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

Abstract

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

Description

TECHNICAL FIELD

[0001] The present invention relates to controlling a wind turbine. In particular, the present invention relates to reducing the amplitude of fore-aft oscillations of a wind turbine tower, and in particular to determining a gain and applying it to a collective pitch offset signal for adjusting the pitch of wind turbine rotor blades to reduce the fore-aft tower oscillation amplitude during transient load events. BACKGROUND

[0002] Wind turbines known in the art have a tower supporting a nacelle and a rotor having a plurality of pitch-adjustable rotor blades. Wind turbines are prone to vibrations or oscillations, such as tower, nacelle or rotor blade movements. It is known that certain types of oscillations can be damped (suppressed) by active pitch of the rotor blades or by adjusting the generator torque. Control strategies for adjusting the blade pitch can be used to maximize the energy production of the wind turbine while minimizing the loads experienced by the various components of the wind turbine.

[0003] Rotor blades can be adjusted as part of a collective pitch control routine, where each rotor blade is adjusted simultaneously in the same way. In particular, collective pitch control of the rotor blades can be used to suppress or reduce oscillations or vibrations of the wind turbine tower in the fore-aft direction of the wind turbine, and can be referred to as fore-aft tower damping (FATD) control. In particular, FATD control uses collective pitch oscillations to create a thrust offset that is in phase opposition to the tower motion. FATD control can generally be used to reduce tower fatigue loads.

[0004] Tuning of wind turbine FATD control can be considered as a trade-off between the amplitude of the damping of the tower oscillations that reduce the tower loads and the level of pitch actuation activity. More aggressive tuning to increase the amplitude of the tower oscillation damping results in increased pitch actuation activity, which can result in high pitch fatigue, for example pitch bearing wear. It can also result in marginal stability of the FATD control feature and / or some power loss if the FATD control feature is activated below rated wind speed. On the other hand, less aggressive tuning that limits the amplitude of the tower oscillation damping can result in insufficient reduction of the tower oscillations, which means that the tower loads remain at undesirably high levels.

[0005] It is against this background that the present invention is made. SUMMARY

[0006] According to an aspect of the application, there is provided a method of controlling a wind turbine. The wind turbine comprises a tower and a rotor having a plurality of pitch-adjustable rotor blades. The method comprises obtaining a thrust signal indicative of an estimate of a thrust experienced by the rotor. The method comprises obtaining a collective pitch offset signal for reducing an amplitude of oscillations 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 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 dependence on the gain-adjusted collective pitch offset signal.

[0007] If the estimated thrust is less than a lower thrust threshold, the gain can be determined to be a minimum gain value. Optionally, the minimum gain value can be 1.

[0008] If the estimated thrust is greater than or equal to an upper thrust threshold that is greater than the lower thrust threshold, the gain can be determined to be a maximum gain value.

[0009] The gain can be determined to increase from a minimum gain value at the lower thrust threshold to a maximum gain value at the upper thrust threshold. Optionally, the increase can be a linear increase.

[0010] The method can 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 can comprise determining the gain based on the velocity signal.

[0011] If: the estimated thrust is greater than or equal to the lower thrust threshold and less than an upper thrust threshold that is 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 that is greater than the lower velocity threshold; or, the estimated thrust 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 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, the gain can be determined to be greater than the minimum gain value and less than the maximum gain value.

[0012] Determining the gain can comprise determining a first gain based on the thrust signal and determining a second gain based on the velocity signal. The gain can be determined by multiplying the first gain and the second gain.

[0013] The first gain can be determined to linearly increase 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 can be determined to linearly increase from a second minimum gain value at the lower velocity threshold to a second maximum gain value at the upper velocity threshold.

[0014] If the estimated thrust is less than the lower thrust threshold or the tower top velocity is less than the lower velocity threshold, the gain can be determined to be a minimum gain value. Optionally, the minimum gain value can be 1.

[0015] The gain can only be determined as the maximum gain value if the estimated thrust is greater than or equal to an upper thrust threshold and the tower top speed is greater than or equal to an upper speed threshold.

[0016] If the gain is determined as the maximum gain value, the method can comprise maintaining the gain at the maximum gain value for at least a first predefined time period.

[0017] If the gain is determined as being greater than the minimum gain value and less than the maximum gain value, the method can comprise maintaining the gain at a value greater than the minimum gain value for at least a second predefined time period.

[0018] Optionally, the first predefined time period can be equal to the second predefined time period.

