Gain-adjusted collective pitch control for reducing fore-aft oscillation amplitude of a wind turbine tower

Abstract

The invention relates to controlling a wind turbine that has a tower and a rotor with a plurality of pitch-adjustable rotor blades. The invention involves obtaining a thrust force signal indicative of an estimation of thrust force experienced by the rotor, and obtaining a collective pitch offset signal for reducing oscillation of a top of the tower in a fore-aft direction of the wind turbine. The invention involves determining a gain based on the thrust force signal, applying the gain to the collective pitch offset signal to obtain a gain-adjusted collective pitch offset signal, and controlling the rotor blades in accordance with the gain- adjusted collective pitch offset signal.

Description

[0001] GAIN-ADJUSTED COLLECTIVE PITCH CONTROL FOR REDUCING FORE-AFT

[0002] OSCILLATION AMPLITUDE OF A WIND TURBINE TOWER

[0003] TECHNICAL FIELD

[0004] The invention relates to controlling a wind turbine. In particular, the invention relates to reducing the amplitude of fore-aft oscillations of a wind turbine tower and, specifically, to determining and applying a gain to a collective pitch offset signal for adjusting pitch of the wind turbine rotor blades to reduce fore-aft tower oscillation amplitude during transient loading events.

[0005] BACKGROUND

[0006] Wind turbines as known in the art have a tower supporting a nacelle and a rotor with a plurality of pitch-adjustable rotor blades. A wind turbine is prone to vibrations or oscillations, such as tower, nacelle, or rotor blade movement. It is known that certain types of oscillations 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.

[0007] Rotor blades may be adjusted as part of a collective pitch control routine, in which each of the rotor blades is adjusted in the same way at the same time. In particular, collective pitch control of the rotor blades may be used to dampen or reduce oscillations or vibrations of the wind turbine tower in a fore-aft direction of the wind turbine, and may be referred to as fore-aft tower damping (FATD) control. Specifically, FATD control uses a collective pitch oscillation to generate a thrust offset in an opposite phase to that of the tower motion. FATD control may typically be used for reducing tower fatigue loads.

[0008] Tuning wind turbine FATD control may be regarded as a trade-off between the magnitude of the damping of the tower oscillations to reduce tower loads, and the level of pitch actuation activity. More aggressive tuning to increase the magnitude of the tower oscillation damping leads to increased pitch actuation activity, which can lead to high pitch fatigue, e.g. pitching bearing wear. It may also lead to marginal stability of the FATD control feature and / or some power loss if the FATD control feature is activated at below rated wind speeds. On the other hand, less aggressive tuning to limit the magnitude of the tower oscillation damping may result in insufficient reduction of tower oscillations, meaning that tower loading remains at undesirably high levels.

[0009] It is against this background to which the present invention is set.

[0010] SUMMARY OF THE INVENTION

[0011] According to an aspect of the invention there is provided a method of controlling a wind turbine. The wind turbine comprises a tower and a rotor with a plurality of pitch-adjustable rotor blades. The method comprises obtaining a thrust force signal indicative of an estimation of thrust force experienced by the rotor. The method comprises obtaining a collective pitch offset signal for reducing oscillation amplitude of a top of the tower in a fore-aft direction of the wind turbine. The method comprises determining a gain based on the thrust force signal. The method comprises applying the gain to the collective pitch offset signal to obtain a gain-adjusted collective pitch offset signal. The method comprises controlling the rotor blades in accordance with the gain-adjusted collective pitch offset signal.

[0012] If the estimated thrust force is less than a lower thrust threshold then the gain may be determined to be a minimum gain value. Optionally, the minimum gain value may be one.

[0013] If the estimated thrust force is greater than or equal to an upper thrust threshold, greater than the lower thrust threshold, then the gain may be determined to be a maximum gain value.

[0014] The gain may be determined to increase from the minimum gain value at the lower thrust threshold to the maximum gain value at the upper thrust threshold. Optionally, this increase may be a linear increase.

[0015] The method may comprise obtaining a velocity signal indicative of a velocity of the top of the tower in the fore-aft direction of the wind turbine. The method may comprise determining the gain based on the velocity signal.

[0016] The gain may be determined to be greater than a minimum gain value and less than a maximum gain value if: the estimated thrust force is greater than or equal to a lower thrust threshold and less than an upper thrust threshold, greater than the lower thrust threshold, and the tower top velocity is greater than or equal to a lower velocity threshold and less than an upper velocity threshold, greater than the lower velocity threshold; or, the estimated thrust force is greater than or equal to the lower thrust threshold and less than the upper thrust threshold, and the tower top velocity is greater than the upper velocity threshold; or, the estimated thrust force is greater than the upper thrust threshold, and the tower top velocity is greater than or equal to the lower velocity threshold and less than the upper velocity threshold.

[0017] Determining the gain may comprise: determining a first gain based on the thrust force signal; and, determining a second gain based on the velocity signal. The gain may be determined by multiplying together the first gain and the second gain.

