Patent Description:
Wind turbines as known in the art include a wind turbine tower supporting a nacelle and a rotor with a number of (typically, three) pitch-adjustable rotor blades mounted thereto. A wind turbine is prone to vibrations, such as tower, nacelle, or rotor blade movement. It is known that certain types of vibrations may be damped by active pitching of the rotor blades or adjusting generator torque. Control strategies for adjusting blade pitch can be used to maximise energy production of a wind turbine while minimising loads experienced by various components of the wind turbine.

As the rotor of a wind turbine rotates, the wind turbine tower may oscillate or vibrate in one or more directions, e.g. side-to-side or fore-aft. The tower may have various modes of natural vibration frequencies. One such mode is a first mode that corresponds to the side-to-side vibrational frequency of the tower. In the case of a floating wind turbine system, e.g. an offshore system, then a natural vibration frequency may be a coupled mode between the tower and a platform on which the wind turbine floats.

The rotational frequency of the rotor - i.e. the frequency with which a complete rotation of the rotor is completed - may be referred to as a 1P frequency. The frequency at which a rotor blade passes the tower for a wind turbine with three blades may be referred to as 3P, which may then result in 3P frequency content in the tower.

In certain wind turbine control schemes, e.g. to maximise energy production, it may be necessary to operate the wind turbine at a rotor speed that results in the 3P frequency coinciding with a natural frequency such as a tower first mode or a coupled mode. This coincidence of frequencies can cause excitations of the tower, resulting in tower fatigue. In certain systems, this can be the case when the rotor approaches its nominal speed. As a wind turbine rotor may be operated at nominal speed for significant periods of wind turbine operation, e.g. in a full-load region above a rated wind speed, then a margin between the 3P frequency and a natural mode may be relatively small for significant periods of wind turbine operation, meaning that the tower may accrue significant fatigue through prolonged excitations local to a natural mode.

<CIT> discloses a method for blade load reduction control of a rotor of a wind turbine. The rotor includes a plurality of blades, and a pitch angle of each blade is controllable by an actuator. The method includes measuring mechanical load parameters on the rotor, providing control for a collective pitch blade setting based on a rotor speed; providing individual pitch control including transforming the mechanical load parameters from a rotational reference frame to a mechanical load on the rotor in a fixed reference frame; determining from the mechanical load two multi-blade pitches; correcting the multi-blade pitches to corrected multi-blade pitches using actuator limitations; inversely transforming the corrected multi-blade pitches to an individual pitch deviation for each blade in the rotational reference frame; adding up for each blade, the individual pitch deviation to the collective pitch to form an individual pitch; and setting each blade to the respective individual pitch.

<CIT> discloses a rotor control system for actuating pitch of pitch adjustable rotor blades of a wind turbine in order to reduce edgewise blade vibrations. The system comprises a pitch actuation unit being arranged to receive an edgewise load signal and apply m-blade coordinate transformations, such as the Coleman transformations, to the edgewise load signal. Based on a selected signal component at either a backward whirling frequency or a forward whirling frequency, a modified modification signal is obtained.

<CIT> discloses blade monitoring of a wind turbine by actively promoting blade vibrations by imposing a pitch actuation signal. A method of operating a wind turbine is disclosed where for each blade of a wind turbine, vibrations of the blade are actively promoted by imposing a pitch actuation signal to the pitch actuator, and at least one parameter relating to the blade vibration is determined.

According to an aspect of the invention there is provided a controller for a wind turbine having a plurality of rotor blades, e.g. three rotor blades. The controller is for adjusting pitch of the rotor blades. The controller is configured to receive sensor data, from a flap loading sensor of each of the three rotor blades, indicative of flap loading on each of the respective rotor blades. The controller is configured to obtain, based on the received sensor data, a flap loading vector in a rotor coordinate frame of the wind turbine. The controller is configured to apply an m-blade coordinate transformation to the flap loading vector to obtain first and second components in a fixed coordinate frame of the wind turbine, where the first and second components are mutually orthogonal. The controller is configured to determine first and second 3P components in the fixed coordinate frame based on the obtained first and second components, the first and second 3P components being indicative of 3P frequency content of a tower of the wind turbine. The controller is configured to apply, using a gain-scheduled control module of the controller, a control action to the first and second 3P components to obtain respective first and second 3P control components for mitigating the 3P frequency content of the tower. An activation gain for the control action is determined by the gain-scheduled control module based on a comparison of the 3P frequency to a natural frequency of the tower. The controller is configured to apply an inverse m-blade coordinate transformation to the first and second 3P control components to obtain pitch reference offset values for the respective rotor blades in the rotor coordinate frame. The controller is configured to transmit a control signal to adjust pitch of the rotor blades based on the obtained pitch reference offset values.

