Controlling wind turbine rotor blade pitch according to dynamic maximum pitch amplitude value

Abstract

The invention relates to adjusting the pitch of a wind turbine rotor blade. The wind turbine controller includes a collective pitch control module for determining a collective pitch reference, and one or more pitch offset control modules. The controller receives a collective pitch signal indicative of a collective pitch angle of the rotor blades; receiving a power signal indicative of a power output of the wind turbine; and for each of the one or more pitch offset control modules, determining a respective dynamic maximum pitch amplitude value based on the received collective pitch signal and the power signal. And each variable pitch offset control module determines a corresponding variable pitch reference offset value according to the corresponding dynamic maximum variable pitch amplitude value. The controller determines an overall pitch reference based on the collective pitch reference and one or more pitch reference offset values.

Description

Technical Field

[0001] The present invention relates to controlling the pitch of rotor blades of a wind turbine, and in particular to controlling the pitch based on one or more dynamic maximum pitch amplitude values. Background Art

[0002] Wind turbines known in the art include a wind turbine tower supporting a nacelle and a rotor, on which are mounted a plurality (typically three) of rotor blades with adjustable pitch. Wind turbines are susceptible to vibrations, such as movement of the tower, nacelle, or rotor blades. It is known that certain types of vibrations can be damped (suppressed) by actively pitching the rotor blades or adjusting the generator torque. Control strategies for adjusting blade pitch can be used to maximize the energy production of the wind turbine while minimizing the loads experienced by the various components of the wind turbine.

[0003] The rotor blades may be adjusted as part of a collective pitch control routine, wherein each rotor blade is adjusted in the same manner at the same time, wherein such a collective pitch controller may be used, for example, to control the wind turbine rotor speed. The wind turbine may include one or more pitch modification or offset controllers, each pitch modification or offset controller being used to determine a respective offset of a collective pitch reference output by the collective pitch controller. Each pitch offset controller may be used to mitigate respective frequency content associated with a component of the wind turbine associated with extreme or fatigue component loads in order to reduce the load. The pitch offset obtained from each pitch offset controller may be in the form of a collective pitch offset, wherein the same offset value is applied to each rotor blade, or may be in the form of an individual or cyclic pitch offset, wherein each blade has its own respective offset value applied. The collective pitch reference and one or more pitch offset signals may be combined to obtain an overall pitch reference for collective and / or individual pitch adjustment of the rotor blades.

[0004] The pitch system of a wind turbine may be a hydraulic pitch system for actuating the rotor blades according to an overall pitch reference. The amount of pitch control that can be performed is limited by the pump capacity of the hydraulic pitch system. The demand on the pump system is a function of the amplitude and frequency of the pitch adjustments. In particular, high-amplitude and high-frequency pitch adjustments place high demands on the pump system. Typically, each pitch offset controller has a corresponding maximum pitch amplitude that defines the maximum offset adjustment amplitude allowed by the controller. This is to ensure that the demand on the hydraulic pitch system does not exceed the amount that the pump system can deliver. However, constraining the amplitude adjustment of each pitch offset controller in this way may limit the effectiveness of the corresponding controller in alleviating certain loads and may not effectively or efficiently utilize the total capacity of the pump system. In addition, constraining the amplitude adjustment of each pitch offset controller in this way may cause fretting wear of the blade bearings during periods of relatively small movements of the pitch angle, thereby creating lubrication problems.

[0005] The present invention is set just in view of this background. Summary of the Invention

[0006] According to one aspect of the present invention, a controller for a wind turbine having a rotor and two or more rotor blades is provided. The controller is configured to adjust the pitch of the rotor blades. The controller includes a collective pitch control module for determining a collective pitch reference for the rotor blades. The controller includes one or more pitch offset control modules. The controller is configured to receive a collective pitch signal indicating a collective pitch angle of the rotor blades. The controller is configured to receive a power signal indicating a power output of the wind turbine. For each of the one or more pitch offset control modules, the controller is configured to determine a corresponding dynamic maximum pitch amplitude value based on the received collective pitch signal and the received power signal. Each pitch offset control module is configured to determine a corresponding pitch reference offset value based on the corresponding dynamic maximum pitch amplitude value. The controller is configured to determine an overall pitch reference based on the collective pitch reference and the one or more pitch reference offset values. The controller can be configured to transmit a control signal to adjust the pitch of the rotor blades based on the overall pitch reference.

[0007] The one or more pitch offset control modules may include one or more pitch-yaw individual pitch offset control modules, each pitch-yaw individual pitch offset control module being configured to mitigate loads experienced by one or more components of the wind turbine. The one or more pitch-yaw individual pitch offset control modules may include a 1P pitch offset control module configured to: receive an out-of-plane load signal indicative of an out-of-plane load on each of the respective rotor blades from one or more sensors of the rotor blades; and determine a 1P pitch reference offset value for each respective rotor blade based on the received out-of-plane load signal to target a 1P frequency content in the received out-of-plane load signal. The one or more pitch-yaw individual pitch offset control modules may include a 2P pitch offset control module configured to: receive an out-of-plane load signal; and determine a 2P pitch reference offset value for each respective rotor blade based on the received out-of-plane load signal to target a 2P frequency content in the received out-of-plane load signal.

[0008] The controller can be configured to determine a statistical dispersion parameter of an out-of-plane (e.g., flapping) load for each of the rotor blades, the statistical dispersion parameter being indicative of a turbulence level in a wind farm in which the wind turbine is operating. The determination can be based on one or more of: a received out-of-plane (e.g., flapping) load signal; or, signals received from nacelle- or tower-based sensors, such as measurements of fore-aft acceleration or main axis signals (pitching and yaw moments) of the wind turbine tower. For each of the one or more pitch offset control modules, the controller can be configured to determine a corresponding dynamic maximum pitch amplitude value based on the determined statistical dispersion parameter indicative of a turbulence level.

[0009] If the turbulence level is greater than a first turbulence level indicating a high turbulence level, the corresponding dynamic maximum pitch amplitude value of the one or more pitch-yaw individual pitch offset control modules can be determined to be greater than if the turbulence level is less than a second threshold turbulence level, the second threshold turbulence level being less than or equal to the first turbulence level. Optionally, the second threshold turbulence level can be less than the first turbulence level. Further optionally, the corresponding dynamic maximum pitch amplitude value of the one or more pitch-yaw individual pitch offset control modules can be determined to decrease monotonically from the second threshold turbulence level to the first turbulence level. For example, the corresponding dynamic maximum pitch amplitude value can decrease linearly from the second threshold turbulence level to the first turbulence level.

