U.S. patent application number 13/811393 was filed with the patent office on 2013-10-17 for method and device for determining a bending angle of a rotor blade of a wind turbine system.
The applicant listed for this patent is Boris Buchtala, Christian Eitner, Felix Hess, Martin Voss. Invention is credited to Boris Buchtala, Christian Eitner, Felix Hess, Martin Voss.
Application Number | 20130272874 13/811393 |
Document ID | / |
Family ID | 44628258 |
Filed Date | 2013-10-17 |
United States Patent
Application |
20130272874 |
Kind Code |
A1 |
Hess; Felix ; et
al. |
October 17, 2013 |
METHOD AND DEVICE FOR DETERMINING A BENDING ANGLE OF A ROTOR BLADE
OF A WIND TURBINE SYSTEM
Abstract
A method for determining a bending angle of a rotor blade of a
wind turbine system includes step of reading in an acceleration
signal which represents an acceleration of the rotor blade acting
essentially perpendicularly with respect to a rotor plane. In
addition, the method includes a step of determining the bending
angle of the rotor blade of the wind turbine using the acceleration
signal.
Inventors: |
Hess; Felix; (Ludwigsburg,
DE) ; Voss; Martin; (Stuttgart, DE) ;
Buchtala; Boris; (Muehlacker, DE) ; Eitner;
Christian; (Muenchen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hess; Felix
Voss; Martin
Buchtala; Boris
Eitner; Christian |
Ludwigsburg
Stuttgart
Muehlacker
Muenchen |
|
DE
DE
DE
DE |
|
|
Family ID: |
44628258 |
Appl. No.: |
13/811393 |
Filed: |
July 2, 2011 |
PCT Filed: |
July 2, 2011 |
PCT NO: |
PCT/EP2011/003293 |
371 Date: |
March 29, 2013 |
Current U.S.
Class: |
416/1 ;
416/61 |
Current CPC
Class: |
F03D 7/0224 20130101;
F05B 2270/807 20130101; F05B 2260/80 20130101; F01D 25/00 20130101;
Y02E 10/72 20130101; F03D 17/00 20160501; Y02E 10/723 20130101;
F05B 2270/331 20130101 |
Class at
Publication: |
416/1 ;
416/61 |
International
Class: |
F01D 25/00 20060101
F01D025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 24, 2010 |
DE |
10 2010 032 120.6 |
Claims
1. A method for determining a bending angle of a rotor blade of a
wind turbine system comprising: reading in at least one
acceleration signal that represents a first acceleration acting on
the rotor blade in a fashion substantially perpendicular to a rotor
plane; and determining the bending angle of the rotor blade of the
wind turbine system by using the at least one acceleration
signal.
2. The method as claimed in claim 1, wherein: reading in the at
least one acceleration signal includes acquiring an acceleration
profile, determining the bending angle of the rotor blade includes
determining a spectrum from the acceleration profile, and
determining the bending angle includes using the determined
spectrum.
3. The method as claimed in claim 2, wherein: determining the
bending angle of the rotor blade includes comparing the determined
spectrum with a provided spectrum, and using a result of the
comparison.
4. The method as claimed in claim 2, wherein reading in the at
least one acceleration signal includes actively exciting vibration
of the rotor blade.
5. The method as claimed in claim 1, wherein determining the
bending angle of the rotor blade includes subjecting the at least
one acceleration signal to at least one of lowpass filtering and
Kalman filtering.
6. The method as claimed in claim 1, wherein: determining the
bending angle includes using an information item relating to at
least one of: a distance of a first acceleration sensor providing
the at least one acceleration signal from a rotor axis, an
inclination angle of the rotor axis from horizontal, and an
acceleration of a tower head of the wind turbine system, and
determining the bending angle includes using an information item
relating to rotational speed and rotary position of a rotor.
7. The method as claimed in claim 1, wherein determining the
bending angle includes considering a time profile of the first
acceleration at a position of the rotor blade.
8. The method as claimed in claim 6, further comprising reading in
a further acceleration signal that represents a second
acceleration, acting substantially in a longitudinal direction of
the rotor blade, at a location of the first acceleration sensor,
wherein determining a bending angle of the rotor blade of the wind
turbine system includes using the further acceleration signal.
9. The method as claimed in claim 1, further comprising one of:
determining individual incidence angles of the rotor blades based
on the bending angle, and determining individual incidence angles
of the rotor blades based on loading, determined from the bending
angle, of the rotor blade or the rotor blades.
10. A device for determining a bending angle of a rotor blade of a
wind turbine system, the device comprising: an interface configured
to read in at least one acceleration signal that represents an
acceleration acting on the rotor blade, and a unit configured to
determine the bending angle of the rotor blade of the wind turbine
system by using the acceleration signal.
11. A computer program product having program code for carrying out
a method for determining the bending angle of a rotor blade of a
wind turbine system, the computer program product comprising: a
mechanism configured to read in at least one acceleration signal
that represents a first acceleration acting on the rotor blade in a
fashion substantially perpendicular to a rotor plane; and a
mechanism configured to determine the bending angle of the rotor
blade of the wind turbine system by using the at least one
acceleration signal, wherein the computer program product is
configured to determine the bending angle when the program is
executed on one of a control unit and a device.
Description
[0001] The present invention relates to a method and a device for
determining a bending angle of a rotor blade of a wind turbine
system in accordance with the independent patent claims.