The activation gain may be greater than zero if a ratio between the 3P frequency and the natural frequency is within a prescribed range. Optionally, the prescribed range may include a ratio equal to <NUM>. Further optionally, the activation gain may be zero if the ratio is not within the prescribed range.

The activation gain may be a constant value if the ratio between the 3P frequency and the natural frequency is within a prescribed sub-range of the prescribed range. The activation gain may reduce from the constant value to zero from an extreme value of the prescribed sub-range to an extreme value of the prescribed range. Optionally, the reduction may be a linear reduction.

The controller may be configured to obtain acceleration data indicative of an acceleration of the tower of the wind turbine. The controller may be configured to estimate an amplitude of tower accelerations based on the obtained acceleration data. The activation gain may be determined based on the estimated amplitude of tower accelerations.

The controller may be configured to obtain a current rotor speed (or generator speed) of the wind turbine. The gain-scheduled control module may ensure that a maximum allowable pitch reference offset amplitude is applied to the control action. By setting a maximum allowable pitch reference offset amplitude it may be ensured that the control action does not exceed a capacity of the hydraulic pumps used for the pitch actuation. The maximum allowable pitch reference offset amplitude of the gain-scheduled control module may be determined based on a comparison of the current rotor speed to a nominal rotor speed of the wind turbine (or a comparison of the current generator speed to a nominal generator speed of the wind turbine).

The allowable pitch reference offset amplitude may be greater for lower values of a ratio between the current rotor speed and the nominal rotor speed. Optionally, the allowable pitch reference offset amplitude is a first pitch reference offset amplitude when the ratio is greater than a prescribed value. Optionally, the allowable pitch reference offset amplitude is a second pitch reference offset amplitude greater than the first pitch reference offset amplitude when the ratio is less than the prescribed value.

The allowable pitch reference offset amplitude may be indicative of a pitch rate at which pitch of the rotor blades is adjusted. Alternatively, or in addition, the allowable pitch reference offset amplitude may be indicative of an amplitude limit to which pitch of the rotor blades is adjustable.

Determining the first and second 3P components may comprise applying a transform to the first and second components based on a phase that moves at the 3P frequency.

Obtaining the flap loading vector may comprise applying a first filter to the received sensor data to remove content in the received sensor data indicative of an imbalance in a rotor of the wind turbine.

The controller may be configured to apply a second filter to remove content in the first and second components indicative of a steady-state contribution in a tilt or yaw direction of the wind turbine. The controller may be configured to apply the second filter prior to determining the first and second 3P components.

The m-blade coordinate transformation may be a Coleman transformation. The inverse m-blade coordinate transformation may be an inverse Coleman transformation. The m-blade transformation is also referred to in the art as a multi-blade transformation. In an embodiment the m-blade coordinate transformation is a Coleman transformation. However, other transformations may also fall into the category of m-blade coordinate transformations, hereunder so-called d-q transformation and Park transformation or similar transformations. It is within the abilities of the skilled person to determine an alternative transformation which may not strictly be a Coleman transformation, but which operates in an equivalent manner.

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

According to another aspect of the invention there is provided a method for adjusting rotor blade pitch of a wind turbine. The method comprises receiving sensor data, from a flap loading sensor of each of three rotor blades of the wind turbine, indicative of flap loading on each of the respective rotor blades. The method comprises obtaining, based on the received sensor data, a flap loading vector in a rotor coordinate frame of the wind turbine. The method comprises applying an m-blade coordinate transformation to the flap loading vector to obtain first and second mutually orthogonal components in a fixed coordinate frame of the wind turbine. The method comprises determining first and second 3P components in the fixed coordinate frame based on the obtained first and second components, the first and second 3P components being indicative of 3P frequency content of a tower of the wind turbine. The method comprises applying, using a gain-scheduled control module of the controller, a control action to the first and second 3P components to obtain respective first and second 3P control components for mitigating the 3P frequency content of the tower. An activation gain for the control action is determined by the gain-scheduled control module based on a comparison of the 3P frequency to a natural frequency of the tower. The method comprises applying an inverse m-blade coordinate transformation to the first and second 3P control components to obtain pitch reference offset values for the respective rotor blades in the rotor coordinate frame. The method comprises transmitting a control signal to adjust pitch of the rotor blades based on the obtained pitch reference offset values.

