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NEED FOR CONTROL SYSTEM.
.The main objective of the use of WINDturbine is generating electricity.
.The power output P, from a wind turbine
is given by the epression!
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. Electrical energy generated form a
wind turbine varies as the cube of
wind speed.
.The power _ wind velocity contains
four regions:
1) Wind speed < cut _ in speed:
at this region the eciency is not
accepted.
!) Wind speed "cut _in speed:at this region the wind turbine
starting to wor#.
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$) Wind speed "rated speed :
at this speed the generator wor#ing
at speci%ed rated power.
&) Wind speed '" cut _out _speed:
that may causes damage the wind
turbine
and causes the damage of the
generator.
. (o we need to control system for:
1) capture rated power.
!) *rotect the wind turbine from high
wind turbine.
$) When generator disconnected
suddenly prevent the rotor from
runaway.
&) +irection control.
Drag & lift forces
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.,ift is de%ned to be the component
of this force that is perpendicular to
the oncoming -ow direction.
.+rag is de%ned to be the
component of this force that is in thesame direction the oncoming -ow
direction.
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"ngle of attac# is the angle between the
lifting body$s reference line and theoncoming %ow.
. The critical angle of attac# is the angle
of attac# which produces maimum lift
coe&cient.
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Power control
. y far the most e/ective way of
in-uencing the aerodynamic angle
of attac# and thus the input
power is by mechanically
ad0usting the rotor blade pitch
angle for this purpose in general
the rotor blade is turned about its
longitudinal ais with the aid of
actively controlled actuators.
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.*ower control by changing the
aerodynamic angle of attac# of
the rotor can be achieved by two
methods:
2) the conventional approach is by
ad0usting the angle of attac# of
the blade to a smaller angle in
order to reduce power input.
3onversely increasing the angleof attac# increases the power
input.
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. 22) The other possibility is to
change the blade pitch angle to a
larger angle of attac# up to theso4called critical aerodynamic
angle of attac# at which point the
air-ow separates at the surface of
the rotor blades thus limiting the
aerodynamic power input.
. This e/ect is #nown as a stall.
Active itc! control
. 2n general the rotor blade is
turned about its longitudinal ais
with the aid of
actively controlled actuators..There is two types of active pitch
control.
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2) F"ll#san itc! control$ rotating
all each blade about its ais in
the direction which reduces the
angle of attac#.22) Partial#san itc! control$ %
a'("sting t!e itc! of onl )*
to +,- of t!e %la'e lengt! is
sucient from the point of
view of aerodynamic eciency.
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la'e itc!ing 'rive
The main distinguishing feature of
blade pitching systems is the type of
drive.
.there is two types:
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2) 5ydraulic drives 22) electrical
drives
/'ra"lic la'e Pitc!
Sste0s
.(upply of the actuators is commonly
housed at a %ed location in thenacelle so that the supply lines must
be routed through the gearbo and
the hollow rotor shaft into the hub.
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. To reach the rotating hub from the
stationary nacelle a sealed hydraulic
rotary transmissionlead through is re6uired.
.Three hydraulic actuators are
installed outside the rotor hub. The
hydraulic supply and return lines for
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the actuators are routed through the
hollow rotor shaft and through the
gearbo into the rear part of thenacelle to the pressure supply
system located there which consists
of a motor4driven pump and pressure
accumulators.
. The actuators are controlled by
means of control valves via a change
in mass -ow or control pressure.
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. To avoid having to use a rotary lead
through one can either locates the
entire hydraulic system in therevolving hub or the actuators have
to be installed in a %ed position in
the nacelle.
2n the latter case the pitch
ad0ustment motion must be
transmitted into the revolving hub
by means of mechanical transmission
elements
.for eample a connecting rod The
hydraulic actuator wor#s against astrong spring so that in a
brea#down involving the complete
loss of system pressure the rotor
blades are forced into the feathered
position by the spring thus causing
the rotor to stop.