[0010] The one or more pitch offset control modules may include one or more further pitch offset control modules distinct from the one or more pitch-yaw individual pitch offset control modules, each further pitch offset control module for mitigating loads experienced by one or more components of the wind turbine.

[0011] The one or more additional pitch offset control modules may include a swirl (whirl, vortex) mode pitch offset control module configured to determine a swirl mode pitch reference offset value for each respective rotor blade to mitigate swirl mode caused by edgewise vibration of the rotor blade. The determination may be based on at least one of: a received tower dynamics signal indicating dynamics of a tower of the wind turbine; and an edgewise load signal from one or more sensors on the rotor blades indicating edgewise loads on each of the respective rotor blades.

[0012] If the received collective pitch signal and the received power signal indicate that the operating point of the wind turbine is within a slew threshold distance of an off-rated region of a power curve of the wind turbine, the dynamic maximum pitch amplitude value for the slew mode pitch offset control module may be determined to be lower than if the operating point is in the rated region of the power curve and further from the off-rated region than the slew threshold distance.

[0013] The dynamic maximum pitch amplitude value of the slewing mode pitch offset control module can be determined as a function of the difference between the edge frequency of the rotor blade and the nP frequency of the rotor, where n is a positive integer. Optionally, the dynamic maximum pitch amplitude value of the slewing mode pitch offset control module can be larger for smaller values ​​of the difference between the edge frequency and the nP frequency.

[0014] If the wind turbine enters the safe operating mode, the dynamic maximum pitch amplitude value of the slew mode pitch offset control module may be determined to be increased.

[0015] The one or more additional pitch offset control modules may include a high-frequency collective pitch offset control module configured to: receive a tower load signal indicating a tower load in a fore-aft direction of the wind turbine; and determine a high-frequency collective pitch reference offset value for the rotor blades based on the received tower load signal to reduce fatigue in the wind turbine tower caused by high-frequency collective content greater than 2P frequency content.

[0016] If the received collective pitch signal and the received power signal indicate that the operating point of the wind turbine is within a high-frequency collective threshold distance of a non-rated region of the power curve of the wind turbine, the dynamic maximum pitch amplitude value for the high-frequency collective pitch offset control module may be determined to be higher than if the operating point is in the rated region of the power curve and further from the non-rated region than the high-frequency collective threshold distance.

[0017] The dynamic maximum pitch amplitude value of the high-frequency collective pitch offset control module can be determined as a function of the difference between the natural mode frequency of the wind turbine tower and the 3P frequency of the rotor. Optionally, the dynamic maximum pitch amplitude value of the high-frequency collective pitch offset control module can be larger for smaller values ​​of the difference between the natural mode frequency and the 3P frequency.

[0018] The corresponding dynamic maximum pitch amplitude values ​​of the one or more additional pitch offset control modules may be determined based on the corresponding determined dynamic maximum pitch amplitude values ​​of the one or more pitch-yaw individual pitch offset control modules. Optionally, if the turbulence level is greater than a first turbulence level, the corresponding dynamic maximum pitch amplitude values ​​of the one or more additional pitch offset control modules may be less than if the turbulence level is less than a second turbulence level.

[0019] According to another aspect of the invention, there is provided a wind turbine comprising a controller as defined above.

[0020] According to another aspect of the present invention, a method for a wind turbine having a rotor and two or more rotor blades is provided. The method is for adjusting the pitch of the rotor blades. The method comprises: determining a collective pitch reference for the rotor blades; receiving a collective pitch signal indicative of a collective pitch angle of the rotor blades; receiving a power signal indicative of a power output of the wind turbine; for each of one or more pitch offset control modules of a controller of the wind turbine, determining a corresponding dynamic maximum pitch amplitude value based on the received collective pitch signal and the received power signal, and determining a corresponding pitch reference offset value based on the corresponding dynamic maximum pitch amplitude value; and determining an overall pitch reference based on the collective pitch reference and the one or more pitch reference offset values, and transmitting a control signal to adjust the pitch of the rotor blades based on the overall pitch reference.

[0021] According to another aspect of the present invention, there is provided a non-transitory computer-readable storage medium having stored thereon instructions which, when executed by one or more processors, cause the one or more processors to perform the method defined above. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Examples of the present invention will now be described with reference to the accompanying drawings, in which:

[0023] Figure 1 schematically illustrates a wind turbine according to an aspect of the present invention;

[0024] Figure 2 Schematically illustrates an embodiment of the present invention Figure 1 Controllers for wind turbines;

[0025] Figure 3 Schematically illustrates Figure 2 A pitch offset control module of a controller;

[0026] Figure 4 (a)-4(c) show how to use Figure 3 An example of using a pitch offset control module to suppress accumulation of backswing content; and

[0027] Figure 5 Shown by Figure 2 The controller performs the steps of the method. DETAILED DESCRIPTION

[0028] Figure 1An example of a wind turbine 10 is illustrated in a schematic diagram. The wind turbine 10 includes a tower 102, a nacelle 103 disposed at or on top of the tower 102, and a rotor 104 operably coupled to a generator housed within the nacelle 103. In addition to the generator, the nacelle 103 houses other components required to convert wind energy into electrical energy, as well as various components required to operate, control, and optimize the performance of the wind turbine 10. The rotor 104 of the wind turbine 10 includes a central hub 105 and three rotor blades 106 projecting outwardly from the central hub 105. In addition, the wind turbine 10 includes a control system or controller ( Figure 1 10). The controllers may be located within the nacelle 103, in the tower 102, or distributed at multiple locations within (or outside) the turbine 10 and communicatively connected to one another. The rotor blades 106 are pitch-adjustable. The rotor blades 106 may be adjusted according to a collective pitch setting, wherein each blade is set to the same pitch value. The rotor blades 106 may also be adjusted according to individual pitch settings, wherein each blade 106 may be provided with an individual pitch set point.

[0029] In some examples, wind turbine 10 includes blade load sensors placed at or near the root 109 of each blade in such a manner that the sensors detect the load in blade 106. Blade load signals from such sensors can be used to determine how to adjust the pitch of each of the individual blades 106. Depending on the placement and type of sensors, loads can be detected in the flapping direction (in-plane / out-of-plane) or the edgewise direction 108 (in-plane). For example, such sensors can be strain gauge sensors or optical Bragg sensors. When the sensors are placed on rotating blades 106, such load signals for each adjustable rotor blade 106 are measured in the rotating reference frame (reference system) of rotor 104.

