DK201770590A1 - Pitch alignment error detection - Google Patents

Abstract

A method for determining an offset error in a pitch angle sensing system of a wind turbine is disclosed. A controller of the wind turbine measures a blade load while sweeping a pitch of a wind turbine blade through a range of pitch angles. The controller generates an estimated blade load based on pitchmeasurements outputted by the pitch angle sensing system when sweeping the pitch of the wind turbine blade. The controller identifies an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.

Description

(¹⁹) DANMARK (10)

DK 2017 70590 A1

(12)

PATENTANSØGNING

Patent- og Varemærkestyrelsen (51) Int.CI.: F03D 17/00 (2016.01) F03D 7/00 (2006.01) (21) Ansøgningsnummer: PA 2017 70590 (22) Indleveringsdato: 2017-07-24 (24) Løbedag: 2017-07-24 (41) Aim. tilgængelig: 2018-07-12 (45) Publiceringsdato: 2018-07-17 (71) Ansøger:

VESTAS WIND SYSTEMS A/S, Hedeager 42, 8200 Århus N, Danmark (72) Opfinder:

Dan Hilton, Tingbakken 31,8883 Gjern, Danmark

Kristian Kiib, Løgten Østervej 141,8541 Skødstrup, Danmark (74) Fuldmægtig:

Vestas Wind Systems A/S IPR Department, Hedeager 42, 8200 Århus N, Danmark (54) Titel: PITCH ALIGNMENT ERROR DETECTION (56) Fremdragne publikationer:

WO 2009/059606 A2

EP 2910776 A1

EP 2693049 A2 (57) Sammendrag:

A method for determining an offset error in a pitch angle sensing system of a wind turbine is disclosed. A controller of the wind turbine measures a blade load while sweeping a pitch of a wind turbine blade through a range of pitch angles. The controller generates an estimated blade load based on pitchmeasurements outputted by the pitch angle sensing system when sweeping the pitch ofthe wind turbine blade. The controller identifies an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.

Fortsættes...

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PITCH ALIGNMENT ERROR DETECTION

BACKGROUND

Field of the Invention

The invention relates to a pitch angle sensing system in a wind turbine and, more specifically, to an approach for verifying the accuracy of a pitch angle sensing system to improve reliable operation of the wind turbine.

Description of the Related Art

Wind turbines are designed to operate reliably and safely under a wide range of wind conditions. To do so, the sensing systems and control systems need to function in the correct way and be accurately installed and configured. An error in the installation or configuration of one system may cause an effect in subsequent, dependent systems within the wind turbine.

For example, there is a need to control a pitch angle of the rotor blades of the wind turbine which relates to an angle-of-attack of the wind turbine blades. An accurate pitch angle measurement is needed to ensure proper turbine performance since the accuracy of the pitch angle measurement directly affects the ability of a controller of the wind turbine to actively control the angle of attack of the blades.

In some instances, the true angle-of-attack differs from what is measured by the wind turbine control system. The true angle-of-attack being greater than that measured by the wind turbine control system can lead to higher loads, uneven loads on the hub and main shaft, and potential damage to the blades. The true angle-of-attack being less than what is measured by the control system can lead to lost production and uneven loads on the hub and main shaft of the wind turbine.

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Embodiments presented herein illustrate how to mitigate or overcome at least some of the above-mentioned problems.

SUMMARY

According to an aspect of the present invention there is provided a method for determining an offset error in a pitch angle sensing system of a wind turbine. A controller of the wind turbine measures a blade load while sweeping a pitch of a wind turbine blade through a range of pitch angles. The controller generates an estimated blade load based on pitch measurements outputted by the pitch angle sensing system when sweeping the pitch of the wind turbine blade. The controller identifies an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.

According to another aspect of the invention, there is provided a wind turbine comprising: a wind turbine blade mounted to a hub, a blade load sensor corresponding to the wind turbine blade, a pitch angle sensing system configured to output a pitch; and a processor. The processor is configured to measure a blade load using the blade load sensor. The processor is further configured to generate an estimated blade load based on a pitch angle measurement outputted by the pitch angle sensing system when measuring the blade load. The processor is further configured to identify an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.

