BACKGROUND
Embodiments of the present disclosure relate to wind turbines, and more
particularly to reducing tower oscillations in wind turbines.
Modern wind turbines operate in a wide range of wind conditions. These wind
conditions can be broadly divided into two categories - below rated speeds and above rated
speeds. To produce power in these wind conditions, wind turbines may include sophisticated
control systems such as pitch controllers and torque controllers. These controllers account e for changes in the wind conditions and accompanying changes in wind turbine dynamics.
For example, pitch controllers generally vary the pitch angle of rotor blades to account for the
changes in wind conditions and turbine dynamics. During below rated wind speeds, wind
power may be lower than the rated power output of the wind turbine. In this situation, the
pitch controller may attempt to maximize the power output by pitching the rotor blades
substantially perpendicular to the wind direction. Alternatively, during above rated wind
speeds, wind power may be greater than the rated power output of the wind turbine.
Therefore, in this case, the pitch controller may restrain wind energy conversion by pitching
the rotor blades such that only a part of the wind energy impinges on the rotor blades. By
controlling the pitch angle, the pitch controller thus controls the velocity of the rotor blades
and in turn the energy generated by the wind turbine.
Along with maintaining rotor velocity, pitch controllers may also be employed to
reduce tower oscillations. Tower oscillations or vibrations occur due to various disturbances,
such as turbulence, inefficient damping, or transition between the two wind conditions.
Moreover, the tower may vibrate along any degree of freedom. For example, the tower may
vibrate in a fore-aft direction (commonly referred to as tower nodding), in a side-to-side
direction (commonly referred to as tower naying), or along its longitudinal axis (commonly
referred to as torsional vibration).
Tower nodding is usually caused by aerodynamic thrust and rotation of the rotor
blades. Every time a rotor blade passes in front of the tower, the thrust of the wind impinging
on the tower decreases. Such continuous variation in wind force may induce oscillations in
the tower. Moreover, if the rotor velocity is such that a rotor blade passes over the tower
each time the tower is in one of its extreme positions (forward or backward), the tower
oscillations may be amplified. Typically, the oscillations in the fore-aft direction are
automatically minimized due to aerodynamic damping. Aerodynamic damping relies on the
fact that the top of the tower constantly oscillates in the fore-aft direction. When the top of
the tower moves upwind (or forward), the rotor thrust is increased. This increase in rotor
thrust pushes the tower back downwind. The downwind push in turn aids in dampening the
tower oscillations. Similarly, when the top of the tower moves downwind, the rotor thrust
may be decreased. This decrease in rotor thrust pushes the tower back upwind. The upwind
push also aids in dampening the tower oscillations.
e Although aerodynamic damping aids in reducing oscillations considerably, if the
rotor velocity is synchronized with the tower oscillations, the results may be detrimental for
wind turbine components. In such instances, the tower may oscillate at a high rate causing
mechanical strain and possible damage to the tower. Moreover, such synchronization may
amplify the rotor velocity at tower resonance frequency, thereby potentially damaging
generators and/or drivetrains connected to the rotor blades. As the amplification of tower
oscillations is dependent on the rotor velocity, pitching the rotor to adjust its velocity may
prevent amplification of the tower oscillations. Accordingly, by pitching the rotor blades, the
pitch controller may control the rotor velocity and prevent amplification of the tower
oscillations.
Typically, the pitch controller utilizes two separate control loops for the two
m functions - controlling the rotor velocity and reducing the tower oscillations. A rotor
velocity control loop is employed to determine a pitch angle to control rotor velocity and a
tower-damping control loop is used to compute a pitch angle to reduce tower oscillations.
Often, these feedback loops operate relatively independently of each other. For example, the
rotor velocity control loop may determine the pitch angle based on rotor velocity, wind speed,
and current pitch angle. The tower-damping control loop, on the other hand, may determine
the pitch angle based on tower deflection, tower top velocity, tower top acceleration, current
pitch angle, and wind speed. Because of this independence, the currently available rotor
velocity control loops may compute a pitch angle to maintain rotor speed that may
disadvantageously induce tower oscillations instead of reducing them. Moreover, these rotor
velocity control loops may cause energy amplification in the rotor near tower resonance
frequencies. Such amplification may increase oscillations in the tower and increase the
fatigue load placed on the wind turbine. Over time, such fatigue loads may reduce the life of
wind turbine parts and increase the expenses associated with wind turbines.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with aspects of the present disclosure, a method for reducing tower
oscillations in a wind turbine is presented. The method includes obtaining a rotor velocity.
Furthermore, the method includes obtaining one or more parameters associated with a tower
of the wind turbine. Further, the method includes determining a modified rotor velocity
based on the one or more parameters. Moreover, the method includes determining a first
pitch angle based on the modified rotor velocity. In addition, the method includes pitching a one or more blades of the wind turbine based on the first pitch angle to reduce the tower
oscillations.
In accordance with another aspect of the present disclosure, a pitch control system
is presented. The pitch control system includes a tower unit configured to determine one or
more parameters associated with a tower of a wind turbine. Further, the pitch control system
includes a decoupling unit configured to determine a modified rotor velocity based on the one
or more parameters. Additionally, the pitch control system includes a controller configured
to determine a first pitch angle based on the modified rotor velocity.
