Determining aerodynamic rotor imbalance in a wind turbine

Abstract

The present disclosure is related to methods (100, 200) for determining an aerodynamic imbalance in a wind turbine rotor (18). A method (100) comprises determining (110) a fore-aft oscillation of the wind turbine (10), determining (120) an azimuth angle of a blade (22) of the wind turbine (10), and based on the fore-aft oscillation and based on the azimuth angle, determining (130) a magnitude of the aerodynamic imbalance and determining a location of the aerodynamic imbalance. A controller (36) suitable for carrying out such methods (100, 200) and a wind turbine (10) comprising such a controller (36) are also provided.

Description

GENERAL ELECTRIC RE OVABLES ESPANA S.L. DECEMBER 21 , 2023GE 700597- WO- 1 P5293PC00DETERMINING AERODYNAMIC ROTOR IMBALANCE IN A WIND TURBINEFIELD

[0001] The present disclosure relates to methods for determining an aerodynamic imbalance in a wind turbine rotor, as well as for reducing, e.g. compensating, the aerodynamic imbalance. The present disclosure more particularly relates to methods, controllers and wind turbines for determining a magnitude and a location of an aerodynamic imbalance, particularly also for reducing the aerodynamic imbalance.BACKGROUND

[0002] Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft to a generator, either directly ("directly driven" or "gearless") or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid.

[0003] Modern wind turbines have increasingly larger rotor diameters to capture more energy throughout their lifetime and reduce the cost of energy. An imbalance in the rotor blades can have a significant impact on the loads , and particularly the fatigue loads suffered by the wind turbine. An imbalance can cause both a fore-aft oscillation of the wind turbine as a side- to-side oscillation of the wind turbine.

[0004] An imbalance in the rotor of the wind turbine can be caused by a variety of factors, including in particular a mass imbalance (i.e. not all the rotor blades have the same weight), and an aerodynamic imbalance i.e. not all the rotor blades are mounted in the same default pitch position with respect to the rotor hub. This means that during operation of the wind turbine, the angle of attack of the wind turbine blades can be slightly different. Blade erosion or dirt accumulation may also lead to aerodynamic imbalance in the wind turbine rotor.

[0005] Thus, imbalances in the rotor can lead to uneven loading and vibrations, and may also increase wear and tear on turbine components and may reduce energy output.Furthermore, wind turbines subjected to rotor imbalance may require maintenance more frequently.

[0006] An imbalance in the rotor can be detected by measuring the loads on the blades. E.g. the use of strain gauges in the blades and on the hub are known to determine e.g. flapwise and edgewise loads on the blades. Loads may also be measured on the rotor shaft, but this is limited to wind turbines including a rotor shaft. For example, at least some direct-drive wind turbines may lack a rotor shaft. And some wind turbines may lack load sensors, e.g. on the rotor shaft. If present, load sensors are not generally reliable throughout the lifetime of the wind turbine.

[0007] Besides detecting rotor imbalance, the rotor imbalance should preferably be reduced or eliminated. The present disclosure aims at determining both a magnitude and a location of a rotor imbalance in a wind turbine.SUMMARY

[0008] In an aspect of the present disclosure, a method for determining an aerodynamic imbalance in a rotor of a wind turbine is provided. The method comprises determining a fore- aft oscillation of the wind turbine with rotor speed (1 P) frequency and determining an azimuth angle of the rotor of the wind turbine. The method further comprises determining an indication of a magnitude of an aerodynamic imbalance and a location of the aerodynamic imbalance based on the determined fore-aft oscillation and on the determined azimuth angle.

[0009] According to this aspect, a fore-aft oscillation with 1P frequency and an azimuth angle may be monitored and used for determining both a magnitude and a location, e.g. an angular location, of the aerodynamic imbalance. An aerodynamic imbalance in the rotor will cause a 1 P fore-aft oscillation due to a different aerodynamic thrust caused by the blade. The use of the azimuth angle enables determining the location (in the rotor plane) of the aerodynamic imbalance because a misaligned blade will provide the same or similar contribution to the aerodynamic thrust and imbalance throughout each rotation. By knowing the location of the aerodynamic imbalance, besides knowing its magnitude, measures can be taken to reduce the aerodynamic imbalance. Accordingly, the wind turbine may continue to operate with reduced loads.

[0010] In a further aspect, a controller for a wind turbine is provided. The controller is configured to carry out any of the methods provided herein. The controller may be configured to receive a signal of a fore-aft oscillation of the wind turbine, receive a signal of an azimuth angle of the rotor of the wind turbine and based on the signal of the fore-aft oscillation andbased on the signal of the azimuth angle, determine a magnitude of the aerodynamic imbalance and determine a location of the aerodynamic imbalance.

[0011] In a further aspect, a wind turbine comprising such a controller is provided.