2n wind turbines with partial blade
pitching
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the pitch mechanism must be
installed in the outer blade area.
Electrical la'e Pitc!
Sste0s
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.The power supply for emergencypitch ad0ustment consists of two
batteries which are also located in
the rotor hub.
The %rst one of these was Enercon
where each rotor blade on theirmedium4si7ed range of turbines 8E4
&9) has its own electric pitch motor
mounted on the outside -ange ring
8igs. ;.1). =ther manufacturers
place the electric pitching drivecompletely inside the rotor hub
Stall control
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.a stall is a reduction in the lift
coecient generated by an airfoil as
angle of attac# increases.. This occurs when the critical angle
of attac# of the airfoil is eceeded.
. Where the angle of attac# increases
beyond a certain point such that the
lift begins to decrease.
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. low separation %eginsto occur atsmall angles of attac# while attac!e'
-ow over the wing is still dominant.
>s angle of attac# increases the
separated regions on the top of the
wing increase in si7e and hinder thewing?s ability to create lift.
>t the critical angle of attac#
separated -ow is so dominant that
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further increases in angle of attac#
produce lesslift and vastly more
drag.
Yaw sste0
.@aw system has been used with all
hori7ontal ais wind turbines toorient the nacelle and the rotor as.
Aertical ais wind turbines do not
need a yaw system since their
vertical rotors can face the wind
from any direction.
.The wind direction changes.
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F"nction
.the @aw system has two main
functions:
1)orienting the rotor and
the nacelle into the wind.
!) (ome turbines also
use active yaw as a means of power
control.
. @awing the rotor with respect to the
wind direction it reduces the
e/ective swept area with respect to
the wind direction.
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1ariations in win'
'irection
.variations of wind direction in time
can be divided into the following
categories:
. 2nter4annual
. >nnual.+iurnal
.(hort4term 8gusts and turbulence).
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. 2nter4annual:2nter4 annual
variations in wind direction occur
over time scales greater than oneyear.
. >nnual:(igni%cant variations in
seasonal or monthly.
. +iurnal 8time of day):large wind
variations also can occur on a
diurnal or daily time scale.
. (hort4term:(hort4 term wind
direction variations of interest
include turbulence and gusts.
2in' 'irectioninstr"0entation
.due to continuous change at wind
direction (o we need a winddirection sensor.
. Wind direction is normally
measured via
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the use of a wind vane.
. Wind vanes usually produce signals
bycontact closures or by
potentiometers.
.=ne type of wind that the use of
self4synchronous motors.
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.The transmitter T is mechanically
connected to a wind vane while the
receiver B is connected to a pointer
on an indicating instrument.
.oth rotors are connected to thesame source of ac.
.When the induced voltages are the
same and opposing each other
there will be no current -ow and no
rotor tor6ue.
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. When the transmitter rotor is
moved voltage magnitudes become
unbalanced causing currents to -owand a tor6ue to be produced on the
receiver rotor.
. This causes the receiver rotor to
turn until it again is in alignment
with the transmitter rotor.
.=ther type of wind vane which
wor#s well for digital data system.
.2t consists of potentiometer a
voltage source A and an analog to
digital converter.
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. The potentiometer is oriented so
the output voltage Advalue is
changes due to wind directionchanges.
. The >C+ converter converts Adto a
digital form for recording.
.Each digital number represents a
range of wind direction.
Tes
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'a
wTy
pe
'a
wTy
pe
y aw i n
gb yf a n (t a i l
w he e l
y aw i n
gb yf a n (t a i l
w he e l
awing % fan#tailw!eels
.@awing with the help of a fan4tail is
the simplest method.. @awing with the aid of a fan4tail
wheel can still be found in some
smaller turbines.
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.the fantail in the cap of the turbine.
. the fantail parallel to the air-ow
at this case it did not turn ..if the wind shifted it would begin to
turn the fantail .
