1. 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.

2. 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.

3. Commercial Turbine Control
Generator Torque
Nacelle Yaw
Blade Pitch
Control Actions

4. Control of Flexible Modes

5. 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

6. 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?

7. Controller Structure

8. 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

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

10. Disturbance model

11. 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

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

13. 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

14. 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.

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

16. Controller Structure

17. 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
fatigue DEL
mean
fatigue DEL 944.4
mean
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

18. Results
Probability Density Functions: 18

19. Results 19

20. Results

21. Results
Tower Bending Moments: 21

22. Results
Power Spectral Densities:

23. Results
Pitch Rate and Generator Torque:

24. 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.

25. 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

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

27. Questions?
Dr. Alan D. Wright 27