[0019] Obtaining the speed signal can comprise obtaining a measured acceleration signal indicative of a measured acceleration of the top of the tower in a fore-aft direction of the wind turbine; obtaining a further estimated thrust signal indicative of an estimated thrust experienced by the rotor, the further estimated thrust signal being obtained based on a blade flapping load signal indicative of a measured flapping load on a rotor blade obtained, and providing the further estimated thrust signal as an input to a limited observer model describing a 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 limited observer model, and providing the error signal as an input to the observer model as part of a feedback loop; and determining the speed signal indicative of an estimated speed of the top of the tower in the fore-aft direction of the wind turbine using the limited observer model.

[0020] The further estimated thrust signal can be obtained based on a determined quasi-static thrust signal indicative of a quasi-static thrust experienced by the rotor, the quasi-static thrust signal being determined using a limited blade element model.

[0021] Optionally, the estimated thrust signal can be equal to the further estimated thrust signal.

[0022] Optionally, the further estimated force signal is obtained by adding the blade flapping load signal indicative of a measured flapping load on a rotor blade obtained to a determined quasi-static thrust signal indicative of a quasi-static thrust experienced by the rotor.

[0023] The collective pitch offset signal can comprise a first collective pitch offset signal determined based on an obtained position signal indicative of a position of a top of the tower in a fore-aft direction of the wind turbine, and a second collective pitch offset signal determined based on a further velocity signal indicative of a 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 can be gain adjusted by applying the determined gain.

[0024] According to another aspect of the present invention, there is provided a non-transitory computer-readable storage medium having stored thereon instructions which, when executed by one or more processors, cause the one or more processors to perform the method defined above.

[0025] According to another aspect of the present invention, there is provided a controller for controlling a wind turbine. The wind turbine comprises a tower and a rotor having a plurality of pitch-adjustable rotor blades. The controller is configured to obtain an estimate of a thrust experienced by the rotor. The controller is configured to obtain a collective pitch offset signal for reducing an amplitude of oscillations 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. 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 dependence on the gain-adjusted collective pitch offset signal.

[0026] According to another aspect of the present invention, there is provided a wind turbine comprising a controller as defined above. BRIEF DESCRIPTION OF DRAWINGS

[0027] Examples of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a wind turbine according to an example of the present invention; Figure 2 schematically illustrates how to determine a collective pitch offset signal in Figure 1 a wind turbine according to an aspect of the present invention; Figure 3 shows a schematic graph illustrating a gain indicative of a collective pitch offset signal to be applied to a collective pitch offset signal determined by Figure 2 a controller of Figure 1 a wind turbine of Figure 4 schematically illustrates an example of how to determine a top of tower velocity in Figure 3 ; and Figure 5 shows a collective pitch offset signal determined by Figure 2the steps of the method performed by the controller. DETAILED DESCRIPTION

[0028] The present invention recognises that it would be beneficial to have a relatively aggressive tuning / high gain on the fore-aft tower damping (FATD) control feature / routine of a wind turbine in transient load situations. In particular, transient load situations can be extreme events, such as a gust, where the loads on the tower and other wind turbine components can be higher than in other conditions / situations. It is beneficial to have a relatively high gain on the FATD control feature during high load events in order to be able to quickly reduce the amplitude of the tower oscillations in a short time period. By boosting the FATD control only in such transient load situations, e.g. during extreme events, or at least reserving the most aggressive intervention of the FATD control for such situations, then the FATD control can have a relatively minimal impact on the pitch and tower fatigue; however, the FATD control will advantageously have a relatively greater impact on the tower extreme loads. Thus, the present invention advantageously provides for increasing the gain of the FATD control only / mainly in situations where there is a risk of reducing extreme tower loads. Further advantages of the present invention will become apparent from the following description.

[0029] Figure 1 An example of a wind turbine 10 is illustrated in a schematic view. The wind turbine 10 includes a tower 102, a nacelle 103 disposed at or on top of the tower 102, and a rotor 104 operably coupled to a generator housed within the nacelle 103. In addition to the generator, the nacelle 103 houses other components required to convert wind energy into electrical energy and various components required to operate, control, and optimize the performance of the wind turbine 10. The rotor 104 of the wind turbine 10 includes a central hub 105 and three rotor blades 106 projecting outwardly from the central hub 105. In addition, the wind turbine 10 includes a control system or controller (not shown in the figure). The controller can be placed inside the nacelle 103, in the tower 102, or distributed at multiple locations inside (or outside) the turbine 10 and communicatively connected to each other. The rotor blades 106 are pitch adjustable. The rotor blades 106 can be adjusted according to collective pitch settings, where each blade is set to the same pitch value. The rotor blades 106 can additionally be adjusted according to individual pitch settings, where each blade 106 can be set with an individual pitch setpoint. Figure 1