According to another aspect of the invention, there is provided a non-transitory computer readable storage medium including computer-readable program code that, when executed by a computer processor, perform an operation. The operation may include measuring a blade load using a blade load sensor of a wind turbine blade while causing the wind turbine blade to sweep a pitch through a range of pitch angles. The operation may further include generating 2

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2017E00055DK an estimated blade load based on pitch measurements outputted by a pitch angle sensing system of a wind turbine when sweeping the pitch of the wind turbine blade. The operation may further include identifying an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

Figure 1 illustrates a diagrammatic view of a wind turbine, according to an embodiment described in this present disclosure.

Figure 2 illustrates a diagrammatic view of the components internal to the nacelle and tower of a wind turbine, according to an embodiment described in this present disclosure.

Figure 3 illustrates Tip-Chord (TC) marks located on the root of a blade of the wind turbine of Figure 1.

Figure 4 illustrates a controller for operating a wind turbine according to an embodiment described in the present disclosure.

Figure 5 is a flow chart of a method for determining an offset error in a pitch angle sensing system of a wind turbine generator according to an embodiment described in the present disclosure.

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Figure 6 is a flow chart of a method for determining an offset error in a pitch angle sensing system of a wind turbine generator according to another embodiment described in the present disclosure.

Figure 7 is a flow chart of a method for troubleshooting pitch errors of a wind turbine blade of a wind turbine generator, according to an embodiment described in this present disclosure.

Figure 8 presents a simulated phase offset between blade loads and expected loads as a function of pitch angle offset errors.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

DESCRIPTION OF EXAMPLE EMBODIMENTS

A wind turbine uses one or more rotors comprising multiple blades to convert kinetic energy of the wind into electrical energy. An error in the installation or configuration of the wind turbine, e.g., by having incorrectly calibrated pitch angles relative to load on a wind turbine blade, can lead to potential damage to the wind turbine blade.

In one embodiment, a controller of a wind turbine may execute during commissioning or troubleshooting of the wind turbine to identify an incorrectly calibrated pitch angle system. In operation, a controller of the wind turbine generator rotates a blade until it is parallel to the ground and sweeps the pitch angle of the blade from a maximum angle to a minimum angle. During the pitch sweep, the controller measures blade loads using a blade load sensor and further determines estimated blade loads. The controller then calculates a pitch angle offset between the measured blade loads and the estimated blade loads. A non-zero pitch angle offset indicates a pitch measurement error or an

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2017E00055DK incorrect blade load sensor position. This procedure can be repeated for each of the blades.

The set of offset errors in pitch angle can be employed during troubleshooting or commissioning of the wind turbine system to aid an operator to correctly correlate measured pitch angles to actual pitch angles of the blades. This procedure may be employed automatically or manually when commissioning new wind turbine generators or whenever a pitch alignment error is suspected. Field technicians and support engineers may invoke this procedure as well as avoid having to make manual measurements/inspections until necessary. For example, the support engineer may access a control system of a wind turbine behaving abnormally and perform the procedure described above to check the pitch alignment remotely. This could result in a correction of the pitch calibration without the support engineer having to visit the turbine. This could also enable better fault diagnosis prior to visiting the turbine and thus reduce the downtime and service cost.

EXAMPLE EMBODIMENTS

Fig. 1 illustrates a diagrammatic view of a horizontal-axis wind turbine generator 100. The wind turbine generator 100 typically comprises a tower 102 and a wind turbine nacelle 104 located at the top of the tower 102. A wind turbine rotor 106 may be connected with the nacelle 104 through a low speed shaft extending out of the nacelle 104. The wind turbine rotor(s) 106 comprises three rotor blades 108 mounted on a common hub 110 which rotate in a rotor plane, but may comprise any suitable number of blades, such as one, two, four, five, or more blades. The blades 108 (or airfoil) typically have an aerodynamic shape with a leading edge 112 for facing into the wind, a trailing edge 114 at the opposite end of a chord for the blades 108, a tip 116, and a root 118 for attaching to the hub 110 in any suitable manner.

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For some embodiments, the blades 108 may be connected to the hub 110 using pitch bearings 120 such that each blade 108 may be rotated around its longitudinal axis to adjust the blade’s pitch. The pitch angle of a blade 108 relative to the rotor plane may be controlled by linear actuators, hydraulic actuators, or stepper motors, for example, connected between the hub 110 and the blades 108.