In accordance with yet another aspect of the present disclosure, a wind turbine is
presented. The wind turbine includes a rotor having one or more rotor blades and a tower
operatively coupled to the rotor. Further, the wind turbine includes a pitch control system for
reducing tower oscillations in the wind turbine. The pitch control system includes a rotor unit
configured to determine a rotor velocity, a tower unit configured to determine at least one of
a tower top velocity and a second pitch angle, a decoupling unit configured to determine a
modified rotor velocity based on at least one of the tower top velocity and the second pitch
angle, and a controller configured to determine a first pitch angle based on the modified rotor
velocity.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present disclosure will be
better understood when the following detailed description is read with reference to the
accompanying drawings in which like characters represent like parts throughout the
drawings, wherein:
FIG. 1 is a diagrammatical representation of forces and motions experienced by a
wind turbine;
FIG. 2 is a diagrammatical representation of an exemplary pitch control system,
according to aspects of the present disclosure;
FIG. 3 is a graph illustrating energy amplification in rotor velocity of a
conventional wind turbine at different wind speeds;
FIG. 4 is a graph illustrating energy amplification in rotor velocity of a wind
0 turbine employing the exemplary pitch control system of FIG. 2 at different wind speeds,
according to aspects of the present disclosure;
FIG. 5 is a diagrammatical representation of another exemplary pitch control
system, according to aspects of the present disclosure;
FIG. 6 is a graph illustrating energy amplification in rotor velocity of a
conventional wind turbine with a tower-damping unit at different wind speeds;
FIG. 7 is a graph illustrating energy amplification in rotor velocity of a wind
turbine employing the exemplary pitch control system of FIG. 5 at different wind speeds,
according to aspects of the present disclosure;
• FIG. 8 is a flowchart illustrating an exemplary method for reducing tower
oscillations in a wind turbine using the pitch control system of FIG. 2, according to aspects of
the present disclosure; and
FIG. 9 is a flowchart illustrating an exemplary method for reducing tower
oscillations in a wind turbine using the pitch control system of FIG. 5, according to aspects of
the present disclosure.
DETAILED DESCRIPTION
The following terms, used throughout this disclosure, may be defined as follows:
Tower dynamics - refers to the mechanics concerned with the motion of a wind
turbine tower under the action of various forces such as wind and rotor movement.
Rotor Dynamics - refers to the mechanics concerned with the motion of the rotor
under the action of various forces such as wind, tower movement, and inertia.
Fore-aft oscillations - refers to tower oscillations in a direction parallel to the
wind direction.
Tower top velocity - refers to the velocity of the tower oscillations experienced at
the top end of a wind turbine tower.
Tower top acceleration - refers to the acceleration of the tower oscillations • experienced at the top of the wind turbine tower.
Tower deflection - refers to the change in position of the top of the wind turbine
tower with respect to a reference position.
Tower resonance - refers to the tendency of a wind turbine to oscillate with
maximum amplitude at tower resonant frequencies.
First mode resonance frequency - refers to the resonant frequency of a first
structural mode of the wind turbine tower where the mode dynamics are characterized by a
second order spring-mass-damper system.
Embodiments of the present disclosure are related to an exemplary system and
method for reducing tower oscillations in a wind turbine. More particularly, the present
disclosure relates to an exemplary rotor velocity control loop that uses a pitch control system
as an actuator. Moreover, the rotor velocity control loop determines a pitch angle that
reduces tower oscillations. To this end, the rotor velocity control loop includes a decoupling
unit that addresses the interdependence between rotor dynamics and tower dynamics using
model based methods to reduce oscillations induced in the tower fore-aft direction at above
rated speeds.
Moreover, embodiments of the present disclosure are described with reference to a
land-based three-blade wind turbine. It will be understood, however, that such a reference is
merely exemplary and that the systems and methods described here may just as easily be
implemented in floating wind turbines, offshore wind turbines, 2-blade wind turbines, or 4-
blade wind turbines without departing from the scope of the present disclosure.
FIG. 1 is a diagrammatical representation that illustrates forces and motions
experienced by a wind turbine 100. The wind turbine 100 includes a tower 102, a rotor 104,
one or more rotor blades 106, and a nacelle 108. The tower 102 may be coupled to the
ground, to the ocean floor, or to a floating foundation using any known securing means, such
as bolting, cementing, welding, and so on.
Further, in FIG. 1 reference numeral 110 is generally representative of wind. The
wind 1 10 may have a mean speed (u). As the wind 1 10 blows in the indicated direction, an
aerodynamic torque (MJ is placed on the rotor blades 106 causing the rotor blades 106 to * rotate in a direction that is substantially perpendicular to the wind direction. This motion of
the rotor blades 106 is represented in FIG. 1 by an angular rotor velocity (w,) of the rotating
blades 106. Further, the nacelle 108 may include a gearbox (not shown) and a generator (not
shown). The gearbox may increase the speed of the rotor blades 106 and the generator may
convert the rotation of the rotor blades 106 into electricity, thus converting the energy of the
wind 110 into electricity. Alternatively, the nacelle 108 may include a direct drivetrain (not
shown). In such cases, inclusion of the gearbox may be circumvented.
Moreover, due to an aerodynamic thrust (FJ of the wind 110 and the rotation of
the rotor blades 106, the tower 102 may oscillate in a fore-aft direction. Reference numeral
114 is generally representative of the fore-aft oscillations. It will be understood that in
addition to the fore-aft oscillations 114, the tower 102 may also experience other oscillations. e Example oscillations include side-to-side oscillations, torsional oscillations, twisting
oscillations, and the like. These oscillations are not illustrated in FIG. 1.
The wind turbine 100 may employ a sensing device to detect the fore-aft
oscillations 114. For example, an oscillation velocity detector (not shown) or an oscillation
deflection detector (not shown) may be employed. Alternatively, an accelerometer 112 may
be employed in the wind turbine 100 to detect the acceleration of the fore-aft oscillations 114.
In some embodiments, the accelerometer 112 may be disposed within the nacelle 108 or at
the top of the tower 102. In other instances, the accelerometer 112 may be positioned at the
center of the tower 102.
Furthermore, to reduce the fore-aft tower oscillations 114 and to control the rotor
velocity, the wind turbine 100 may include an exemplary pitch control system 116 that may
include a rotor velocity control loop (not shown). In some embodiments, the pitch control
system 116 may also include a tower-damping control loop (not shown). Depending on the
mean or effective speed of the incoming wind 1 10, the exemplary pitch control system 1 16
may be configured to determine the pitch angle of the rotor blades 106 to maximize output
power (within the rated limits) and/or minimize tower oscillations. As noted previously,
some of the previously known pitch controllers may tend to increase tower oscillations,
instead of decreasing them. This increase in tower oscillations may be because conventional
pitch controllers fail to account for the interdependence between rotor dynamics and tower
dynamics. a Tower dynamics for the wind turbine 100, in one example, may be represented by
a second order linear equation:
where, xf, is the tower top acceleration, Cf, is the velocity-damping constant of the tower
102, of, is the first mode tower resonant frequency, xfa is the tower top velocity, and Xfa is
the tower deflection. Further, K is an inverse of a generalized mass for the first mode, Fz is
the aerodynamic thrust, o, is the angular velocity, 8 is the pitch angle, and v, is the effective
wind speed.