According to another aspect of the invention there is provided a non-transitory, computer-readable storage medium storing instructions thereon that when executed by one or more processors cause the one or more processors to execute the method defined above.

Examples of the invention will now be described with reference to the accompanying drawings, in which:.

<FIG> illustrates, in a schematic view, an example of a wind turbine <NUM>. The wind turbine <NUM> includes a tower <NUM>, a nacelle <NUM> disposed at the apex of, or atop, the tower <NUM>, and a rotor <NUM> operatively coupled to a generator housed inside the nacelle <NUM>. In addition to the generator, the nacelle <NUM> houses other components required for converting wind energy into electrical energy and various components needed to operate, control, and optimise the performance of the wind turbine <NUM>. The rotor <NUM> of the wind turbine <NUM> includes a central hub <NUM> and three rotor blades <NUM> that project outwardly from the central hub <NUM>. Moreover, the wind turbine <NUM> comprises a control system or controller (not shown in <FIG>). The controller may be placed inside the nacelle <NUM>, in the tower <NUM> or distributed at a number of locations inside (or externally to) the turbine <NUM> and communicatively connected to one another. The rotor blades <NUM> are pitch-adjustable. The rotor blades <NUM> can be adjusted in accordance with a collective pitch setting, where each of the blades are set to the same pitch value. In addition, the rotor blades <NUM> are adjustable in accordance with individual pitch settings, where each blade <NUM> may be provided with an individual pitch setpoint.

In some examples, the wind turbine <NUM> includes blade load sensors placed at, or in the vicinity of, each blade root <NUM> in a manner such that the sensor detects loading in the blade <NUM>. Blade load signals from such sensors may be used to determine how to adjust the pitch of each of the individual blades <NUM>. Depending on the placement and the type of sensor, loading may be detected in the flap (flapwise) direction <NUM> (in/out of plane) or in the edge (edgewise) direction <NUM> (in-plane). Such sensors may be strain gauge sensors or optical Bragg-sensors, for instance. As the sensors are placed on the rotating blades <NUM>, such load signals for each of the adjustable rotor blades <NUM> are measured in the rotating reference frame of the rotor <NUM>.

Shown within each blade <NUM> is a blade load sensor <NUM>. In different examples, more than one blade load sensor may be provided in each blade. The blade load sensors <NUM> are arranged to measure flap loading on the blades <NUM>. The blade load sensors <NUM> are shown to be placed in the blades towards the root end. In embodiments, the blade load sensors are placed in the actual root section.

<FIG> schematically illustrates an example of an overall controller <NUM> (of the wind turbine <NUM>) that includes a feedback speed controller or control block <NUM> implemented to determine individual pitch actuation signals capable of reducing blade loads experienced by the rotor blades <NUM>. In the illustrated implementation, the speed controller <NUM> minimises a speed error (ω - ωref) between the actual rotor speed, ω, and a reference rotor speed, ωref, in order to output a requested power P (in the form of a power setpoint) and a collective pitch reference, θcol. The collective pitch reference as determined by the speed controller <NUM>, in view of the rotor speed, may also take further sensor values into account. This is referred to in <FIG> as a measurement set, ms, being input into the speed controller <NUM>. The feedback speed controller <NUM> may be implemented by a PI (proportional-integral), PID (proportional-integral-derivative), or similar control scheme. In one example, the speed controller <NUM> may alternatively be a model predictive controller which, based on minimising a cost function, is arranged to determine the collective pitch reference and/or the power reference.

<FIG> further illustrates a control block or controller <NUM>, of the overall controller <NUM>, which may be referred to as a pitch actuation unit (PAU). In the pitch actuation unit <NUM>, pitch modification signals (Δθ<NUM>, Δθ<NUM>, Δθ<NUM>) are being determined based on one or more input signals. In some examples, the input signals include blade load signals from load sensors <NUM> in the blades <NUM>.