[0002] Wind energy systems are controlled via the adjustment of the
rotor blades about their longitudinal axis, and by the generator
torque. Controlled variable for the pitch control is the rotor
rotational speed, and the manipulated variables are the pitch
angles of the rotor blades. The collective pitch control CPC is
used with conventional systems. Here, the three rotor blades are
all adjusted with the same pitch angle. In the case of wind energy
systems with a horizontal axis and at least two rotor blades,
synchronous adjustment of the blade angles is used to control the
rotational speed above the rated wind speed so that by changing the
incidence angle the aerodynamic lift, and thus the drive torque,
are reduced in such a way that the system can be operated in the
region of the rated rotational speed. Given wind speeds above the
switchoff speed, this blade adjustment mechanism is additionally
used as a brake by setting the blades in a fashion nose into the
wind so that the rotor can no longer supply any appreciable drive
torques. Because of asymmetric aerodynamic loads, pitch and yaw
torques on the nacelle are produced in the case of this collective
blade adjustment. The asymmetric loads result, for example, from
wind shears in a vertical direction (boundary layers), yaw angle
errors, gusts and instances of turbulence, damming of the flow at
the tower, etc. A known approach to reducing these asymmetric
aerodynamic loads consists in adjusting the incidence angle of the
blades individually (Individual Pitch Control=IPC). This control
approach requires determination of the bending torques (in
particular, flapping bending torques) that prevail at the rotor
blade root. The bending torques then serve as controlled variable
for the individual blade adjustment. Strain gauge sensors that are
applied at the rotor blade root can be used to determine the
bending torques. The problems in the case of the strain gauge
sensors consist in the application and risk of breakage, and in the
short service life.
[0003] Other methods such as are disclosed, for example, in WO
2008/041066 or DE 197 39 164 B4 determine the pitching and yawing
torques by measuring the nacelle acceleration via gyrometers or by
means of sensors that use distance measurements to measure the
deformations, occurring during the loads, of system parts, and
thereby determine the loads. The blade bending torques are very
well suited as controlled variable from the point of view of the
IPC. However, it has not so far been possible to find any
measurement technique suitable for continuous use. Fiber Bragg
sensors laminated into the blades to measure torques cannot be
exchanged in the case of a defect, while strain gauge sensors
bonded on have a far too short service life. Both methods
additionally have the problem that the measurement is performed
only locally on the blade. Local inhomogeneities in the laminate
therefore lead to measuring errors, and so an inference concerning
the global stress state in the blade root, and thus the torque
acting there, is always affected by errors.
[0004] It is therefore the object of the present invention to
provide a method and a device that enable an improved determination
of the load of a rotor blade of a wind turbine system.
[0005] This object is achieved by the subject matter of the
independent patent claims. Advantageous refinements follow from the
subject matters of the subclaims, and from the following
description.
[0006] The present invention provides a method for determining a
bending angle of a rotor blade of a wind turbine system, the method
having the following steps: [0007] reading in at least one
acceleration signal that represents an acceleration acting on the
rotor blade, and [0008] determining the bending angle of the rotor
blade of the wind turbine system by using the acceleration
signal.
[0009] Furthermore, the present invention produces a device for
determining a bending angle of a rotor blade of a wind turbine
system, the device having the following features: [0010] an
interface for reading in an acceleration signal that represents an
acceleration acting on the rotor blade, and [0011] a unit for
determining the bending angle of the rotor blade of the wind
turbine system by using the acceleration signal.
[0012] Also advantageous is a computer program product having
program code that is stored on a machine readable carrier such as a
semiconductor memory, a hard disk memory or an optical memory, and
that is used to carry out the method according to one of the
abovedescribed embodiments when the program is executed on a
control unit or a device.
[0013] The invention is based on the finding that the bending of
the rotor blade of the wind turbine system is related in a
predetermined fashion to a bending torque of this rotor blade at
the blade root. In order to determine the bending, use is made, in
particular, of an acceleration or an acceleration signal that is
measured in a fashion substantially perpendicular to the rotor
plane. The acceleration in a longitudinal direction of the blade
can also be used as additional acceleration signal. Denoted in this
case as rotor plane is a virtual or actual plane in which the rotor
blades rotate about the rotor axis of the wind turbine system. This
means that the acceleration used in the present approach represents
an acceleration in the direction of the rotor axis. Given knowledge
of this predetermined relationship, it is possible in this case to
use the acceleration of the rotor blade, or at least of a part of
the rotor blade, to infer the bending torque present at the blade
root of this rotor blade so that a conventional control unit can
further be used to determine the blade pitch angle by using a
modified control parameter. It is not mandatory in this case to
infer the bending torque present at the blade root; rather, it is
also possible to directly calculate the incidence angle to be set
on the basis of the measured acceleration or acceleration read in.
In this case, the acceleration determined is thus used to determine
the blade deflection (that is to say a value beta), the IPC pitch
angle or the incidence angle of the rotor blades being determined
therefrom. The preset control can therefore use the bending torque
(or the bending angle), and then determines the so-called pitch
angle of the rotor blade. It is also possible in this case to use
the term "incidence angle" for the term "pitch angle". Thus, the
incidence angle or the individual incidence angle for the rotor
blade can be determined from the bending angle.
[0014] The present invention offers the advantage that conventional
control units can continue to be used such that there is no need
for a cost intensive redevelopment of a control unit for
controlling the incidence angles of the rotor blades of the wind
turbine system. At the same time, the sensor variables used can be
provided by using sensors that are substantially more robust
against aging phenomena and measuring errors. Because wind turbine
systems are designed for a long running time and, in particular, it
is very cost intensive to exchange rotor blades, the abovementioned
advantage is gaining even further in importance. A simple and cost
effective retrofitting is also possible with the approach presented
here.
[0015] In accordance with a favorable embodiment of the present
invention, a profile of the acceleration can be acquired in the
read-in step, in the determination step a spectrum being determined
from the acceleration profile, and the bending angle being
determined by using the determined spectrum. Such an embodiment of
the present invention offers the advantage that relatively small
measuring errors can be compensated by using a spectrum that has
been determined over a specific period and was subsequently
transformed into the frequency domain. It is possible in this case
to utilize the fact that the revolution of the rotor blade or the
rotor blades periodically gives rise to physical influences at
specific positions in the pitch circle of the rotor blade,
specifically exactly when the rotor blade again reaches the
specific position in the following revolution.
