1. Active Control Systems for Wind Turbines
Avishek Kumar
Supervisor: Dr Karl Stol
The University of Auckland

2. Overview Wind Turbines Power Extraction Traditional Control
Modern Control
Future of Control

3. Horizontal Axis Wind Turbines
Source: US Department of Energy

4. Large Wind Turbines 5MW (1400-1500 households) 126m Blade Span
12.1 rpm
Power controlled by blade pitch
Onshore and offshore

6. Ideal turbine (max. 60% efficient)
Power Capture
Power
[kW]
Pwind
 w3
Ideal turbine (max. 60% efficient)
Torque Control
Pitch Control
Prated
Region 1
Region 2
Region 3
Wind Speed, w
[m/s]
wcut-in
wrated
wcut-out
Source: Dr. Karl Stol, UoA

7. Power Generation Control Objectives
Region 1:
Turbine is stopped
Region 2:
Maintain constant tip speed ratio to produce maximum power below rated wind speed.
Region 3:
Maintain rated rotor speed and power.
Power Coefficent, Cp
(Blade Tip Speed)/Wind Speed, λ

8. Classical Control
Use collective pitch and/or generator torque to adjust rotor speed depending on the region.
The torque and pitch controllers work separately.

9. Loads Become more significant as turbines get larger
By reducing loads we can also decrease cost of energy by:
Increasing the lifetime of turbines
Reducing structural material
Reducing maintenance

10. Loads High Winds Stochastic Winds Vertical Wind Shear and Cross Winds
Inertia and Gravity
Tower Interference
Wind Turbulence and Gusts
Source: E Hau

11. Modern Control Traditional Control Modern Control
Single control objective
Multiple control objective
Single input single output
Multi input multi output
Controllers work
separately
Single centralised
controller

12. Modern Control Objectives for the Wind Turbine
Maintain Rotor Speed
Keep the best tip speed ratios in Region 2
Not exceed rated velocity in Region 3
Have smooth power output
Reducing DYNAMIC loading on the turbine.
Blade flap
Tower fore-aft vibration
Drive train torsional vibration

13. Individual Blade Pitching
With modern control (MIMO) we can control the load on each blade individually
This now allows mitigation of ASSYMETRIC loading:
Wind shear
Tower shadow
Inertial loads
Turbulence across the swept area

14. Simulation Results
Stol, Zhao, Wright (2006), Individual Blade Pitch Control for the Controls
Advanced Research Turbine (CART), J. of Solar Energy Eng.,
Transactions of the ASME, v 128, n 4, Nov, 2006, p

15. Model based control tresats a nonlinear system as a linear one
The Problems
Model based control tresats a nonlinear system as a linear one y x

16. The Solution Allows control over the entire operating envelope
More predictable behaviour
Nonlinear Control
Increase in the performance of control
Increase in safety

17. Current State of Nonlinear Control
Variety of nonlinear controllers are being explored
Simulations show successful SISO power control
Very little work has been conducted with multiple control objectives systems
No work has been conducted (publicly) with Individual Blade Pitching

18. My Research What: Why: Nonlinear control Individual blade pitching
Multiple control objectives
Why:
Reduce cost of energy
More predictable turbine behaviour
Safer turbine behaviour

19. Summary Wind turbines are getting bigger
Loads are increasing cost of energy
Modern control (Linear) can mitigate loads AND maintain rated power
Using individual blade pitch we can mitigate ASSYMETRIC loads
Modern Linear Control is only optimal about it’s operating region
Nonlinear control aims to apply all the above benefits over the entire operating envelope

20. Questions?

21. Wind Energy Facts Wind accounts for currently 2% of our electricity.
Global increase of 25% a year for the last 5 years.
321 MW either running or being commissioned in NZ.
Current COE is 5.5-7c/kWhr (2005)
New Zealand Wind Energy Association. (2007, June 28 - last update). [Online]. Available: [2007, July 11]