The PAU <NUM> determines pitch modification signals, or pitch reference offset values, (Δθ<NUM>, Δθ<NUM>, Δθ<NUM>) for each rotor blade <NUM>. These offsets are superimposed onto the collective pitch reference to provide resulting or overall pitch modification signals (θA, θB, θC) that can be applied to the pitch actuators of the rotor blades <NUM> individually.

In the example shown in <FIG>, a collective pitch reference for the pitch-adjustable rotor blades <NUM> is being determined based on a rotor speed, and a resulting pitch modification signal is applied to the pitch-adjustable rotor blades <NUM>. The resulting pitch modification signal is applied to the pitch-adjustable rotor blades <NUM> individually, and for each individual blade <NUM> is based on a signal of the collective pitch reference and the respective individual pitch modification signal. In one example, the individual pitch modification signal is being applied in a cyclic manner. Thus, pitch actuation signals are determined for each pitch adjustable rotor blade <NUM> based on the pitch modification signal for each rotor blade <NUM>.

The described controller <NUM> may be in the form of any suitable computing device, for instance 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 customer processors and memory. The one or more functional units may use a common computing substrate (for example, they may run on the same server) or separate substrates, or one or both may themselves be distributed between multiple computing devices. A computer memory may store instructions for performing the methods performed by the controller, and the processor(s) may execute the stored instructions to perform the method.

The present invention is directed to reducing levels of fatigue in the wind turbine tower <NUM>. In particular, the invention is directed to reducing tower fatigue that arises as a result of 3P frequency content in the tower, i.e. vibrations caused by the blade passing frequency of the wind turbine (the frequency with which a blade passes the tower in a three-blade wind turbine during operation). Specifically, the invention is aimed at reducing tower fatigue resulting from excitations that occur when the 3P frequency coincides with, or is in relatively close proximity to, a natural frequency mode, e.g. the tower first mode, or a coupling mode between the wind turbine tower and a platform in the case of a floating wind turbine system.

The present invention provides for controlling pitch of the wind turbine rotor blads to reduce tower fatigue resulting from 3P frequency content. In particular, the present invention advantageously utilises the fact that cyclic disturbances in a rotational coordinate plane describing rotor and blade rotation of the wind turbine appear in a fixed coordinate plane describing tower movement of the wind turbine. Specifically, the invention utilises the fact that 2P cyclic disturbances in a rotational frame appear at 3P in a fixed frame. In this way, pitch adjustment of the blades to reduce 2P cyclic disturbances can be used to reduce 3P tower vibrations, and therefore reduce tower fatigue.

In the described example, the PAU <NUM> of <FIG> is used to determine the pitch adjustment values, or pitch reference offset values, Δθ<NUM>, Δθ<NUM>, Δθ<NUM> for each rotor blade <NUM> to reduce tower fatigue caused by 3P frequency content. The PAU <NUM> takes as input signals from the blade load sensors <NUM>.

<FIG> schematically illustrates functional or processing blocks/modules/units of the PAU <NUM>. The input signal <NUM> is sensor data from the blade load sensors <NUM>. In the described example, the blade load sensors <NUM> are flap load sensors that indicate loading on the respective blade <NUM> in the flapwise direction. As mentioned above, in the described example each blade <NUM> has a flap load sensor <NUM>. The PAU <NUM> may receive data from each of the sensors <NUM>. The flap load sensor signals are referred to in the figure as Mabc, where the suffix abc refers to the the three blades, i.e. blade a, etc..

The sensor data received from the flap load sensors <NUM> is used to obtain a three-dimensional vector, where each value of the vector indicates a flap bending moment associated with a respective one of the three blades <NUM>. The flapwise load signals are measured in a rotating or rotor reference frame. The rotating reference frame may be in a rotor rotational plane of the wind turbine <NUM>, and may be centred at the rotor <NUM>, for instance.

The flapwise load signals may be coordinate transformed by an m-blade (multi-blade) coordinate transformation. The transformation takes the three rotating signals into a fixed reference frame along a first reference direction d and a second reference direction q. The transformation makes it possible to identify 3P frequency content in the tower <NUM> from the cyclic disturbances in the blade flapwise load signals.