[0016] It is particularly advantageous when in the determination
step the determined spectrum is compared with a provided spectrum,
the bending angle being determined by using a result of comparison
between the determined spectrum and the provided spectrum. Such an
embodiment of the present invention offers the advantage of the
possibility of a good and reliable determination of the provided
spectrum. By way of example, it is possible in this case to
determine a mean value from a multiplicity of recorded spectra, it
then also being possible for such a provided spectrum to map
certain variations of the environmental conditions.
[0017] In accordance with a particular embodiment of the present
invention, in the read-in step a movement of the rotor blade in a
fashion substantially perpendicular to the rotor blade can be
actively effected. Such an embodiment of the present invention
offers the advantage that spectra already to be expected for
specific, frequently occurring scenarios can be measured or
calculated and stored in a memory. For example, in this case a
specific bending angle can be assigned to each spectrum stored in
the memory. In practical use, it is possible in this way to
determine the bending angle very easily numerically or by
circuitry, since in essence a comparison of the determined spectrum
with one or more spectra from the memory needs to be performed in
order to obtain an already very accurate magnitude for the bending
angle from the result of comparison when the determined spectrum
corresponds approximately to a specific spectrum to which this
bending angle is assigned.
[0018] In accordance with a further embodiment of the present
invention, in the determination step the acceleration signal can be
subjected to lowpass filtering and/or Kalman filtering. Such an
embodiment of the present invention offers the advantage that
filtering provides a smoothing of the measured value read in, which
increases the stability of the control method for the incidence
angle. In particular, high-frequency signal interference components
are filtered out in this case, leaving the pure useful signal which
carries the desired information, to be evaluated, relating to the
gravitation and centrifugal force.
[0019] Furthermore, in a further embodiment of the invention it is
also possible in the determination step for determining the bending
angle to make use of an information item relating to bending
stiffness or to an approximation of the bending stiffness, an
information item relating to a distance of an acceleration sensor
providing the acceleration signal from a rotor axis, an inclination
angle of the rotor axis from the horizontal, and/or an acceleration
of a tower head of the wind turbine system. Such an embodiment of
the present invention offers the advantage that a very precise
estimate of the bending torque occurring at the blade root is
possible hereby so that a slight change in the parametrization of
control units already in use is necessary. This is relevant, in
particular, because the control units currently in use determine
the control of the incidence angle for a rotor blade on the basis
of an occurring bending torque, and so the control variable can be
very easily exchanged.
[0020] In order to obtain a determination of the bending angle of
the rotor blade that is as precise as possible, in the
determination step it is possible to determine a time profile of
the acceleration at a position of the rotor blade from the
acceleration signal, and to determine the bending angle or the
blade deflection of the rotor blade by using the determined
profile. The time profile can in this case extend over a rotor
blade revolution about the rotor axis. Such an embodiment of the
present invention determines the bending of the rotor blade by
virtue of the fact that the position relative to the gravitational
acceleration, which acts periodically during the measurement of the
acceleration of the rotor blade in a fashion now amplifying and now
reducing the measured sensor signal, is determined.
[0021] In accordance with another embodiment of the present
invention, in the read-in step it is possible to read in a further
acceleration signal that is measured in the direction of the
longitudinal axis of the rotor blade. In this case, in the
determination step of the bending angle of the rotor blade of the
wind turbine system is determined by using the further acceleration
signal.
[0022] The invention is explained in more detail in exemplary
fashion below with the aid of the attached drawings, in which:
[0023] FIG. 1 shows an illustration of a uniform definition of
designations of the possible movements on a wind energy system;
[0024] FIG. 2 shows a block diagram of a control unit for the
individual incidence angles of a rotor blade of a wind energy
system, in the case of which an exemplary embodiment of the present
invention can be used;
[0025] FIG. 3 shows a diagram of the principle of the relevant
variables for a positioning of the sensor on a rotor blade;
[0026] FIG. 4 shows a diagram illustrating the relationship between
a blade deflection and a root bending torque, plotted against
time;
[0027] FIG. 5 shows an illustration of a sensor coordinate system
on a rotor blade;
[0028] FIG. 6 shows an illustration showing the measurement
principle and the processing of the sensor signal obtained;
[0029] FIG. 7 shows a diagram in which a sensor signal subjected to
lowpass filtering and a reference signal are illustrated;
[0030] FIG. 8 shows a diagram illustrating the relationship between
a blade deflection and a useful signal of the acceleration
measurement of a sensor that is positioned at a distance of r=10 m
from the rotor hub; and
[0031] FIG. 9 shows a flowchart in accordance with an exemplary
embodiment of the present invention as method.
[0032] Identical or similarly acting elements can be provided in
the following figures by identical or similar reference symbols.
Furthermore, the figures of the drawings, their description and the
claims include numerous features in combination. It is clear here
to a person skilled in the art that these features can also be
considered individually, or can be brought together to form further
combinations that are not explicitly described here. Furthermore,
in the following description the invention may be explained by
using different measurements and dimensions, although the invention
is not to be understood as confined to these measurements and
dimensions. Furthermore, inventive method steps can be executed
repeatedly and in a sequence other than that described. If an
exemplary embodiment includes an "and/or" conjunction between a
first feature/step and a second feature/step, this can be read to
the effect that, in accordance with one embodiment, the exemplary
embodiment has both the first feature/the first step and the second
feature/the second step and, in accordance with a further
embodiment, the exemplary embodiment has either only the first
feature/the first step or only the second feature/the second
step.
[0033] A particular aim of the invention is to provide a
possibility of using a control method to minimize the yaw and pitch
torques on the nacelle, which result from asymmetric aerodynamic
loads. Manipulated variables are advantageously the individual
incidence angles of the blades of the wind turbine system. An
important aspect in this case is that, in accordance with the
approach presented here, the controlled variables are determined
via acceleration sensors on the rotor blades. To this end, there is
installed in at least one rotor blade at least one acceleration
sensor that can measure accelerations in the flapping direction
(that is to say perpendicular to the rotor plane). This offers the
advantage that it is possible hereby to use point sensors that can
easily be applied in the blades, are easy to exchange, and do not
acquire static errors such as stresses owing to temperature
differences and the inhomogeneous blade material. In addition, in
some circumstances the sensors are already present if condition
monitoring of the blades is installed.