[0012] In yet another aspect, a method for operating a wind turbine is provided. The method comprises determining a signal of a fore-aft acceleration of the wind turbine and determining a signal of an azimuth angle of a blade of the wind turbine. The method further comprises, based on the signal of the fore-aft acceleration and based on the signal of the azimuth angle, determining a magnitude of an aerodynamic imbalance and determining a location of the aerodynamic imbalance at 1 P frequency in a rotor plane of the wind turbine. The method further comprises, based on the determined magnitude and location of the aerodynamic imbalance, operating the wind turbine for reducing the magnitude of the aerodynamic imbalance. According to this aspect, the wind turbine may be operated with reduced loads by avoiding aerodynamic imbalance and particularly without affecting the annual energy production of the wind turbine.

[0013] Determination of the fore-aft oscillation may be based on any “movement signal”. Throughout this disclosure, the term “movement signal” should be understood as a signal that is at least partially representative of a movement, in this case in particular of the fore-aft oscillation. Thus, a movement signal may be, for example, a signal indicating a movement or deformation, a position or a signal comprising acceleration values.

[0014] Additional objects, advantages and features of embodiments of the present disclosure will become apparent to those skilled in the art upon examination of the description, or may be learned by practice.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 schematically illustrates a perspective view of one example of a wind turbine;

[0016] Figure 2 illustrates an example of a hub and a nacelle of a wind turbine;

[0017] Figure 3 shows a flowchart of a method for determining an aerodynamic imbalance in a wind turbine;

[0018] Figure 4 shows a schematic representation of an example of a plane of a wind turbine rotor and a vector of imbalance;

[0019] Figure 5 shows a schematic representation of an example of the method in figure

[0020] Figure 6 shows a flowchart of a method for operating a wind turbine.DETAILED DESCRIPTION OF EXAMPLES

[0021] Reference will now be made in detail to embodiments of the disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation, not as a limitation. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the teaching. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0022] Figure 1 is a perspective view of an example of a wind turbine 10. In the example, the wind turbine 10 is a horizontal-axis wind turbine. Alternatively, the wind turbine 10 may be a vertical-axis wind turbine. In the example, the wind turbine 10 includes a tower 15 that extends from a support system 14 on a ground 12, a nacelle 16 mounted on tower 15, and a rotor 18 that is coupled to nacelle 16. The rotor 18 includes a rotatable hub 20 and at least one rotor blade 22 coupled to and extending outward from the hub 20. In the example, the rotor 18 has three rotor blades 22. In an alternative embodiment, the rotor 18 includes more or less than three rotor blades 22. The tower 15 may be fabricated from tubular steel to define a cavity (not shown in figure 1) between a support system 14 and the nacelle 16. In an alternative embodiment, the tower 15 is any suitable type of a tower having any suitable height. According to an alternative, the tower can be a hybrid tower comprising a portion made of concrete and a tubular steel portion. Also, the tower can be a partial or full lattice tower.

[0023] The rotor blades 22 are spaced about the hub 20 to facilitate rotating the rotor 18 to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. The rotor blades 22 are mated to the hub 20 by coupling a blade root portion 24 to the hub 20 at a plurality of load transfer regions 26. The load transfer regions 26 may have a hub load transfer region and a blade load transfer region (both not shown in figure 1). Loads induced to the rotor blades 22 are transferred to the hub 20 via the load transfer regions 26.

[0024] In examples, the rotor blades 22 may have a length ranging from about 15 meters (m) to about 90 m or more. Rotor blades 22 may have any suitable length that enables the wind turbine 10 to function as described herein. For example, non-limiting examples of blade lengths include 20 m or less, 37 m, 48.7 m, 50.2m, 52.2 m or a length that is greater than 91m, e.g. about 120 m. As wind strikes the rotor blades 22 from a wind direction 28, the rotor 18 is rotated about a rotor axis 30. As the rotor blades 22 are rotated and subjected to centrifugal forces, the rotor blades 22 are also subjected to various forces and moments. As such, the rotor blades 22 may deflect and / or rotate from a neutral, or non-deflected, position to a deflected position.

[0025] Moreover, a pitch angle of the rotor blades 22, i.e., an angle that determines an orientation of the rotor blades 22 with respect to the wind direction, may be changed by a pitch system 32 to control the load and power generated by the wind turbine 10 by adjusting an angular position of at least one rotor blade 22 relative to wind vectors. Pitch axes 34 of rotor blades 22 are shown. During operation of the wind turbine 10, the pitch system 32 may particularly change a pitch angle of the rotor blades 22 such that the angle of attack of (portions of) the rotor blades are reduced, which facilitates reducing a rotational speed and / or facilitates a stall of the rotor 18.

[0026] In the example, a blade pitch of each rotor blade 22 is controlled individually by a wind turbine controller 36 or by a pitch control system 80. Alternatively, the blade pitch for all rotor blades 22 may be controlled simultaneously by said control systems.

[0027] Further, in the example, as the wind direction 28 changes, a yaw direction of the nacelle 16 may be rotated about a yaw axis 38 to position the rotor blades 22 with respect to wind direction 28.

[0028] In the example, the wind turbine controller 36 is shown as being centralized within the nacelle 16, however, the wind turbine controller 36 may be a distributed system throughout the wind turbine 10, on the support system 14, within a wind farm, and / or at a remote-control center. The wind turbine controller 36 includes a processor 40 configured to perform the methods and / or steps described herein. Further, many of the other components described herein include a processor.