The m-blade coordinate transformation may be in the form of a Coleman transformation. The Coleman transformation may be defined as follows: <MAT> where Mabc is the bending moment vector (in the rotor coordinate frame), which in the described example is in the flap direction (but in different examples could be in the edge direction), ψ is the (1P) phase, and Mdq is a vector in the fixed coordinate frame. When Mabc is the vector containing the out-of-plane blade root bending moments and the phase is set as the rotor azimuth, the directions d, q are tilt and yaw directions; however, in general, the Coleman transformation simply transforms the rotating signals into mutually orthogonal first and second components in the fixed reference frame. In addition to indicating tilt and yaw loads, the fixed coordinate frame can be used to capture information relating to excitation of the tower <NUM>.

Returning to <FIG>, the blade flap bending moments Ma, Mb, Mc may be pre-processed or filtered prior to undergoing the coordinate transformation to remove signal content that is not needed for the present purpose. In particular, the functional block <NUM> may remove frequency content in the input signal <NUM> related to imbalances in the rotor <NUM>. The block <NUM> may be a high-pass filter tuned to not introduce any gain or phase distortion at 2P in the rotating frame. This high-pass filter may be applied in equal measure to all three of the blade load signals.

The functional unit <NUM> then applies the m-blade transformation to the (possibly prefiltered) flap bending moment vector Mabc to obtain the vector Mdq including the first and second components Md, Mq, as described above. As a result of this transformation, 1P content (i.e. rotor rotational frequency content) in the input signal appears at 0P in the transformed signal. Also, 3P content in the input signal corresponds to the collective flap load, and this disappears in the transformed signal. On the other hand, 2P and 4P content in the input signal (in the rotor frame) appears at 3P in the transformed signal (in the fixed frame); 5P and 7P content in the input signal (in the rotor frame) appears at 6P in the transformed signal (in the fixed frame). In particular, a 2P disturbance in the flap direction of the rotor ABC frame (rotational coordinate frame) generates an excitation in the tower <NUM> at 3P.

As the described example is aimed at targeting 3P content, then the first and second components obtained from the m-blade transformation may be input to the functional unit <NUM> of the PAU <NUM>, which is for removing content in the transformed signal (in the fixed reference frame) related to steady-state tilt and yaw contributions. The unit or block <NUM> may be a high-pass filter tuned such that it does not introduce any phase or gain distortion at 3P (in the fixed frame). This high-pass filter may be applied in equal measure to both of the orthogonal signals.

At the functional unit <NUM>, a further transform may be applied to first and second components. As the focus in the present approach is on 3P frequency content, then the transformation may be based on a phase of the 3P content. In one example, the following transformation is applied: <MAT> where ψ<NUM>P is the 3P phase, and Md,<NUM>P, Md,<NUM>P are first and second 3P components. In the figure Md, Mq is marked as d<NUM>q<NUM>, and Md,<NUM>P, Md,<NUM>P are marked as d<NUM>q<NUM>.

In this frame of reference, the 4P content in the original input signal (in the rotor frame) now appears at 6P, and so can be removed by application of a low-pass filter. This low-pass filter may be applied in equal measure to both of the orthogonal signals.

Once the contribution of the 2P content (in the rotor frame) to 3P frequency content in the fixed frame has been isolated in this manner, then a control action for counteracting it, or mitigating its effect, is determined as part of a general control action module <NUM>. Application of the control action may include application of a gain-scheduled control module or control unit as part of the module <NUM>. As illustrated in <FIG>, the module <NUM> may include a PI (proportional integral) controller 306a to be applied to the respective 3P components.

The control scheme of the described example is gain-scheduled as a different system response is desired and appropriate for different operating points of the wind turbine <NUM>. As outlined above, 2P cyclic disturbances in the rotor (abc) coordinate frame generate excitations in the tower <NUM> at 3P in the fixed frame. The response of the tower <NUM> can become large if the 3P frequency is approximately equal to a natural frequency, such as a tower first mode or coupled mode. On the other hand, if there is a sufficient margin between the 3P and natural frequencies, then the tower excitation response may be relatively small. As such, the efficacy of performing control actions (i.e. blade pitch adjustment) to reduce flap load content at 2P may be relatively low. Given that there is fatigue accrued by the blade bearing in order to adjust blade pitch, then taking action to reduce flap load content at 2P may be economic only when the 3P frequency is approximately equal to a natural frequency in the fixed coordinate frame, in the sense of achieving an overall reduction in wind turbine component fatigue.