a The effective wind speed (v,) refers to the effective speed of the wind at the hub
height of the wind turbine 100. Because the wind 1 10 is distributed spatially and temporally,
the wind speed varies significantly at different points over the area swept by the rotor blades
106, and therefore different portions of the wind turbine 100 may experience different wind
speeds. The effective wind speed (v,) is representative of the difference between the mean
wind speed (u) and the tower top velocity (x~,) as depicted in equation (2):
The left-hand side of equation (1) indicates that the motion of the tower 102 may
be dependent on the tower top acceleration (jif.,), tower top velocity (xfa), tower
deflection (Xf ,), resonant frequency (of ,), and velocity-damping constant (Cf ,). In
addition, the right-hand side of equation (1) illustrates that the aerodynamic thrust
(F,) experienced by the tower 102 may be a function of the angular velocity (or), the pitch
angle (O), and the effective wind speed (v,). Further, the aerodynamic thrust (F,) may be a
function of the mean wind speed (u) and the tower top velocity (xfa) as these parameters
affect the effective wind speed (v,).
Moreover, rotor dynamics for the wind turbine 100 may also be represented by a
first order linear equation:
where, J, is a moment of inertia of the rotor 104, hr is the rate of change in the angular
velocity of the rotor, N is gearbox ratio, and Tg is the generator reaction torque.
It will be noted that both the rotor dynamics and the tower dynamics depend on
the effective wind speed (v,). Further, it will be noted that the effective wind speed (v,) is a
function of the tower top velocity (xfa). Therefore, it is evident from equations ( I ) and (3)
that the tower dynamics and the rotor dynamics are dependent on each other. In fact, these
dynamics are related to each other because of the tower top velocity (xfa), rotor velocity
(o,),an d pitch angle (8).
Conventional pitch controllers typically assume that the rotor dynamics and the
tower dynamics are independent. Consequently, these pitch controllers generally ignore the
tower top velocity while computing the pitch angle for controlling the rotor velocity andlor
damping the tower oscillations. Moreover, because of this exclusion, conventional pitch
controllers may cause instability in the rotor dynamics and energy amplification in the rotor
velocity at frequencies close to the tower resonance. In one embodiment, the exemplary pitch
control system 1 16 may be configured to employ the towe; top velocity in the computation of
the pitch angle. More particularly, the exemplary pitch control system 116 may be
configured to deduct the effects of the tower top velocity from the rotor velocity. By
including the tower top velocity and compensating for this value in the computation of the
pitch angle, the exemplary pitch control system 116 may advantageously decouple the rotor
dynamics and the tower dynamics.
FIG. 2 illustrates an exemplary embodiment 200 of the pitch control system 116
of FIG. 1, according to aspects of the present disclosure. The pitch control system 200 of
FIG. 2 includes a rotor velocity control loop. Further, the pitch control system 200 may
include a rotor unit 202, a tower unit 204, and a controller 206. Moreover, the pitch control
system 200 may also include a decoupling unit 208. In one embodiment, the controller 206
may be disposed in a feedback loop of the rotor unit 202 and the decoupling unit 208 may be
disposed at an output of the rotor unit 202 and the tower unit 204.
The rotor unit 202 may be configured to determine a rotor velocity (a,). In one
embodiment, the rotor unit 202 may be configured to determine the rotor velocity (or) by
directly measuring the angular speed of the rotor 104 (see FIG. 1) using a sensing device such
as a speedometer or an angular velocity meter. Alternatively, the rotor unit 202 may be
configured to determine the rotor velocity (a,) by determining a power output of the wind
turbine 100 (see FIG. 1) or the rotation speed of a generator. It may be noted that these
values are proportional to the rotor velocity. Accordingly, determination of any of these
parameters may aid the rotor unit 202 in determining the rotor velocity. It will be understood
that various models and measurement means may be employed to determine the rotor
velocity and any of these models or means may be employed to determine the rotor velocity
without departing from the scope of the present disclosure.
The tower unit 204 may be configured to determine one or more parameters
associated with the tower 102. These parameters may be representative of the tower
dynamics. For instance, in one embodiment of the pitch control system 200, the tower unit
a 204 may be configured to determine the tower top velocity (xfa). The tower top velocity
(xfa) may be estimated using the tower top acceleration (xfa) AS previously noted, the
accelerometer 112 (see FIG. 1) may be employed to sense the tower top acceleration and
communicate this information to the tower unit 204. The tower unit 204 may be configured
to perform any known computation to determine the tower top velocity (xfa) For instance,
the tower unit 204 may be configured to determine the tower top velocity (xfa) by
performing an integration operation on the tower top acceleration (xfa). Alternatively, the
tower unit 204 may determine the tower top velocity (xfa) from the tower acceleration
(xfa) using a model based estimator such as a Kalman Filter.
In other embodiments, the tower top velocity (xfa) may be estimated by a
deflection sensor that detects a degree of deflection of the tower 102 about a determined rest
position. By measuring the deflection at various instances of time, the tower top velocity
(xf,) may be determined. In another embodiment, the tower unit 204 may be configured to
perform a differentiation operation on the tower deflection to determine the tower top
velocity (xfa). In yet another embodiment, the tower top velocity (xfa) may be directly
sensed by a velocity sensor. It will be understood that the tower unit 204 may perform
various other functions and operations without departing from the scope of the present
disclosure. For example, the tower unit 204 may maintain and continuously update a model
of the tower dynamics.
'. In accordance with aspects of the present disclosure, the decoupling unit 208 may
be configured to determine a modified rotor velocity based on parameters of the tower 102.
To this end, the decoupling unit 208 may include a computing unit 21 0 and a subtracting unit
212. The computing unit 210 may be configured to receive the parameters associated with
the tower 102. By way of example, the computing unit 210 may be configured to receive the
tower top velocity from the tower unit 204. Furthermore, the computing unit 210 may be
configured to determine a rotor velocity component based on the tower top velocity
(hereinafter referred to as the "first rotor velocity component"). The first rotor velocity
component may be representative of the effect of the tower top velocity on the rotor velocity.