[0029] As used herein, the term “processor” is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific, integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. It should be understood that a processor and / or a control system can also include memory, input channels, and / or output channels.

[0030] Figure 2 is an enlarged sectional view of a portion of the wind turbine 10. In the example, the wind turbine 10 includes the nacelle 16 and the rotor 18 that is rotatably coupled to the nacelle 16. More specifically, the hub 20 of the rotor 18 is rotatably coupled to an electricgenerator 42 positioned within the nacelle 16 by the main shaft 44, a gearbox 46, a high-speed shaft 48, and a coupling 50. In the example, the main shaft 44 is disposed at least partially coaxial to a longitudinal axis (not shown) of the nacelle 16. A rotation of the main shaft 44 drives the gearbox 46 that subsequently drives the high-speed shaft 48 by translating the relatively slow rotational movement of the rotor 18 and of the main shaft 44 into a relatively fast rotational movement of the high-speed shaft 48. The latter is connected to the generator 42 for generating electrical energy with the help of a coupling 50. Furthermore, a transformer 90 and / or suitable electronics, switches, and / or inverters may be arranged in the nacelle 16 in order to transform electrical energy generated by the generator 42 having a voltage between 400V to 1000 V into electrical energy having medium voltage (10 - 35 KV). Said electrical energy is conducted via power cables from the nacelle 16 into the tower 15.

[0031] The gearbox 46, generator 42 and transformer 90 may be supported by a main support structure frame of the nacelle 16, optionally embodied as a main frame 52. The gearbox 46 may include a gearbox housing that is connected to the main frame 52 by one or more torque arms 103. In the example, the nacelle 16 also includes a main forward support bearing 60 and a main aft support bearing 62. Furthermore, the generator 42 can be mounted to the main frame 52 by decoupling support means 54, in particular in order to prevent vibrations of the generator 42 to be introduced into the main frame 52 and thereby causing a noise emission source.

[0032] Optionally, the main frame 52 is configured to carry the entire load caused by the weight of the rotor 18 and components of the nacelle 16 and by the wind and rotational loads, and furthermore, to introduce these loads into the tower 15 of the wind turbine 10. The rotor shaft 44, generator 42, gearbox 46, high speed shaft 48, coupling 50, and any associated fastening, support, and / or securing device including, but not limited to, support 52, and forward support bearing 60 and aft support bearing 62, are sometimes referred to as a drive train 64.

[0033] In some examples, the wind turbine may be a direct drive wind turbine without gearbox 46. Generator 42 operate at the same rotational speed as the rotor 18 in direct drive wind turbines. They therefore generally have a much larger diameter than generators used in wind turbines having a gearbox 46 for providing a similar amount of power than a wind turbine with a gearbox.

[0034] The nacelle 16 may also include a yaw drive mechanism 56 that may be used to rotate the nacelle 16 and thereby also the rotor 18 about the yaw axis 38 to control the perspective of the rotor blades 22 with respect to the wind direction 28.

[0035] For positioning the nacelle 16 appropriately with respect to the wind direction 28, the nacelle 16 may also include at least one meteorological measurement system 58 which may include a wind vane and anemometer. The meteorological measurement system 58 can provide information to the wind turbine controller 36 that may include wind direction 28 and / or wind speed. In the example, the pitch system 32 is at least partially arranged as a pitch assembly 66 in the hub 20. The pitch assembly 66 includes one or more pitch drive systems 68 and at least one sensor 70. Each pitch drive system 68 is coupled to a respective rotor blade 22 (shown in figure 1) for modulating the pitch angle of a rotor blade 22 along the pitch axis 34. Only one of three pitch drive systems 68 is shown in figure 2.

[0036] In the example, the pitch assembly 66 includes at least one pitch bearing 72 coupled to hub 20 and to a respective rotor blade 22 (shown in figure 1) for rotating the respective rotor blade 22 about the pitch axis 34. The pitch drive system 68 includes a pitch drive motor 74, a pitch drive gearbox 76, and a pitch drive pinion 78. The pitch drive motor 74 is coupled to the pitch drive gearbox 76 such that the pitch drive motor 74 imparts mechanical force to the pitch drive gearbox 76. The pitch drive gearbox 76 is coupled to the pitch drive pinion 78 such that the pitch drive pinion 78 is rotated by the pitch drive gearbox 76. The pitch bearing 72 is coupled to pitch drive pinion 78 such that the rotation of the pitch drive pinion 78 causes a rotation of the pitch bearing 72.

[0037] Pitch drive system 68 is coupled to the wind turbine controller 36 for adjusting the pitch angle of a rotor blade 22 upon receipt of one or more signals from the wind turbine controller 36. In the example, the pitch drive motor 74 is any suitable motor driven by electrical power and / or a hydraulic system that enables pitch assembly 66 to function as described herein. Alternatively, the pitch assembly 66 may include any suitable structure, configuration, arrangement, and / or components such as, but not limited to, hydraulic cylinders, springs, and / or servomechanisms. In certain embodiments, the pitch drive motor 74 is driven by energy extracted from a rotational inertia of hub 20 and / or a stored energy source (not shown) that supplies energy to components of the wind turbine 10.