[0030] ​In some examples, the wind turbine 10 comprises blade load sensors placed at or near each blade root 109 in such a way that the sensors detect a load in the blade 106. The blade load signals from such sensors can be used to determine how to adjust the pitch of each of the individual blades 106. Depending on the placement and type of the sensors, the load can be detected in the flap (in-plane / out-of-plane) or edge (along-edge) direction 108 (in-plane). For example, such sensors can be strain gauge sensors or optical Bragg sensors. When the sensors are placed on the rotating blades 106, such load signals of each adjustable rotor blade 106 are measured in the rotating reference frame (frame of reference) of the rotor 104.

[0031] Figure 2 Elements of an example of a controller 20 of the wind turbine 10 are schematically illustrated, which controller 20 is implemented to determine a pitch actuation signal that enables maximization of the power generation of the wind turbine 10 and reduction or mitigation of the loads experienced by one or more components, such as the tower 102, the 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 minimizes a speed error (ω - ω ref ) between an actual rotor speed ω and a reference rotor speed ω ref , in order to output a requested power P (in the form of a power setpoint) and a collective pitch reference θ col In view of the rotor speed, the collective pitch reference determined by the speed controller 202 can also take into account further sensor values. This is referred to in Figure 2 as measured sets ms, which are input into the speed controller 202. The feedback speed controller 202 can be implemented by a PI (proportional-integral), PID (proportional-integral-derivative) or similar control scheme. In one example, the collective pitch control module 202 can instead be a model predictive controller that is arranged to determine the collective pitch reference and / or the power reference based on minimizing a cost function.

[0032] Figure 2 A control block / module or pitch offset controller 204 of the controller 20 is also illustrated. In the pitch actuation unit 204, a pitch modification signal or pitch reference offset value is determined based on one or more input signals 205. In the described example, the controller 204 is or comprises a front- and-attitude-tower-damping (FATD) controller or control feature. The FATD controller 204 is used to reduce / counteract the loads on the top of the tower 102 and / or the nacelle 103 in the fore-aft direction of the wind turbine 10, i.e. perpendicular to the plane of the rotor 104, for example, in the Figure 1oscillations / vibrations in the outboard-inboard direction (i.e. in the direction out of the page in Fig. 1). In particular, the FATD controller 204 determines and outputs a collective pitch offset signal or reference θ off 206. The collective pitch offset signal 206 is to be combined (e.g. added) to a collective pitch reference θ col from the speed controller 202 to obtain a collective pitch control signal θ A 207. 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.

[0033] However, according to examples of the present application, prior to adding the collective pitch offset signal 206 to the collective pitch reference θ col a gain is determined and applied, e.g. via a multiplier, to the collective pitch offset signal 206 obtained from the FATD controller 204 to obtain a gain-adjusted collective pitch offset signal θ gain 208. It is this gain-adjusted collective pitch offset signal 208 that is added to the collective pitch reference θ col from the speed controller 202 to obtain the collective pitch control signal 207.

[0034] A gain scheduling control block or controller 209 determines and outputs a gain 210 to apply to the collective pitch offset signal 206. The gain 210 can generally be in the form of a factor or multiplier 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 further detail below.

[0035] Referring back to the FATD controller 204, the controller 204 can determine the collective pitch offset signal 206 based on one or both of: a signal indicative of a speed 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 signals 205 to the FATD 204 can include the tower top fore-aft speed and / or position.

[0036] In the example where the FATD controller 204 uses fore-aft position of the nacelle or tower top, the position signal can be obtained in any suitable manner. For example, the position can be determined based on a measured acceleration signal indicative of nacelle or tower top movement, where the acceleration signal can be obtained from an accelerometer positioned at the top of the tower 102 or in / on the nacelle 103. The acceleration signal can then be appropriately double integrated in order to obtain the position. Typically, any suitable filter (e.g. a leaky filter) that integrates the relevant signal can be applied to obtain velocity and position from acceleration. The position signal can be obtained in different ways, such as a GPS signal, an inclinometer, an inertial measurement unit (IMU), or a Kalman filter.