To determine the first rotor velocity component, the computing unit 210 may utilize a linear
model of the rotor dynamics. The rotor dynamics may be represented by the following first
order linear equation:
or approximations thereof, where is the partial derivative of the aerodynamic torque with
6%
respect to the rotor velocity, % is the partial derivative of the aerodynamic torque with
respect to the pitch angle, and is the partial derivative of the aerodynamic torque with
Sv
respect to the mean wind velocity.
Further, a linear model of the rotor dynamics may be represented by the following
equation:
or approximations thereof, where 6hrf is the rate of change of the first rotor velocity
component and SGrf is the first rotor velocity component.
It may be noted that all the variables in equation (9, with the exception of the first
rotor velocity component, may be detected andlor stored by the rotor unit 202 and/or the
tower unit 204. The values of these variables may be communicated to the computing unit
210. The computing unit 210 may be configured to compute the first rotor velocity
component based on the values of these variables.
0 Moreover, in one example, the computing u$t 21 0 may be implemented as one or
more digital filters. In another example, the computing unit 210 may be implemented as a
general-purpose computing device. The general-purpose computing device may be
selectively activated or reconfigured by a decoupling meansfunit. For example, the
computing device may store the rotor dynamics and the linearized model of the rotor
dynamics in a non-transitory computer readable storage medium, such as, but not limited to,
any type of disk, memory, magnetic card, optical card, or any type of media suitable for
storing electronic instructions. Further, the computing device may store instructions or
programs configured to compute the first rotor velocity component.
As described previously, the decoupling unit 208 may further include the
subtracting unit 212 that may be configured to receive the rotor velocity (ar) from the rotor
a unit 202 and the first rotor velocity component (6Grf) from the computing unit 210.
Moreover, the subtracting unit 212 may be configured to subtract the first rotor velocity
component (6Grf) from the rotor velocity (ar) to obtain a modified rotor velocity. The
modified rotor velocity may be representative of the rotor velocity that is devoid of the
effects of the tower top velocity.
The controller 206 may be configured to receive the modified rotor velocity,
process this value, and generate a pitch angle value (60) corresponding to the modified rotor
velocity (hereinafter referred to as a "first pitch angle"). To process this value, in one
embodiment, the controller 206 may include a lookup table (LUT) that includes previously
computed pitch angle values corresponding to various rotor velocities. The modified rotor
velocity may be compared with the stored rotor velocities to determine a corresponding first
12
pitch angle. Alternatively, the controller 206 may include a threshold rotor velocity. In this
case, the modified rotor velocity may be compared with a threshold rotor velocity. Further,
the controller 206 may be configured to generate an error signal indicative of any deviation of
the modified rotor velocity from the threshold rotor velocity. The controller 206 may further
include a LUT to store pitch angle values corresponding to various error values. By
performing a lookup in such a table, the controller 206 may be configured to determine an
appropriate first pitch angle. In other embodiments of the controller 206, the first pitch angle
may be computed in real time by utilizing one or more known wind turbine models that may
be stored in an associated LUT.
In some instances, independent pitching of the rotor blades 106 may further
0 reduce the oscillations and increase the efficiency of the wind turbine 100. In such instances,
the controller 206 may be configured to independently determine first pitch angles for each
rotor blade 106. Techniques for such computations may include receiving modified rotor
velocities corresponding to each rotor blade 106 separately or receiving a single modified
rotor velocity. In case of individual modified rotor velocities, the controller 206 may be
configured to perform a simple lookup in the LUT to determine the individual first pitch
angles. Otherwise, the controller 206 may be configured to utilize one or more wind turbine
models to determine the individual first pitch angles. For example, during the turbine design
phase, various calculations may be carried out to determine a model for defining the rotor
velocity attained at various individual pitch angles and wind speeds. The results of such
computations may be stored in the controller 206. Subsequently, during operation, the
controller 206 may be configured to perform a lookup to determine the individual first pitch
@ angles that may be utilized to attain the modified rotor velocity. Alternatively, the controller
206 may be configured to supply the modified rotor velocity, previous pitch angles, and
current wind speed to the model to determine the individual first pitch angles. It will be
understood that various pitch angle controllers are currently employed in wind turbines and
that any of these pitch controllers may be utilized to implement the controller 206 without
departing from the scope of the present disclosure. The controller 206 may be any of the
controllers known in the art, such as a proportional controller, a proportional integral
controller, a proportional-integral-derivative controller, a linear-quadratic regulator, or a
linear-quadratic Gaussian regulator without departing from the scope of the present
disclosure.
In some embodiments, the rotor unit 202 may include a pitch actuator 214 for
pitching the rotor blades 106 based on the first pitch angle determined by the controller 206.
As described previously, the controller 206 may be configured to generate and transmit
substantially similar first pitch angles for the blades in the wind turbine 100 to the pitch
actuator 214. Alternatively, the controller 206 may transmit independent first pitch angles to
the pitch actuator 214. The pitch actuator 214, in turn, may include any actuation mechanism
to adjust the pitch angle of the rotor blades 106. For example, the pitch actuator 214 may be
a hydraulic system that receives pitch angle values in the form of voltage signals and pitches
the rotor blades 106 by actuating a pitch cylinder (not shown) at a variable rate.
Alternatively, the pitch actuator 214 may be an electrical, electronic, or electro-mechanical
system without departing from the scope of the present disclosure.
It may be noted that FIG. 2 illustrates the decoupling unit 208 and the controller
206 as separate hardware units. However, it will be understood that in some embodiments,
the controller 206 may be designed as a multi-input and multi-output (MIMO) controller that
includes the functionality of the decoupling unit 208 and/or the rotor and tower units 202 and
204. In embodiments where the controller 206 includes the decoupling unit 208, the tower
top velocity and the rotor velocity may be directly provided to the controller 206. The
controller 206, in turn, may include the computing unit 210 and the subtracting unit 212 to
compute the first rotor velocity component and subtract this value from the detected rotor
velocity, respectively. Based on the subtraction, the controller 206 may determine the
modified rotor velocity.
a FIGS. 3 and 4 are graphs 300, 400 schematically illustrating simulated energy
amplification in rotor velocity of a wind turbine, at various wind speeds. Further, these
graphs 300, 400 illustrate the energy amplification of the rotor velocity using pitch angle as
an actuator. More particularly, graph 300 illustrates the effect of a conventional pitch control
system (without the decoupling unit) on the energy amplification in the rotor velocity of a
conventional wind turbine at different wind speeds and frequencies. Graph 400 illustrates the
effect of the exemplary pitch control system 200 of FIG. 2 on the energy amplification in the
rotor velocity of the wind turbine 100 at different wind speeds and frequencies.