[0030] Figure 2 Schematically illustrates elements of an example of an overall controller 20 of a wind turbine 10 implemented to determine pitch actuation signals capable of maximizing power generation of the wind turbine 10 and reducing or alleviating loads experienced by one or more components (e.g., tower 102, rotor blades 106, etc.). In the illustrated embodiment, a collective pitch control module in the form of a speed controller (control module / block) 202 of the overall controller 20 compares an actual rotor speed ω to a reference rotor speed ω. ref The speed error between (ω-ω ref ) is minimized so that the output request power P (in the form of a power set point) and the collective pitch reference θ col Given the rotor speed, the collective pitch reference determined by the speed controller 202 may also take into account additional sensor values. Figure 2The measurement set ms is referred to as the input to the speed controller 202. The feedback speed controller 202 can be implemented by a PI (proportional integral), PID (proportional integral derivative) or similar control scheme. In one example, the collective pitch control module 202 can alternatively be a model predictive controller that is arranged to determine a collective pitch reference and / or a power reference based on minimizing a cost function.

[0031] Figure 2 Also illustrated is a control block / module or controller 204 of the overall controller 20, which may be referred to as a pitch actuation unit (PAU). In the pitch actuation unit 204, a pitch modification signal or pitch reference offset value is determined based on one or more input signals 205. As will be described in more detail below, the PAU 204 includes one or more pitch reference offset control modules. Each pitch reference offset control module determines a respective pitch modification signal or pitch reference offset value that is intended to mitigate a respective type of load experienced by the wind turbine 10, such as a load resulting from a particular frequency content exhibited by one or more components of the wind turbine 10. The pitch modification signal determined by each respective pitch reference offset control module may be an individual pitch modification signal for each respective rotor blade 106, or may be a collective pitch modification signal. (A combination of these) offset or modification signals θ1, θ2, θ3 are superimposed on the collective pitch reference from the collective pitch control module 202 to provide a resulting or overall pitch modification signal or pitch reference θ that may be applied collectively and / or individually to the pitch actuators of the rotor blades 106. A ,θ B ,θ C The pitch reference offset control module is described in more detail below.

[0032] The controller 20 described may be in the form of any suitable computing device, such as one or more functional units or modules implemented on one or more computer processors. Such functional units may be provided by suitable software running on any suitable computing substrate using conventional or custom processors and memory. One or more functional units may use a common computing substrate (e.g., they may run on the same server) or separate substrates, or one or both substrates themselves may be distributed among multiple computing devices. The computer memory may store instructions for executing the method performed by the controller, and the processor may execute the stored instructions to perform the method.

[0033] Figure 3 The PAU 204 of the overall controller 20 in the described example is schematically illustrated in more detail. In particular, in the described example, Figure 3 The PAU 204 is illustrated as including four pitch reference offset control modules or pitch offset controllers.

[0034] The first of these pitch offset controllers may be a so-called 1P controller 31. The 1P controller is a separate pitch controller that targets a 1P frequency content. The 1P frequency is the rotational frequency of the wind turbine rotor 104, i.e., the frequency at which a full rotation of the rotor 104 is completed. The 1P controller 31 may receive as input a load signal 311 from one or more blade sensors of the wind turbine 10. The 1P controller 31 determines an out-of-plane moment based on the received signal. In one example, the input load signal is in the form of a blade flap load measurement, in which gravity is taken into account in a prescribed manner to determine the out-of-plane moment. In another example, the input load signal may include flap and edge load measurements to obtain the out-of-plane moment. More generally, the input load signal may be considered a blade out-of-plane load signal. In the case of three rotor blades 106, a three-dimensional vector is obtained, in which each value of the vector indicates an out-of-plane blade root bending moment associated with a respective one of the three blades 106.

[0035] The 1P controller 31 determines a 1P pitch reference offset value or signal for each respective rotor blade based on the received out-of-plane load signal, in particular based on the out-of-plane moment. The determined 1P pitch reference offset value is used to target the 0P tilt-yaw load in the fixed reference frame. This can also be used to target the 1P flapping load at relatively low collective pitch angles. In order to determine the 1P offset signal, the 1P controller 31 can transform the received load signal from the rotating or rotor reference frame into the fixed reference frame, for example via an m-blade (multi-blade) coordinate transformation such as the Coleman transformation. This results in the 1P frequency content in the input signal appearing at 0P in the transformed signal. The control action for counteracting this content or mitigating its effects is determined, for example, via a PI (proportional-integral) control routine, and the inverse m-blade transformation can then be applied to obtain a control component in the rotor frame, in particular in the form of a pitch modification signal that is applied to the pitch angle setting of the respective rotor blade to counteract the 1P content. The output signal from the 1P controller 301 is therefore an individual pitch modification signal θ 1Pa ,θ 1Pb ,θ 1Pc .

[0036] The second of the pitch offset controllers may be a so-called 2P controller 32. In a manner corresponding to the 1P controller 31, the 2P controller 32 is a separate pitch controller for targeting a 2P frequency content. A 2P cyclic disturbance in the rotor coordinate frame (rotating reference frame) is twice the frequency of rotor rotation. This disturbance appears at 3P in the fixed coordinate frame (coordinate system). The 2P controller 32 may therefore be used to cancel out vibration modes at 3P in the fixed frame, where 3P is the frequency at which the rotor blades pass the tower in a three-bladed wind turbine, i.e. three times each time the rotor 104 makes a full rotation. Similar to the 1P controller 31, in the 2P controller 32, an input signal 321 in the form of blade sensor data is received from the blade load sensors to obtain a three-dimensional out-of-plane blade root bending moment vector.

[0037] An m-blade transformation may be applied by the 2P controller 32 to obtain a component in a fixed frame. In order to target the 2P content in the input signal (as required in the 2P controller 32), a further transformation may therefore be applied to isolate this content in the transformed signal. This is required because not only does the 2P content from the input signal 321 appear at 3P in the transformed signal, but also the 4P content in the input signal is included. A further transformation may be a rotation with an angular velocity of 3P. The 2P content in the input signal 321 then appears at 0P in the reference frame of this further transformation. Once the 2P content has been isolated in this way, a control action is determined to counteract it or mitigate its effects, in particular to obtain a control component in this reference frame. The inverse transformation back to the rotor frame is then applied to the control component to obtain the pitch modification signal θ 2Pa ,θ 2Pb ,θ 2Pc , with the pitch angle setting applied to the corresponding rotor blades to offset the 2P content.