Fig. 2 illustrates a diagrammatic view of typical components internal to the nacelle 104 and tower 102 of a wind turbine generator 100. When the wind 200 pushes on the blades 108, the rotor 106 spins and rotates a low-speed shaft 202. Gears in a gearbox 204 mechanically convert the low rotational speed of the low-speed shaft 202 into a relatively high rotational speed of a high-speed shaft 208 suitable for generating electricity using a generator 206.

A controller 210 may sense the rotational speed of one or both of the shafts 202, 208. If the controller decides that the shaft(s) are rotating too fast, the controller may signal a braking system 212 to slow the rotation of the shafts, which slows the rotation of the rotor 106 - i.e., reduces the revolutions per minute (RPM). The braking system 212 may prevent damage to the components of the wind turbine generator 100. The controller 210 may also receive inputs from an anemometer 214 (providing wind speed) and/or a wind vane 216 (providing wind direction). Based on information received, the controller 210 may send a control signal to one or more of the blades 108 in an effort to adjust the pitch 218 of the blades. By adjusting the pitch 218 of the blades with respect to the wind direction, the rotational speed of the rotor (and therefore, the shafts 202, 208) may be increased or decreased. Based on the wind direction, for example, the controller 210 may send a control signal to an assembly comprising a yaw motor 220 and a yaw drive 222 to rotate the nacelle 104 with respect to the tower 102, such that the rotor 106 may be positioned to face more (or, in certain circumstances, less) upwind.

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Figure 3 shows TC marks 308 located on the root 118 of a blade 108 in the vicinity of fasteners 306 for coupling the root 118 of the blade 108 to the rotor hub 110. TC marks 308 are marks on the root 118 of the blade 108 that indicate a reference axis 304 of the blade 108 parallel to the chord at the tip of the blade 108. This location is also the zero point for measuring the pitch angle 218 of the blade 108. A blade load sensor 302 is positioned 90 degrees from the reference axis 310 since the blade load sensor 302 is a flap load sensor.

Accurate measurement of the pitch (hereinafter “pitch angle) 218 of the blades 108 relies on proper alignment/installation of the blades 108 relative to the hub 110 as well as proper alignment and calibration of blade load sensors 302. The blades 108 and the blade load sensors 302 are aligned according to the TC marks 308 which act as the reference. Root causes for poor pitch angle measurements are alleviated by employing embodiments of the present disclosure. These root causes can include:

• The TC mark 308 may be incorrectly indicated during blade manufacturing.

• The blade 108 can be incorrectly mounted I installed on the hub 110.

• The calibration of the pitch angle sensor is incorrectly performed.

• Installation of blade load sensor 302 can be incorrectly mounted I installed in the blade.

Figure 4 illustrates a controller 210 for operating a wind turbine according to an embodiment described in the present disclosure. The controller 210 includes a processor 404 and memory 406. The processor 404 can represent one or more processing elements which each can include one or more processing cores. The memory 406 can include volatile memory elements, nonvolatile memory elements, and combinations thereof.

The memory 406 includes a blade load estimator 409, a phase angle estimator 410, and a pitch controller 416. In one embodiment, the blade load estimator

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409, the offset error estimator 410, and pitch controller 416 are software components whose functions are executed using the processor 404. However, in other embodiments, the blade load estimator 409, the offset error estimator

410, and the pitch controller 416 may be formed entirely from hardware elements or from a combination of hardware and software elements.

Embodiments of the present disclosure provide a turbine test function (TTF), executed by the processor 404 of the controller 210 that may execute during commissioning or troubleshooting of the wind turbine. In operation, the wind turbine generator 100 locks or stops the rotor 106 with one blade 108 oriented horizontally. The pitch angle controller 416 causes the horizontal blade 108 to sweep through a pitch angle 218 from a maximum angle (e.g., 90 degrees) to a minimum angle (e.g., zero degrees). During the pitch sweep, the controller 210 collects measured blade loads 408 from the blade load sensor 302. In one embodiment, the measured blade loads 408 are collected in parallel with the blade load estimator 409 determining estimated blade loads 412, which are recorded to memory 406 along with the measured blade loads 408. The blade load estimator 409 may determine the estimated blade loads 412 at least in part from pitch angle measurements 418 generated by the pitch angle controller

416. The estimated blade loads 412 may further be determined from turbine parameters and from azimuth angle measurements of the rotor 106. The offset error estimator 410 then calculates pitch angle offsets 414 between the measured blade loads 408 and the estimated blade loads 412. Non-zero pitch angle offsets 414 indicate a pitch measurement error or an incorrect blade load sensor position. This procedure is repeated for each of the blades 108.