[0034] Recourse may be made to the illustration in accordance with
FIG. 1 in the interest of standard definition of the following
variables used regarding the possible movements of a wind turbine
system. Here, a wind turbine system is understood as a system
having a tower on which a nacelle is fastened. This nacelle
contains a generator that is coupled to a rotor, the rotor having
two rotor blades in the example illustrated in FIG. 1. In this
case, the tower can execute a tower longitudinal bending 100 and a
tower transverse bending 110 given an incident flow of wind and a
transmission of forces of the rotor onto the nacelle and the tower.
Further, the tower can execute a tower torsion 120 about its
vertical axis. A movement of the tower about its vertical axis is
also denoted as yawing 130 of the wind turbine system. Furthermore,
it is also possible for forces to act on the tower or the wind
turbine system, which leads to a rolling 140, that is to say a
rolling movement about the rotor axis of the wind turbine system.
If the effect of wind on the wind turbine system is to induce a
movement that acts both perpendicular to the vertical axis of the
tower and also to the rotor axis, the wind turbine system is said
to pitch 150. The rotor blades can, on the one hand, execute a
pivoting movement 160 or a flapping movement 170, or twist
internally, this equally being denoted as torsion 180, referred now
to the rotor blades. The pivoting movement 160 corresponds in this
case to a desired movement of the rotor blades about the rotor
axis, the flapping movement 170 denoting a movement, in particular
of the tips of the rotor blades, out of the rotor plane, that is to
say in the direction of extent of the rotor axis. Such a definition
of movements of a wind turbine system follows the definition from
the book of E. Hau, entitled "Wind turbine systems", in which
corresponding controlled variables are named for the yaw and pitch
torque of the nacelle. The flapping movement leads to bending
torques on the blade root, and is the cause of yaw and pitch
torques of the nacelle.
[0035] An important aspect of the present invention can be seen in
that it is possible to make use of acceleration signals from
sensors on the blade, and to process these signals within a control
method in order to reduce the yaw and pitch torques on the nacelle
via the individual adjustment of the blade incidence angles.
[0036] A system in the case of which the present invention can be
used in accordance with one exemplary embodiment is illustrated in
a simplified fashion as a block diagram in FIG. 2. The system 200
for controlling the wind turbine system 210 comprises in this case
a unit 220 for operational control, and a unit 230 for controlling
the individual incidence angle 235 (.beta..sub.IPC1,2,3) for each
of the rotor blades of the wind turbine system 210. From a sensor
of the wind turbine system 210, the unit 220 for operational
control (also denoted as CPC; CPC=Collective Pitch Control)
receives a signal for outputting, in particular a signal relating
to the rotational speed .omega. of the rotor of the wind turbine
system 210. The signal can now be used, on the one hand, by the
unit 220 for operational control to determine a generator torque
240 that is to be set, and to make this available for controlling
the wind turbine system 210 and, on the other hand, to determine
for all rotor blades a common incidence angle 242 (.beta..sub.CPC)
for which the wind turbine system has an optimum output efficiency.
From at least one sensor in or on a rotor blade of the wind turbine
system 210, the control unit 230 (also denoted as IPC controller)
for the individual incidence angle 235 receives a signal relating
to an acceleration a.sub.1 of this rotor blade at that position at
which the sensor is arranged. In particular, the control unit 230
for the individual incidence angle 235 can receive signals relating
to accelerations a.sub.1,2,3 from a plurality of, for example from
all rotor blades, and in this case it can provide for each rotor
blade for which it receives a sensor signal a corresponding signal
.beta..sub.IPC1,2,3 for setting the individual incidence angle 235
of the relevant rotor blade. In this way, the signal relevant to
the common incidence angle can be corrected for each individual
rotor blade in order to take account of local wind inhomogeneities.
Furthermore, the shear of the wind also leads to asymmetric loads.
The signal relating to the common incidence angle 242 can then, for
example, be combined additively with the different signals relating
to the individual incidence angles 235 for the relevant rotor
blades, thus yielding a control signal 250 for the individual
relevant rotor blades of the wind turbine system 210. This
adjustment of the incidence angle of the individual rotor blades of
the wind turbine system 210 in accordance with the desired
incidence angles is subsequently set by an actuator 255. Under the
influence of varying wind conditions 260, the rotor blades are then
deflected in a flapping direction with different degrees of
intensity, this deflection or the acceleration occurring in this
case being measured in turn by the appropriate sensors, and being
fed via the sensor signals 265 to the operational control unit 220
and to the control unit 230 for the individual incidence angle. In
this way, the control loop for controlling the individual incidence
angles is closed.
[0037] A very simple modification of already existing control
systems for the individual incidence angles of the rotor blades of
a wind turbine system can be implemented by using acceleration
signals that represent an acceleration of the individual rotor
blades in the flapping direction. Specifically, conventional wind
turbine systems mostly use the bending torques at the blade root of
the rotor blades to set the individual incidence angles of the
relevant rotor blades. However, since a simple relationship between
a bending torque at the blade root of a rotor blade and an
associated bending of the rotor blades in a flapping direction can
mostly be detected, or is known, it is possible by means of a
signal of a substantially more robust acceleration sensor to use
for the control of the incidence angle of the rotor blade an
adequately useful signal that represents the acceleration of the
rotor blade or of a part of the rotor blade in a flapping direction
by employing this signal to determine the bending angle of the
rotor blade in accordance with the invention. Two variants can now
be conceived in order to obtain and to process a signal relating to
a bending angle of a rotor blade that can be processed well and is
as free from interference as possible.