[0038] The pitch assembly 66 may also include one or more pitch control systems 80 for controlling the pitch drive system 68 according to control signals from the wind turbine controller 36, in case of specific prioritized situations and / or during rotor 18 overspeed. In the example, the pitch assembly 66 includes at least one pitch control system 80 communicatively coupled to a respective pitch drive system 68 for controlling pitch drive system 68 independently from the wind turbine controller 36. In the example, the pitch control system 80 is coupled to the pitch drive system 68 and to a sensor 70. During normal operation of the windturbine 10, the wind turbine controller 36 may control the pitch drive system 68 to adjust a pitch angle of rotor blades 22.

[0039] According to an embodiment, a power generator 84, for example comprising a battery and electric capacitors, is arranged at or within the hub 20 and is coupled to the sensor 70, the pitch control system 80, and to the pitch drive system 68 to provide a source of power to these components. In the example, the power generator 84 provides a continuing source of power to the pitch assembly 66 during operation of the wind turbine 10. In an alternative embodiment, power generator 84 provides power to the pitch assembly 66 only during an electrical power loss event of the wind turbine 10. The electrical power loss event may include power grid loss or dip, malfunctioning of an electrical system of the wind turbine 10, and / or failure of the wind turbine controller 36. During the electrical power loss event, the power generator 84 operates to provide electrical power to the pitch assembly 66 such that pitch assembly 66 can operate during the electrical power loss event.

[0040] In the example, the pitch drive system 68, the sensor 70, the pitch control system 80, cables, and the power generator 84 are each positioned in a cavity 86 defined by an inner surface 88 of hub 20. In an alternative embodiment, said components are positioned with respect to an outer roof surface of hub 20 and may be coupled, directly or indirectly, to the outer roof surface.

[0041] According to an aspect of the disclosure, a method 100 for determining an aerodynamic imbalance in a rotor 18 of a wind turbine 10 is provided. The method is shown in the flow chart of figure 3. The method comprises, at block 110, determining a fore-aft oscillation of the wind turbine 10. The method further comprises, at block 120, determining an azimuth angle 0(t) of a blade 22 of the wind turbine 10. The method further comprises, at block 130, based on the signal of the fore-aft oscillation and based on the signal azimuth angle, determining a magnitude of the aerodynamic imbalance and determining a location of the aerodynamic imbalance.

[0042] Movement signals relating to the fore-aft oscillation may be obtained from a variety of sources. In particular, movement signals may be signals regarding accelerations. Such signals may be obtained from accelerometers installed in the wind turbine, e.g. in the rotor or the nacelle of the wind turbine or the top of the tower.

[0043] An aerodynamic imbalance may be represented as a vector in the plane of the rotor 18. Figure 4 shows a schematic representation of an example of a plane of a wind turbine rotor 18 and a vector of imbalance 101. Imbalance 101 therefore has a certain magnitude or amplitude and a certain direction. The signals of the fore-aft movement, and in particular of thefore-aft acceleration and the azimuth angle 0 may be used for determining the magnitude and the direction of the imbalance 101.

[0044] Determining a fore-aft acceleration may be performed directly, i.e. from direct measurements of fore-aft acceleration, or indirectly, i.e. from measurements of another parameter and subsequent calculation of the fore-aft acceleration based on the measurements. Determining may in some examples comprise measuring a fore-aft acceleration, e.g. with one or more accelerometers. The accelerometers may be installed in the wind turbine 10, for example in the rotor 18 or the nacelle 16 or the top of the tower 15.

[0045] Determining an azimuth angle may likewise be performed directly or indirectly. In some examples, one or more azimuth angle sensors may be used for determining the azimuth angle. For example, one or more encoders may be provided. The encoders may be arranged with a rotor shaft or e.g. with a generator rotor.

[0046] The azimuth angle may have a reference position at which this angle is 0°. For example, the reference position for the azimuth angle may be an imaginary line 102 between the hub (i.e. the center of the rotor 18) and a highest point reached by the tip of the blades 22 in the plane of the rotor 18, see figure 4. An azimuth angle for the rotor may be regarded to be 0° when a specific blade (e.g. “blade 1”) is in the 12 o’clock position. In other examples, the reference 102 for the azimuth angle may be a different one.

[0047] The steps of determining 110 a signal of a fore-aft oscillation and determining 120 a signal of an azimuth angle may be performed at overlapping times, specifically simultaneously. For example, they may be determined during a certain finite time period. A finite time period may be referred to as “time window”. The time window may be predefined. In some examples, the time window may be between 1 minute (including at least a few revolutions of the wind turbine rotor 18) and 15 minutes, e.g. between 3 minutes and 10 minutes. In other examples, other time windows may be used.