A trouble-shooter may desire to obtain a set of offset values over sweep of pitch angles. To this effect, the method of Figure 5 may be executed by the turbine test function (TTF).

Figure 5 is a flow chart of a method 500 for determining an offset error in a pitch angle sensing system of a wind turbine generator 100 according to an 8

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2017E00055DK embodiment described in this present disclosure. The wind turbine generator 100 includes a plurality of wind turbine blades 108 mounted to a hub 110. The wind turbine generator 100 further includes a blade load sensor 302 corresponding to the wind turbine blade 108. The wind turbine generator 100 further includes a pitch angle controller 416 configured to output a pitch angle measurement 418. An offset error estimator 410 estimates blade loads 412 and when compared to measured blade loads 408 stored in memory 406, determines pitch angle offsets 414 or phase errors in the estimated blade load 412.

In some embodiments, this procedure may be performed while the wind turbine generator 100 is idling. In the context of wind turbine generators 100, idling refers to the wind turbine generator 100 not producing power but the rotor 106 permits the wind turbine blades 108 to rotate freely. In another embodiment, one or more wind turbine blades 108 not under test may be feathered, i.e., pitched at a maximum pitch angle (e.g. greatest angle of attack) to slow the rotor 106 or bring the rotor 106 to a complete stop.

At block 505, the controller 210 automatically rotates the rotor 106 until a blade 108 is parallel to the ground. At block 510, in one embodiment, the controller 210 locks the wind turbine rotor 106. Although the wind turbine rotor 106 may be locked, this is not a requirement.

At block 515, the controller 210 sweeps a pitch of a wind turbine blade 108 through a range of pitch angles 218. In one embodiment, the controller 210 moves the blade 108 to a maximum pitch position (e.g., the pitch limit) and moves the blade 108 either continuously or at discrete intervals to a minimum pitch position. In one embodiment, the range of pitch angles is approximately 90 degrees.

During the sweep, at block 520, the controller 210 measures a blade load using a blade load sensor 302. In an embodiment, the blade load sensor 302 may be 9

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2017E00055DK a strain gauge. By means of calculations and calibration, a measured blade strain is converted into a measured blade load. For example, the blade load sensor 302 measures the blade load at a predefined pitch change - e.g., each time the pitch changes by 0.5 degrees - or at a predefined time period - e.g., every 100 milliseconds. In this manner, the controller 210 can measure a plurality of blade load values while pitching the blade 108. In one embodiment, the controller 210 uses the plurality of blade load values to a generate a blade load curve which maps the blade load to different pitch angles in the range of pitch angles used when sweeping the blade.

At block 525, the controller 210 generates an estimated blade load based on pitch measurements outputted by the pitch angle sensing system 210 when sweeping the pitch of the wind turbine blade 108. As mentioned above, the pitch measurements may be off from the actual pitch angle (or angle of attack) of the blade 108. For example, the pitch angle sensing system 210 may be calibrated assuming that the TC mark is properly aligned with the rotor (and that the TC mark is the actual zero position of the blade 108). However, as discussed above, the true zero position of the blade 108 may be misaligned resulting in the pitch angle sensing system 210 to output inaccurate pitch measurements.

In one embodiment, the estimated blade load is further generated from a measured azimuth angle of the wind turbine blade 108. In another embodiment, the estimated blade load may be indicative of a blade self-weight moment at an associated pitch angle and azimuth angle. In another embodiment, the estimated blade load may also be derived from the azimuth angle, aerodynamic forces estimated from wind speed, and the pitch angle.

At block 530, the offset error estimator 410 identifies an offset error in the pitch angle sensing system based on a phase difference between the measured blade load 408 and the estimated blade load 412. In an embodiment, the offset

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2017E00055DK error estimator 410 performs the identification in response to commissioning or troubleshooting the wind turbine generator 100. In another embodiment, the offset error estimator 410 may determine the offset error for each point of pitch angle after having fitted the estimated blade load 412 values to a first curve (e.g., using the least squares method) and having fitted the measured blade load 408 to a second curve. The resulting set of phase offsets determined by comparing the first curve to the second curve can be used to calibrate an overall pitch angle of the wind turbine blade 108.