Graph 300 illustrates that there is significant energy amplification at the tower
resonance frequency (generally indicated by reference numeral 302). In essence, such
amplification occurs because conventional pitch controllers do not account for the tower top
velocity while determining the pitch angle to control the rotor velocity.
To circumvent the shortcomings of the conventional pitch controllers, the
exemplary decoupling unit 208 of FIG. 2 may be configured to prevent energy amplification
and reduce fore-aft oscillations 114 (see FIG. 1) at tower resonance frequencies. In
particular, the decoupling unit 208 may be configured to determine a rotor velocity
component that results from the tower oscillations. Additionally, the decoupling unit 208
may be configured to deduct this component from the rotor velocity. Consequently, the
effects of the tower oscillations on the rotor velocity may be substantially minimized.
Accordingly, wind speed and pitch angle may be the only factors that affect the modified
a rotor velocity. Graph 400 illustrates this statement. It will be appreciated that the energy
amplification of FIG. 3 is not present in FIG. 4. Therefore, introduction of the exemplary
decoupling unit 208 in the pitch control system 200 aids in minimizing energy amplification
and subsequent tower oscillations.
FIG. 5 is a diagrammatical representation of another exemplary embodiment 500
of the pitch control system 1 16 of FIG. 1. In this embodiment, the pitch control system 500
includes a rotor velocity control loop and a tower-damping control loop. Accordingly, the
pitch control system 500 includes a rotor unit 502, a tower unit 504, and a controller 506.
These units function substantially similar to the similarly named units described with
reference to FIG. 2. Furthermore, the pitch control system 500 may include a tower-damping
unit 508, a decoupling unit 510, and an adder 512. The tower-damping unit 508 may be
e coupled between an output of the tower unit 504 and an input of the rotor unit 502. Also, the
decoupling unit 5 10 may be coupled at an output of the rotor unit 502, tower unit 504, and the
tower-damping unit 508. Further, the adder 512 may be coupled between an output of the
controller 506 and the tower-damping unit 508, and an input of the rotor unit 502.
The tower-damping unit 508 may be configured to reduce the oscillations in the
tower 102 of FIG. 1. As previously noted with reference to FIG. 1, these oscillations are
typically caused by disturbances in the wind 110, operation of the rotor blades 106, or any
other such factors. During operation of the wind turbine 100, a lift and a drag act on the rotor
blades 106. The drag acts as a thrust in the front-rear direction of the tower 102, thereby
inducing fore-aft oscillations 1 14. Moreover, the magnitude of the thrust varies depending on
the wind speed and the pitch angle. Accordingly, by controlling the pitch angle, the thrust in
the front-rear direction may be adjusted, which in turn regulates the fore-aft oscillations 114.
With continuing reference to FIG. 5, in accordance with some aspects of the
present disclosure, the tower-damping unit 508 may be configured to calculate a pitch angle
for generating a desired thrust on the rotor blades 106. In one example, the desired thrust
may be representative of the thrust that may be applied on the rotor blades 106 to
substantially minimize or cancel the oscillations of the tower 102. Further, the towerdamping
unit 508 may determine the pitch angle based on the detected tower top
acceleration. Subsequently, the adder 512 may add the pitch angle for damping (hereinafter
referred to as the "second pitch angle") with the first pitch angle to generate a combined pitch
angle. The combined pitch angle may be employed to pitch the rotor blades 106.
Despite reducing oscillations caused by the aerodynamic thrust (FJ, conventional
tower dampers may introduce energy amplification in the rotor at tower resonance. This
amplification may occur because conventional pitch controllers ignore the effects of the
second pitch angle on the first pitch angle while computing the first pitch angle. In
accordance with aspects of the present disclosure, embodiments of the pitch control system
500 account for the effects of the second pitch angle on the first pitch angle. In particular, the
pitch control system 500 may be configured to deduct these effects along with the effects of
the tower top velocity from the rotor velocity to determine a modified rotor velocity. By
minimizing and/or removing the effects of the second pitch angle and the tower top velocity
from the rotor velocity, embodiments of the pitch control system 500 aid in reducing or
I, eliminating the possibility of energy amplification at tower resonance frequencies in the rotor
104 (see FIG. I).
To obtain the modified rotor velocity, the decoupling unit 5 10 may be configured
to determine components of rotor velocity based on one or more parameters associated with
the tower 102, such as the tower top velocity and the second pitch angle. More particularly,
the decoupling unit 5 10 may be configured to determine a component of rotor velocity due to
the second pitch angle (hereinafter referred to as the "second rotor velocity component") in
addition to the first rotor velocity component. Accordingly, the decoupling unit 5 10 may be
configured to receive the tower top velocity from the tower unit 504 and the second pitch
angle from the tower-damping unit 508. In one embodiment, the decoupling unit 510 may
include a computing unit 514 and a subtracting unit 516. The computing unit 514 may be
configured to determine the first rotor velocity component and the second rotor velocity
component using a linearized model of the rotor dynamics, in one example. Accordingly, in
this embodiment, the linearized model may include the second pitch angle in addition to the
tower top velocity. The linearized model of the rotor dynamics may be represented by the
following equation:
or approximations thereof, where, 6GrC is a combination of the first rotor velocity component
and the second rotor velocity component, &arics the rate of change of the combination of the
first and second rotor velocity components, and 60,,, is the second pitch angle.
4b The computing unit 514 may be configured to retrieve the second pitch angle and
the tower top velocity from the tower-damping unit 508 and the tower unit 504, respectively.
Based on these values, the computing unit 5 14 may be configured to determine a combination
of the first and second components of the rotor velocity due to tower oscillations and tower
damping. To determine the modified rotor velocity, the subtracting unit 516 may be
configured to deduct the combination of the first and second rotor velocity components from
the rotor velocity.