As such, an activation strategy module 306b of the controller <NUM> may be implemented to determine an activation gain for any mitigation control action to be taken, where the activation gain is determined based on the level of excitation generated in the wind turbine tower <NUM> as a result of 3P frequency content. In particular, the 3P frequency of the tower <NUM> is compared to an obtained natural frequency of the tower <NUM>, e.g. a tower first frequency mode. An activation gain of the control signal is then determined based on this comparison. The natural frequency may for instance be a defined parameter of the wind turbine <NUM> that is hard-coded into the controller <NUM>.

In one example, the activation gain may be determined based on a difference between the 3P and natural frequencies. For instance, if the difference is below a prescribed threshold value - indicating that the 3P and natural frequencies approximately coincide - then the activation gain may be set such that the control action to reduce tower 3P fatigue is 'activated'. This may involve setting an activation gain to a certain value, e.g. one. On the other hand, if the difference between the 3P and natural frequencies is above the threshold value, then the activation gain may be set such that a control action to reduce tower 3P fatigue is 'deactivated' (i.e. it is determined that tower 3P fatigue reduction is uneconomic or not needed). This may involve setting the activation gain to zero, for instance.

In another example, the activation gain may be determined based on a ratio between the 3P and natural frequencies. For instance, if the ratio is sufficiently close to one - indicating that the 3P and natural frequencies are approximately equal - then the activation gain may be set such that the control action to reduce tower 3P fatigue is activated.

After the functional unit <NUM> and before the functional unit <NUM>, a functional unit arranged for adjusting the signal phase may be arranged. The PI controller 306a may alter the phase leading to non-linear lags in the system, and such lags may advantageously be compensated. The phase adjustment may be based on the operating point, e.g. by using a look-up table.

<FIG> shows a plot that schematically illustrates how an activation gain may be determined based on a ratio between 3P and natural frequencies. The activation gain may also be regarded as an amplitude being applied to a control action for mitigating or reducing 3P frequency content in the tower <NUM>. As illustrated in the example of <FIG>, if the ratio between the 3P and natural frequencies is sufficiently close to one, then the activation gain is set to one (i.e. the maximum). <FIG> also illustrates that, in the described example, as the ratio moves further away from one, the activation is ramped downwards until the activation gain becomes zero (i.e. the tower 3P reduction is deactivated) for values of the ratio distant from one (i.e. when the 3P and natural frequencies are sufficiently different from one another).

In <FIG>, a range [-a, a] of ratio values may be defined in which the activation gain is non-zero, where the range [-a, a] includes a ratio value of one. Values of the ratio outside of the range [-a, a] may then result in an activation gain of zero. A sub-range [-b, b] within the range [-a, a] may also be defined, where the sub-range [-b, b] also includes the ratio value of one. The activation gain may be set to a constant value, e.g. one, for values of the ratio in the sub-range [-b, b]. Values of the ratio outside of the sub-range [-b, b] but inside the range [-a, a] may then be set to a value less than the constant value in the sub-range (e.g. less than one), but greater than zero. As illustrated in <FIG>, the activation gain may vary linearly with the ratio value in this range. Purely as an illustrative example, the values a and b may be <NUM> and <NUM>, respectively; however, it will be understood that these values may be tuned to any suitable values.

In some examples, the activation gain may be further modified from the value determined as outlined above. In particular, one or more further considerations may be taken into account when setting the gain value for the present control scheme, i.e. when determining to what extent the present control scheme seeks to dampen tower 3P frequency content. This further refinement may be performed with the aim of limiting fatigue of the blade bearings. For instance, the energy in the tower <NUM> at 3P may be taken into consideration for this purpose.