[0037] The collective pitch offset signal 206 determined by the FATD 204 based on fore-aft position of the nacelle 103 or tower top can advantageously result in stabilization of the nacelle 103 and can prevent the problem of unwanted coupling between the controller 20 (including the speed controller 202) and the tower 102.

[0038] In the example where the FATD controller 204 uses fore-aft velocity of the nacelle or tower top, the velocity signal can be obtained in any suitable manner. For example, the velocity can be the center of mass velocity of the nacelle 103, the velocity of a suitable sensor, or the velocity of other fixed point deemed to represent movement of the nacelle 103 or top of the tower 102 in the fore-aft direction. In practice, the velocity can be determined based on a measured acceleration signal indicative of nacelle or tower top movement, the acceleration being integrated to obtain the velocity, as for the position.

[0039] Now returning to the gain scheduling controller 209, as mentioned above, it is desirable to have relatively aggressive intervention from the FATD controller 204 during transient events, such as extreme load cases, e.g. gusts, by means of applying relatively high gains to the collective pitch offset signal 206. This means that the gain scheduling controller 209 needs to be able to detect the onset of a transient event and react quickly to it. In practice, the transient event, where the tower 102 can end up in an extreme deflected position (with associated high loads), can typically be detected earlier than the actual extreme deflection occurs. Thus, determining the gain 210 based on detection of a transient event, such as an extreme load case, can result in at least partly preventing extreme deflection of the tower 102.

[0040] For example during a gust, the rapidly increasing wind speed will result in an increase in rotor thrust. High rotor thrust is associated with high forces at the top of the tower 102. In the described example, the gain scheduling controller 209 therefore determines the gain 210 based on the estimated rotor thrust, i.e. the estimated thrust experienced by the rotor 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 the estimated rotor thrust allows the controller 20 to react quickly to transient events to damp fore-aft tower oscillations by ramping up the gain 210 applied to the collective pitch offset signal 206.

[0041] When the wind speed rapidly increases, the increase in rotor thrust can be followed by an increase in the speed of the top of the tower 102 or the nacelle 103 in the fore-aft direction. High tower top (forward or aft) speed is associated with high tower deflection and high loads. In the described example, the gain scheduling controller 209 therefore additionally determines the gain 210 based on the tower top speed or nacelle speed in the fore-aft direction, i.e. the input signal 211 comprises a tower top speed signal. In particular, in the described example, the gain scheduling scheme (which can also be considered a triggering / activation scheme for the FATD controller 204) is implemented such that the gain applied to the collective pitch offset signal 206 is increased as required. By scheduling the gain 210 based on the tower top speed as well as the rotor thrust, the number of activations of the FATD controller 204 is advantageously reduced, thereby reducing pitch fatigue compared to the case where the gain 210 is scheduled based on the rotor thrust alone.

[0042] A relatively high tower top speed is associated with a relatively high tower top deflection which would be expected to follow later in the oscillation cycle. Therefore, determination of the tower top speed can be used to identify them before the potential high load scenario occurs, which means that early or preventative action can be taken.

[0043] The described controller 20 can be in the form of any suitable computing device, for example one or more functional units or modules implemented on one or more computer processors. Such functional units can be provided by suitable software running on any suitable computing substrate using conventional or custom processors and memory. The one or more functional units can use a common computing substrate (for example, they can run on the same server) or separate substrates, or one or both of the substrates can themselves be distributed between multiple computing devices. The computer memory can store instructions for performing the method performed by the controller, and the processor can execute the stored instructions to perform the method.

[0044] Figure 3 An activation scheme 30 for the FATD controller 204 according to the described example is schematically illustrated. In particular, Figure 3to indicate how the gain scheduling controller 209 determines the gain 210 based on the rotor thrust and the tower top speed. In particular, Figure 3 to indicate three activation regions 301, 302, 303: a least active (or optionally, deactive) region 301, a partially active region 302, and a fully active region 303.

[0045] The different activation regions 301, 302, 303 are defined or distinguished with reference to respective thresholds relating to the rotor thrust and the tower top speed. Each threshold can be predefined. As Figure 3 As shown, in the described example, each of the rotor thrust and the tower top speed has two thresholds associated therewith. The thrust thresholds can be referred to as a first or lower thrust threshold 304 and a second or upper thrust threshold 305, the upper threshold 305 being greater than the lower threshold 304. In a corresponding manner, the tower top speed thresholds can be referred to as a first or lower speed threshold 306 and a second or upper speed threshold 307, the upper threshold 307 being greater than the lower threshold 306.

