1. European Wind Energy Conference and Exhibition 2006 Athens, Greece EWEC’06 Athens 27 February-2 March 20061/16 Investigation of the Stability Bounds of Wind Turbines in view of Passive Instability Suppression V.A.Riziotis and S.G.Voutsinas National Technical University of Athens School of Mechanical Engineering Fluids Section E.S. Politis and P. K. Chaviaropoulos Centre for Renewable Energy Sources Wind Energy Section

2. EWEC’06 Athens 27 February-2 March 2006 2/16 NTUA Overview of the Presentation

3. EWEC’06 Athens 27 February-2 March 2006 3/16 NTUA

4. EWEC’06 Athens 27 February-2 March 2006 4/16 NTUA isolated blade analysis 2% improvement in power production 1 st lead-lag 1 st flap

5. EWEC’06 Athens 27 February-2 March 2006 5/16 NTUA full wind turbine analysis

6. EWEC’06 Athens 27 February-2 March 2006 6/16 NTUA ReferenceModified 1Modified 2 Freq(hz) red(%)Freq(hz)red(%) 1st flap0.981 0.00.980-0.1 1st lag1.9281.614-16.31.225-36.5 2nd flap2.8782.866-0.42.856-0.8

7. EWEC’06 Athens 27 February-2 March 2006 7/16 NTUA 1 st flap 2 nd cyclic flap 1 st cyclic lead-lag

8. EWEC’06 Athens 27 February-2 March 2006 8/16 NTUA 1 st flap 2 nd cyclic flap 1 st cyclic lead-lag

9. EWEC’06 Athens 27 February-2 March 2006 9/16 NTUA 1 st flap 2 nd cyclic flap 1 st cyclic lead-lag Reference modified 2 regressive lead-lag mode shape

10. EWEC’06 Athens 27 February-2 March 2006 10/16 NTUA isolated blade analysis FOIL2W: 2D viscous-inviscid interaction model wind speed 12m/s 1 st lead-lag mode

11. EWEC’06 Athens 27 February-2 March 2006 11/16 NTUA introduction of a soft connection of the nacelle with the tower k yaw = 5x10 5 Nm/rad

12. EWEC’06 Athens 27 February-2 March 2006 12/16 NTUA Effect from the introduction of a soft yaw connection on the damping of the rotor lead- lag modes Application to a Pitch-Variable Speed wind turbine

13. EWEC’06 Athens 27 February-2 March 2006 13/16 NTUA 2 nd flap 1 st lag Reference soft yaw regressive lead-lag mode shape

14. EWEC’06 Athens 27 February-2 March 2006 14/16 NTUA Effect from the introduction of a soft yaw connection on the damping of the rotor lead- lag modes Application to an Active Stall wind turbine

15. EWEC’06 Athens 27 February-2 March 2006 15/16 NTUA Conclusions

16. EWEC’06 Athens 27 February-2 March 2006 16/16 NTUA Acknowledgements European Commission & Greek Secretariat for Research and Technology for co-funding this work under contract NNK5-CT2002-00627 (STABCON project) STABCON project partners for publishing of the paper