[0048] In figure 5, which shows a schematic representation of an example of the method of figure 3, the signal of a fore-aft acceleration is illustrated as x”(t) and the signal of the azimuth angle is illustrated as 0(t). As mentioned before, the fore-aft acceleration may be derived directly from accelerometers mounted on the wind turbine but could also be derived from measurements of e.g. the position or the speed of the nacelle.

[0049] The signals thus comprise a plurality of points determined during a corresponding period of time. The frequency at which the determinations are performed may also be adjusted appropriately. For example, a plurality of determinations may be performed in one second.

[0050] The magnitude of the aerodynamic imbalance and the location of the aerodynamic imbalance may be determined at 1 P frequency, i.e. at a frequency at which the wind turbine blades rotate.

[0051] The signal of the fore-aft acceleration may be considered to be a periodic signal depending on the azimuth angle and comprising a plurality of harmonics of the rotational speed of the wind turbine rotor. Specifically, in the presence of an imbalance, e.g. aerodynamic imbalance, the fore-aft acceleration may be assumed as periodic and may therefore be expressed as a sum of harmonics of the rotational speed w(t) as follows:

[0052] where Anand 0nare the amplitude and phase of the nthharmonic, respectively, and where cn(t) = n0(t) is the rotational speed.

[0053] In this regard, a fore-aft acceleration may be indicative of aerodynamic imbalance. The amplitude and phase of the fore-aft acceleration, specifically at 1P frequency, may thus be indicative of an amplitude and phase of the aerodynamic imbalance. An aerodynamic imbalance may lead to movement or vibrations in the fore-aft direction because the aerodynamic thrust provided by the blades, e.g. due to pitch misalignment, will vary with the rotation of the blades. For example, the thrust will be different when a “different” blade moves at a lower side of the rotor plane, e.g. between an azimuth angle of 90° to 270°, than when it moves at a higher side of the rotor plane, e.g. between an azimuth angle of 270° to 90°.

[0054] Also, simulations by the inventors have shown for example that the pitch misalignment of one or more blades, i.e. a pitch angle being different from a desired pith angle, generally has a linear relationship with the amplitude at 1P frequency of the fore-aft acceleration. Simulations by the inventors have also shown that mass imbalance has a negligible effect in the amplitude and in the phase of the fore-aft acceleration at 1 P frequency. Therefore, determining an amplitude and a phase at 1 P frequency of for example a fore-aft acceleration may be a suitable and reliable option for determining a magnitude and a direction of aerodynamic imbalance.

[0055] In this manner, the magnitude and the location of the aerodynamic imbalance, in particular in the rotor plane, may be determined as an amplitude and as a phase of the harmonic at 1 P frequency of the rotational speed of the wind turbine rotor. I.e., the magnitude of the imbalance 101 would be x"P= A-L and the direction of the imbalance would be 01P= 0rIn other words, determining the magnitude of the aerodynamic imbalance may comprise determining an amplitude of the first harmonic of the signal of the fore-aft acceleration, anddetermining the location of the aerodynamic imbalance may comprise determining a phase of the first harmonic of the signal of the fore-aft acceleration.

[0056] By considering the signal of the fore-aft acceleration as periodic and dependent on the azimuth angle, specifically as a sum of harmonics as indicated above, both the severity (magnitude, amplitude) of the aerodynamic imbalance and its location in the rotor plane may be determined. Therefore, the aerodynamic imbalance may be effectively and precisely reduced, even eliminated, later on.

[0057] The magnitude of the aerodynamic imbalance and the location of the aerodynamic imbalance at 1P frequency, e.g. the amplitude and phase of the fore-aft acceleration at 1 P frequency, may be determined in several ways. In some examples, determining the magnitude of the aerodynamic imbalance may comprise using a filter, optionally a peak filter, for determining a signal of fore-aft acceleration at 1 P frequency, and determining a root mean square (RMS) of the signal. As the signal is (or is close to) a periodic wave, a relationship between the RMS and the amplitude of the signal is known. Therefore, the magnitude of the imbalance may be obtained. In these examples, determining the location of the aerodynamic imbalance may comprise using a filter, optionally a peak filter, for determining a signal of the azimuth angle at 1 P frequency.

[0058] In other words, the time signals of the determined fore-aft acceleration and azimuth angle may be filtered at 1 P frequency for obtaining the time signals of the fore-aft acceleration and azimuth angle at 1 P frequency. Therefore, an amplitude and phase at 1 P frequency may be known at a desired moment of time.

[0059] In other examples, determining the magnitude and the location of the aerodynamic imbalance at 1 P frequency may comprise determining a signal of the fore-aft acceleration as a function of azimuth angle based on the determined signals of the fore-aft acceleration and of the signal azimuth angle, and performing a Fourier transform, specifically a fast Fourier transform (FFT), of the signal of fore-aft acceleration as a function of azimuth angle.

[0060] As the evolution of the fore-aft acceleration with time and the evolution of the azimuth angle with time is known (this is determined at blocks 110 and 120 of method 100), a signal of fore-aft acceleration as a function of azimuth angle x”(0) may be obtained. By changing to the frequency domain, e.g. by applying an FFT, the amplitude of the first harmonic of the fore-aft acceleration and the phase of the azimuth of the first harmonic may be obtained.