[0046] As will be apparent from Figure 3 As will be apparent, in the described example, if the rotor thrust is less than the lower thrust threshold 304, or the tower top speed is less than the lower speed threshold 306, then the activation regime is in the least active region 301. If the rotor thrust is greater than the lower thrust threshold 304, and the tower top speed is greater than the lower speed threshold 306, but the rotor thrust is less than the upper thrust threshold 305, or the tower top speed is less than the upper speed threshold 307, then the activation regime is in the partially active region 302. If the rotor thrust is greater than the upper thrust threshold 305, and the tower top speed is greater than the upper speed threshold 307, then the activation regime is in the fully active region 303.

[0047] Accordingly, the gain scheduling controller 209 determines the gain 210 based on which region of the activation regime 30 the operation of the wind turbine 10 is in. For example, in the least active region 301, the gain can be determined to be a minimum gain value. In some examples, this minimum value can be 1. This would be considered to continue 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 a gain 210 equal to 1.

[0048] In the fully active region 303, the gain can be determined to be a maximum gain value, so as to enable maximum intervention of the FATD controller 204 in this operating condition. This maximum value can be greater than 1, to allow the FATD controller 204 to ramp up its intervention relative to a normal or default level (which can correspond to a gain value equal to 1 ) during transient events. For example, the maximum gain value can be two or any other suitable value.

[0049] In the partial active region 302, the gain 210 can be determined based on specific values of the thrust and the tower top speed (obtained from the input signals 211). In the described example, the gain 210 in the partial active region 302 can increase linearly from a minimum gain value when the thrust is equal to the lower thrust threshold 304 and / or the tower top speed is equal to the lower speed threshold 306 to a maximum gain value when the thrust is greater than or equal to the upper thrust threshold 305 and the tower top speed is greater than or equal to the upper speed threshold 306.

[0050] In practice, this can be implemented in series, where respective gains are determined for the thrust and the tower top speed, which are then combined (e.g. multiplied) together to obtain the (total) gain 210 to be applied to the collective variable pitch offset signal 206. That is, a first (thrust) gain can be determined based on the thrust signal 211 and a second gain (speed) can be determined based on the tower top speed signal 211 (where the first gain and the second gain can be determined in either order), where the first gain and the second gain are multiplied together to obtain the total gain 210. For example, the first gain can increase linearly from a minimum gain value at the lower thrust threshold 304 to a maximum gain value at the upper thrust threshold 305, and the second gain can increase linearly from a minimum gain value at the lower speed threshold 306 to a maximum gain value at the upper speed threshold 307. The determination of the gains can be implemented as a look-up table of gain values. In other words, the respective gains can vary linearly in each axis direction, where these are combined to determine the total gain. It will be appreciated that in different examples, variations other than linear variations can be utilised.

[0051] When the FATD controller 204 has reduced the rotor thrust and the tower top speed back to the lower levels, then the gain can be restored / reduced back to the normal / default value. However, to ensure that the FATD control feature is (fully) effective for the entire duration of the transient event, a hold function can be applied to hold the determined gain value at the higher level for at least a defined duration. Thus, even if the FATD control feature reduces the rotor thrust and the tower top speed to the lower levels while the transient event is ongoing, the gain is held at the higher level for the duration of the event. For example, the defined duration can be of the order of one to two tower fore-aft motion cycles. For example, if the gain is determined to be the maximum gain value when the wind turbine 10 is operating in the full active region 303, if one of the rotor thrust and the tower top speed reduces below the respective upper threshold 305, 307 such that operation is now in the partial active region 302, the gain can still be held at the maximum gain value for the defined period of time. A similar approach can be taken if the wind turbine operation reduces from the partial active region 302 to the lowest active region 301.

[0052] The estimated thrust signal 211 to be used by the scheduling controller 209 can be obtained in any suitable manner. For example, the thrust can be estimated based on a defined equation that depends on a thrust coefficient. In one example, the thrust Ft can be estimated according to: wherein is the air density, is the radius of the rotor 104, is the wind speed, is the pitch angle of the rotor blades 106, is the rotational speed of the rotor 104, and the thrust coefficient is a defined function of the pitch angle and the tip speed ratio .

[0053] In another example, the thrust can be estimated based on a flapping load measurement obtained from a flapping load sensor on the rotor blades and a blade element momentum model (BEM). The measured flapping load can be obtained by combining (e.g., averaging) the measured flapping load signals obtained from the sensors of each rotor blade 106. Alternatively, the measured flapping load can be based on the flapping load signal from only one of the blade load sensors. The blade flapping load measurement is indicative of the bending moment at the root 109 of the rotor blade 106.