[0038] In a first variant, a natural frequency analysis of the
determined accelerations or of the acceleration signals derived
therefrom can be carried out. To this end, use is made of the
natural vibrations of the (rotor) blade. The excitation during the
operation of the system is performed by aerodynamically induced
vibrations or via an additionally fitted shaker, that is to say a
unit which actively sets the rotor blade vibrating. In this case,
the acceleration sensors continuously acquire and store signals,
and determine the amplitude spectrum of the natural vibrations
after a specific measuring time (at most 1 s). This frequency
spectrum is, for example, compared with desired spectra that are
stored in the control/regulation device and are associated with
specific loading states of the blade. Loading at the blade is
reduced by adjusting the blade angle, and this is controlled by
comparison with the desired spectra. The desired spectra are
preliminarily determined by measurements on the blade without and
with loadings, or else determined from calculations via natural
frequency analysis. The advantage of this variant consists in that
it is also possible to make use of the already available
measurement technique for condition monitoring, which has already
integrated the sensors, the acquisition of measured values, and the
preparation and evaluation of the acceleration signals. Desired
spectra already stored are also associated therewith. Desired
spectra for loading cases that are stored in the control or
regulation device should be added thereto for such an application
scenario. Said spectra can be determined by measurements on blade
test stands. It is probably easier to carry out reference
measurements before mounting on the blade, and to carry out similar
measurements on the rotor blade after mounting in the case of wind
speeds below the startup speed. The deviations relating to these
spectra in the case of loading are calculated via simulation
starting from these spectra and the blade data, and stored as
desired spectra.
[0039] A second variant for the use of the approach presented here
is to be seen in the use of data from a direct acceleration
measurement and evaluation thereof. In this case, the bending angle
of the rotor blade is determined from the measured accelerations.
The control aim is then to set the same bending angles at all rotor
blades. The blade angles are once again manipulated variables.
Because of aerodynamic effects such as turbulence and vortex
shedding, vibrations of the blade are always excited, but they are
of higher frequency than the vibrations to be removed by control in
the region of the first natural frequency of blade and tower.
Consequently, for control purposes the measured acceleration should
be filtered by means of a lowpass filter. The lower half of the
rotor blade is advantageous for the position of the first
(acceleration) sensor, since the blade tip can be excited to
vibrate strongly owing to the tapering and the transverse flows
prevailing there, which also drive the tip vortex.
[0040] The following aspects may be adduced as advantages of the
two abovedescribed variants. Firstly, it is possible to use known
and possibly already present measuring devices and, if appropriate,
data determined by the condition monitoring of the blades.
Furthermore, there is no need for any application of strain gauges
or the like, in the case of which it is not known in the prior art
where and how they are to be exactly fitted. In addition, the
temperature compensation for these sensors has not yet been
satisfactorily resolved in technical terms. In addition, an
acceleration sensor can easily be replaced in the case of a defect.
This is impossible with strain sensors that have been laminated in.
The signals supplied by strain sensors are possibly not indicative,
since they acquire only the local strain. Again, when the
abovedescribed approach is used there is no occurrence of errors
owing to static loadings such as temperature stresses, excessive
local stresses owing to the inhomogeneous material, ice coating
(with simultaneous use of condition monitoring) etc., something
which greatly increases the reliability of the control by making
use of the variable of the bending angle which is calculated from
the blade acceleration.
[0041] In other words, this means that the approach presented here
constitutes a use as additional control function with the pitch
drives of the applicant. On the basis of current market trends,
future drives should be able to adjust the blades individually.
[0042] A further important aspect of the present invention consists
in enabling, on the basis of an acceleration sensor (DCU), an
improved IPC-suited measurement method in which the sensor has a
longer service life, a simple exchangeability of the sensor system
is ensured, and a variable equivalent to the global stress state in
the blade root is acquired as far as possible.
[0043] In the case of the approach presented below, a substantial
aspect resides in the use of a signal of an acceleration sensor
that measures the acceleration of the rotor blade in the direction
of the rotor axis. The acceleration sensor should be able to
measure stationary acceleration. The measurement of the
acceleration in the blade is known according to the prior art, and
is used, inter alia, for condition monitoring. A twofold
integration of this measured acceleration would provide the current
blade deflection. However, this method has a drift that corrupts
the calculated results over a longer time. This measured variable
is therefore not suitable for IPC control.
[0044] In accordance with an exemplary embodiment, the invention
presented here presents a measurement concept that enables a signal
evaluation suitable for the IPC control. An online signal
evaluation can be performed on the basis of sensor data of a
mono-axial acceleration sensor. A possible use resides, for
example, in the field of IPC control, or with experimental
measurements on wind energy systems. The control of the blade
angles of the rotor blades of a wind energy system requires the
blade deflection in a flapping direction (that is to say
perpendicular to the rotor plane given 0 degree pitch setting). In
order to determine this variable, the blade deflection can be
measured directly via strain gauge sensors on the rotor blade root.
An alternative sensor concept for the measurement of the blade
deflection is the use of acceleration sensors whose measurement
equation is described by the so-called navigation equation (3),
which reads as follows:
a .fwdarw. = 2 r .fwdarw. t 2 | i + g .fwdarw. = v .fwdarw. i t | i
+ g .fwdarw. , ( 3 ) ##EQU00001##
a corresponding to the measured acceleration, and g to the
gravitational acceleration.
[0045] Given appropriate lowpass filtering of the local
acceleration, the sensor signal can be used to estimate the
projection of the gravitational vector, and thus the pitch angle of
the sensor coordinate system. The orientation of the sensor can be
used to infer the deflection of the rotor blade, and thus the
corresponding flapping bending torque. The diagram from FIG. 4
shows a measured relationship for the deflection of the rotor blade
and the corresponding flapping bending torque, in which time is
represented on the abscissa, and the profile of the blade
deflection (dashed line) and of the blade root bending torque
(continuous line) is represented on the ordinate. It is to be seen
in this case from FIG. 4 that the profiles for the measured blade
deflection and the measured blade root bending torque correspond to
one another such that in order to control the individual incidence
angle of the rotor blade it is also possible to use the blade
deflection, and thus also the acceleration that leads to the
relevant blade deflection.