[0018] The first gain may be determined to increase linearly from a first minimum gain value at the lower thrust threshold to a first maximum gain value at the upper thrust threshold. The second gain may be determined to increase linearly from a second minimum gain value at the lower velocity threshold to a second maximum gain value at the upper velocity threshold.

[0019] If an or the estimated thrust force is less than a or the lower thrust threshold or the tower top velocity is less thana or the lower velocity threshold, then the gain may be determined to be a or the minimum gain value. Optionally, the minimum gain value may be one.

[0020] The gain may be determined to be a or the maximum gain value only if the estimated thrust force is greater than or equal to an or the upper thrust threshold and the tower top velocity is greater than or equal to an or the upper velocity threshold.

[0021] If the gain is determined to be the maximum gain value, then the method may comprise holding the gain at the maximum gain value for at least a first predefined time period.

[0022] If the gain is determined to be greater than the minimum gain value and less than the maximum gain value, then the method may comprise holding the gain at a value greater than the minimum gain value for at least a second predefined time period.

[0023] Optionally, the first predefined time period may be equal to the second predefined time period. Obtaining the velocity signal may comprise: obtaining a measured acceleration signal indicative of measured acceleration of the top of the tower in the fore-aft direction of the wind turbine; obtaining a further estimated thrust force signal indicative of estimated thrust force experienced by the rotor, the further estimated thrust force signal being obtained based on an obtained blade flap load signal indicative of measured flap loading on the rotor blades, and providing the further estimated thrust force signal as input to a defined observer model describing motion of the top of the tower; determining an error signal based on a difference between the measured acceleration signal and an estimated acceleration signal obtained using the defined observer model, and providing the error signal as input to the observer model as part of a feedback loop; and, determining, using the defined observer model, the velocity signal indicative of estimated velocity of the top of the tower in the fore-aft direction of the wind turbine.

[0024] The further estimated thrust force signal may be obtained based on a determined quasistatic thrust force signal indicative of quasi-static thrust force experienced by the rotor, the quasi-static thrust force signal being determined using a defined blade element model.

[0025] Optionally, the estimated thrust force signal may be equal to the further estimated thrust force signal.

[0026] Optionally, the further estimated force signal is obtained by adding the obtained blade flap load signal indicative of measured flap loading on the rotor blades to the determined quasi- static thrust force signal indicative of quasi-static thrust force experienced by the rotor.

[0027] The collective pitch offset signal may comprise a first collective pitch offset signal and a second collective pitch offset signal, the first collective pitch offset signal being determined based on an obtained position signal indicative of a position of the top of the tower in the fore-aft direction of the wind turbine, and the second collective pitch offset signal being determined based on an obtained further velocity signal indicative of velocity of the top of the tower in the fore-aft direction of the wind turbine. One or both of the first collective pitch offset signal and the second collective pitch offset signal may be gain-adjusted by application of the determined gain.

[0028] 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 perform the method defined above. According to another aspect of the invention there is provided a controller for controlling a wind turbine. The wind turbine comprises a tower and a rotor with a plurality of pitch- adjustable rotor blades. The controller is configured to obtain an estimation of thrust force experienced by the rotor. The controller is configured to obtain a collective pitch offset signal for reducing oscillation amplitude of a top of the tower in a fore-aft direction of the wind turbine. The controller is configured to determine a gain based on the estimated thrust force. The controller is configured to apply the gain to the collective pitch offset signal to obtain a gain-adjusted collective pitch offset signal. The controller is configured to control the rotor blades in accordance with the gain-adjusted collective pitch offset signal.

[0029] According to another aspect of the invention there is provided a wind turbine comprising a controller as defined above.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Examples of the invention will now be described with reference to the accompanying drawings, in which:

[0032] Figure 1 is a schematic diagram of a wind turbine in accordance with an example of the invention;

[0033] Figure 2 schematically illustrates a controller of the wind turbine of Figure 1 in accordance with an aspect of the invention;

[0034] Figure 3 shows a schematic plot indicating a gain to be applied to a collective pitch offset signal determined by the controller of Figure 2, the gain being a function of a thrust force and a tower top velocity of the wind turbine of Figure 1 ;

[0035] Figure 4 schematically illustrates an example of how the tower top velocity in Figure 3 is determined; and,

[0036] Figure 5 shows the steps of a method performed by the controller of Figure 2 in accordance with an aspect of the invention. DETAILED DESCRIPTION

[0037] The present invention recognises that it would be beneficial to have relatively aggressive tuning I high gains on a fore-aft tower damping (FATD) control feature I routine of a wind turbine during transient load cases. In particular, the transient load cases may be extreme events, such as wind gusts, where loading on the tower and other wind turbine components may be higher than in other conditions I cases. It is beneficial to have relatively high gains on a FATD control feature to be able to quickly reduce the amplitude of tower oscillations for a brief period during high loading events. By ramping up FATD control only in such transient load cases, e.g. during extreme events - or at least reserving the most aggressive interventions of FATD control for such cases - then FATD control may have relatively minimal impact on pitch and tower fatigue; however, FATD control will advantageously have relatively large impact on tower extreme loads. The invention therefore beneficially provides for increasing the gains of FATD control only I primarily in cases where it is necessary to reduce a risk of extreme tower loads. Further advantages of the invention will become apparent from the following description.