[0054] As known in the art, the BEM decomposes the rotor blade into a number of small elements (along its span) and then determines the forces and moments acting on each of these elements. These forces are then integrated over the entire blade to obtain the forces and moments experienced by the rotor blade. In the present context, the BEM provides a signal of the rotor thrust in units of N, i.e., a mapping signal in units of 1 / m, that can map the sum of the measured flapping loads (moments / torques) in units of Nm. Specifically, the BEM can calculate the forces and moments on the rotor blade 106 based on the lift and drag curves of the individual blade elements of the blade, which are then summed up to the (entire) rotor 104. The forces and moments are expressed at the center of the rotor hub / intersection of the rotor blades 106; however, as mentioned above, the blade load sensors are positioned at or near the root of the rotor blades 106 (e.g., at most a few meters from the root), which is at a certain radius or distance from the center of the rotor hub. This can be used to obtain the rotor thrust from the blade flapping loads measured by the blade load sensors of the rotor blades 106.

[0055] The speed signal 211 to be used by the dispatch controller 209 can also be obtained in any suitable manner. For example, the tower top speed in the forward and backward directions can be determined in the same manner as described above for use by the FATD controller 204. That is, the tower top speed can be determined based on the integration of an acceleration signal measuring acceleration in the forward and backward directions, for example, from an accelerometer located at the top of the tower 102 or in the nacelle.

[0056] Alternatively, the velocity signal can be obtained based on an observer model approach 211. Figure 4 The diagram schematically illustrates how the fore-and-aft velocity of the tower top is estimated according to estimation scheme 40 using an observer modeling approach. Scheme 40 includes an observer block 41 having a defined observer model 411, which can be any suitable model known in the art for describing the motion of the top of the tower 102 or the nacelle 103. For example, the model can be a one-dimensional (linear) spring-damper model known in the art; however, more complex models can also be used. As is known, the model defines a state vector consisting of the states of the system being modeled. These are a set of variables used to describe the dynamics of the system, which are embodied in the state-space matrix. In, and another (input) vector representing the external input of the system. For example, wind speed changes are processed through an input matrix. Influence state dynamics. As is known, the model defines the dynamics based on the actions on the state vector. The output matrix Add to the action on the input vector feedforward matrix The obtained output vector Model 411 also includes an error term. Multiply by gain .

[0057] The observer model 411 provides an estimate of the tower top acceleration 412 in the forward and backward directions as output. The error between the estimated acceleration 412 and the measured acceleration signal 413 is determined at the processing block 414. The measured acceleration signal indicates the measured acceleration in the fore-and-aft direction at the top of the tower or nacelle 103, and can be obtained from an accelerometer located in the nacelle 103 or at the top of the tower 102. Error Multiply by the gain at processing block 415 Furthermore, the gain-adjusted error signal 416, as part of the feedback loop, is fed back to the observer model 411.

[0058] The observer model 411 also takes the estimated thrust signal 417 as input. The estimated thrust signal 417 can be obtained in any suitable manner. Figure 4In the illustrated example, the thrust is estimated based on measured blade flap loads from blade load sensors on the rotor blades, as described above. In particular, the sum of the blade flap bending moments 418 obtained from the blade load sensors on each of the rotor blades 106 is used to obtain a rotor thrust 420 at processing block 419. In some examples, this can be used as the estimated thrust signal 417. However, in the described example, the estimate of the thrust 417 is obtained from two separate sources. In addition to the blade flap load measurements, a quasi-static estimate of the thrust 421 is also obtained. In order that the two estimates can be combined to obtain the estimated thrust 417 for input into the observer model 411, the output thrust 420 from 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 signal 417. In practice, a low-pass filter can be applied to the quasi-static thrust estimate 421 before it is added to the high-pass filtered thrust estimate 423. The quasi-static estimate 421 can be obtained using a blade element momentum model 424, as described above.

[0059] Basing the estimate of the thrust on blade flap load measurements means that transient events, such as rapid changes in wind speed, can be detected quickly. When used to determine the gain to be applied to the FATD feature, this means that the determined gain can be increased quickly in response to transient events so that the fore-aft tower damping can quickly and aggressively intervene to reduce the fore-aft oscillation amplitude caused by the transient event. By using only the high frequency content of the thrust estimate obtained based on blade flap load measurements and combining this with a quasi-static estimate of the thrust, a more accurate overall thrust estimate can be obtained (as the quasi-static estimate can be more accurate for low frequency content).