[0046] Assuming negligible torsion, it suffices to take account of
the x-component of the sensor signal in order to determine the
blade deflection. The x-component points in the direction of the
normal vector on the blade surface, and lies in the bending
flapping direction to the extent that no blade torsion is present.
A sensor coordinate system 500 in the rotor blade, such as is shown
in FIG. 5, is assumed to this end. In this case, the z-component is
oriented in the direction of the rotor blade end, the x-component
in a normal to the rotor plane, and the y-component in a pivoting
direction of the rotor blade. Furthermore, a coordinate system 510
in the hub of the rotor, and a coordinate system 520 in the rotor
shaft can be used for the conversion of the sensor acceleration
values, as is further described in more detail below.
[0047] In order to determine the blade deflection, the first step
to this end is to transform the coordinates of the tower into the
rotor axis, the coordinates of the rotor axis into the rotor blade,
and from the rotor blade into the bent rotor blade. The following
transformation matrices can be used to this end:
M tower into rotor axis = ( cos .LAMBDA. 0 - sin .LAMBDA. 0 1 0 sin
.LAMBDA. 0 cos .LAMBDA. ) = M ST , M rotor axis into rotor blade =
( 1 0 0 0 cos .OMEGA. sin .OMEGA. 0 sin .OMEGA. cos .OMEGA. ) = M
BS , and ##EQU00002## M rotor blade into deflected rotor blade = (
cos .beta. 0 - sin sin .OMEGA. 0 1 0 sin .beta. 0 cos .beta. ) = M
B ' B , ##EQU00002.2##
.LAMBDA. representing the inclination angle of the rotor axis
relative to a horizontal, .OMEGA. representing the rotor azimuth
angle about the rotor axis, and .beta. representing the torsion
angle of the rotor blade at the location of the sensor from the
rotor plane. In this case, a projection of the gravitational
acceleration
g _ T = ( 0 0 - g ) T ##EQU00003##
can be described in the following way:
g _ T = M B ' B M BS M ST g _ B ' = g ( cos .beta. sin .LAMBDA. +
sin .beta. cos .OMEGA. cos .LAMBDA. - sin .OMEGA. cos .LAMBDA. sin
.beta. sin .LAMBDA. - cos .beta. cos .OMEGA. cos .LAMBDA. ) .
##EQU00004##
[0048] Furthermore, a measurement equation of the acceleration
sensors can be specified as follows:
a _ = ( t ) t [ ( t ) f r _ ] + g _ , ##EQU00005##
from which it follows that:
a Sensor ' = ( .omega. 2 cos .beta. sin .beta. r s + r s .beta. _ +
g ( cos .beta. sin .LAMBDA. + sin .beta. cos .OMEGA. cos .LAMBDA. )
0 r s ( 2 .omega. sin .beta. .beta. - .omega. . cos .beta. ) - g (
sin .OMEGA. cos .LAMBDA. ) - .omega. 2 ( cos .beta. ) 2 r s - r s
.beta. _ 2 + g ( sin .beta. sin .LAMBDA. - cos .beta. cos .OMEGA.
cos .LAMBDA. ) ) B ' ##EQU00006##
the first column of the above-specified matrix representing the
centripetal acceleration, the second column of the above-specified
formula the measured accelerations on the basis of the rotation of
the sensor coordinate system, and the third column of the above
specified formula the gravitational acceleration.
[0049] If the tower head acceleration is not to be neglected, it is
necessary to expand the sensor equation by a.sub.tower head,
where
a tower head = ( + a x ( cos .beta. cos .LAMBDA. - sin .beta. sin
.LAMBDA. sin .OMEGA. ) + a y sin .beta. sin .OMEGA. + a x sin
.LAMBDA. sin .OMEGA. + a y cos .OMEGA. + a x ( sin .beta. cos
.LAMBDA. + cos .beta. sin .LAMBDA. cos .OMEGA. ) - a y cos .beta.
sin .OMEGA. ) . ##EQU00007##
[0050] Consequently, it therefore holds for the measured total
acceleration a.sub.sensor of the sensor that:
a.sub.sensor=a.sub.sensor'+a.sub.tower head.
[0051] The components based on the rotation of the sensor
coordinate system can be filtered by a lowpass filter and thereby
eliminated.
[0052] In accordance with the exemplary embodiment of the present
invention presented here, it is possible to implement two
measurement concepts or measurement methods. A mono-axially
measuring acceleration sensor on the rotor blade is used for the
first method. What is measured in this case is the acceleration
that is, for example, directed normally onto the rotor blade
surface. This acceleration is denoted as a.sub.x, sensor and can be
expressed as follows, neglecting the tower head acceleration:
a.sub.x,Sensor=.omega..sup.2cos .beta.sin
.beta.r.sub.s+r.sub.s{umlaut over (.beta.)}+g(cos .beta.sin
.LAMBDA.+sin .beta.cos .OMEGA.cos .LAMBDA.),
the term g(cos .beta.sin .LAMBDA.+sin .beta.cos .OMEGA.cos
.LAMBDA.) periodically repeated with the angle .OMEGA..
Furthermore, when .beta. is small, and thus sin .beta. tends to 0,
the trajectory of the gravitational acceleration can be used
uniquely to determine .beta..
[0053] Without blade deflection, it therefore holds that .beta.=0.
It follows herefrom that
(a.sub.x).sub.sensor,filtered=+gsin .LAMBDA.=const.