At block 535, if the pitch angle sensing system 210 has not yet performed measurements for all of the blades 108 of the wind turbine generator 100, then at block 540, the controller 210 unlocks the wind turbine rotor 106 and execution returns to block 505 for remaining blades 108. That is, the controller 210 may permit the rotor 106 to rotate until a different blade 108 is horizontal with the ground and then repeat the method shown in Figure 5. If, at block 535, the pitch offset of the last of the blades 106 of the wind turbine generator 100 has been determined, then the procedure terminates.

In an embodiment, the sweep of measured pitch angles may range from a maximum pitch angle (e.g., 90 degrees) of the wind turbine blade 108 to a minimum pitch angle (e.g., 0 degrees) of the wind turbine blade 108. In another embodiment, the wind turbine generator 100 may be operated in an idling mode before performing any measurements. In an embodiment, the method 500 may be performed when an azimuthal angle of the wind turbine blade 106 is parallel with the ground. One advantage of performing the method 500 when the blade is parallel with the ground is that the signal for the blade load sensor may be at a maximum when sweeping the pitch in contrast to when the blade is perpendicular to the ground when the blade load sensor signal is at a minimum.

A trouble-shooter may desire to obtain one pitch angle to obtain one offset value for calibration instead of going through a sweep of pitch angles to

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2017E00055DK measure a plurality of blade loads to identify the pitch angle offset. To this effect, the method of Figure 6 may be performed by the controller 210.

Figure 6 is a flow chart of a method 600 for determining an offset error in a pitch angle sensing system of a wind turbine generator 100 according to another embodiment described in this present disclosure. At block 605, the controller 210 measures a blade load using a blade load sensor 302. At block 610, the controller 210 generates an estimated blade load based on a pitch angle measurement when measuring the blade load. At block 615, the controller 210 identifies an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.

Because the accuracy of the pitch angle sensing system 210 is important to the proper operation of and reliability of the wind turbine generator, it may be desired to have service personnel inspect the wind turbine generator. Unfortunately, if the root cause of pitch errors is because the blade is installed incorrectly, then there is a large risk that relying purely on software may not be enough to correct for mechanical errors. For example, the blade is bolted on to the hub via evenly distributed bolts. This makes it possible to install the blade one or more bolt holes wrong, resulting in a pitch offset. The root cause of this may be bad installation but it can also be that the TC mark is in the wrong place so that following correct installation procedure still yields a pitch offset on the blade. If the offset is significant enough, it may not be possible for the pitch system to obtain some pitch angles within the required range and thus the blade needs to be removed and reinstalled in the correct position/angle.

Accordingly, there may be no need for employing the service personnel to inspect the wind turbine blade assembly for small deviations in calibrated pitch angle from actual pitch, but not for large pitch angle offset values. In such circumstances, the method 700 of Figure 7 may be employed as described below.

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Figure 7 is a flow chart of a method 700 for troubleshooting pitch errors of a wind turbine blade 108 of a wind turbine generator 100, according to an embodiment described in this present disclosure. At block 705, the controller 210 of the wind turbine generator 100 detects an error associated with a pitch angle system. In one embodiment, detecting an error may be in response to a trigger event indicating there is a problem with the wind turbine. For example, the trigger event can be a discrepancy between the estimated and measured blade load during energy production that is not present or is significantly smaller in idle mode. In another example, blade loads may be too large during energy production.

At block 710, the controller 210 may perform the method 500 or 600 to determine one or more pitch angle offset errors for one or more pitch angles/measured loads. At block 715, if the determined pitch angle offset error exceeds a threshold value (e.g., more than 5 degrees), then at block 720, the controller 210 sends a message to a user interface indicating that personnel ought to be dispatched to the wind turbine generator 100 to correct for the pitch angle error. If, at block 715, the determined pitch angle offset error is below a threshold value, then at block 725, the controller 210 automatically corrects for the pitch angle offset error.

The personnel may be notified to inspect the wind turbine generator 100 if value(s) of the offset are above a specified threshold value as detected using the method of either Figure 5 or 6. The personnel may inspect the turbines, and may determine that the error is in the TC mark location(s) - e.g., a minor alignment error. If the error is determined to be minor, then the wind turbine generator 100 may be turned on line and the personnel may employ the controller to compensate for the alignment as shown in block 725. However, after inspection the personnel may conclude that the offsets are due to a major problem in the wind turbine generator hardware, e.g., the blade load sensor.

