Field testing of individual pitch control on the NREL CART-2 wind turbine

1. Field testing of individual pitch control on the NREL CART-2 wind turbine E. Bossanyi* and A Wright+ *Garrad Hassan & Partners Ltd., +NREL Presented at: EWEC 2009, Marseille

2. Context Work carried out as part of the EU 6th Framework integrated project “UPWIND”. Over many years, simulations have demonstrated that Individual Pitch Control (IPC) can be an effective means of reducing wind turbine fatigue loading. To date, no publishable field test data is available to prove that IPC actually lives up to expectations in reality. Using the public-domain CART-2 and CART-3 research turbines at NREL, this project provides a way to achieve the field validation which is needed to provide the confidence to design new turbines which rely on the load reductions which IPC can provide.

3. The NREL Turbines • Two turbines available: 42m diameter, 660 kW • Although not representative of modern multi-MW designs, turbines are adequate for proof of principle • Research turbines – advantage of being very accessible with minimum fuss • CART-2: 2-bladed – aiming for measurements in spring 2009 • Turbine ready and operational • Relevant: 2-Bladed turbines still a definite option for large offshore machines • Uses conventional strain gauges, but very robust & well calibrated • Fast pitch actuator – should be suitable for IPC • CART-3: 3-bladed – aiming for measurements in spring 2010 • Turbine ready but awaiting completion of the control system by NREL

4. Programme • July 2008: Principles agreed: • CART-2 testing Spring 2009 (should already be underway) • CART-3 testing one year later • October - November 2008: CART-2 algorithm design completed and tested in simulations. • Delay awaiting final DOE signature of confidentiality agreement. Once this is in place: • Transfer of algorithm to NREL • Further NREL proving simulations • Still hoping to start field tests before end of current wind season! • Summer 2009: CART-3 algorithm design to start. • Spring 2010: CART-3 field testing.

5. CART-2 turbine • Teetered rotor, but teeter will be locked with teeter brake. Principle is to use IPC instead of teetering to reduce out of plane blade & hub loads. • Bladed model completed. • CART-2 control algorithm designed by GH: • Optimal power production over nominal speed range • Speed regulation by interacting torque and collective pitch control • Drive train damping filter in torque controller • F/A tower damping by collective pitch control • 1P individual pitch control to reduce rotating and non-rotating loads • 2P individual pitch control not required • Simulation testing completed.

6. Simulation at 18 m/sTower F/A damping: Tower base bending moment (My) • Significant reduction of fore-aft tower vibration and fatigue loading at 1st tower frequency. Tower 1st fore-aft mode

7. Simulation at 18 m/s IPC: Blade root out of plane bending moment (My) • 1P out of plane loads virtually eliminated on rotating components. • Teeter hinge no longer needed! Rotation frequency (1P)

8. Simulation at 18 m/s IPC: Rotating hub moment (My) • 1P out of plane loads virtually eliminated on rotating components. • Teeter hinge no longer needed! Rotation frequency (1P)

9. Simulation at 18 m/s IPC: Yaw moment (Yaw bearing Mz): non-rotating frame • 1P (rotating) becomes 0P and 2P on non-rotating components. • 0P compensates for wind shear and direction change: reduces peak loads. • Large reduction in 2P fatigue loading. Low frequency (0P) Blade passing frequency (2P)

10. Simulation at 24 m/s IPC: Yaw moment (Yaw bearing Mz): non-rotating frame • Reduction at 0P helps to reduce peak yaw & nod moments. • Reduced 3P also helps. • Reduced 3P means lower fatigue loads.

11. Simulation at 18 m/s Pitch activity • IPC requires extra pitch action • Additional pitching is concentrated at 1P Rotation frequency (1P)

12. Simulation at 18 m/s Pitch activity

13. IPC: 2 Blades vs 3 blades • Control action calculated in non-rotating frame: two axes, just the same for any number of blades. • Number of blades is embodied in the rotational transformations: • For 2-blades, non-rotating fatigue loads (2P) are reduced. • For 3 blades, non-rotating fatigue loads are at 3P. Reducing these requires additional (2nd harmonic) IPC at 2P (rotating), to reduce non-rotating loads at 1P and 3P. (Higher harmonics are also possible, but probably unnecessary.)

14. 2nd Harmonic IPC • The CART-3 turbine will provide the opportunity for field testing of both 1P and 2P IPC: • The CART-3 algorithm is not yet designed, but a similar algorithm has already been designed for the Upwind 5MW reference turbine.

15. 1st and 2nd Harmonic IPCUpwind 5MW reference turbine (3 bladed) • Rotating loads reduced at: • 1P • 2P • Non-rotating loads reduced at: • 0P (and 2P) • 3P (and 1P) 1st harmonic IPC 2nd harmonic IPC

16. 1st and 2nd Harmonic IPCUpwind 5MW reference turbine (3 bladed) • Extra pitch action at: • 1P • 2P

17. Conclusions • IPC can replace teeter hinge on 2-bladed turbine (reduces 1P loads in rotating components). • IPC on 2-bladed turbine also reduces 2P fatigue loading on non-rotating components. • On 3-bladed turbine, 2nd harmonic (2P) IPC is needed to reduce the 3P fatigue loads on non-rotating components. • CART-2 field measurements to start soon, with 1P IPC and tower fore-aft damping. • CART-3 field measurements next year, with 1P and 2P IPC and tower fore-aft damping. • 2P IPC proven in simulations for 5MW Upwind turbine.

18. Acknowledgements This work has been carried out under the 6th Framework Integrated Project “Upwind”. The support of the European Commission is gratefully acknowledged.