Control of wind turbines and wind farms Norcowe 2015 PhD Summer school Single Turbine Control

Transcription

1 of wind and wind farms Norcowe 2015 PhD Summer school Single Turbine August, 2015 Department of Electronic Systems Aalborg University Denmark

2 Outline Single Turbine Why is Historic Stall led in partial Why is Historic Stall led in partial

3 Single Turbine Why is Historic Stall led in partial 2 Objectives for the lecture This part of the course should give understanding of: Why wind turbine is? How the dierent methods work. The dierent possibilities modern wind turbine oers.

4 Why is Historic Stall led in partial 3 Subject for the lecture The subject here is top level of turbine speed and power. Many other single turbine task exist which is not covered here e.g. Yaw. Internal generator power electronic. Internal pitch

5 Why is Historic Stall led in partial 4 Approach Focus on horizontal axis wind (HAWT). Take a historical perspective moving from Dutch wind mills to modern wind. Avoid all the physical and mathematical details. Explain the the basic functionality/physics. Skip the actual modeling and design part.

6 Why is Single Turbine Why is Historic Stall led in partial 5 Basic turbine physics - one inertia model The wind gives a driving torque in the front end of the rotor. In the back end of the drive train a generator, grinder or similar gives a braking torque. Breaking torque T g Inertia I I ω = T r T g, Rotational speed ω Driving wind torque T r P w = 1 ρav 3, T r = P w C P

7 Why is Historic Stall led 6 With no active This will not give a safe rotational speed at all wind conditions (except for stall led ). Especially in high wind over-speed will occur. The basic task for wind turbine is to: secure a suitable rotational speed under all wind and load conditions. in partial

8 Historic Single Turbine Why is Historic Stall led in partial 7 Dutch wind mill ( ) The Dutch wind mill had sails on the blade. The rotor most be stopped for the sails to be manually led.

9 Single Turbine The Danish klapsejler ( ) Why is Historic 8 Stall led led the blade via connected aps (klap in Danish). I These aps (klapper) was in partial collectively led in with the connection though the hub. I The Danish klapsejler

10 Stall led - the Danish turbine Single Turbine Why is Historic Stall led in partial 9 Passive stall (1973-now) The generator is a asynchronous generator with short circuited rotor resulting in constant speed within a few percent. The rotor is geometrically designed to stall at rated generator power thus limiting the power in wind above rated.

11 Why is Historic Stall led in partial 10 Stall explained from aerodynamic When the pitch β and rotational speed ω r is constant the angle of attack θ b is increasing with wind speed v a. Lift L and driving force F t increases with angle of attack from below zero to approximately 10 degrees where it suddenly drops when the blade stalls. L v bt = ω r r(1 + a t) F t v ba = va(1 aa) v b F a D θ b β φ

12 Why is Historic Stall led in partial 11 Active stall (1990?-now) With passive stall the power curve is hard to get straight. The air density is varying up to approximately 25%. Therefore a passive stall turbine will have a power curve that is mostly under rated generator power. The power curve can be straightened out by slowly varying pitch to adjust the stall level.

13 Single Turbine Why is Historic Stall led in partial 12 constant speed ( ) The rst variable pitch was really a stall led turbine with pitching blades. The asynchronous generator with few percent slip compared with the pitching rates resulted in to large variation in load and power.

14 Why is Historic Stall led in partial 13 variable speed (1995-now) Variable speed was a revolution in wind turbine technology. The generator power is made lable independent of generator speed in a speed rage. In full load, the speed variation can be increased from e.g. 1% for asynchronous generators to e.g. 10% for variable speed machines. This speed variation can be achieved with the pitch ler. Also variable speed gives the opportunity to run more on maximum eciency in partial load.

15 Why is Historic Stall led in partial 14 Variable speed technology Variable speed can be realized in a number of ways. ling a resistor in the generator rotor using light communication thus avoiding a physical link. The most common is to use the double fed induction generator (DFIG) technology.

16 Why is Historic Stall led in partial 15 Full stator power conversion can also be used. Presently (2015) the latest technology is a permanent magnet generator combined with a gear-less design.