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In such circumstances, the wind turbine generator may pitch incorrectly even if the controller compensates for the misalignment since the blade load sensor has malfunctioned which can make the problem worse. In such circumstances, the personnel may fix the hardware or may de-commission the wind turbine generator 100 altogether.

Figure 8 presents a simulated phase offset between blade loads and expected loads as a function of pitch angle offset errors.

In the preceding, reference is made to embodiments presented in this disclosure. However, the scope of the present disclosure is not limited to specific described embodiments. Instead, any combination of the features and elements provided above, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the aspects, features, embodiments and advantages described herein are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s).

As will be appreciated by one skilled in the art, the embodiments disclosed herein may be embodied as a system, method or computer program product. Accordingly, aspects may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, microcode, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon.

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The present invention may be a system, a method, and/or a computer program product. The computer program product may include a computerreadable storage medium (or media) (e.g., a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.

Aspects of the present disclosure are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments presented in this disclosure. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figs, illustrate the architecture, functionality and operation of possible implementations of systems, methods and computer program products according to various embodiments. In this regard, each block in the flowchart or block diagrams may represent a module, segment or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the

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2017E00055DK blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose 5 hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

In view of the foregoing, the scope of the present disclosure is determined by the claims that follow.

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Claims (15)

1. WHAT IS CLAIMED IS:

   1. A method for determining an offset error in a pitch angle sensing system of a wind turbine, comprising:

   measuring a blade load while sweeping a pitch of a wind turbine blade through a range of pitch angles;

   generating an estimated blade load based on pitch measurements outputted by the pitch angle sensing system when sweeping the pitch of the wind turbine blade; and identifying an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.
2. 2. The method of claim 1, wherein the plurality of measured pitch angles range from a maximum pitch angle of the wind turbine blade to a minimum pitch angle of the wind turbine blade.
3. 3. The method of claims 1 or 2, wherein said identifying is performed in response to commissioning or troubleshooting the wind turbine.
4. 4. The method as in any one of the preceding claims, wherein the estimated blade load is further generated from a measured azimuth angle of the wind turbine blade and aerodynamic forces derived from wind speed.
5. 5. The method as in any one of the preceding claims, further comprising repeating said measuring, said generating, and said identifying for remaining blades of a plurality of blades of the wind turbine.
6. 6. The method as in any one of the preceding claims, wherein the measured blade load is a measured strain.

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7. 7. The method as in any one of the preceding claims, wherein said measuring, said generating, and said identifying are performed with a fixed azimuth angle.
8. 8. The method as in any one of the preceding claims, further comprising operating the wind turbine in an idling mode before said measuring.
9. 9. The method as in any one of the preceding claims, wherein the estimated blade load signal is indicative of a blade self-weight moment at an associated pitch angle.
10. 10. The method as in any one of the preceding claims, further comprising performing said measuring, said generating, and said identifying when an azimuthal angle of the wind turbine blade is parallel with the ground.
11. 11. A wind turbine comprising:

    a wind turbine blade mounted to a hub;

    a blade load sensor corresponding to the wind turbine blade;

    a pitch angle sensing system configured to output a pitch; and a processor configured to:

    measure a blade load using the blade load sensor;

    generate an estimated blade load based on a pitch angle measurement outputted by the pitch angle sensing system when measuring the blade load; and identify an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.
12. 12. The wind turbine of claim 11, wherein the measured pitch angle ranges from a maximum pitch angle of the wind turbine blade to a minimum pitch angle of the wind turbine blade.

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13. 13. The wind turbine of claims 11 or 12, wherein said identifying is performed in response to commissioning or troubleshooting of the wind turbine.
14. 14. The wind turbine as in any one of the preceding claims, wherein the estimated blade load is further generated from a measured azimuth angle of the wind turbine blade.
15. 15. A non-transitory computer readable storage medium including computerreadable program code that, when executed by a computer processor, perform an operation, the operation comprising:

    measuring a blade load using a blade load sensor of a wind turbine blade while causing the wind turbine blade to sweep a pitch through a range of pitch angles;

    generate an estimated blade load based on pitch measurements outputted by a pitch angle sensing system of a wind turbine when sweeping the pitch of the wind turbine blade; and identify an offset error in the pitch angle sensing system based on a phase difference between the measured blade load and the estimated blade load.

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