[0038] Figure 1 illustrates, in a schematic view, an example of a wind turbine 10. The wind turbine 10 includes a tower 102, a nacelle 103 disposed at the apex of, or atop, the tower 102, and a rotor 104 operatively coupled to a generator housed inside the nacelle 103. In addition to the generator, the nacelle 103 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 10. The rotor 104 of the wind turbine 10 includes a central hub 105 and three rotor blades 106 that project outwardly from the central hub 105. Moreover, the wind turbine 10 comprises a control system or controller (not shown in Figure 1). The controller may be placed inside the nacelle 103, in the tower 102 or distributed at a number of locations inside (or externally to) the turbine 10 and communicatively connected to one another. The rotor blades 106 are pitch-adjustable. The rotor blades 106 can be adjusted in accordance with a collective pitch setting, where each of the blades are set to the same pitch value. The rotor blades 106 may additionally be adjustable in accordance with individual pitch settings, where each blade 106 may be provided with an individual pitch setpoint.

[0039] In some examples, the wind turbine 10 includes blade load sensors placed at, or in the vicinity of, each blade root 109 in a manner such that the sensor detects loading in the blade 106. Blade load signals from such sensors may be used to determine how to adjust the pitch of each of the individual blades 106. Depending on the placement and the type of sensor, loading may be detected in the flap (flapwise) direction (in / out of plane) or in the edge (edgewise) direction 108 (in-plane). Such sensors may be strain gauge sensors or optical Bragg-sensors, for instance. As the sensors are placed on the rotating blades 106, such load signals for each of the adjustable rotor blades 106 are measured in the rotating reference frame of the rotor 104.

[0040] Figure 2 schematically illustrates elements of an example of a controller 20 of the wind turbine 10 implemented to determine pitch actuation signals capable of maximising power generation and reducing or mitigating loads experienced by one or more components of the wind turbine 10, e.g. tower 102, rotor blades 106, etc. In the illustrated implementation, a collective pitch control module in the form of a speed controller (control module / block) 202 of the controller 20 minimises a speed error (® - ®ref) between the actual rotor speed, co, and a reference rotor speed, ®ref, in order to output a requested power P (in the form of a power setpoint) and a collective pitch reference, 0COi- The collective pitch reference as determined by the speed controller 202, in view of the rotor speed, may also take further sensor values into account. This is referred to in Figure 2 as a measurement set, ms, being input into the speed controller 202. The feedback speed controller 202 may be implemented by a PI (proportional-integral), PID (proportional-integral-derivative), or similar control scheme. In one example, the collective pitch control module 202 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.

[0041] Figure 2 further illustrates a control block / module or pitch offset controller 204, of the controller 20. In the pitch actuation unit 204, pitch modification signals, or pitch reference offset values, are determined based on one or more input signals 205. In the described example, the controller 204 is, or comprises, a fore-aft tower damping (FATD) controller or control feature. The FATD controller 204 is for reducing I counteracting oscillations I vibrations of the top of the tower 102, and / or the nacelle 103, in a fore-aft direction of the wind turbine 10, i.e. the direction perpendicular to a plane of the rotor 104 (e.g. in a direction in-out of the page in Figure 1). In particular, the FATD controller 204 determines and outputs a collective pitch offset signal or reference, 0Off, 206. The collective pitch offset signal 206 is to be combined, e.g. added, to the collective pitch reference 9COioutput from the speed controller 202 to obtain a collective pitch control signal, 0A, 207 which is to be used to control the rotor blades 106. In particular, the controller 20 sends the collective pitch control signal 207 to a pitch system of the wind turbine 10 to control the pitch bearings such that the pitch of the rotor blades 106 is adjusted in accordance with the collective pitch control signal 207.

[0042] However, in accordance with examples of the invention, prior to the collective pitch offset signal 206 being added to the collective pitch reference 0coi, a gain is determined and applied, e.g. via multiplication, to the collective pitch offset signal 206 obtained from the FATD controller 204 to obtain a gain-adjusted collective pitch offset signal, 0gain, 208. It is this gain-adjusted collective pitch offset signal 208 that is added to the collective pitch reference 0COifrom the speed controller 202 to obtain the collective pitch control signal 207.

[0043] A gain scheduling control block or controller 209 determines and outputs the gain 210 for applying to the collective pitch offset signal 206. The gain 210 may typically be in the form of a factor or multiple to be applied to the output from the FATD controller 204. The gain 210 is determined based on one or more input signals 211. This will be described in greater detail below.

[0044] Referring back to the FATD controller 204, this controller 204 may determine the collective pitch offset signal 206 based one or both of: a signal indicative of a velocity of the nacelle 103 or top of the tower 102 in the fore-aft direction; and, a signal indicative of a position of the nacelle 103 or top of the tower 102 in the fore-aft direction. That is, the input signal 205 to the FATD 204 may include the tower top fore-aft velocity and / or position.