[0054] The result with .beta..noteq.0 is a rotational frequency
component (sin .beta.cos .OMEGA.cos .LAMBDA.), it being possible to
determine the bending angle from the following relationship:
2 g ( sin .beta. cos .LAMBDA. ) = A .beta. = arc sin ( A 2 g cos
.LAMBDA. ) . ##EQU00008##
[0055] The change in the projection trajectory of the gravitational
acceleration is therefore determined by the deflection of the rotor
blade (.beta..noteq.0), and can be used to determine .beta.. The
first measurement method of the blade deflection on the basis of a
mono-axially measuring acceleration sensor offers advantages with
reference to a sensor that can be used cost effectively, and to a
simpler evaluation of the sensor signals than in the case of the
use of a plurality of sensor signals. However, it must be adduced
as a disadvantage of this measurement method for the bending angle
that a smaller useful signal is available, because only the signal
amplitude can be used to determine .beta..
[0056] Variables as explained in more detail with reference to FIG.
3 can be used for the second method for determining the bending
angle. Here, the acceleration of the rotor blade 300 in the
direction of the rotor axis 310 is considered, the acceleration
sensor being arranged at the distance r therefrom. The following
accelerations are measured by the local rotation of the rotor blade
at the location of the acceleration sensor by the angle .beta.
owing to the deflection of the rotor blade from the rotor plane
(320):
a.sub.x,Sensor=.omega..sup.2cos .beta.sin
.beta.r.sub.s+r.sub.s{umlaut over (.beta.)}+g(cos .beta.sin
.LAMBDA.+sin .beta.cos .OMEGA.cos .LAMBDA.)+a.sub.x(cos .beta.cos
.LAMBDA.-sin .beta.sin .LAMBDA.sin .OMEGA.)+a.sub.ysin .beta.sin
.OMEGA.
and
a.sub.x,Sensor=-.omega..sup.2(cos .beta.).sup.2r.sub.s-r.sub.s{dot
over (.beta.)}.sup.2+g(sin .beta.sin .LAMBDA.-cos .beta.cos
.OMEGA.cos .LAMBDA.)+a.sub.x(sin .beta.cos .LAMBDA.+cos .beta.sin
.LAMBDA.cos .OMEGA.)-a.sub.ycos .beta.sin .OMEGA..
[0057] The first term (that is to say the first product) of the two
equations is constant in this case with reference to the angle
.beta.. The second term (that is to say the second product) is
negligible when the acceleration sensor signal is subjected to
lowpass filtering. The last term (that is to say the last product)
is periodic with the angle .OMEGA..
[0058] By way of example, a constant component of
.omega..sup.2r.sub.s=58 m/s.sup.2 can be obtained given an angular
velocity of .omega.=1.7 rad/s (which corresponds to a system
rotational speed of 15 rpm) and a distance of the sensor r.sub.s=20
m. The rotational frequency component is g=9.81 m/s.sup.2 in this
case. Consequently, the acceleration in the z-direction fluctuates,
by way of example, from 68 m/s.sup.2 to 48 m/s.sup.2 within a
revolution of the rotor blade above the rotor axis. All variables
except .beta. are now known in the filtered equation for
a.sub.z,sensor (that is to say the second term is filtered out).
The equation can therefore be solved numerically for the desired
torsion angle .beta..
.omega. 2 r s cos 2 .beta. .beta. = arc cos a z .omega. 2 r s
##EQU00009##
[0059] The above-specified equation for a.sub.x,sensor specifies
how the acceleration measured in the x-direction is composed of the
known and unknown variables. If this acceleration is additionally
measured, the accuracy of the determination of .beta. can be
increased. In particular, the use of a Kalman filter can lead to
better results. In this case, a model of the rotor blade is
simulated in the Kalman filter and the deflection is determined
therefrom. The simulation is updated and/or corrected (for example,
by means of a predictor, corrector method) in each time step with
the aid of the two measurements (a.sub.x, a.sub.z). The blade
deflection can be determined directly from the blade inclination
with the aid of a model for the blade bending (that is to say the
bending line). The blade root bending torque then also follows from
the model for the blade bending. The bending stiffness EI so also
requires to be known for this. Since the known IPC controllers use
only the differences in the blade root bending torques of the
blades for control purposes, there is no need for an absolutely
accurate value, and an approximate value suffices for the bending
stiffness EI. Such a previously mentioned measurement concept could
also be established simply by the presence of an acceleration
sensor in the rotor blade which is arranged for measuring the
acceleration in a z-direction.
[0060] This the application of the above-described equations, it is
then possible to use the acceleration signals to infer the bending
angle of the rotor blade, which is then further used to control the
incidence angle of the rotor blade. In particular, in this case the
application of the second method has the advantage that because of
a constant centrifugal force there is present over a rotor
revolution a constant useful signal from which the g-projection can
then be calculated or also used to determine .beta..
[0061] The projection component of the gravitational acceleration,
which is to be ascribed purely to the flapping bending of the rotor
blade, should be determined in order to be able to determine the
blade deflection. To this end, it is possible to filter out the
change in projection of the gravitational acceleration, which
results from the rigid body movement of the system. Two degrees of
freedom determine the rigid body movement: the rotation about the
rotor axis and the blade angle adjustment about the pitch axis. In
addition, the rotation of the azimuth bearing could also be
considered, but this is neglected in this consideration.
[0062] The projection component responsible for the blade
deflection is therefore yielded from:
{right arrow over (g)}.sub.Bending={right arrow over
(g)}.sub.Mess-{right arrow over (g)}.sub.RBFilter (4)
g.sub.RBFilter corresponding to the projection vector of the
gravitational acceleration, which is calculated on the basis of the
rigid body movement. g.sub.Mess is the gravitational acceleration
component measured by the sensor. g.sub.Bending is the
appropriately filtered signal, which is to be ascribed purely to
the elastic deformation of the rotor blade (that is to say
corresponds to the bending in a flapping direction). Equation (4)
can be used to filter out the projection component of the
gravitational vector, which is not to be ascribed to the
deflection. The calculation of the projection of the gravitational
acceleration, which results from the rigid body movement, may be
gathered from the following equation (5).