According to one embodiment, the decoupling unit 5 10 may be implemented as
one or more digital filters or a computing device - one for determining the first rotor velocity
component and the other for determining the second rotor velocity component. Alternatively,
m the decoupling unit 510 may be implemented as a single digital filter or computing device
that may be configured to determine both the first and second rotor velocity components
simultaneously.
The other units, such as the controller 506 and the rotor unit 502, may function in
a manner that is substantially similar to the operation of their counterparts as described with
reference to FIG. 2. For instance, the rotor unit 502 may be configured to communicate the
detected rotor velocity to the subtracting unit 516. Similarly, the controller 506 may be
configured to determine the first pitch angle and provide this value to the adder 512.
Furthermore, the adder 5 12, in turn, may be configured to receive the first pitch angle and the
second pitch angle and combine these two values to determine a combined pitch angle. This
combined pitch angle may be communicated to a pitch actuator 518. Further, the pitch
actuator 5 18 may be configured to pitch the rotor blades according to the communicated pitch
angle.
FIGs. 6 and 7 are graphs 600, 700 schematically illustrating simulated energy
amplification in rotor velocity of a wind turbine. Further, these graphs 600, 700 illustrate
energy amplification using pitch angle as an actuator. More particularly, FIG. 6 illustrates
the effect of a conventional pitch controller (with a tower damping loop) on the energy
amplification in the rotor velocity at different wind speeds and frequencies. FIG. 7 illustrates
the effect of the exemplary pitch control system 500 of FIG. 5 on the energy amplification in
the rotor velocity at different wind speeds and frequencies.
Graph 600 illustrates that there is significant energy amplification at the tower
@ resonance frequency, generally represented by reference numeral 602. It may be noted that
the energy amplification in this case is not as severe as in FIG. 3 due to the inclusion of a
tower-damping loop in this conventional pitch controller. Graph 700 illustrates that the peak
of the energy amplification indicated in FIG. 6 is significantly reduced by implementing the
decoupling unit 510 of the pitch control system 500. Therefore, by introducing the
decoupling unit 5 10, energy amplification at tower resonance frequencies may be prevented
and excessive tower oscillations because of amplitude amplification may be circumvented.
FIG. 8 is a flow chart 800 that illustrates an exemplary method for reducing
oscillations in a wind turbine. The method will be described with reference to FIGs. 1-2.
The method begins at step 802 where a rotor velocity of a wind turbine, such as the wind
turbine 100, is determined. In one embodiment, the rotor unit 202 may be configured to
determine the rotor velocity by directly measuring the rotor velocity using a sensor, such as
an anemometer, a speedometer, a rotational velocity meter, and so on. Alternatively, the
rotor unit 202 may be configured to determine the rotor velocity by measuring an output
power or generator speed of the wind turbine 100. In this case, the rotor velocity may be
estimated as the velocity that generates the corresponding output power or generator speed.
Subsequently, at step 804, one or more parameters associated with a tower, such
as the tower 102, may be determined. More particularly, a tower top velocity may be
determined. In one embodiment, the tower unit 204 may be configured to determine the
tower top velocity based on a tower top acceleration. The accelerometer 112 coupled to the
wind turbine 100 may be employed to determine the acceleration of the tower deflections.
Based on this detected value, the tower unit 204 may compute the tower top velocity. By
way of example, the tower unit 204 may perform an integration operation on the tower top
acceleration to determine the tower top velocity. Alternatively, the tower velocity may be
determined from available measurements such as tower acceleration using a model-based
estimator such as a Kalman filter. In other embodiments, a velocity sensor or a deflection
sensor may be installed on the wind turbine 100 to measure the tower top velocity or the
tower deflection, respectively. In case the tower deflection is detected, the tower unit 204
may be configured to perform a differentiation operation on the tower deflection to determine
the tower top velocity. Furthermore, one or more of the sensors may be coupled to the tower
unit 204 such that the measured parameter value may be directly provided to the tower unit
204.
Furthermore, at steps 806 and 808, a modified rotor velocity may be computed.
To this end, a first rotor velocity component may be computed, as indicated by step 806. The
computing unit 210 may be configured to utilize a linearized model of the rotor dynamics as
represented by equation (5) to determine the modified rotor velocity. By substituting the
tower top velocity and other variable values in equation (9, the computing unit 210 may
determine the first rotor velocity component.
At step 808, the first rotor velocity component may be subtracted from the rotor
velocity obtained at step 802 to determine the modified rotor velocity. In one embodiment,
the subtracting unit 212 may be configured to perform this operation. The subtracting unit
2 12 may be a digital computing device or an electric hardware device without departing from
e the scope of the present disclosure. In case of a hardware device, the computing unit 210
may be configured to output an electrical signal corresponding to the first rotor velocity
component. Similarly, the rotor unit 202 may convert the rotor velocity into an electrical
signal. These signals (i.e., the first rotor velocity component and the rotor velocity) may then
be subtracted in the subtracting unit 2 12. In the case of a digital computing device, the digital
values for the rotor velocity and the first rotor velocity component may be provided to the
subtracting unit 212 where these may be subtracted to determine the modified rotor velocity.
Subsequently, at step 810, a first pitch angle may be generated based on the
modified rotor velocity. The subtracting unit 212 may be configured to communicate the
modified rotor velocity to the controller 206. The controller 206, in turn, may be configured
to determine the corresponding first pitch angle. As described previously, the controller 206
I may be configured to perform this operation by utilizing any one of a number of known
I technologies. For instance, the controller 206 may include a prepopulated LUT that includes
pitch angle values corresponding to various rotor velocities. Alternatively, the controller 206
may be configured to store a determined threshold rotor velocity, such as a rotor velocity that
generates rated power output. The controller 206 may subsequently compare the modified
rotor velocity with the threshold rotor velocity to generate an error signal. Furthermore, the
controller 206 may also include a LUT that stores pitch angles corresponding to various error
signals. Accordingly, the controller 206 may be configured to compare the generated error
signal with the error signals in the LUT to determine an appropriate first pitch angle.
Furthermore, in some wind turbines, the controller 206 may be configured to generate first
pitch angle values for the rotor blades 106 individually so that each rotor blade 106 may be
@ pitched at a different angle. In other embodiments, the controller 206 may generate one first
pitch angle for all the rotor blades 106.