[0045] In examples in which the nacelle or tower top position in the fore-aft direction is used by the FATD controller 204, the position signal may be obtained in any suitable manner. For instance, the position may be determined based on a measured acceleration signal indicative of the nacelle or tower top movement, where the acceleration signal may be obtained from an accelerometer positioned at the top of the tower 102 or in / on the nacelle 103. The acceleration signal may then be integrated twice as appropriate in order to obtain position. In general, any suitable filter that integrates the relevant signal may be applied to obtain velocity and position from acceleration, e.g. leaky filters. The position signal may be obtained in different ways, such as a GPS signal, an inclinometer, an inertial measuring unit (IMU), or a Kalman filter.

[0046] Basing the collective pitch offset signal 206 determined by the FATD 204 on the fore-aft position of the nacelle 103 or tower top may beneficially result in stabilisation of the nacelle 103, and may guard against problems with undesired couplings between the controller 20, including the speed controller 202, and the tower 102.

[0047] In examples in which the nacelle or tower top velocity in the fore-aft direction is used by the FATD controller 204, the velocity signal may be obtained in any suitable manner. The velocity may for instance be a centre-of-mass velocity of the nacelle 103, the velocity of an appropriate sensor, or the velocity of other fixed points deemed to represent the movement of the nacelle 103 or top of the tower 102 in the fore-aft direction. Indeed, the velocity may, like the position, be determined based on a measured acceleration signal indicative of the nacelle or tower top movement, the acceleration being integrated to obtain the velocity.

[0048] Now referring back to the gain scheduling controller 209, as described above it is desired to have relatively aggressive intervention from the FATD controller 204 - by means of relatively high gains applied to the collective pitch offset signal 206 - during transient events, such as extreme load cases, e.g. wind gusts. This means that the gain scheduling controller 209 needs to be able to detect and react quickly to the onset of transient events. Indeed, a transient event - where the tower 102 may end up in an extreme deflected position (with associated high loading) - can typically be detected earlier than the extreme deflection actually occurs. Hence, determining the gain 210 based on the detection of transient events such as extreme load cases may result in at least partially preventing extreme deflection of the tower 102.

[0049] A rapidly increasing wind speed, e.g. during a wind gust, will cause an increase in rotor thrust. A high rotor thrust is associated with a high force at the top of the tower 102. In the described example, the gain scheduling controller 209 therefore determines the gain 210 based on estimated rotor thrust, i.e. the estimated thrust force experienced by the rotor

[0050] 104. The input signal 211 to the gain scheduling controller 209 therefore comprises an estimated rotor thrust signal. Advantageously, scheduling the gain 210 based on estimated rotor thrust allows for the controller 20 to react quickly to transient events to dampen fore- aft tower oscillations by ramping up the gain 210 applied to the collective pitch offset signal 206.

[0051] An increase in rotor thrust when the wind speed has increased rapidly may then be followed by an increase in the velocity of the top of the tower 102 or the nacelle 103 in the fore-aft direction. A high tower top (forward or backward) velocity is associated with high tower deflection and high loads. In the described example, the gain scheduling controller 209 therefore determines the gain 210 additionally based on tower top velocity or nacelle velocity in the fore-aft direction, i.e. the input signal 211 comprises a tower top velocity signal. In particular, in the described example a gain scheduling scheme - which may also be regarded as a trigger I activation scheme of the FATD controller 204 - is implemented such that gains applied to the collective pitch offset signal 206 are increased as necessary. By scheduling the gain 210 based on tower top velocity as well as rotor thrust, the number of activations of the FATD controller 204 is beneficially reduced - thereby reducing pitch fatigue - compared to if the gain 210 is scheduled based on rotor thrust only.

[0052] A relatively high tower top velocity is associated with an expectation that a relatively high tower top deflection will follow later in an oscillation cycle. As such, determination of tower top velocity can be used to identify potential high loading scenarios before they occur, meaning that early or preventative action may be taken.

[0053] The described controller 20 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 custom 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.

[0054] Figure 3 schematically shows an activation scheme 30 for the FATD controller 204 according to the described example. In particular, Figure 3 indicates how the gain 210 is determined by the gain scheduling controller 209 based on rotor thrust force and tower top velocity. Specifically, Figure 3 indicates three activation regions 301 , 302, 303: a minimally active (or, optionally, deactivated) region 301 , a partially active region 302, and a fully active region 303.

[0055] The different activation regions 301 , 302, 303 are delineated or separate with reference to respective threshold values related to the rotor thrust force and tower top velocity. Each of the threshold values may be predefined. As indicated in Figure 3, in the described example each of the rotor thrust force and tower top velocity has two threshold values associated therewith. The thrust force thresholds may be referred to as a first or lower thrust threshold value 304 and a second or upper thrust threshold value 305, the upper threshold value

[0056] 305 being greater than the lower threshold value 304. In a corresponding manner, the tower top velocity thresholds may be referred to as a first or lower velocity threshold value

[0057] 306 and a second or upper velocity threshold value 307, the upper threshold value 307 being greater than the lower threshold value 306.