{right arrow over
(g)}.sub.RBFilter=T.sub.Blade.sub.--.sub.HubT.sub.z(.beta.)T.sub.Hub.sub.-
--.sub.RotorT.sub.x(.alpha.){right arrow over (g)}.sub.RotorCOS
(5)
where [0063] T.sub.Blade.sub.--.sub.Hub corresponds to a
transformation matrix for a transformation into the blade segment
COS, [0064] T.sub.z(.beta.) represents a rotation by .beta. (that
is to say a pitch angle of the rotor blade) referred to the z-axis
of the blade bearing COS, [0065] T.sub.Hub.sub.--.sub.Rotor
corresponds to a transformation matrix for a transformation into
the blade bearing COS, and [0066] T.sub.x(.alpha.) corresponds to a
rotation by .alpha. (that is to say an azimuth angle of the rotor)
referred to the x-axis of the rotor COS.
[0067] In this case, {right arrow over (g)}.sub.RotorCOS denotes
the gravitational vector expressed in the inertial rotor axis
coordinate system. The rotor axis is upwardly inclined by
approximately 5.degree. by the so-called shaft angle.
[0068] The measurement principle in accordance with the first
method is illustrated in the two partial figures of FIG. 6. In this
case, the left-hand partial figure illustrates a measurement
principle and an associated measurement signal, in the case of
which the wind turbine system and/or the rotor blades are
deflected. By contrast, the right-hand partial figure from FIG. 7
illustrates a measurement principle and an associated measurement
signal in the case of application of this measurement principle, it
being possible to infer the elastic deformation of the rotor blade
from the variation in the amplitude.
[0069] The diagrams of FIGS. 7 and 8 represent the sensor signal
(dashed line 700) at the blade tip (that is to say at a distance of
r=36 m from the blade root) and the sensor signal subjected to
lowpass filtering (continuous line 710) plotted against time,
whereas FIG. 8 represents the profile of a sensor signal subjected
to lowpass filtering. The reference signal corresponds in each case
to the projection of the gravitational acceleration into the sensor
coordinate system. It is to be seen that the lowpass filtering
enables determination of the amplitude of the projection of the
gravitational vector. The amplitude of the projection is relevant
for the evaluation of the blade deflection.
[0070] Furthermore, there is a correlation between the amplitude of
the blade deflection and the filtered sensor signal. Here, it is
necessary to take account of the change in the g-projection, which
may be ascribed to the deflection of the rotor blade. This means
that a relatively large amplitude signal is also to be expected
given a relatively large distance of the sensor from the blade
root. This information can also be obtained from the amplitudes of
the variable {right arrow over (g)}.sub.Bending. Consequently, the
blade root (flapping) bending torques can be determined directly
with the aid of the blade acceleration sensors in the course of an
appropriate calibration. It may be shown that a better evaluation
is possible on the basis of the larger blade bending given a
distance of r=20 meters of the acceleration sensor relative to the
rotor hub. There is an analogous result given a distance of r=36
meters of the acceleration sensor relative to the rotor hub, the
simulation results not being illustrated here.
[0071] However, there is still a problem in that the useful signal,
that is to say the change in the inclination of the gravitational
vector on the basis of the blade bending, is relatively small in
relation to the interference signal the more closely the sensor is
applied on the blade root. It is correspondingly more advantageous
to measure further out on the blade, for example at a position of
r=36 meters from the blade hub, because of the larger deflection by
comparison with the measurement points located relatively close to
the blade bearing at, for example, r=10 and r=20 meters.
Furthermore, the measurement of the rotor rotational speed and of
the pitch angle are important for the application of the
measurement concept presented here. However, this is currently
prior art for wind energy systems, and is used for conventional
control methods.
[0072] Furthermore, the approach presented here enables an already
well matured range of acceleration sensors (for example MM3, DCU)
of the applicant to be used for the sensor signal evaluation
described here, and said approach is capable of future use within a
larger scope in the field of the control of wind energy
systems.
[0073] In accordance with a further exemplary embodiment, the
present invention comprises a method 900 for determining an
incidence angle of a rotor blade of a wind turbine system as
illustrated in the form of a flowchart in FIG. 9. The method 900
has a step of reading in 910 an acceleration signal that represents
an acceleration of the rotor blade acting substantially
perpendicular to a rotor plane of the wind turbine system.
Furthermore, the method 900 comprises a step of determining 920 the
incidence angle of the rotor blade of the wind turbine system by
using the acceleration signal.
LIST OF REFERENCES
[0074] 100 Tower longitudinal bending
[0075] 110 Tower transverse bending
[0076] 120 Tower torsion
[0077] 130 Yawing
[0078] 140 Rolling
[0079] 150 Pitching
[0080] 160 Pivoting movement
[0081] 170 Flapping movement
[0082] 180 Torsion
[0083] 200 System for controlling the wind turbine system
[0084] 210 Wind turbine system
[0085] 220 Operational control unit
[0086] 230 Control unit for the individual incidence angle
[0087] 235 Individual incidence angles (.beta..sub.IPC1,2,3)
[0088] 240 Generator torque
[0089] 242 Common incidence angle (.beta..sub.CPC)
[0090] 250 Control signal
[0091] 255 Actuator
[0092] 260 Local wind conditions
[0093] 265 Sensor signals
[0094] 300 Rotor blade
[0095] 310 Rotor axis
[0096] 320 Rotor plane
[0097] 500 Coordinate system in the rotor blade
[0098] 510 Coordinate system in the rotor hub
[0099] 520 Coordinate system in the rotor shaft
[0100] 700 Reference signal
[0101] 710 Filtered sensor signal
[0102] 900 Method for determining a bending angle of a rotor
blade
[0103] 910 Reading in an acceleration signal
[0104] 920 Determining the bending angle of the rotor blade of the
wind turbine system
* * * * *