1. Wind Turbine Aerodynamics Section 2 – Power Control E-Learning UNESCO ENEA Casaccia - February 26 2007 Fabrizio Sardella

2. 2 THE CONTROL OF THE POWER The wind speed is continuously changing in a way that we can’t predict or control. When the wind speed increases, the power increases according to the already known formula: Consequently the loads on the turbine increase. To prevent failures without designing the turbine for any wind speed value (it would be not economically convenient), it is necessary to limit the power and design the turbine according to this limit. The most used systems to limit the power are: - The pitch control - The stall control

3. 3 THE CONTROL OF THE POWER The Pitch Control In this case the blades are rotated around their longitudinal axis by some hydro-mechanical or electro-mechanical devices. The blade rotation induces a variation in the angle of attack and then a variation in the aerodynamic characte- ristics of the blade. W Nominal position Stop position Starting position

4. 4 THE CONTROL OF THE POWER The global effect can be represented in the plot on the left side, while the way how the blade is rotated is represented in the figure on the right side. The blade rotation allows to start and stop the rotor, to have the maximum C p at a given wind speed (Opti-Tip) and to limit the power when necessary. β = 0° β = 2° β+β+ β = -1.5° W Nominal position Stop position Starting position

5. 5 THE CONTROL OF THE POWER The Stall Control In this case the drop of aerodynamic performances is obtained by increasing the angle of attack as the wind speed increases, until the air flow is not any more able to remain adherent to the profile. In this way the profile stalls and the power drops. Fixed RPM are required V∞V∞ V∞V∞ STALL W1W1 RR 11 α1α1 W2W2 22 α2α2  TIP

6. 6 THE CONTROL OF THE POWER The big advantage of this system, with respect to the pitch system, is the extreme simplicity in mechanics and electronics (the stall is a passive control), the disadvantages are a power curve which drops after the rated wind speed, the vibrations induced by the stall and impossibility to use variable speed. Furthermore the start and stop of the turbine is more complicated than in a pitch controlled WTG. The power curves obtained with the two kind of controls are represented in the figure on the left side.

7. 7 THE FIXED RPM WTGs’+PITCH We have already described the behaviour of a fixed RPM machine below rated power: the pitch system acts to maximize the value of the Cp. Above rated power, when the controller reads that the power output has exceeded the nominal value, a pitch actuation is performed. Under a control point of view it is possible to say that a classic pitch control applied to a fixed RPM machine, due to the unavoidable delays in pitch actuation, is only able to keep the power close the nominal value, going up and down around the nominal value. In other words the pitch control, which is a mechanical system, is not able to follow the fast wind speed changes (gusts) and the resulting power control is not so effective; so the loads control.

8. 8 THE FIXED RPM WTGs’+PITCH From a dynamical point of view the behaviour of a fixed RPM machine is similar to a clamped disc (rotor) connected to an elastic element (main shaft). An increase in aerodynamic torque is immediately translated in an increase of mechani- cal reaction torque and so in increased stresses on the drive train and in the structure. Rotor Main Shaft Gearbox Generator

9. 9 THE OPTI-SLIP ® SYSTEM The Opti-Slip ® System is a Vestas patent which allows to change a bit the generator RPM in specific conditions. The system is based on changes in electrical parameters which are much more fast then the mechanical actuations. In particular the response of the system to the gusts is very effective: the rotor increase its rotational speed and accumulates part of the energy contained in the gust in rotational energy, giving the time to the pitch actuation to reach the adequate position. Looking at the aerodynamics it is possible to observe how a rotational speed increase can limit the increase or reduce the angle of attack, so that the power increase is only due to the RPM increase. W1W1 1R1R α1α1 W2W2 α2α2  2R2R TIP

10. 10 THE OPTI-SLIP ® SYSTEM In the following plot is represented a V47 Power Curve and the ranges where the Opti-Tip ® and the Opti-Slip ® systems work. The optimal power control means also an optimal loads control so Vestas has been able to pass from V39 500kW to V47 660kW without substantial changes in the WTG structure, only improving the power control by the Opti-Slip ®. The Opti- Slip ® system can be also considered as Opti Tip Opti Slip + pitch regulation the first step towards the variable speed machines.

11. 11 THE OPTI-SLIP ® SYSTEM Below rated power Above rated power

12. 12 THE VARIABLE SPEED WTGs’ In terms of power control the Opti-Speed ® together with the pitch system works conceptually as the Opti-Slip ®, but it is much more effective. From a dynamical point of view in a variable speed machine it is possible to adjust the reaction torque in the generator by means of the converter. This gives a smoother response to the gusts and a much lower stress variation. Rotor Main Shaft Gearbox Generator Converter

13. 13 THE ACTIVE STALL TM Also in the case of the stall control a big improvement has been introduced: the Active-Stall TM. It consists in the possibility to change the initial position of the blade so that W Nominal position Stop position it is possible to change the point at which the stall occurs. The resultant power curve is very similar to the one obtained with the pitch control. Furthermore it is possible to stop the machine as a pitch controlled machine by rotating the blade at -95 degrees with respect to the nominal position. In this case the blade presents the trailing edge to the wind.

14. 14 THE INDIVIDUAL PITCH The individual pitch has been introduced on the V90 3MW; it gives the possibility to pitch the blades with different angles each, based on the reading of the blade deflection. In this way it is possi- ble to compensate part of the not uniform wind/load distribution on the rotor which is the origin of extra loads in yaw and tilt. This kind of control is superimposed to the already existing Opti-Tip ® and Pitch controls. The result is lighter nacelles and lighter towers (the weight of a V90 3MW is approximately the same as a V80 2 MW). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1819 20 21 22 23 24 25 26 27 28 29 30 31 34 35 32 33 Flap moment 16 m/s - 6 deg yaw error With load sensors No load sensors Optimal