[0058] As is apparent from Figure 3, in the described example if the rotor thrust is less than the lower thrust threshold 304, or the tower top velocity is less than the lower velocity threshold 306, then the activation scheme is in the minimally active region 301 . If the rotor thrust is greater than the lower thrust threshold 304, and the tower top velocity is greater than the lower velocity threshold 306, but the rotor thrust is less than the upper thrust threshold 305, or the tower top velocity is less than the upper velocity threshold 307, then the activation scheme is in the partially active region 302. If the rotor thrust is greater than the upper thrust threshold 305, and the tower top velocity is greater than the upper velocity threshold 307, then the activation scheme is in the full active region 303.

[0059] The gain scheduling controller 209 therefore determines the gain 210 based on which region of the activation scheme 30 operation of the wind turbine 10 is in. For instance, in the minimally active region 301 the gain may be determined to be a minimum gain value. In some examples, this minimum value may be one. This would be regarded as continuing with normal or default operation of the FATD controller 204 as the collective pitch offset signal 206 obtained from the FATD controller 204 would in this case be multiplied by the gain 210 equal to one.

[0060] In the fully active region 303 the gain may be determined to be a maximum gain value in order to effect maximum intervention of the FATD controller 204 in such operating conditions. This maximum value may be greater than one, to allow the FATD controller 204 to ramp up its intervention, relative to a normal or default level (which may correspond to a gain value equal to one), during transient events. For instance, the maximum gain value may be two or any other suitable value.

[0061] In the partially active region 302, the gain 210 may be determined based on the specific values of thrust force and tower top velocity (obtained from the input signals 211). In the described example, the gain 210 in the partially active region 302 may increase linearly from the minimum gain value when the thrust force equals the lower thrust threshold 304 and / or when the tower top velocity equals the lower velocity threshold 306 to the maximum gain value when the thrust force is greater than or equal to the upper thrust threshold 305 and the tower top velocity is greater than or equal to the upper velocity threshold 306.

[0062] In practice, this may be implemented in series, with respective gains being determined for thrust force and tower top velocity, before combining, e.g. multiplying, these together to obtain the (overall) gain 210 to be applied to the collective pitch offset signal 206. That is, a first (thrust) gain may be determined based on the thrust force signal 211 , and a second gain (velocity) may be determined based on the tower top velocity signal 211 (where the first and second gains may be determined in either order), with the first and second gains being multiplied together to obtain the overall gain 210. For instance, the first gain may increase linearly from the minimum gain value at the lower thrust threshold 304 to a maximum gain value at the upper thrust threshold 305, and the second gain may increase linearly from the minimum gain value at the lower velocity threshold 306 to a maximum gain value at the upper velocity threshold 307. The determination of gain may be implemented as a look up table of gain values. In other words, a respective gain may vary linearly in each axis direction, with these being combined to determine the overall gain. It will be understood that in different examples, variation other than linear variation may be utilised.

[0063] When the FATD controller 204 has reduced the rotor thrust and tower top velocity back to lower levels then the gain may be reverted I decreased back to a normal I default value. However, in order to ensure that the FATD control feature is (sufficiently) active throughout the duration of a transient event, a hold function may be applied to hold the determined gain value at a higher level for at least a defined duration of time. Hence, even if the FATD control feature reduces rotor thrust and tower top velocity to lower levels while a transient event is ongoing, then the gain is held at a higher level for the duration of the event. The defined duration may for instance be of the order of one to two tower fore-aft motion cycles. For instance, if the gain is determined to be the maximum gain value as the wind turbine 10 is operating in the fully active region 303, then if one of the rotor thrust and tower top velocity is reduced to below the respective upper threshold value 305, 307 such that operation is now in the partially active region 302, then the gain may still be held at the maximum gain value for a defined time period. A similar approach may be taken if wind turbine operation is reduced from the partially active region 302 to the minimally active region 301. The estimated thrust force signal 211 to be used by the scheduling controller 209 may be obtained in any suitable way. For instance, the thrust force may be estimated based on a defined equation that depends on a thrust coefficient. In one example, the thrust force Ftmay be estimated according to: where p is air density, R is a radius of the rotor 104, V is wind speed, 6 is pitch angle of the rotor blades 106, is rotational speed of the rotor 104, and the thrust coefficient Ctis a defined function of pitch angle 6 and tip speed ratio OJR / V.

[0064] In another example, the thrust force may be estimated based on flap load measurements obtained from flap load sensors on the rotor blades and on a blade element momentum model (BEM). The measured flap load may be obtained by combining, e.g. averaging, the measured flap load signal obtained from the sensor of each rotor blade 106. Alternatively, the measured flap load may be based on a flap load signal from only one of the blade load sensors. The blade flap load measurement is indicative of the bending moment at the root 109 of the rotor blade 106.