[0038] The 1P and 2P controllers 31 , 32 may be collectively referred to as a pitch-yaw controller or a pitch-yaw individual pitch offset control module.

[0039] The PAU 204 may include an additional pitch reference offset control module in the form of a so-called slewing mode pitch offset control module or slewing mode controller 33. In particular, the slewing mode controller 33 is used to determine a pitch modification signal that is used to mitigate or offset the edgewise slewing content caused by the edgewise vibrations of the rotor blades. As the rotor of a wind turbine rotates, the oscillations of the blades relative to their edgewise axes may cause the blades to move in the same plane as the plane of rotation of the rotor. It will be appreciated that the rotor shaft is effectively mounted at one end and is unsupported at the hub end to which the blades are attached. When the edgewise oscillations of the blades excite the rotor with forces transverse to its longitudinal axis, then under resonant conditions, this may cause the rotational axis of the rotor shaft to describe an unstable mode of motion. This phenomenon may be referred to as "slewing." This phenomenon is typically more likely to occur at higher wind speeds.

[0040] The phase difference between the edgewise oscillations of the blades determines whether rotation occurs in the same direction as the rotor rotation (which may be referred to as "forward (positive) rotation" or "forward rotation mode"), or whether rotation occurs in the direction opposite to the direction of rotor rotation (which may be referred to as "backward (reverse) rotation" or "backward rotation mode").

[0041] Gyroscopic mode vibrations can be detected from various types of sensor measurements received as input signals 331 to gyroscopic mode controller 33. In one example, input signal 331 is in the form of edgewise load measurements from blade sensors. In another example, gyroscopic mode vibrations can be detected by monitoring the behavior of nacelle 103 or the upper portion of tower 102, as the gyroscopic rotation of the rotor shaft applies lateral forces to nacelle 103 via rotor 104, thereby causing nacelle 103 to sway from side to side. In such an example, input signal 331 can thus be in the form of measurements from an accelerometer located at the top of tower 102 or in nacelle 103, and can indicate the dynamics of wind turbine tower 102, such as lateral (side-to-side) movement of tower 102.

[0042] The slewing mode controller 33 determines a slewing mode pitch reference offset value for each respective rotor blade 106 to mitigate the slewing mode caused by blade edgewise vibration, in particular in the form of a separate pitch modification signal θ applied to the pitch angle setting of the respective rotor blade 106. WMa ,θ WMb ,θ WMcIn the example where the input signal 331 is a blade edge load measurement, an m-blade coordinate transform can be applied to the input signal to obtain a transformed signal in a fixed frame. This transform makes it possible to distinguish between backward rotation and forward rotation, both of which are different manifestations of the edge 1st order mode. In particular, when the phase is set to the rotor azimuth angle in the transform, backward rotation occurs at the edge frequency minus 1P, while forward rotation occurs at the edge frequency plus 1P. Another approach is to apply two counter-rotating Coleman transform operations, the first operation on the rotation angle at the positive edge frequency and the second operation on the rotation angle at the negative edge frequency. In the first operation, the backward rotation mode is driven to 0 Hz and the forward rotation mode is driven to twice the edge frequency. In the second operation, the forward rotation mode is driven to 0 Hz and the backward rotation mode is driven to twice the edge frequency. This large frequency separation can be beneficial for filtering purposes. The edge frequencies can be treated as known parameters that can be accessed via a lookup table or the like in a memory module. Control action may be applied to the transformed signal to mitigate the slewing content and then transformed back into the rotor frame via an inverse m-blade transformation to obtain the pitch modification signal θ WMa ,θ WMb ,θ WMc .

[0043] The PAU 204 may include an additional collective pitch reference offset control module in the form of a so-called high-frequency collective pitch offset control module or high-frequency collective controller 34. The high-frequency collective controller 34 is used to determine a pitch modification signal that is used to reduce fatigue levels in the wind turbine tower 102 caused by high-frequency content in tower motion. Specifically, in the case of a floating platform wind turbine system (e.g., an offshore system), the controller 34 reduces tower fatigue caused by excitation that occurs when the 3P frequency coincides with or is relatively close to the frequency content of the coupling mode resulting from the coupling between the tower 102 and the floating platform. The 3P frequency is the frequency at which the rotor blades pass the tower 102 of a wind turbine having three blades. The high-frequency collective controller 34 may also be used to reduce tower fatigue caused by excitation that occurs when the 3P frequency coincides with or is relatively close to the natural mode of the tower 102 of an onshore wind turbine.

[0044] Controller 34 provides control of high-frequency content by controlling or adjusting the phase lead / lag of a collective pitch reference offset used to control or adjust the pitch of wind turbine rotor blades 106. High-frequency content can be defined as frequency content greater than 2P, i.e., having a higher frequency than 2P. The targeted oscillation is in the fore-aft direction of the tower. Therefore, the input signal 341 to high-frequency collective controller 34 is a signal indicating tower loading in the fore-aft direction of wind turbine 10. In one example, this is in the form of acceleration measurement signals from one or more accelerometers at the top of tower 102 or in nacelle 103, indicating fore-aft acceleration of tower 102 and nacelle 103. High-frequency collective controller 34 can generate additional signals based on the input signals, particularly in quadrature with the input signals, for example using a second-order generalized integrator (SOGI). A phase shift is then applied to the mutually orthogonal signals. The optimal phase of the collective 3P pitch reference offset (i.e., the phase best suited to reducing tower fatigue) can vary with the operating point of wind turbine 10. A collective pitch reference offset value or signal θ for the rotor blades 106 is then determined based on one of the mutually orthogonal signals. HFC , to reduce high-frequency collective content.

[0045] Each pitch offset controller 31, 32, 33, 34 has a respective maximum pitch amplitude that defines the maximum offset adjustment amplitude allowed by that controller. This is to ensure that the hydraulic pitch system of wind turbine 10, which is used to actuate or adjust the pitch of the rotor blades according to the overall pitch reference output by overall controller 20, does not require more pitch adjustments than can be delivered by the hydraulic pump system of wind turbine 10. Since larger pitch adjustments place higher demands on the pump system, defining a maximum pitch amplitude (amplitude saturation value) for each pitch offset controller ensures that the pump capacity is not exceeded, where the pump capacity reflects the rate at which fluid can be pumped into the pump system.