[0061] Still in other examples, determining the magnitude and the location of the aerodynamic imbalance at 1 P frequency may comprise determining a first modulated signal of the fore-aft acceleration by multiplying the determined signal of the fore-aft acceleration by asine of the signal of the azimuth angle, and determining a second modulated signal of the fore- aft acceleration by multiplying the determined signal of the fore-aft acceleration by a cosine of the signal of the azimuth angle. These steps are illustrated in the example of figure 5. The first modulated signal has been represented by x”(t)sin0(t), and the second modulated signal has been represented by x”(t)cos0(t).

[0062] The method may further comprise filtering the first modulated signal and the second modulated signal for eliminating harmonics above the OP frequency, i.e. above the first harmonic of the non-modulated signals. In this way, components of the first and second modulated signals which are not constant may be eliminated. It should be noted that the components at OP frequency of the first and second modulated signals, i.e. the constant components, correspond to the 1 P component of the signal which has not yet been modulated. The filter may for example be a low-pass filter. A filter has been represented by the letter “F” in figure 5. After this filtering, it may be obtained that x”(t)sin0(t) « y cos (0 (represented as C1 in figure 5) and that x”(t)cos0(t) « y sin (0 (represented as C2 in figure 5). From these two equations, the values of Ax= x"Pand 0i —1Pmay be obtained, as the values of C1 and C2 are known.

[0063] In some examples, instead of eliminating the harmonics above OP frequency of the first and second modulated signals, the determined signal of the fore-aft acceleration, i.e. the non-modulated signal, may be filtered for eliminating harmonics above the 1 P frequency. Then the filtered signal may be multiplied by sin0(t) and by cos0(t) to obtain two modulated signals.

[0064] The values of A-L and 0! obtained in this manner may have a particularly good precision. For example the value of 0! obtained in this way may be more reliable than the value of 0! obtained by using a filter such as a peak filter and determining a RMS value.

[0065] Still other ways of determining the amplitude and phase of the first harmonic of the fore-aft acceleration may be used.

[0066] As the magnitude and direction of the imbalance 101 have been determined, imbalance may be reduced, e.g. compensated. The method may further comprise operating the wind turbine 10 for reducing the aerodynamic imbalance based on the magnitude and the location of the aerodynamic imbalance. The wind turbine may be operated in several manners to achieve this. Ideally, the operation may not reduce, or barely reduce, power production. I.e., instead of de-rating the wind turbine, the wind turbine may be operated in a different manner.

[0067] However, if the determined magnitude of the fore-aft acceleration at 1 P frequency is excessively high, e.g. if it reaches or exceeds a predefined threshold, the speed of rotationof the blades may be reduced and the wind turbine may even be stopped in some examples. An alarm may also be emitted if such a threshold is reached or exceeded. In some examples, more than one threshold may be predefined. For example, a first threshold may be a threshold which requires reducing rotational speed and emitting a warning signal, and a second threshold may be a threshold which requires stopping the wind turbine and emitting an alert signal.

[0068] In some examples, operating may comprise modifying a pitch angle of at least one of the blades of the wind turbine. In some examples, this may be performed such that a collective pitch, i.e. a sum of the pitch angles of the blades of the wind turbine, is zero or close to zero. The pitch offset provided to the wind turbine blades may be such that the overall performance of the wind turbine rotor is not affected. This may mean that the behavior of the wind turbine rotor does not vary from the behavior designed for the rotor. For example, a thrust and a torque at a given rotor speed and at a given wind speed may not be affected.

[0069] The method may further comprise determining an indication of pitch misalignment Ap based on the magnitude of the aerodynamic imbalance. Pitch misalignment may be a relevant cause of aerodynamic imbalance. The lack of a symmetrical pitch in the blades may introduce asymmetric loads in the rotor that may be translated into vibrations, e.g. nacelle vibrations, at 1P frequency. The aerodynamic imbalance may be particularly intense in the fore-aft direction, as mentioned before.

[0070] An indication of pitch misalignment may refer to how much a pitch angle of one or more blades deviates from the expected or desired values. For example, if a blade has a pitch angle which deviates +1° from its “correct” value and another blade has a pitch angle which deviates -2° from its “correct” value, the total indication of pitch misalignment may be 3°.

[0071] Determining the indication of pitch misalignment may comprise using predefined data linking the magnitude of the aerodynamic imbalance and the indication of pitch misalignment. The predefined data may be stored in any suitable data structure, e.g. a lookup table or other. The data structure may include information about wind speed, as the effect of pitch misalignment on aerodynamic imbalance may vary depending on a prevailing wind speed.

[0072] A relationship between the magnitude of the aerodynamic imbalance and the indication of pitch misalignment may be obtained based on simulations and / or based on experimental data. The relationship may specifically be linear. I.e., an increase in pitch misalignment may linearly increase the aerodynamic imbalance and the fore-aft acceleration for a given wind speed.