[0065] As is known in the art, a BEM breaks down a rotor blade into several small elements (along its span) and then determines the forces and moments acting on each of these elements. These forces are then integrated across the entire blade to obtain the forces and moments experienced by the rotor blade. In the present context, a BEM provides a signal that can map a sum of measured flap loads (moment / torque) of unit Nm into a rotor thrust force of unit N, i.e. the mapping signal of unit 1 / m. Specifically, the BEM may calculate forces and moments on the rotor blade 106 based on lift and drag curves for individual blade elements of the blades, which are then summed to the (full) rotor 104. The forces and moments are represented at a centre of the rotor hub I an intersection point of the rotor blades 106; however, as mentioned above, the blade load sensors are positioned at or near to (e.g. up to a few metres from) a root of the rotor blades 106, which is some radius or distance out from the centre of the rotor hub. This may be used to obtain rotor thrust force from the blade flap loads measured by the blade load sensors of the rotor blades 106.

[0066] The velocity signal 211 to be used by the scheduling controller 209 may also be obtained in any suitable way. For instance, the tower top velocity in the fore-aft direction may be determined in the same way as described above for use by the FATD controller 204. That is, the tower top velocity may be determined based on integrating an acceleration signal indicative of measured acceleration in the fore-aft direction, e.g. from an accelerometer located at the top of the tower 102 or in the nacelle.

[0067] Alternatively, the velocity signal 211 may be obtained based on an observer model approach. Figure 4 schematically illustrates how the tower top fore-aft velocity may be estimated according to an estimation scheme 40 that uses an observer model approach. The scheme 40 includes an observer block 41 with a defined observer model 411 , which may be any suitable model known in the art for describing motion of the top of the tower 102 or the nacelle 103. For instance, the model may be a one-dimensional (linear) spring damper model as is known in the art; however, more complex models may also be used. As is known, the model defines a state vector x consisting of the states of the system being modelled, i.e. a set of variables used to describe the system dynamics, which are embodied in a state space matrix A, and another (input) vector u representing external inputs to the system, e.g. wind speed variation, which affect the state dynamics through an input matrix B. Also as is known, the model defines an output vector Y that is obtained based on an output matrix C acting on the state vector x added to a feedforward matrix D acting on the input vector u The model 411 further includes an error term e multiplied by a gain K.

[0068] The observer model 411 provides as output an estimation of tower top acceleration 412 in the fore-aft direction. The error e between the estimated acceleration 412 and a measured acceleration signal 413 is determined at processing block 414. The measured acceleration signal is indicative of a measured acceleration of the tower top or nacelle 103 in the fore- aft direction, and may be obtained from an accelerometer positioned in the nacelle 103 or at the top of the tower 102. The error e is multiplied by a gain K at the processing block 415, and the gain-adjusted error signal 416 is fed back into the observer model 411 as part of a feedback loop.

[0069] The observer model 411 also takes as input an estimated thrust force signal 417. The estimated thrust force signal 417 may be obtained in any suitable manner. In the example illustrated in Figure 4, the thrust force is estimated based on measured blade flap loads from blade load sensors on the rotor blades, as described above. In particular, the sum of blade flap bending moments 418 obtained from the blade load sensor on each rotor blade 106 is used to obtain rotor thrust force 420 at processing block 419. In some examples, this could be used as the estimated thrust force signal 417. However, in the described example, the estimate of thrust force 417 is obtained from two separate sources. In addition to the blade flap load measurements, a quasi-static estimate of thrust force 421 is obtained. In order that the two estimates can be combined to obtain the estimated thrust force 417 for input into the observer model 411 , the output thrust force 420 from the processing block 419 is high-pass filtered at processing block 422, and this high-pass filtered estimate 423 is combined, e.g. added, to the quasi-static estimate 421 to obtain the estimated thrust force signal 417. Indeed, a low-pass filter may be applied to the quasi- static thrust force estimate 421 prior to the addition to the high-pass filtered thrust force estimate 423. The quasi-static estimate 421 may be obtained using a blade element momentum model 424, as described above.

[0070] Basing the estimation of thrust force on blade flap load measurements means that transient events, e.g. rapid changes in wind speed, can be detected quickly. When used for determining the gain to be applied to the FATD feature, this means that the determined gain can be increased rapidly in response to a transient event so that fore-aft tower damping can quickly and aggressively intervene to reduce fore-aft oscillation amplitude caused by the transient event. By using only the high frequency content of the thrust force estimate obtained based on the blade flap load measurements, and combining this with a quasi-static estimate of thrust force, a more accurate overall thrust estimate may be obtained (as the quasi-static estimate may be more accurate for low frequency content).

[0071] The use of a tower top velocity signal obtained using an observer approach that estimates rotor thrust based on blade flap load measurements allows for quick increases in the gain for FATD control to enable quick damping of tower oscillations that may occur during transient event load cases. This may be particularly useful in Extreme Coherent Gust with Direction change (ECD) load cases. The loads are reduced both during the initial part of the event, but also more dominantly during the second swing back of the tower (rebound load peak), where it has the potential to significantly reduce or even remove this load peak.