[0060] The use of the tower top velocity signal obtained using the observer method of estimating the rotor thrust based on blade flap load measurements allows for a quick increase in the gain of the FATD control to achieve a quick damping of the tower oscillations that can occur during transient event load cases. This can be particularly useful in extreme coherent gusts with an event change of direction (ECD) load case. The load decreases during the initial part of the event but also more predominantly during the second revolution back of the tower (rebound load peak) where it has the potential to significantly reduce or even cancel out this load peak.

[0061] A speed signal 425 indicative of an estimated speed (or velocity) of the top of the tower in the fore-aft direction can then be obtained from the observer model 411. Using an observer model to obtain the speed signal can beneficially provide a more accurate speed estimate than other methods. In particular, the observer method can advantageously maintain stability when the gain on the FATD controller output is ramped up, meaning that a higher gain can be applied / potentially applied when using this method relative to some other methods. In practice, this more accurate speed estimate allows for more accurate tuning of the triggering / activation threshold used to determine the gain. A position signal 425 indicative of an estimated position of the top of the tower in the fore-aft direction can also be obtained from the observer model 411.

[0062] Figure 5 The steps of the method 50 performed by the controller 20 according to the described examples are summarised. At step 501, the method involves obtaining a thrust signal indicative of an estimate of the thrust experienced by the rotor 104. The rotor thrust can be obtained in any suitable manner, for example according to one or more of the methods outlined above.

[0063] In some examples, at this step, the method 50 also involves obtaining a speed signal indicative of a speed of the top of the tower 102 or nacelle 103 in the fore-aft direction of the wind turbine 10. The fore-aft tower top speed can be obtained in any suitable manner, for example according to one or more of the methods outlined above.

[0064] At step 502, the method 50 involves obtaining a collective pitch offset signal for reducing an amplitude of oscillations of the top of the tower 102 or nacelle 103 in the fore-aft direction of the wind turbine 10. The collective pitch offset signal can be obtained from a fore-aft tower damping (FATD) controller or control feature of the wind turbine 10. In some examples, the method 50 can be seen as comprising determining the collective pitch offset signal by the FATD controller.

[0065] At step 503, the method 50 involves determining a gain based on the thrust signal, and applying the gain to the collective pitch offset signal to obtain a gain-adjusted collective pitch offset signal. In examples where the gain is determined based on the thrust signal alone, the gain can be determined to be a minimum gain value if the rotor thrust is below a lower threshold, and / or the gain can be determined to be a maximum gain value if the rotor thrust is greater than an upper threshold. The gain can increase, for example in a linear manner, from the minimum value to the maximum value between the lower threshold and the upper threshold.

[0066] In examples where a speed signal is also obtained, then at step 503, the gain is determined based on both the thrust signal and the speed signal. The gain can be determined to be a minimum gain value if the estimated rotor thrust is less than a lower thrust threshold; or the fore-aft tower top speed is less than a lower speed threshold. The gain can be determined to be a maximum gain value only if both of the following are met: the estimated rotor thrust is greater than an upper thrust threshold; and, the fore-aft tower top speed is greater than an upper speed threshold. The gain can vary between the minimum and maximum values when both the rotor thrust and the speed are greater than the respective lower thresholds and at least one of the rotor thrust and the speed is less than the respective upper threshold.

[0067] It will be appreciated that in further examples, the gain can be determined based on further signals / parameters in addition to the rotor thrust and the fore-aft tower top speed.

[0068] In some examples, the minimum gain value can be 1. In such examples, this corresponds to continuing normal or default operation of the FATD controller 204.

[0069] In some examples, the FATD controller 204 determines the collective pitch offset signal based on both the fore-aft position of the tower top or nacelle 103 and the fore-aft speed. This can be implemented as determining a first signal based on the obtained tower top position, determining a second signal based on the obtained tower top speed, and then combining the first signal and the second signal to obtain the collective pitch offset signal. The first signal and / or the second signal can be subject to the gain applied prior to combination with the other of the first and second signals. In such examples, the gain determined at step 503 as part of the present method 50 can be applied only to one of the first and second signals, or can be applied to the (overall) collective pitch offset signal.