[0073] Therefore, as a relationship between the magnitude of the aerodynamic imbalance and the indication of pitch misalignment may be known in advance, the magnitude of the aerodynamic imbalance determined during method 100 may be translated into an indication of pitch misalignment which may then be reduced, in particular by pitching one or more of the blades.

[0074] Modifying the pitch angle of at least one of the blades may comprise determining a pitch angle for the at least one of the blades based on the location of the aerodynamic imbalance and the determined indication of misalignment of pitch angle. A pitch angle for at least one of the blades may be determined in several manners. In some examples, trigonometrical relationships may be used.

[0075] In some of these examples, determining the pitch angle may comprise determining a first vector component and a second vector component by projecting a vector having an amplitude given by the indication of pitch misalignment p and a direction given by the location of the aerodynamic imbalance 0! on two perpendicular axes in the plane of the rotor 18. The axes may correspond to the axes around which a moment may occur. For example, one of the axes may be linked to a pitch moment and the other axis may be linked to another pitch moment. One of the axes may be aligned with a blade. The other axis may be perpendicular to it. The vector of the indication of pitch misalignment may therefore be projected or decomposed into the first and second components in the rotor plane. These two axes in the rotor plane form a first coordinate system.

[0076] Determining the pitch angle may further comprise projecting the first vector component and the second vector component on axes in the plane of the rotor given by the blade direction for obtaining pitch setpoints for each blade. The directions along which the blades extend (spanwise direction of the blades) define three axes in the rotor plane which form a second coordinate system. The first and second components may therefore be transformed into components along the directions of each blade.

[0077] After operating the wind turbine for reducing the aerodynamic imbalance, the fore- aft acceleration and the azimuth angle may be determined 110, 120 again to check whether aerodynamic imbalance is still present, and if so, its magnitude and location may be determined 110, 120 and the wind turbine may then be operated again for reducing it.

[0078] The method 100 of this aspect may be particularly performed while the wind turbine is in a normal mode of operation, i.e. while wind turbine is in a power-production mode, where the wind turbine rotor 18 is rotated at a sufficient speed for generating power and supplying it to the electrical grid. The method may be performed in real time.

[0079] In a further aspect of the disclosure, a wind turbine controller 36 for determining an aerodynamic imbalance in a wind turbine rotor is provided. The controller is configured to receive a signal of a fore-aft oscillation, e.g. fore-aft acceleration, of the wind turbine, receive a signal of an azimuth angle of a blade of the wind turbine, and based on the signal of the fore- aft oscillation and based on the signal of the azimuth angle, determine 130 a magnitude of the aerodynamic imbalance and determine a location of the aerodynamic imbalance.

[0080] The controller may be a main controller of the wind turbine 10. The controller may comprise a processor and a memory. Instructions and data may be stored in the memory. The processor may be configured to read instructions from the memory and execute them. The controller may be in communication with other systems of the wind turbine 10. For example, the controller may be communicatively coupled with a pitch drive system 68 for modifying the pitch angle of one or more of the blades 22 of the wind turbine 10. The controller may also be communicatively coupled with one or more sensors for receiving measured data, e.g. with an accelerometer providing fore-aft acceleration data.

[0081] Details and explanations of the methods 100 and 200 (see below) are applicable to, and may be combined with, this aspect, and vice versa.

[0082] In a further aspect of the disclosure, a wind turbine is provided. The wind turbine 10 comprises the controller of the previous aspect. The wind turbine may in particular be an offshore wind turbine. The details and explanations of the previous aspects are applicable to this aspect as well.

[0083] In a further aspect of the invention, a method 200 for operating a wind turbine 10 is provided. The method 200 is shown in the flowchart of figure 6. Method 200 comprises, at block 210, determining a signal of a fore-aft acceleration of the wind turbine, and at block 220, determining a signal of an azimuth angle of a blade of the wind turbine. The method further comprises, based on the signal of the fore-aft acceleration and based on the signal of the azimuth angle, at block 230, determining a magnitude of an aerodynamic imbalance and determining a location of the aerodynamic imbalance at 1 P frequency in a rotor plane of the wind turbine, and at block 240, based on the determined magnitude and location of the aerodynamic imbalance, operating the wind turbine for reducing the magnitude of the aerodynamic imbalance. The imbalance may be reduced without reducing, or barely reducing, a power output of the wind turbine.

[0084] The signal of the fore-aft acceleration may be considered to be a periodic signal depending on the azimuth angle and comprising a plurality of harmonics of the rotational speed of the wind turbine rotor.

[0085] Determining the magnitude and the location of the aerodynamic imbalance at 1 P frequency in the rotor plane may comprise determining an amplitude and a phase of the first harmonic of the signal of the fore-aft acceleration.

[0086] Determining the amplitude and the phase of the first harmonic of the signal of the fore-aft acceleration may comprise, in some examples, obtaining a first signal by multiplying the signal of the fore-aft acceleration by the cosine of the signal of the azimuth angle, and obtaining a second signal by multiplying the signal of the fore-aft acceleration by the sine of the signal of the azimuth angle. Determining may further comprise eliminating the harmonics above the 1 P frequency of the first and second signals, and determining the amplitude and the phase of the first harmonic based on the first and second filtered signals.