[0072] A velocity signal 425 indicative of estimated velocity (or speed) of the tower top in the fore- aft direction can then be obtained from the observer model 411 . The use of the observer model to obtain the velocity signal may beneficially provide a more accurate estimate of velocity than other approaches. In particular, the observer approach may advantageously retain stability when the gain on the FATD controller output is ramped up, meaning that higher gains may be applied I possible when this approach is used, relative to some other approaches. Indeed, this more accurate estimate of velocity allows for more accurate tuning of the trigger / activation thresholds used to determine the gain. A position signal 425 indicative of estimated position of the tower top in the fore-aft direction can also be obtained from the observer model 411.

[0073] Figure 5 summarises the steps of a method 50 performed by the controller 20 in accordance with the described example. At step 501 , the method involves obtaining a thrust force signal indicative of an estimation of thrust force experienced by the rotor 104. The rotor thrust force may be obtained in any suitable way, for instance according to one or more of the approaches outlined above.

[0074] In some examples, at this step the method 50 also involves obtaining a velocity signal indicative of a velocity of the nacelle 103 or the top of the tower 102 in the fore-aft direction of the wind turbine 10. The fore-aft tower top velocity may be obtained in any suitable way, for instance according to one or more of the approaches outlined above.

[0075] At step 502, the method 50 involves obtaining a collective pitch offset signal for reducing oscillation amplitude of the nacelle 103 or the top of the tower 102 in the fore-aft direction of the wind turbine 10. The collective pitch offset signal may be obtained from a fore-aft tower damping (FATD) controller or control feature of the wind turbine 10. The method 50 may in some examples be regarded as including the determination of the collective pitch offset signal by the FATD controller.

[0076] At step 503, the method 50 involves determining a gain based on the thrust force signal, and applying the gain to the collective pitch offset signal to obtain a gain-adjusted collective pitch offset signal. In examples in which the gain is determined based only on the thrust force signal, the gain may be determined to be a minimum gain value if the rotor thrust is below a lower threshold, and / or the gain may be determined to be a maximum gain value if the rotor thrust is greater than an upper threshold. The gain may increase, e.g. in a linear manner, from the minimum to maximum values between the lower and upper thresholds.

[0077] In examples in which the velocity signal is also obtained, then at step 503 the gain is determined based on both the thrust force signal and the velocity signal. The gain may be determined to be a minimum gain value if either: the estimated rotor thrust is less than a lower thrust threshold; or, the fore-aft tower top velocity is less than a lower velocity threshold. The gain may be determined to be a maximum gain value only if both: the estimated rotor thrust is greater than an upper thrust threshold; and, the fore-aft tower top velocity is greater than an upper velocity threshold. The gain may vary between the minimum and maximum values when both the rotor thrust and velocity are greater than the respective lower thresholds and at least one of the rotor thrust and velocity is less than the respective upper threshold.

[0078] It will be understood that in further examples, the gain may be determined based on further signals I parameters in addition to the rotor thrust and fore-aft tower top velocity.

[0079] In some examples, the minimum gain value may be one. In such examples, this corresponds to continuing normal or default operation of the FATD controller 204.

[0080] In some examples, the FATD controller 204 determines the collective pitch offset signal based on both the fore-aft position and the fore-aft velocity of the tower top or nacelle 103. This may be implemented as determining a first signal based on the obtained tower top position, determining a second signal based on the obtained tower top velocity, and then combining the first and second signals to obtain the collective pitch offset signal. The first and / or second signal may be subject to an applied gain prior to being combined with the other of the first and second signal. In such examples, the gain determined at step 503 as part of the present method 50 may be applied to only one of the first and second signals, or may be applied to the (overall) collective pitch offset signal.

[0081] At step 504, the method 50 involves controlling the rotor blades 106 in accordance with the gain-adjusted collective pitch offset signal. As described above, this may involve combining I applying the collective pitch offset signal with a collective pitch reference obtained from a speed controller, and sending the collective pitch control signal to a pitch system of the wind turbine 10 to control the pitch bearings such that the pitch of the rotor blades 106 is adjusted in accordance with the collective pitch control signal.

[0082] Many modifications may be made to the described examples without departing from the scope of the appended claims.

[0083] In the above-described example, the pitch actuation controller only includes a fore-aft tower damping controller. However, it will be understood that in different examples the pitch actuation controller may include further pitch offset controllers that output collective or individual pitch control signals for controlling the wind turbine rotor blades, e.g. side-to- side tower damping (SSTD) control. Different gains may be applied to the output signals of these different pitch offset controllers.

Claims

CLAIMS1. A method of controlling a wind turbine, the wind turbine comprising a tower and a rotor with a plurality of pitch-adjustable rotor blades, the method comprising: obtaining a thrust force signal indicative of an estimation of thrust force experienced by the rotor; obtaining a collective pitch offset signal for reducing oscillation amplitude of a top of the tower in a fore-aft direction of the wind turbine; determining a gain based on the thrust force signal, and applying the gain to the collective pitch offset signal to obtain a gain-adjusted collective pitch offset signal; and, controlling the rotor blades in accordance with the gain-adjusted collective pitch offset signal.

2. A method according to Claim 1 , wherein if the estimated thrust force is less than a lower thrust threshold then the gain is determined to be a minimum gain value.