[0046] Previously, these maximum pitch amplitudes have been defined as static (constant) values ​​that are appropriately allocated between the different pitch offset controllers based on the total capacity of the pump system, i.e., a reserve has been allocated to each controller. In an illustrative example, a 1P pitch offset controller may have its amplitude constrained to 5 degrees, a 2P pitch offset controller may have its amplitude constrained to 1 degree, and further pitch offset controllers (e.g., slewing mode controller, high frequency collective controller) may have their amplitude constrained to appropriate static values. The pump capacity reserve allocated to each pitch offset controller may alternatively be expressed as a proportion / percentage of the overall pump capacity. However, this may not result in the most efficient use of the pitch offset controllers to reduce or offset extreme or fatigue loads on one or more wind turbine components. In particular, in the event that one such pitch offset controller is deactivated or implemented to implement a relatively small pitch adjustment, the unused capacity may be able to be used by another pitch offset controller that is not implemented most efficiently due to the corresponding maximum pitch amplitude constraint imposed on it. That is, reserving capacity in a static manner means that the pump capacity may not be allocated optimally, resulting in hindered load relief capacity.

[0047] An advantage of the present invention is that a dynamic maximum pitch amplitude value is determined for each pitch offset controller of a wind turbine. In particular, each dynamic maximum pitch amplitude value is determined based on an operating point of the wind turbine. Specifically, each dynamic maximum pitch amplitude value is determined based on the collective pitch angle of the rotor blades and the power output of the wind turbine. Since the requirements for operation of different types of pitch offset controllers or intervention from different types of pitch offset controllers are different at different operating points of the wind turbine, for a given operating point, a corresponding maximum pitch amplitude value can be determined to relax the regulation constraints on those pitch offset controllers whose operation is required at the operating point, i.e. the maximum pitch amplitude value is increased. At the same time, in order to ensure that the pump capacity is not exceeded, the regulation constraints on other pitch offset controllers whose operation is not required or is required to a lesser extent at the operating point are tightened, i.e. the maximum pitch amplitude value is reduced.

[0048] Scheduling a dynamic maximum pitch amplitude value based on wind turbine power output is advantageous because it ensures uniqueness of the operating point (e.g., compared to using generator speed). In some examples, each pitch offset controller may have a default maximum pitch amplitude value associated with it, with deviations (increases or decreases) from the default value enforced for certain operating points. Other advantages associated with the present invention will become apparent from the following description.

[0049] return Figure 2, the overall controller 20 includes a dynamic maximum pitch amplitude module 206 or a dynamic amplitude saturation module for determining the dynamic maximum pitch amplitude of each pitch offset controller 31, 32, 33, 34. In some examples, the dynamic maximum pitch amplitude module 206 can be considered as part of the PAU 204. The dynamic maximum pitch amplitude module 206 takes as input a signal 207 indicative of the power output of the wind turbine. For example, this can be in the form of a measured and / or estimated electrical power output of the wind turbine 10. The dynamic maximum pitch amplitude module 206 also takes as input a signal 208 indicative of the collective pitch angle of the rotor blades 106. In one example, this can be a collective pitch reference θ output by the speed controller 202. col .

[0050] The wind turbine operating point is determined based on the input measurement values / signals 207, 208. The dynamic maximum pitch amplitude module 206 then determines a (different) dynamic maximum pitch amplitude value for each respective pitch offset controller 31, 32, 33, 34. For example, this determination may be performed by means of a lookup table in a storage device accessible to the module 206, wherein a given operating point corresponds to a given value for each of the respective pitch offset controllers 31, 32, 33, 34. Figure 3 Each pitch offset controller 31, 32, 33, 34 is provided with a corresponding determined dynamic maximum pitch amplitude value 312, 322, 332, 342. The pitch modification signal determined by each pitch offset controller 31, 32, 33, 34 is constrained by the corresponding dynamic maximum pitch amplitude value 312, 322, 332, 342.

[0051] In an example of the present invention, higher priority may be given to controllers that handle extreme loads on one or more wind turbine components. In particular, individual pitch-yaw pitch offset controllers 31 and 32 are used to reduce extreme loads, and therefore, these controllers may be given priority in ensuring that these controllers 31 and 32 are allocated sufficient pump capacity to reduce extreme loads. Extreme loads may become more problematic at higher wind speeds. Therefore, at higher wind speeds, controlling extreme pitch and yaw loads requires the greatest effort / intervention from controllers 31 and 32. Therefore, for operating points corresponding to higher wind speeds, the dynamic maximum pitch amplitude of one or both of the pitch-yaw pitch offset controllers 31 and 32 may be increased. In other words, the dynamic maximum pitch amplitude of the pitch-yaw pitch offset controllers 31 and 32 may be determined to be greater for higher wind speeds than the dynamic maximum pitch amplitude of these controllers 31 and 32 determined for lower wind speeds. In this context, higher wind speeds may refer to wind speeds within the rated operating region of wind turbine 10 (e.g., on the power curve of wind turbine 10) and may be relatively far from the derated operating region. An increase in the dynamic maximum pitch amplitude of the pitch-yaw pitch offset controllers 31, 32 at higher wind speeds may result in a corresponding decrease in the determined dynamic maximum pitch amplitude of other pitch offset controllers of the wind turbine 10 (e.g., the slewing mode and / or high frequency collective controllers 33, 34) to ensure that the overall pump capacity is not exceeded.

[0052] An important aspect of the present invention is that dynamic maximum pitch amplitude module 206 considers the turbulence level of the wind near wind turbine 10 when determining the corresponding dynamic maximum pitch amplitude value. The most extreme load conditions experienced by wind turbine 10 may occur under conditions of relatively high wind speed and relatively high turbulence. Therefore, under such conditions, the highest priority may be given to pitch-yaw pitch offset controllers 31, 32 by virtue of the highest dynamic maximum pitch amplitude.

[0053] Intervention of the pitch-yaw pitch offset control can also be beneficial for reducing extreme loads (e.g., extreme flapping loads) at other wind speeds, particularly at wind speeds near the transition point between the derated and rated operating regions of wind turbine 10. This can also be referred to as the wind speed at the "knee" of the power curve. It can be observed that the pitch offset amplitude required by the pitch-yaw controllers 31, 32 to mitigate extreme loads at such wind speeds is often below the amplitude saturation level, e.g., below the default maximum pitch amplitude value of these controllers 31, 32. Therefore, under such conditions, it may not be necessary to increase the maximum value above the default value to achieve effective extreme load control. However, an exception may be under moderate wind speed conditions, e.g., near the "knee" of the power curve, paired with relatively high turbulence levels, where greater pitch-yaw pitch offset intervention would be beneficial. Therefore, under such conditions, the dynamic maximum pitch amplitude of the pitch-yaw pitch offset controllers 31, 32 can be determined to be larger, e.g., relative to conditions of moderate wind speed and moderate or low turbulence levels.