[0087] Operating the wind turbine for reducing the magnitude of the aerodynamic imbalance may comprise determining a pitch setpoint for each blade of the wind turbine. Determining the pitch setpoint may comprise deriving a pitch offset from the magnitude of the aerodynamic imbalance and determining the pitch setpoint for each blade of the wind turbine based on the pitch offset.

[0088] Details and explanations of all the previous aspects may be applied and combined with this aspect, and vice versa. Therefore, the methods described herein may be performed by a wind turbine controller of a wind turbine, e.g. the main controller of the wind turbine, and the wind turbine controller and the wind turbine, e.g. an offshore wind turbine, may be configured to perform the methods described herein.

[0089] This written description uses examples to disclose the teaching, including the preferred embodiments, and also to enable any person skilled in the art to practice the teaching, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Aspects from the various embodiments described, as well as other known equivalents for each such aspects, can be mixed and matched by one of ordinary skill in the art to construct additional embodiments and techniques in accordance with principles of this application. If reference signs related to drawings are placed in parentheses in a claim, they are solely for attempting to increase the intelligibility of the claim, and shall not be construed as limiting the scope of the claim.

Claims

CLAIMS1. A method (100) for determining an aerodynamic imbalance in a rotor (18) of a wind turbine (10), the method comprising: determining (110) a fore-aft oscillation of the wind turbine (10) with rotor speed, 1P, frequency; determining (120) an azimuth angle of the rotor (18) of the wind turbine (10); and determining (130) an indication of a magnitude of an aerodynamic imbalance and a location of the aerodynamic imbalance based on the determined fore-aft oscillation and on the determined azimuth angle.

2. The method of claim 1 , further comprising operating the wind turbine (10) to reduce the aerodynamic imbalance.

3. The method of claim 2, comprising applying a pitch offset to one or more wind turbine blades (22) of the wind turbine (10).

4. The method of claim 3, wherein the pitch offset applied to the wind turbine blades (22) is such that an aerodynamic performance of the rotor (18) is not affected.

5. The method of claim 3 or claim 4, comprising determining a pitch misalignment, and wherein the pitch misalignment for the one or more wind turbine blades (22) is determined based on the magnitude of the aerodynamic imbalance and on the location of the aerodynamic imbalance.

6. The method of claim 5, wherein determining the pitch misalignment based on the magnitude of the aerodynamic imbalance is based on simulations of the wind turbine (10) with rotor (22) blades with a pitch offset.

7. The method of claim 6, wherein determining the pitch offset comprises: determining a first vector component and a second vector component by projecting a vector having an amplitude given by the indication of pitch misalignment and a direction given by the location of the aerodynamic imbalance on two perpendicular axes in a plane of the rotor; andprojecting the first vector component and the second vector component on axes in the plane of the rotor given by a blade direction for obtaining pitch setpoints for each blade (22).

8. The method of any of claims 1 - 7, wherein determining the fore-aft oscillation of the wind turbine (10) comprises determining an acceleration of the wind turbine (10), and optionally of a nacelle (16) of the wind turbine (10).

9. The method of claim 8, wherein: determining the indication of the magnitude of the aerodynamic imbalance comprises using a filter for determining a signal of fore-aft acceleration at 1P frequency, and determining a root mean square of the signal; and wherein determining the location of the aerodynamic imbalance comprises using a filter for determining a signal of the azimuth angle at 1P frequency.

10. The method of claim 8, wherein determining the indication of the magnitude and the location of the aerodynamic imbalance comprises determining a signal of the fore-aft acceleration as a function of azimuth angle based on determined signals of the fore-aft acceleration and of the azimuth angle, and performing a Fourier transform of the signal of fore- aft acceleration as a function of azimuth angle.

11. The method of claim 8, wherein determining the indication of the magnitude and the location of the aerodynamic imbalance comprises: determining a first modulated signal of the fore-aft acceleration by multiplying a determined signal of the fore-aft acceleration by a sine of a signal of the azimuth angle; and determining a second modulated signal of the fore-aft acceleration by multiplying the determined signal of the fore-aft acceleration by a cosine of the signal of the azimuth angle.

12. The method of claim 11 , further comprising filtering the first modulated signal and the second modulated signal for eliminating harmonics above a OP frequency.

13. The method of claim 12, wherein the filter is a low-pass filter.

14. A wind turbine controller (36) for determining an aerodynamic imbalance in a wind turbine rotor (18), the controller being configured to: receive a signal of a fore-aft oscillation of the wind turbine (10); receive a signal of an azimuth angle of a blade (22) of the wind turbine (10); and based on the signal of the fore-aft oscillation and based on the signal of the azimuth angle, determine a magnitude of the aerodynamic imbalance and determining a location of the aerodynamic imbalance.

15. The controller of claim 14, further configured to send signals to the one or more blades (22) to change a pitch angle of the blades (22) based on the determined magnitude and location of the aerodynamic imbalance.

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