sub-optimal power capture in variable speed turbines. Adaptive generator torque control is one method of eliminating this sub-optimality; however, before adaptive control can become widely used in the wind industry, it must be proven to be safe. This paper analyzes the stability of an adaptive torque control law and the gain adaptation law in use on the Controls Advanced Research Turbine (CART) at the National Renewable Energy Laboratorys National Wind Technology Center.

Controls Advanced Research Turbine: Lessons Learned during Advanced Controls Testing

This paper describes some of the problems encountered while conducting tests on the Controls Advanced Research Turbine installed at the National Wind Technology Center. It also discusses some of the peculiarities of the C-code used to control the CART and discusses the fault protection routine, Turbine safety, and some of its failures.

Testing controls to mitigate fatigue loads in the controls Advanced Research Turbine

Wind turbines are complex, nonlinear, dynamic systems forced by aerodynamic, gravitational, centrifugal, and gyroscopic loads. The aerodynamics of wind turbines is nonlinear, unsteady, and complex. Turbine rotors are subjected to a complicated three-dimensional (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, maximize energy capture, and add active damping to maintain stability for these dynamically active structures operating in a complex environment. Researchers at the National Renewable Energy Laboratory are designing, implementing, and testing advanced controls to maximize energy extraction and reduce structural dynamic loads. These control designs are based on a linear model of the turbine that is generated by specialized modeling software. This paper describes testing of a control algorithm to mitigate blade, tower, and drivetrain loads using advanced state-space control methods. The controller uses independent blade pitch to regulate the turbines speed in Region 3, mitigate the effects of shear across the rotor disk, and add active damping to the towers first fore-aft bending mode. Additionally, a separate generator torque control loop is designed to add active damping to the towers first side-side mode and the first drivetraintorsion mode. This paper discusses preliminary implementation and field tests of this controller in the Controls Advanced Research Turbine at the National Renewable Energy Laboratory. Also included are preliminary comparisons of the performance of this controller to results from a typical baseline Proportional-Integral-Derivative controller designed with just Region 3 speed regulation as the goal.

Advanced Wind Turbine Controllers Attenuate Loads when Upwind Velocity Measurements are Inputs

Sophisticated control algorithms for wind turbines provide means of increasing energy capture and mitigating extreme and fatigue loads. By including wind speed measurements upwind of the rotor, these control algorithms can include a feed-forward loop that anticipates the wind turbine response. Continuous-wave, monostatic, coherent lidar provides the capability of obtaining such measurements. Simulations show that a lidar device mounted on the turbine hub, aligned with a turbine blade, and rotating with the rotor produces a linear vertical shear measurement. The inclusion of this measurement in a disturbance accommodating controller (DAC) reduces blade flap damage equivalent loads when compared to a DAC that requires blade tip flap deflection measurements.