[0054] In one example, if the turbulence level is greater than a first turbulence level indicating a high turbulence level, the corresponding dynamic maximum pitch amplitude value of the pitch-yaw controller 31, 32 is determined to be greater than the case where the turbulence level is less than a second threshold turbulence level, the second threshold turbulence level being less than or equal to the first turbulence level. In one example, the first threshold level and the second threshold level are equal. In a more preferred example, the first threshold level and the second threshold level are not equal, and the maximum amplitude value is ramped up from the second threshold to the first threshold, for example, in a monotonic manner. This can, for example, be implemented as a linear interpolation or increase of the maximum amplitude from the second threshold to the first threshold, optionally wherein the maximum amplitude is constant for turbulence values ​​greater than the first threshold and / or less than the second threshold.

[0055] In a corresponding manner, at lower turbulence levels, there may be little or no need for intervention of the pitch-yaw controllers 31, 32. Therefore, the maximum amplitude values ​​of these controllers 31, 32 under such conditions may be reduced in order to free up pump capacity for other pitch offset controllers (e.g., controllers addressing fatigue load issues).

[0056] Turbulence can be determined based on data obtained from blade load sensors of wind turbine 10. In particular, this can be based on flapping moment sensor data obtained from the blade load sensors. Specifically, statistical dispersion parameters (e.g., standard deviation) of the sensor data are indicative of the turbulence level of the wind farm, and such parameters can be determined to ascertain the turbulence level. The determination of turbulence can be performed by dynamic maximum pitch amplitude module 206 or a different portion of overall controller 20.

[0057] Edge turning tends to be a phenomenon most pronounced at high wind speeds and / or high pitch angles for a wind turbine. Therefore, having an increased maximum pitch amplitude value under such conditions may be beneficial; however, as described above, priority may be given to the pitch-yaw pitch offset controllers 31, 32 in terms of pump capacity (and therefore maximum amplitude value), possibly at the expense of the slew mode controller 33. Edge fatigue is dominated by gravity, and the edge turning phenomenon is not critical for the most frequent wind speeds experienced by the wind turbine. Therefore, for operating points near the "knee" of the power curve, it may not be necessary to pre-allocate pump capacity to the slew mode controller 33. In one example, this may be expressed or implemented as determining the dynamic maximum pitch amplitude value of the controller 33 to be a lower value for operating points in the non-rated region of the power curve or in the rated region but within a threshold distance of the non-rated region than for operating points in the rated region but greater than the threshold distance from the non-rated region.

[0058] Note that "direct" excitation of the edge slewing mode occurs during a frequency conflict between the edge frequency of rotor blade 106 and the peak of the associated nP frequency, where n is a positive integer. Thus, the determination of the dynamic maximum pitch amplitude value of slewing mode controller 33 may depend on the determined difference between the edge frequency of rotor blade 106 and the nP frequency of rotor 104. Specifically, the dynamic maximum pitch amplitude value of slewing mode controller 33 may be larger for smaller values ​​of the difference between the edge frequency and the nP frequency, i.e., where the edge frequency and the nP frequency are closer. In one example, the highest value of the dynamic maximum pitch amplitude of slewing mode controller 33 is provided when the backward slewing frequency (i.e., the edge frequency minus 1P) crosses or intersects the 3P frequency (or is within a certain distance of the 3P frequency) and / or when the forward slewing frequency (i.e., the edge frequency plus 1P) crosses or intersects the 6P frequency (or is within a certain distance of the 6P frequency).

[0059] It should also be noted that while the 2P controller 32 can address the pitch-yaw loads, it can also be used to suppress the accumulation of slewing modes. Therefore, the determination of the dynamic maximum pitch amplitude value of the 2P controller 32 can also be based on the frequency of the excitation that causes the slewing mode as described above. Specifically, the dynamic maximum pitch amplitude value of the 2P controller 32 can be increased under such conditions, while the dynamic maximum pitch amplitude value of the 1P controller 31 can be reduced or maintained at a relatively small value under these conditions (which can free up capacity for the 2P controller).

[0060] Figure 4 An example is shown in which the 2P controller 32 is used to suppress the accumulation of the backward rotation mode. In particular, Figure 4 (a) shows a graph of the maximum pitch amplitude value 41 of the 1P controller 31 over time, and Figure 4(b) shows a graph of the maximum pitch amplitude value 42 of the 2P controller 32 over time. When the 3P frequency becomes close to the backward slewing frequency (i.e., the blade edge frequency minus 1P), the dynamic maximum pitch amplitude value 42 of the 2P controller 32 increases to suppress the backward slewing mode, as shown in FIG. Figure 4 To ensure that there is sufficient pump capacity for this increase in the maximum pitch amplitude, the dynamic maximum pitch amplitude value 41 of the IP controller 31 is correspondingly reduced, as shown in (b). Figure 4 As shown in (a). Figure 4 (c) A graph showing how the back rotation content changes over time. In particular, Figure 4 (c) shows a comparison of how the increase in the accumulated backward slewing content when the 3P frequency approaches the backward slewing frequency is suppressed when the maximum pitch amplitude value 42 of the 2P controller 32 increases. Specifically, it can be seen that when the maximum amplitude 42 of the 2P controller 32 increases, the accumulation of the backward slewing content 43 is less than the equivalent accumulation of the backward slewing content 44 if the maximum pitch amplitude value 42 of the 2P controller 32 is not increased, that is, if the static maximum pitch amplitude 44 is used.

[0061] When the wind turbine 10 is operated at a derated value, such as in a safe operating mode, most loads on the wind turbine components are reduced. However, an exception to this is edge loads, which may still be significant in such an operating mode. Therefore, in one example, when the wind turbine 10 enters a safe mode or is otherwise operated at a derated value, the controller 20 may determine to increase the dynamic maximum pitch amplitude of the slewing mode controller 33. This may be accompanied by a corresponding reduction in the dynamic maximum pitch amplitude values ​​of other controllers (such as the pitch-yaw controllers 31, 32 and / or the high-frequency collective controller 34) to ensure that the overall pump capacity is not exceeded. Alternatively, in one example, the 2P pitch-yaw controller 32 may maintain a relatively high dynamic maximum pitch amplitude value (or increase it) in a safe or other derated mode (e.g., in addition to the slewing mode controller 33). In particular, this may be beneficial when safe mode operation is paired with conditions of moderate to high turbulence.

