Field Testing Controls to Mitigate Fatigue Loads in the Controls Advanced Research Turbine Alan D. Wright Lee J. Fingersh National Renewable Energy Laboratory Karl A. Stol University of Auckland 28th ASME Wind Energy Symposium Orlando, Fl. January 5, 2009 I’d like to give you a general overview of some of the more-important codes that we use at the NWTC. Some of these codes were developed by our industry and university partners, some by government labs, and some by us.

Presentation Scope Show design of region 3 generator torque and blade pitch controller. Goals: Region 3 speed regulation active damping of tower side-side and fore-aft motion Describe field implementation and tests in the Controls Advanced Research Turbine. Compare state-space control results to baseline PID control results.

Commercial Turbine Control Generator Torque Nacelle Yaw Blade Pitch Control Actions

Control of Flexible Modes

Region 3 Control Design Control Actuators: Collective blade pitch, generator torque Goals: Collective Blade Pitch Control: Speed regulation Tower fore-aft damping Generator Torque Control: Tower side-side damping Drive-train torsion damping Two separate control loops: Collective blade pitch Generator torque

Questions Can separate control loops be used to add active damping to two closely spaced modes? Tower 1st fore-aft mode (0.87 Hz.) Tower 1st side-side mode (0.88 Hz.) Will these separate loops destabilize each other? Two separate control loops: Collective blade pitch Generator torque Will adding damping reduce tower fatigue loads?

Controller Structure

Full State Feedback Control Regulate rotor-speed in the presence of wind-speed disturbances and stabilize turbine modes. Stabilize flexible modes through full state feedback. Use state estimation to provide the controller with needed states (including wind-speed). Account for uniform wind disturbances 2-Multiple input/single output control loops

Rotor Collective Pitch Control Model Perturbed structural dofs & rates: tower f-a rotor collective flap generator-speed Pitch actuator states

Disturbance model

Open-loop and Closed-loop Pole Locations Open-loop poles Closed-loop poles First flap symmetric mode −3.63 ± 13.81i −3.66 ± 13.85i Tower first f-a mode −0.07 ± 5.52i −1.27 ± 5.32i Generator speed −0.1943 −2.50

Generator Torque Control Design Model Perturbed structural dofs & rates: tower s-s drive-train torsion generator-speed filter states

Open-loop and Closed-loop Pole Locations Open-loop poles Closed-loop poles Tower first s-s mode −0.002 ± 5.54i −0.140 ± 5.54i Generator speed −0.102 Drive train first torsion −0.01 ± 22.47i −1.07 ± 22.45i

Testing Strategy Using two separate uncoupled control loops to add active damping to two closely spaced tower modes Will these two control loops interact and destabilize the turbine? Test the tower f-a damping with collective pitch first. Add tower s-s damping with generator torque.

Controllers: PID State-space1 State-space2 Speed regulation yes Tower f-a damping no Tower s-s damping Drive-train torsion damping

Controller Structure

Results Statistics and Performance Measure Baseline PID Control (Undamped) (5530 sec.) State-Space 1 (tower f-a damping only) (2500 sec.) State-Space 2 (tower f-a and s-s damping) (3700 sec.) Tower fore-aft bending (kNm) mean 1527.5 fatigue DEL 1342.2 mean 1525.6 fatigue DEL 944.4 mean 1498.9 fatigue DEL 928.6 Tower side-side bending (kNm) mean 122.1 fatigue DEL 920.5 mean 293.3 fatigue DEL 926.4 mean 360.1 fatigue DEL 680.7 Blade pitch rate (deg/s) avg mag 2.02 max 15.1 min -15.5 avg mag 2.32 max 14.7 min -13.9 avg mag 2.33 min -12.9 Generator torque (Nm) Std 0.00 max 3524 min 3524 std 23.1 max 3631 min 3423 std 115.8 max 4177 min 2942 17

Results Probability Density Functions: 18

Results 19

Results

Results Tower Bending Moments: 21

Results Power Spectral Densities:

Results Pitch Rate and Generator Torque:

Conclusions Designed and performed field tests of two separate control loops: Rotor collective pitch control active tower f-a damping Region 3 speed regulation generator torque control active tower s-s damping active drive-train torsion damping Field tests demonstrate 30% reduction in tower f-a and 26% reduction in tower s-s fatigue loads compared to simple PID controls. Reasons for lack of drive-train torque load mitigation not resolved. Results showed no undesirable interactions between these separate control loops.

Future Work Resolve issue with drive-train torsion damping and re-test controller. Implement and field-test independent blade pitch for shear mitigation and generator torque control. Investigate alternative sensors for independent pitch control look ahead Lidar additional blade sensors hub or shaft sensors

Acknowledgements Dr. Michael Robinson – NREL management support Garth Johnson, Scott Wilde – CART maintenance and support Marshall Buhl – MCrunch data analysis scripts 26

Questions? Dr. Alan D. Wright 303-384-6928 alan_wright@nrel.gov 27