Following the determination of the first pitch angle, one or more rotor blades 106
may be pitched based on a corresponding first pitch angle, as indicated by step 812. To this
end, the controller 206 may transmit the first pitch angle to the pitch actuator 214. The pitch
actuator 214 may, in turn, be configured to utilize any known actuating means to alter the
pitch angle of the blades. Some examples of pitch actuating means may include hydraulic
means, electrical means, electronic means, and electro-mechanical means.
FIG. 9 is a flow chart 900 illustrating another exemplary method for reducing
oscillations in a wind turbine. This method is described with reference to FIGS. 1 and 5.
0 Similar to the method previously described, this method begins at step 902 by determining
the rotor velocity. Subsequently, at step 904, one or more parameters associated with the
tower 102 may be obtained. The parameters may include tower top velocity and a second
pitch angle. In one example, the tower top velocity may be determined at step 906 and the
second pitch angle may be determined at step 908. To this end, the pitch control system 500
may include the tower-damping unit 508. The tower-damping unit 508 may be configured to
determine the second pitch angle based on a linear model of tower dynamics and the tower
top velocity. As described previously with reference to FIG. 5, the tower-damping unit 508
may be configured to determine the thrust required to reduce the oscillations and determine
the second pitch angle that may aid in generating the desired thrust.
Once the second pitch angle is computed, a modified rotor velocity may be
determined at step 910. To compute the modified rotor velocity, it may be desirable to obtain
the first and second rotor velocity components. Accordingly, the first and second
components of rotor velocity are computed, as indicated by step 912. In one embodiment, for
this computation, the computing unit 514 may be configured to utilize the linearized model of
rotor dynamics provided by equation (6). Using this equation, the computing unit 5 14 may
be configured to determine a combination of the first and second rotor velocity
components (G,,). In this model, the computing unit 5 14 may be configured to employ the
values of the tower top velocity and the second pitch angle to determine the first and second
components of rotor velocity. Subsequently, at step 914, the first and second components of
rotor velocity are subtracted from the rotor velocity obtained at step 902 to determine the a modified rotor velocity. In one embodiment, the combination of the first and second rotor
velocity components (G,,) may be subtracted from the rotor velocity to determine the
modified rotor velocity.
Furthermore, at step 916, a first pitch angle may be generated based on the
modified rotor velocity. More particularly, the modified rotor velocity may be communicated
to the controller 206 and the controller 206 may be configured to generate the first pitch
angle. The first pitch angle and the second pitch angle may be combined in the adder 5 12 to
generate a combined pitch angle, as indicated by step 91 8. This combined pitch angle may be
transmitted to the pitch actuator 214. At step 920, the pitch actuator 51 8 may be configured
to pitch the rotor blades 106 (individually or together) to obtain a desired rotor velocity and to
reduce tower oscillations.
It will be understood that the methods of FIGS. 8 and 9 may be repeated
continuously, periodically, or at determined intervals of time to maintain the desired rotor
velocity and/or minimize tower oscillations. In case of high turbulence or very high speeds,
these methods may not be sufficient to maintain the rotor velocity and/or the tower
oscillations within threshold limits. In such cases, the pitch control system 116 may also be
configured to power off or shut down the wind turbine 100 until the turbulent conditions pass.
Such a measure may be taken to prevent damage to the wind turbine 100.
Furthermore, although the systems and methods described hereinabove decouple
rotor and tower dynamics to reduce fore-aft tower oscillations and maintain effective rotor
velocity, these systems may be utilized to decouple other wind turbine dynamics as well. For
example, the decoupling unit 208 and/or 510 may be utilized in a pitch control system to
decouple rotor blade-flap and tower fore-aft vibrations. Similarly, the decoupling unit 208
andlor 510 may be utilized in a torque controller to decouple blade-edge and drivetrain
dynamics.
In addition, the foregoing examples, demonstrations, and process steps such as
those that may be performed by the system may be implemented by suitable code on a
processor-based system, such as a general-purpose or special-purpose computer. It should
also be noted that different implementations of the present technique may perform some or all
of the steps described herein in different orders or substantially concurrently, that is, in
parallel. Furthermore, the functions may be implemented in a variety of programming * languages, including but not limited to C++ or Java. Such code may be stored or adapted for
storage on one or more tangible, machine-readable media, such as on data repository chips,
local or remote hard disks, optical disks (that is, CDs or DVDs), memory, or other media,
which may be accessed by a processor-based system to execute the stored code. Note that the
tangible media may comprise paper or another suitable medium upon which the instructions
are printed. For instance, the instructions may be electronically captured via optical scanning
of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable
manner if necessary, and then stored in a data repository or memory.
Moreover, the various lookup tables may be incorporated in any data repository
system. For example, these lookup tables may be implemented in a read only memory,
random access memory, flash memory, relational databases, or any other form of memory
without departing from the scope of the present disclosure. Further, these lookup tables may
be stored in a single data repository or in individual data repositories.
Conventional rotor velocity loops typically ignore parameters such as the tower
top velocity ( x ~ , )a nd the pitch angle calculated by the tower-damping loop (Om,) while
determining the pitch angle to control rotor velocity. Such disregard may induce energy
amplification in the rotor at tower resonance frequencies. Sudden energy amplification may
be detrimental for the rotor, drive train, and generator. Moreover, linear analysis reveals that
the interdependence between the rotor dynamics and the tower dynamics results in unstable
rotor dynamics. The exemplary rotor velocity loop of the pitch control system of the present
disclosure effectively reducesleliminates the effects of the tower dynamics on the rotor
dynamics and therefore reduces energy amplification in the rotor at tower resonance.
22
Moreover, the exemplary pitch control system may be employed to stabilize rotor dynamics.
Further, the fatigue loads experienced by the wind turbines may also be reduced such that
fatigue loads are within desired working limits. For example, the systems and methods
described here may reduce tower fatigue by approximately 17%.
While only certain features of the invention have been illustrated and described
herein, many modifications and changes will occur to those skilled in the art. It is, therefore,
to be understood that the appended claims are intended to cover all such modifications and
changes as fall within the true spirit of the present disclosure.