17 Single Turbine Why is Historic Stall led in partial 16 Basic : Keep high energy production while maintaining safe within the basic bonds: keep generator speed and power between minimum and maximum. overview from low to high wind No power production, generator oine Idling Start up Partial load Constant minimum speed Variable speed to achieve optimal Cp tracking Constant maximum speed Full load Constant maximum speed and generator torque or power Shot down/derate in high winds

18 C P surface limited by 0 Why is 0.5 Historic Stall led C P in partial Pitch β 0 0 TSR λ Figure : C p curves for NREL5MW

19 Why is Historic Stall led in partial 18 Q a (Nm) x v (m/s) 10 Aerodynamic torque at rpm β (deg) Figure : Rotor torque curves for NREL5MW

20 Why is Historic Stall led in partial x 106 Gradient of aerodynamic torque at steady state δ Q a /δ β δ Q a /δ v δ Q a /δ ω r * v (m/s)

21 Why is Historic Stall led in partial 20 Q g (Nm) x ω (rad/s) Steady state curve β (deg) Figure : Plot of steady state al curves for NREL5MW

22 in partial Single Turbine Why is Historic Stall led in partial 21 Basic speed and power Partial load Full load Constant optimal pitch Optimal Cp tracking using Feedforward with T g ω 2 or Feedback using the speed reference ω ref v a Constant generator torque or power. Constant maximum speed reference obtained using pitch based on feedback of generator speed. Switching between partial is a challenge. Starting pitching before nominal power and speed is obtained loses energy production. Newer pitching before nominal power and speed is obtained can lead to overspeed in extreme gust situations.

23 β Pitch β ref Why is Historic Stall led in partial 22 β ref T g,ref T g,rated Gen torque T g,ref Wind Turbine Partial load Full load Power P Speed ω Figure : Simplifyed diagram where either partial or full load is active.

24 Single Turbine Why is Historic Stall led in partial 23 c:, r: redundancy, s:supervision Minimal Standard Advanced Futuristic Generator torque c c c c Collective pitch c c c c Cyclic pitch c c Individual pitch r r c Local ow c Generator speed c c c c Nacelle wind speed s s s s Tower acceleration s c c Rotor speed s s c Rotor position s c Blade loading c c Tower position c Blade position c Blade angle attack c Gear stage speeds c LIDAR c

25 Why is Historic Stall led in partial 24 Local ow devices - possibilities Trailing edge aps: Like on airplanes. Vortex generators: Small devices on leeward front end of blade which can move ow closer to the blade and increase lift. Blowing/Suction: Transporting air in/out from the blade surface. Micro tabs: Small spoilers that change the lift and drag.

26 Why is Historic Stall led in partial 25 Figure : Trailing edge aps.

27 Single Turbine Why is Historic Stall led in partial 26 There are many other important besides keeping speed and power at suitable levels. Fatigue Reducing fatigue load is also very important Drive train damping using the generator torque. Tower damping using collective pitch. Blade damping using Cyclic pitch for e.g. yaw error. Individual pitch for spatial variations in general. Trailing edge aps can be used for faster actuation compared to full span pitch.

28 Why is Historic Stall led in partial 27 Noise For onshore wind noise modeling and can be important. Noise increases with turbine loading. Aerodynamic noise is dominating. Low wind gives low noise. Turbines can not be heard in wind above rated because of background noise. Just below rated is the worst case. Here noise can be reduced at the cost of reduced energy production.

29 Why is Historic Stall led in partial 28 Low voltage ride through (LVRT) A short circuit on the transmission net can give low voltage which completely unload the generator. Most net operators demand to keep spinning and connected for short faults e.g. less than 3 seconds. The turbine needs to pitch very fast to avoid over-speeding. At the same time tower oscillations due to sudden tower trust decrease must be avoided.

30 Why is Historic Stall led in partial 29 High wind ride through (HWRT) Many simply shot down completely when wind exceeds some speed typically 25 m/s. This is a problem if a whole farm shots down within a few minutes. This can be avoided by a decreasing the set points for power and perhaps speed above normal shot down. Then the wind speed for complete shot down can be increase to e.g. m/s.

31 Why is Historic Stall led in partial Concurrent Aero-Servo-Elastic Design of wind Increased integration of and design to optimize the COE for example by obtaining more slender. Interesting topics for potential improvement: Wind eld estimation and prediction. LIDAR. Local blade actuators. Active blade and tower. Avoid blade tower collision. Adapting ler tuning to level of turbulence and shear.

32 Thank you for listening Questions are welcome