Active Control Systems
for Wind Turbines
Avishek Kumar
Supervisor: Dr Karl Stol
The University of Auckland
1
Overview
Wind Turbines
Power Extraction
Traditional Control
Modern Control
Future of Control
2
Horizontal Axis Wind
Turbines
Source: US Department of Energy
3
Large Wind Turbines
5MW (1400-1500 households)
126m Blade Span
12.1 rpm
Power controlled by blade pitch
Onshore and offshore
4
5
[kW]
Power
Power Capture
Pwind
 w3
Ideal turbine
(max. 60% efficient)
Torque Control
Pitch Control
Region 3
Region 2
Region 1
Prated
Wind Speed, w
[m/s]
wcut-in
wrated
wcut-out
Source: Dr. Karl Stol, UoA
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
7
Classical Control
Use collective pitch and/or generator torque to
adjust rotor speed depending on the region.
The torque and pitch controllers work
separately.
8
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
9
Loads
High Winds
Stochastic Winds
Vertical Wind Shear
and Cross Winds
Inertia and Gravity
Tower Interference
Wind Turbulence
and Gusts
Source: E Hau
10
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
11
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
12
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
13
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 498-505
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The Problems
Model based control tresats a nonlinear
system as a linear one
y
x
15
The Solution
Allows control over
the entire operating
envelope
More predictable
behaviour
Nonlinear Control
Increase in safety
Increase in the
performance of
control
16
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
17
My Research
What:



Nonlinear control
Individual blade pitching
Multiple control objectives
Why:



Reduce cost of energy
More predictable turbine behaviour
Safer turbine behaviour
18
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
19
Questions?
20
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: http://www.windenergy.org.nz/ [2007, July 11]
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