As illustrated in <FIG>, in one example the activation gain as described in connection with <FIG>, is modified (in the control block <NUM>) by multiplying it by a secondary activation element <NUM> that assesses the 3P content in the tower acceleration signal <NUM>. In particular, acceleration signals may be received relating to the acceleration of the tower <NUM>, e.g. side-to-side <NUM> and/or fore-aft <NUM> tower acceleration. These acceleration signals may be received from one or more accelerometers located in the tower <NUM> or nacelle <NUM> of the wind turbine <NUM>. An estimation of the 3P content in the tower <NUM> may then be determined using a defined wind turbine tower model based on the acceleration signals. A gain value <NUM> based on this 3P content estimation may then be determined. For instance, the determined gain may be higher for higher values of the estimated 3P content. This content-dependent gain <NUM> may then be combined with (multiplied by) the activation gain to obtain the modified activation gain used for applying the control scheme to reduce tower 3P fatigue.

Returning to <FIG>, a maximum pitch reference offset amplitude may be set and be variable for different operating points of the wind turbine <NUM>. In particular, different rotor or generator speeds of the wind turbine <NUM> can influence the demand on a pitch adjustment system, in particular effecting pump capacity of the hydraulic pump. At lower rotor speeds, the pitch system is used to adjust blade pitch less frequently than when the wind turbine <NUM> is being operated at nominal speed. From the perspective of pump capacity of the pitch system, therefore, extreme values (maximum or minimum values) of the allowable pitch amplitude may be increased at lower rotor speeds with having significant effect on the pitch system, as may a rate at which the blade pitch is adjusted. Indeed, such an increase may beneficially assist with lubrication of the pitch system, which is used less frequently at lower speeds.

The allowable maximum pitch reference offset may be set on the Md,<NUM>P, Md,<NUM>P / d<NUM>q<NUM> components after application of the activation gain by imposing a saturation on the components so that the resulting pitch reference offset values after the inverse coordinate transformation are limited by the allowable maximum level.

In view of the above, constraints on the pitch amplitude and/or rate may be defined based on rotor or generator speed. Note that the pitch rate and amplitude limits of a 2P pitch controller is typically defined relative to nominal speeds. When the ratio is close to, or equal to, one - indicating that the wind turbine <NUM> is operating at nominal speed (e.g. in a full-load operating region) - then the allowable pitch reference offset amplitude may be lower than when the ratio is significantly less than one, indicating that the wind turbine <NUM> is operating a lower rotor speed.

The gain-scheduled controller <NUM> may generate a first vector pointing in a first direction in the fixed coordinate frame. The direction of this vector may then be manipulated so that the 3P control components that are then output from the module <NUM> may form a second vector pointing in a second direction in the fixed frame, where the second vector may be determined to counteract the first vector. In some sense, the second vector may be regarded as being opposite to the first vector, such that the control signal is 'opposite' to what the wind turbine is doing, so as to mitigate the tower 3P content. These processing components are therefore aimed at addressing the excitations experienced by the wind turbine tower <NUM> as indicated in the 3P frequency content of the received signal.

<FIG> is a plot illustrating simulation <NUM> equivalent loads obtained by implementing the described control scheme for reducing fatigue in the wind turbine tower <NUM> caused by 3P frequency content. In particular, <FIG> shows a comparison of performance results when the described control scheme is used versus when the described control scheme is not used. Specifically, the circle data points <NUM> correspond to a case in which the described control scheme is used, and the asterisk/star data points <NUM> correspond to a case in which the described control scheme is not implemented. It may be seen that, overall, the tower experiences a lesser degree of excitation than when the control scheme is implemented compared to when it is not.

<FIG> summarises the steps of a method <NUM> performed by the controller <NUM> of the wind turbine <NUM>. At step <NUM>, the method <NUM> involves receiving sensor data indicative of flap loading on each of the respective rotor blades <NUM>. The sensor data may be received from one or more flap loading sensors <NUM> of each of the three rotor blades <NUM>. Alternatively, the flapwise loading on each blade <NUM> may be obtained or determined in any other suitable manner. The flapwise loading on each blade <NUM> is used to obtain a three-dimensional flap loading vector in a rotor coordinate frame of the wind turbine <NUM>.

At step <NUM>, the method <NUM> includes applying an m-blade coordinate transformation to the flap loading vector to obtain first and second mutually orthogonal components in a fixed coordinate frame of the wind turbine <NUM>. The m-blade coordinate transformation may be a Coleman transformation. The first and second components may be in respective d and q directions of a d-q fixed reference frame. Obtaining the flap loading vector may include applying a filter to the flap loading data to remove content related to an imbalance in the wind turbine rotor <NUM>. Alternatively, the flap loading vector may be formed based on the received flap loading data, and then modified to remove the rotor imbalance content.