[0062] High frequency collective pitch control is particularly useful for addressing fatigue problems that commonly occur in floating systems. As a result of the distribution of time spent at each operating point following a known Weibull distribution, it is noted that the need for high frequency collective pitch control is greatest at the inflection points of the power curve. In one example, this can be represented or implemented as determining the dynamic maximum pitch amplitude value of the high frequency collective controller 34 to be higher for operating points in the non-rated region of the power curve or in the rated region but within a threshold distance of the non-rated region than for operating points in the rated region but greater than the threshold distance from the non-rated region. Increasing the dynamic maximum pitch amplitude value of the controller 34 near the inflection point of the power curve in this way can also be beneficial because it can reduce the risk of micro-shredding that occurs due to insufficient lubrication of the blade bearings. The benefit can be significant because the high frequency collective controller 34 can intervene relatively regularly during wind turbine operation because it is addressing fatigue problems.

[0063] In the case of a floating system, such fatigue issues may arise due to the proximity of the 3P frequency to the natural mode between wind turbine tower 102 and the floating platform. Therefore, the amount of pump capacity allocated to high-frequency collective pitch controller 34 can advantageously be set as a function of the margin / difference between the 3P and natural mode frequency content. In particular, this can be expressed or implemented as controller 34 having a dynamic maximum pitch amplitude value that is greater for smaller values ​​of the difference between the natural mode frequency and the 3P frequency than for larger values ​​of the difference.

[0064] Furthermore, in conditions of relatively low or moderate turbulence (which may be considered relatively common conditions and therefore associated with fatigue), the 1P controller 31 may not prioritize pump capacity, but rather means that the high frequency collective controller 34 may be allocated a larger pump capacity (by means of an increased maximum pitch amplitude) in such conditions to reduce fatigue loads.

[0065] Figure 5 The steps of a method 50 implemented by the controller 20 of the wind turbine 10 are summarized. At step 501, the method 50 involves receiving a collective pitch signal indicative of the collective pitch angle of the rotor blades 106, such as a collective pitch reference determined by the speed controller 202. At step 502, the method 50 involves receiving a power signal indicative of the power output of the wind turbine 10. This may be, for example, the measured electrical power output of the wind turbine 10 (to the grid).

[0066] At step 503, method 50 involves determining, for each pitch offset controller 31, 32, 33, 34, a corresponding dynamic maximum pitch amplitude value based on the received collective pitch signal and the received power signal, i.e., based on the wind turbine operating point. In an example comprising at least one pitch-yaw pitch offset controller 31, 32 and at least one additional pitch offset controller (e.g., a slewing mode and / or high-frequency collective controller 33, 34), the dynamic maximum pitch amplitude value of each additional pitch offset controller can be determined based on the dynamic maximum pitch amplitude value determined for each of the pitch-yaw controllers 31, 32. That is, a higher priority can be given to the pitch-yaw controller 31, 32 with respect to pump capacity. Thus, an increase in the maximum amplitude determined for the pitch-yaw controller 31, 32 can automatically result in a decrease in the maximum amplitude of the additional pitch offset controllers 33, 34, regardless of wind conditions.

[0067] The respective dynamic maximum pitch amplitude value may be further determined based on a determined turbulence level of the wind near the wind turbine 10. In particular, under relatively high turbulence conditions, the pitch-yaw controllers 31, 32 may be given higher priority with respect to pump capacity than the further pitch offset controllers 33, 34.

[0068] In general, under conditions of low to moderate wind speeds and low to moderate turbulence, the dynamic pitch amplitude of the 1P controller 31 can be reduced, while the dynamic pitch amplitude of the high frequency collective controller 34 can be increased. Under conditions of low to moderate wind speeds and moderate to high turbulence, the dynamic pitch amplitude of the 1P controller 31 can be increased to suppress the increased flapping extreme loads associated with such conditions. For low to moderate wind speeds, and regardless of the turbulence level, the dynamic pitch amplitude of the slewing mode controller 33 can be reduced (optionally to zero).

[0069] In addition, under conditions of relatively high wind speeds and low to moderate turbulence, the dynamic pitch amplitudes of the respective 1P and 2P controllers 31, 32 may be increased, while the dynamic pitch amplitudes of the high frequency collective controller 34 may be reduced (and some pump capacity maintained for the slewing mode controller 33). Under conditions of relatively high wind speeds and moderate to high turbulence, the dynamic pitch amplitudes of one or more of the 1P, 2P, and slewing mode controllers 31, 32, 33 may be increased. In some cases, the 1P controller 31 may be given a lower priority than the 2P and slewing mode controllers 32, 33, where reduced 1P controller 31 intervention is compensated, for example, by implementing power derating.

[0070] Each pitch offset controller 31, 32, 33, 34 then determines a corresponding pitch reference offset value based on the corresponding dynamic maximum pitch amplitude value. That is, each determined pitch reference offset value is constrained to be less than (in magnitude) the corresponding determined dynamic maximum pitch amplitude value. At step 504, method 50 involves determining an overall pitch reference based on the collective pitch reference and each of the pitch reference offset values. A control signal is transmitted to adjust the pitch of rotor blades 106 based on the overall pitch reference.

[0071] Many modifications may be made to the examples described without departing from the scope of the appended claims.

[0072] In the described examples, the pitch actuation unit includes pitch reference offset control modules in the form of a 1P controller, a 2P controller, a slew mode controller, and a high-frequency collective pitch controller. However, it will be appreciated that different examples may include fewer or more such pitch reference offset control modules, and indeed any suitable combination thereof. For example, in some examples, a lateral (left and right) tower damping controller may be included using pitch, but it will be appreciated that various different types of pitch controllers may be included.

Claims

1. A controller for a wind turbine having a rotor and two or more rotor blades, the controller being configured to adjust the pitch of the rotor blades, the controller comprising: a collective pitch control module for determining a collective pitch reference of the rotor blades; as well as one or more pitch offset control modules; The controller is configured to: receiving a collective pitch signal indicative of a collective pitch angle of the rotor blades; receiving a power signal indicative of a power output of the wind turbine; and For each of the one or more pitch offset control modules, determining a corresponding dynamic maximum pitch amplitude value based on the received collective pitch signal and the received power signal, Each pitch offset control module is configured to determine a corresponding pitch reference offset value according to a corresponding dynamic maximum pitch amplitude value. The controller is configured to determine an overall pitch reference based on the collective pitch reference and the one or more pitch reference offset values, and to transmit a control signal to adjust the pitch of the rotor blades according to the overall pitch reference.