Systems and Methods to Reduce Tower Oscillations in a Wind Turbine
Part List
100 - Wind turbine
102 - Tower
104 - Rotor
106 - Rotor blades
108 - Nacelle
110- Wind
1 12 - Accelerometer
0 1 14 - Fore-aft oscillations
1 16 - Pitch control system
200 - Exemplary embodiment of pitch control system
202 - Rotor unit
204 - Tower unit
206 - Controller
208 - Decoupling unit
2 10 - Computing unit
2 12 - Subtracting unit
2 14 - Pitch actuator
300 - Graph illustrating the effect of a conventional pitch control system on energy captured by a
wind turbine at different wind speeds and frequencies
302 - Energy amplification at the tower resonance frequency
400 - Graph illustrating the effect of the exemplary pitch control system 200 on energy captured
by the wind turbine 100 at different wind speeds and frequencies
500 - Another exemplary embodiment of the pitch control system
502 - Rotor unit
504 - Tower unit
506 - Controller
508 - Tower-damping unit
5 10 - Decoupling unit
5 12 - Adder
5 14 - Computing unit
5 16 - Subtracting unit
5 18 - Pitch actuator
600 - Graph illustrating the effect of a conventional pitch control system (with a tower damping
loop) on energy captured by a wind turbine at different wind speeds and frequencies
602 - Energy amplification at the tower resonance frequency
700 - Graph illustrating the effect of the exemplary pitch control system 500 on energy captured
by the wind turbine 100 at different wind speeds and frequencies
800-8 12 - Exemplary method for reducing tower oscillations in a wind turbine
900-920 - Exemplary method for reducing tower oscillation in a wind turbine having a tower
damping loop

We claim:
1. A method for reducing tower oscillations 114 in a wind turbine 100, the method
comprising:
determining a rotor velocity;
obtaining one or more parameters associated with a tower 102 of the wind turbine
100;
determining a modified rotor velocity based on the one or more parameters;
determining a first pitch angle based on the modified rotor velocity; and
pitching one or more blades 106 of the wind turbine 100 based on the first pitch angle
to reduce the tower oscillations 1 14.
2. The method of claim 1, wherein obtaining the one or more parameters
associated with the tower 102 comprises:
determining a tower top velocity in a fore-aft direction; andlor
determining a second pitch angle.
3. The method of claim 2, wherein determining the modified rotor velocity
comprises:
determining a first rotor velocity component based on the tower top velocity by
utilizing a linear model of rotor dynamics, wherein the linear model is represented by:
( J J ~ D , - 2 so,) = 6Mz -sx,f‘ I
or approximations thereof, where, J, is a moment of inertia of a rotor 104, 6Grf is the first
rotor velocity component, 6ar1 is a rate of change of the first rotor velocity component, and
6xf, is the tower top velocity; and
subtracting the first rotor velocity component from the rotor velocity to obtain the
modified rotor velocity.
4. The method of claim 2, wherein pitching the one or more blades 106
comprises:
combining the first pitch angle and the second pitch angle to obtain a combined pitch
angle; and
pitching the one or more blades of the wind turbine based on the combined pitch
angle to reduce the tower oscillations 114.
5. The method of claim 4, wherein determining the modified rotor velocity
comprises:
determining a first rotor velocity component based on the tower top velocity and a
second rotor velocity component based on the second pitch angle by utilizing a linear model
of rotor dynamics, wherein the linear model is represented by:
or approximations thereof, where J,. is the moment of inertia of a rotor, 6arC is a combination
of the first rotor velocity component and the second rotor velocity component, is rate of
change of the combination of the first rotor velocity component and the second rotor velocity
component, 6xf, is the tower top velocity, and 68,,, is the second pitch angle; and
subtracting the first rotor velocity component and the second rotor velocity
component from the rotor velocity to obtain the modified rotor velocity.
6. A pitch control system 1 16, comprising:
a rotor unit 202 configured to determine a rotor velocity;
a tower unit 204 configured to determine one or more parameters associated with a
tower of a wind turbine;
a decoupling unit 208 configured to determine a modified rotor velocity based on the
one or more parameters; and
a controller 206 configured to determine a first pitch angle based on the modified
rotor velocity.
7. The pitch control system 116 of claim 6, wherein the one or more parameters
comprises a tower top velocity and/or a second pitch angle.
8. The pitch control system 116 of claim 7, wherein the decoupling unit 208
further comprises:
a computing unit 210 configured to determine a first rotor velocity component based
on the tower top velocity; and
a subtracting unit 212 configured to subtract the first rotor velocity component from
the rotor velocity to generate the modified rotor velocity.
9. The pitch control system 1 16 of claim 7, further comprising:
a tower-damping unit 508 configured to determine the second pitch angle; and
an adder 5 12 configured to combine the first pitch angle and the second pitch angle to
generate a combined pitch angle.
10. The pitch control system 1 16 of claim 9, wherein the decoupling unit 5 10
further comprises
a computing unit 5 14 configured to:
receive the tower top velocity from the rotor unit 502;
receive the second pitch angle from the tower-damping unit 504;
determine a first rotor velocity component based on the tower top velocity;
determine a second rotor velocity component based on the second pitch angle;
and
a subtracting unit 5 16 configured to subtract the first rotor velocity component and the
second rotor velocity component from the rotor velocity to determine the modified rotor
velocity value.
1 1. A wind turbine 100, comprising:
a rotor 104 comprising one or more rotor blades 106;
a tower 102 operatively coupled to the rotor 104;
a pitch control system 116 configured to reduce tower oscillations 114 in the wind
turbine 100, the pitch control system 1 16 comprising:
a rotor unit 502 configured to determine a rotor velocity;
a tower unit 504 configured to determine at least one of a tower top velocity
and a second pitch angle;
a decoupling unit 510 configured to determine a modified rotor velocity based
on at least one of the tower top velocity and the second pitch angle, the decoupling
unit comprising:
a computing unit 5 14 to determine at least one of a first rotor velocity
component and a second rotor velocity component based on at least one of the
tower top velocity and the second pitch angle, respectively; and
a subtracting unit 5 16 configured to deduct at least one of the first rotor
velocity component and the second rotor velocity component from the rotor
velocity to obtain the modified rotor velocity; and
a controller 506 configured to determine a first pitch angle based on the
modified rotor velocity.