3. A method according to Claim 2, wherein if the estimated thrust force is greater than an upper thrust threshold, greater than the lower thrust threshold, then the gain is determined to be a maximum gain value.

4. A method according to Claim 3, wherein the gain is determined to increase linearly from the minimum gain value at the lower thrust threshold to the maximum gain value at the upper thrust threshold; optionally, wherein the increase is a linear increase.

5. A method according to Claim 1 , the method comprising: obtaining a velocity signal indicative of a velocity of the top of the tower in the fore- aft direction of the wind turbine; and, determining the gain based on the velocity signal.

6. A method according to Claim 5, wherein if: the estimated thrust force is greater than or equal to a lower thrust threshold and less than an upper thrust threshold, greater than the lower thrust threshold, and the tower top velocity is greater than or equal to a lower velocity threshold and less than an upper velocity threshold, greater than the lower velocity threshold; or,the estimated thrust force is greater than or equal to the lower thrust threshold and less than the upper thrust threshold, and the tower top velocity is greater than the upper velocity threshold; or, the estimated thrust force is greater than the upper thrust threshold, and the tower top velocity is greater than or equal to the lower velocity threshold and less than the upper velocity threshold, then the gain is determined to be greater than a minimum gain value and less than a maximum gain value, and determining the gain comprises: determining a first gain based on the thrust force signal; and, determining a second gain based on the velocity signal, wherein the gain is determined by multiplying together the first gain and the second gain.

7. A method according to Claim 6, wherein the first gain is determined to increase linearly from a first minimum gain value at the lower thrust threshold to a first maximum gain value at the upper thrust threshold, and wherein the second gain is determined to increase linearly from a second minimum gain value at the lower velocity threshold to a second gain value at the upper velocity threshold.

8. A method according to Claim 6 or Claim 7, wherein if the estimated thrust force is less than the lower thrust threshold or the tower top velocity is less than the lower velocity threshold, then the gain is determined to be the minimum gain value.

9. A method according to any of Claims 6 to 8, wherein the gain is determined to be the maximum gain value only if the estimated thrust force is greater than or equal to the upper thrust threshold and the tower top velocity is greater than or equal to the upper velocity threshold.

10. A method according to any of Claims 6 to 9, wherein: if the gain is determined to be the maximum gain value, then the method comprises holding the gain at the maximum gain value for at least a first predefined time period; and / or, if the gain is determined to be greater than the minimum gain value and less than the maximum gain value, then the method comprises holding the gain at a value greater than the minimum gain value for at least a second predefined time period; optionally, wherein the first predefined time period is equal to the second predefined time period.

11. A method according to any of Claims 5 to 10, wherein obtaining the velocity signal comprises: obtaining a measured acceleration signal indicative of measured acceleration of the top of the tower in the fore-aft direction of the wind turbine; obtaining a further estimated thrust force signal indicative of estimated thrust force experienced by the rotor, the further estimated thrust force signal being obtained based on an obtained blade flap load signal indicative of measured flap loading on the rotor blades, and providing the further estimated thrust force signal as input to a defined observer model describing motion of the top of the tower; determining an error signal based on a difference between the measured acceleration signal and an estimated acceleration signal obtained using the defined observer model, and providing the error signal as input to the observer model as part of a feedback loop; and, determining, using the defined observer model, the velocity signal indicative of estimated velocity of the top of the tower in the fore-aft direction of the wind turbine.

12. A method according to Claim 11 , wherein the further estimated thrust force signal is obtained based on a determined quasi-static thrust force signal indicative of quasi-static thrust force experienced by the rotor, the quasi-static thrust force signal being determined using a defined blade element model; optionally, wherein the estimated thrust force signal is equal to the further estimated thrust force signal; further optionally, wherein the further estimated force signal is obtained by adding the obtained blade flap load signal indicative of measured flap loading on the rotor blades to the determined quasi-static thrust force signal indicative of quasi-static thrust force experienced by the rotor.

13. A method according to any previous claim, wherein the collective pitch offset signal comprises a first collective pitch offset signal and a second collective pitch offset signal, the first collective pitch offset signal being determined based on an obtained position signal indicative of a position of the top of the tower in the fore-aft direction of the wind turbine, and the second collective pitch offset signal being determined based on an obtained further velocity signal indicative of velocity of the top of the tower in the fore-aft direction of the wind turbine, and wherein one of the first collective pitch offset signal and the second collective pitch offset signal is gain-adjusted by application of the determined gain.

14. A controller for controlling a wind turbine, the wind turbine comprising a tower and a rotor with a plurality of pitch-adjustable rotor blades, the controller being configured to: obtain an estimation of thrust force experienced by the rotor; obtain a collective pitch offset signal for reducing oscillation amplitude of a top of the tower in a fore-aft direction of the wind turbine; determine a gain based on the estimated thrust force, and apply the gain to the collective pitch offset signal to obtain a gain-adjusted collective pitch offset signal; and, control the rotor blades in accordance with the gain-adjusted collective pitch offset signal.

15. A wind turbine comprising a controller according to Claim 14.

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