Wind Turbine Aerodynamics
Section 2 – Power Control
E-Learning UNESCO
ENEA Casaccia - February 26 2007
Fabrizio Sardella
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:
P 
1
 W3A Cp
2
• 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
2
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 characteristics of the blade.
Stop
position
Starting
position
Nominal
position
W
3
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 Cp at a given wind
speed (Opti-Tip) and to limit the power when necessary.
Stop
position
V47 Cp CURVE
Starting
position
0.6
0.5
β = -1.5°
Cp
0.4
β = 2°
β = 0°
0.3
0.2
Nominal
position
0.1
β+
0.0
0.0
5.0
10.0
TSR
15.0
20.0
W
4
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
α1
TIP
R
1
α2

2
W1
W2
V∞
V∞
STALL
5
THE CONTROL OF THE POWER
Power [kW]
• 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
700
start and stop of the
600
turbine is more
500
complicated than in
a pitch controlled
400
WTG. The power
Pitch Control
300
curves obtained with
Stall Control
200
the two kind of
controls are
100
represented in the
figure on the left
5
7
9
11
13
15 17 19 21 23 25
side.
Wind Speed [m/s]
6
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.
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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 mechanical reaction torque and so in increased stresses on the drive
train and in the structure.
Gearbox
Rotor
Generator
Main
Shaft
8
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
2 R
1R
position. Looking at
the aerodynamics it
TIP
is possible to observe
W1 W
α1
2
how a rotational
α

2
speed increase can
limit the increase
or reduce the angle of attack, so that the power increase is
only due to the RPM increase.
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THE OPTI-SLIP® SYSTEM
Electric Power [kW]
• 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
V47 Power Curve
been able to pass
700
from V39 500kW to
600
Opti Slip +
pitch regulation
V47 660kW without
500
substantial changes
400
Opti Tip
in the WTG structure,
300
only improving the
200
power control by the
100
Opti-Slip®. The Opti0
0
5
10
15
20
25
30
Slip® system can be
Wind Speed [m/s]
also considered as
the first step towards the variable speed machines.
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THE OPTI-SLIP® SYSTEM
Below rated power
Above rated power
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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.
Gearbox
Rotor
Main
Shaft
Generator
Converter
12
THE ACTIVE STALLTM
• Also in the case of the stall control a big improvement has
been introduced: the Active-StallTM. It consists in the
possibility to change the initial position of the blade so that
it is possible to change the
point at which the stall occurs.
The resultant power curve is
Nominal
position
very similar to the one
obtained with the pitch control.
Furthermore it is possible to
stop the machine as a pitch
Stop position
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
W
the wind.
13
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 possible 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).
Flap moment
16 m/s - 6 deg yaw error
1
35
2
34
3
33
4
5
32
6
31
7
30
8
29
28
9
27
10
26
11
25
12
24
13
23
14
22
15
21
16
20
19
18
17
With load sensors
No load sensors
Optimal
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