.this would turn a shaft running from
the fantail
+own to wheels or a gear bo whichwould turn the turbine bac# into the
wind .
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This has the advantage of not
re6uiring a separate power source
or controls.
free Yaw te
. Turbines with free yaw are normally
downwind machines..2f the rotor is positioned downwind
the point of attac# of the total
aerodynamic force of the rotor is
located behind the ais of rotation of
the tower head.
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.the yaw ais so that with the cross
wind force the aerodynamic forces
produce a restoring moment on the
rotor within a very wide yaw angle
range.
.2n turbines with free yaw the yaw
system is normally much simpler.
. =ften there is nothing more than
the yaw bearing..ree yaw machines sometimes
incorporate yaw dampers to limit the
yaw rate.
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Active Yaw te
. Dpwind turbines normally have
some type of active yaw control.
. This usually includes:
. =ne or more yaw motors gears
.bra#e to #eep the turbine stationaryin yaw when it is properly aligned.
. The speed must be reduced so that
the yaw rate is slow and so that
ade6uate tor6ue can be suppliedfrom a small motor.
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Yaw %earing. Begardless of the type of yaw system
all hori7ontal ais wind turbines have
some type of yaw bearing.
.The primary component is a large
bearing that connects the main frame tothe tower.
. 2t serves as a rotatable connection
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between the tower and the nacelle of
the wind turbine.
.2n a turbine with active yaw theyaw bearing includes gear teeth
around its circumference.
.@aw damping is desirable even during
the yawing in order to avoid unwanted
yawing oscillations.
.These re6uirements can be met both by
a
conventional roller bearing.
Yaw %ra3es
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.yaw bra#e system comprising:
. +is# bra#e.
.ra#e caliper.
. 3aliper actuator 8hydraulic).
.When not yawing the machinery is
positively loc#ed by means of several
yaw bra#e calipers acting on a bra#e
disc.
.dis# bra#e is a device for slowing or
stopping the rotation of while it is in
motion.
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'a
wdr
ive
hy
drau
l icsy
ste
)l e
ct ri ca l
syst
em
. To stop the wheel friction material
in the form of bra#e pads is forced
mechanically hydraulicallypneumatically or electromagnetically
against both sides of the disc.
The bra#e caliper is the assembly
which houses the bra#e pads and
pistons.
Yaw 'rive
. The yaw drive is the name given to
the mechanism used to rotate the
nacelle with in order to #eep theturbine facing into the wind.
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. The two choices for the yaw drive
are hydraulic or electrical
components.the hydraulic system name lower
costs smaller si7e and also higher
tor6ue as advantages.
. Each yaw drive consists of powerful
electric or hydraulic motor with its
electric drive and
a large gearbo which increases the
tor6ue.
Yaw Control. There are two aims of control
system:
.1) the deviation of the rotor from
the wind direction the yaw angle is
supposed to be as small as possible
to avoid power loss.
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.!) The yaw control system must not
respond too sensitively to avoid
continuous small yaw movementswhich would reduce the life of the
mechanical components.
. The yaw motion in an active yaw
system is controlled using yaw erroras an input.
.@aw error is monitored by means of
a wind vane mounted on the turbine.
. When the yaw error is outside the
allowed range for some period oftime the drive system is activated
and the turbine is moved in the
appropriate direction.
. The problem is to %nd a practicable
compromise as it is not possible toset up a general rule.
. The situation relating to theW>4F9
turbine will be given as an eample.
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. The wind measuring system of the
turbine provides a mean value of the
wind direction over a period of tenseconds.
.2f the deviation remains below $
degrees the yaw control system willnot be activated.
. 2f the yaw angle determined is
above this value the time until
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correction is determined in
accordance with a pre4programmed
function..2f the wind speed eceeds $F mCs
the rotor will not be yawed.
References
1_ Wind Turbines: undamentals Technologies>pplications Economics GErich5auH
!_ Wind Energy 5andboo# GTonyurtonH
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