[0070] At step 504, the method 50 involves controlling the rotor blades 106 in dependence on the gain-adjusted collective pitch offset signal. As described above, this can involve combining / applying the collective pitch offset signal with a collective pitch reference obtained from a speed controller, and sending a collective pitch control signal to a pitch system of the wind turbine 10 to control pitch bearings such that pitch of the rotor blades 106 is adjusted in dependence on the collective pitch control signal.

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

[0072] In the above examples, the pitch actuation controller comprises only fore-aft tower damping controllers. However, it will be appreciated that in different examples, the pitch actuation controller can comprise further pitch offset controllers, the outputs of which are used to control collective or individual pitch control signals for the wind turbine rotor blades, such as side-to-side tower damping (SSTD) control. Different gains can be applied to the output signals of these different pitch offset controllers.

Claims

1. A method of controlling a wind turbine, the wind turbine comprising a tower and a rotor having a plurality of pitch-adjustable rotor blades, the method comprising: obtaining a thrust signal indicative of an estimate of a thrust experienced by the rotor; obtaining a collective pitch offset signal for reducing an amplitude of oscillations of a top of the tower in a fore-aft direction of the wind turbine; determining a gain based on the thrust 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. The gain is determined to be a minimum gain value if the estimated thrust is less than a lower thrust threshold.

2. The method of claim 1, wherein, The gain is determined to be a maximum gain value if the estimated thrust is greater than an upper thrust threshold that is greater than the lower thrust threshold.

3. The method of claim 2, 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.

4. The method of claim 3, wherein, 5. The method of claim 1, the method comprising: obtaining a speed signal indicative of a speed of a top of the tower in a fore-aft direction of the wind turbine; and determining the gain based on the speed signal. The gain is determined to be greater than a minimum gain value and less than a maximum gain value if: the estimated thrust is greater than or equal to a lower thrust threshold and less than an upper thrust threshold that is greater than the lower thrust threshold, and the tower top speed is greater than or equal to a lower speed threshold and less than an upper speed threshold that is greater than the lower speed threshold; or 6. The method of claim 5, wherein, the estimated thrust is greater than or equal to the lower thrust threshold and less than the upper thrust threshold, and the tower top speed is greater than the upper speed threshold; or the estimated thrust is greater than the upper thrust threshold, and the tower top speed is greater than or equal to the lower speed threshold and less than the upper speed threshold, and determining the gain comprises: determining a first gain based on the thrust signal; and determining a second gain based on the speed signal, wherein the gain is determined by multiplying the first gain and the second gain. 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 speed threshold to a second gain value at the upper speed threshold. The gain is determined to be the minimum gain value if the estimated thrust is less than the lower thrust threshold or the tower top speed is less than the lower speed threshold.

7. The method of claim 6, wherein, The gain is determined to be the maximum gain value only if the estimated thrust is greater than or equal to the upper thrust threshold and the tower top speed is greater than or equal to the upper speed threshold.

8. The method of claim 6 or claim 7, wherein, 10. The method of any of claims 6 to 9, wherein:

9. The method of any one of claims 6-8, wherein, if the gain is determined to be the maximum gain value, the method comprises maintaining the gain at the maximum gain value for at least a first predefined time period; and / or, if the gain is determined to be the minimum gain value, the method comprises maintaining the gain at the minimum gain value for at least a second predefined time period. ​ If the gain is determined to be greater than the minimum gain value and less than the maximum gain value, the method comprises maintaining 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. The method of any one of claims 5 to 10, wherein, Obtaining the velocity signal comprises: obtaining a measured acceleration signal indicative of a measured acceleration of the top of the tower in a fore-aft direction of the wind turbine; obtaining a further estimated thrust signal indicative of an estimated thrust experienced by the rotor, the further estimated thrust signal being obtained based on an obtained blade flap load signal indicative of a measured flap load on the rotor blades, and providing the further estimated thrust signal as input to a defined observer model describing a 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 a velocity signal indicative of an estimated velocity of the top of the tower in the fore-aft direction of the wind turbine using the defined observer model.

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

13. The method of any preceding 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 a further velocity signal indicative of a 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 applying a determined gain.

14. A controller for controlling a wind turbine, the wind turbine comprising a tower and a rotor having a plurality of pitch adjustable rotor blades, the controller being configured to: obtain an estimate of a thrust experienced by the rotor; obtain a collective pitch offset signal for reducing an amplitude of oscillations of a top of the tower in a fore-aft direction of the wind turbine; determine a gain based on the estimated thrust, 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 the controller of claim 14.

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