2. The controller according to claim 1, wherein: The one or more pitch offset control modules include one or more pitch-yaw individual pitch offset control modules, each pitch-yaw individual pitch offset control module being configured to mitigate loads experienced by one or more components of the wind turbine and comprising one or both of the following: a 1P pitch offset control module configured to: receive an out-of-plane load signal from a sensor on one or more of the rotor blades, the out-of-plane load signal indicating an out-of-plane load on each of the respective rotor blades; and determine a 1P pitch reference offset value for each respective rotor blade based on the received out-of-plane load signal to target a 1P frequency content in the received out-of-plane load signal; as well as A 2P pitch offset control module is configured to: receive the out-of-plane load signal; and determine a 2P pitch reference offset value for each corresponding rotor blade based on the received out-of-plane load signal to target the 2P frequency content in the received out-of-plane load signal.

3. The controller according to claim 2, wherein the controller is configured to: determining a statistical dispersion parameter of an out-of-plane load for each of the rotor blades based on the received out-of-plane load signal, the statistical dispersion parameter being indicative of a turbulence level in a wind farm in which the wind turbine operates; and For each of the one or more pitch offset control modules, a corresponding dynamic maximum pitch amplitude value is determined based on the determined statistical dispersion parameter indicative of a turbulence level.

4. The controller according to claim 3, wherein: If the turbulence level is greater than a first turbulence level indicating a high turbulence level, the corresponding dynamic maximum pitch amplitude values ​​of the one or more tilt-yaw individual pitch offset control modules are determined to be greater than the case where the turbulence level is less than a second threshold turbulence level, and the second threshold turbulence level is less than or equal to the first turbulence level; optionally, wherein the second threshold turbulence level is less than the first turbulence level, and wherein the corresponding dynamic maximum pitch amplitude values ​​of the one or more tilt-yaw individual pitch offset control modules are determined to decrease monotonically from the second threshold turbulence level to the first turbulence level.

5. A controller according to any preceding claim, wherein The one or more pitch offset control modules include one or more additional pitch offset control modules distinct from the one or more pitch-yaw individual pitch offset control modules, each additional pitch offset control module for mitigating loads experienced by one or more components of the wind turbine.

6. The controller according to claim 5, wherein: The one or more additional pitch offset control modules include a slewing mode pitch offset control module, which is configured to determine a slewing mode pitch reference offset value for each corresponding rotor blade to mitigate a slewing mode caused by edgewise vibration of the rotor blade, the determination being based on at least one of: a received tower dynamics signal indicative of dynamics of a tower of the wind turbine; and an edgewise load signal from one or more sensors of the rotor blades, the edgewise load signal indicating an edgewise load on each of the corresponding rotor blades.

7. The controller according to claim 6, wherein: If the received collective pitch signal and the received power signal indicate that the operating point of the wind turbine is within a slew threshold distance of a non-rated region of a power curve of the wind turbine, the dynamic maximum pitch amplitude value of the slew mode pitch offset control module is determined to be lower than a case where the operating point is in the rated region of the power curve and is further from the non-rated region than the slew threshold distance.

8. The controller according to claim 6 or claim 7, wherein: The dynamic maximum pitch amplitude value of the slewing mode pitch offset control module is determined as a function of the difference between the edge frequency of the rotor blade and the nP frequency of the rotor, where n is a positive integer; optionally, the dynamic maximum pitch amplitude value of the slewing mode pitch offset control module is larger for smaller values ​​of the difference between the edge frequency and the nP frequency.

9. The controller according to any one of claims 6 to 8, wherein: If the wind turbine enters a safe operating mode, the dynamic maximum pitch amplitude value of the slew mode pitch offset control module is determined to be increased.

10. The controller according to any one of claims 5 to 9, wherein: The one or more additional pitch offset control modules include a high-frequency collective pitch offset control module configured to: receive a tower load signal indicating a tower load in a fore-aft direction of the wind turbine; and determine a high-frequency collective pitch reference offset value for the rotor blades based on the received tower load signal to reduce fatigue in the wind turbine tower caused by high-frequency collective content greater than 2P frequency content.

11. The controller according to claim 10, wherein: If the received collective pitch signal and the received power signal indicate that the operating point of the wind turbine is within a high-frequency collective threshold distance of a non-rated region of a power curve of the wind turbine, the dynamic maximum pitch amplitude value of the high-frequency collective pitch offset control module is determined to be higher than when the operating point is in the rated region of the power curve and is further from the non-rated region than the high-frequency collective threshold distance.

12. The controller according to claim 10 or claim 11, wherein: The dynamic maximum pitch amplitude value of the high frequency collective pitch offset control module is determined as a function of the difference between the natural mode frequency of the wind turbine tower and the 3P frequency of the rotor; Optionally, for a smaller value of the difference between the natural mode frequency and the 3P frequency, the dynamic maximum pitch amplitude value of the high-frequency collective pitch offset control module is larger.

13. A controller according to any one of claims 5 to 12, when dependent on claim 2, wherein: The corresponding dynamic maximum pitch amplitude values ​​of the one or more additional pitch offset control modules are determined based on the corresponding determined dynamic maximum pitch amplitude values ​​of the one or more tilt-yaw individual pitch offset control modules; optionally, when dependent on claim 4, wherein, if the turbulence level is greater than the first turbulence level, the corresponding dynamic maximum pitch amplitude values ​​of the one or more additional pitch offset control modules are less than the case where the turbulence level is less than the second turbulence level.

14. A wind turbine comprising a controller according to any preceding claim.

15. A method for a wind turbine having a rotor and two or more rotor blades, the method for adjusting the pitch of the rotor blades, the method comprising: determining a collective pitch reference for the rotor blades; receiving a collective pitch signal indicative of a collective pitch angle of the rotor blades; receiving a power signal indicative of a power output of the wind turbine; For each of the one or more pitch offset control modules of the controller of the wind turbine, determining a corresponding dynamic maximum pitch amplitude value based on the received collective pitch signal and the received power signal, and determining a corresponding pitch reference offset value according to the corresponding dynamic maximum pitch amplitude value; as well as An overall pitch reference is determined based on the collective pitch reference and the one or more pitch reference offset values, and a control signal is transmitted to adjust the pitch of the rotor blades according to the overall pitch reference.

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