At step <NUM>, the method <NUM> includes determining first and second 3P components in the fixed coordinate frame based on the obtained first and second components. These 3P components are indicative of 3P frequency content of the wind turbine tower <NUM>. The 3P components may be determined by applying a transform to the first and second components. The transform may be based on a phase of the 3P frequency content of the tower.

At step <NUM>, the method <NUM> includes applying a control action to the 3P components to obtain respective 3P control components for mitigating the 3P frequency content of the tower <NUM>. This may include application of a gain-scheduled control module to the 3P components. An activation gain for the control action may be determined based on a difference between the 3P frequency and a natural frequency mode, e.g. tower first mode. The activation gain may be such that the control action is applied only if the difference between the 3P and natural frequencies is small enough, e.g. only if the 3P frequency is within a prescribed amount of the natural frequency. The activation gain may vary depending on the precise difference between the 3P and natural frequencies. In a general sense, the activation gain may be greater when the difference between the 3P and natural frequencies is smaller. The activation gain may be modified in view of additional factors. For instance, an energy in the wind turbine tower <NUM> may be determined based on a tower acceleration signal, with the activation gain being modified based on the estimated energy.

An allowable maximum pitch reference offset in the gain-scheduled control module may vary depending on the operating point of the wind turbine <NUM>. In particular, an allowable maximum pitch reference offset may be determined or set based on the rotation speed of the rotor <NUM> and, specifically, based on the rotor speed relative to a nominal rotor speed. In general, the allowable maximum pitch reference offset may be greater for lower values of the rotor speed relative to the nominal value. A greater allowable maximum pitch reference offset may be reflected by a greater pitch rate of blade pitch adjustment and/or more extreme values of blade pitch being realisable (i.e. greater amplitude levels).

At step <NUM>, the method <NUM> involves applying an inverse m-blade coordinate transformation to the 3P control components (in the fixed reference frame) to obtain pitch reference offset values Δθ<NUM>, Δθ<NUM>, Δθ<NUM> for the respective rotor blades <NUM> in the rotor coordinate frame, i.e. the output of the PAU <NUM> in <FIG>. This inverse transformation may be in the form of an inverse Coleman transformation. At step <NUM>, the method <NUM> involves transmit a control signal to adjust pitch of the rotor blades <NUM> based on the obtained pitch reference offset values Δθ<NUM>, Δθ<NUM>, Δθ<NUM>. For instance, the pitch reference offset values may be combined with a collective pitch reference θcol from the speed control unit <NUM> as shown in <FIG> to obtain pitch modification signals Δθ<NUM>, Δθ<NUM>, Δθ<NUM> that are used by a pitch actuation unit to adjust the pitch of the rotor blades <NUM>.

Claim 1:
A controller (<NUM>) for a wind turbine (<NUM>) having three rotor blades, the controller being for adjusting pitch of the three rotor blades (<NUM>), the controller (<NUM>) being configured to:
receive sensor data, from a flap loading sensor (<NUM>) of each of the three rotor blades (<NUM>), indicative of flap loading on each of the respective rotor blades (<NUM>), and obtain, based on the received sensor data, a flap loading vector in a rotor coordinate frame of the wind turbine (<NUM>);
apply an m-blade coordinate transformation to the flap loading vector to obtain first and second mutually orthogonal components in a fixed coordinate frame of the wind turbine (<NUM>);
determine first and second 3P components in the fixed coordinate frame based on the obtained first and second components, the first and second 3P components being indicative of 3P frequency content of a tower (<NUM>) of the wind turbine (<NUM>);
apply, using a gain-scheduled control module (<NUM>) of the controller (<NUM>), a control action to the first and second 3P components to obtain respective first and second 3P control components for mitigating the 3P frequency content of the tower (<NUM>), wherein an activation gain for the control action is determined by the gain-scheduled control module (<NUM>) based on a comparison of the 3P frequency to a natural frequency of the tower (<NUM>);
apply an inverse m-blade coordinate transformation to the first and second 3P control components to obtain pitch reference offset values for the respective rotor blades (<NUM>) in the rotor coordinate frame; and,
transmit a control signal to adjust pitch of the rotor blades (<NUM>) based on the obtained pitch reference offset values.