Correcting blade pitch in wind turbines

Abstract

A method (40) for correcting blade pitch in a wind turbine (10) having a tower (12) and a plurality of rotor blades (18) is provided. The method (40) includes: receiving (410) sensor output data from one or more wind turbine sensors (141), the sensor output data including data indicating tower (12) excitation for a plurality of different pitch angles for a particular blade (18); determining (420) a corrected pitch reference for the particular blade (18), the corrected pitch reference corresponding to a minimum tower excitation based on the received sensor output data; and sending (430) the corrected pitch reference to a pitch actuator system (24) for the particular blade (18).

Description

Technical Field

[0001] The present invention generally relates to modifying the pitch of rotor blades of a wind turbine. Background Art

[0002] Wind turbines known in the art include a tower supporting a nacelle and a rotor with a number of rotor blades. The rotor blades are usually pitch-adjustable; each blade can be rotated about the longitudinal axis of the blade by a pitch actuator. The pitch of each blade is adjusted to change the angle of attack of the blade, thereby changing the characteristics of the blade relative to the wind. Adjusting the pitch of the rotor blades can adjust the drag and lift experienced by the blades, thereby changing the load experienced by the blades.

[0003] Misalignment of the blades from the intended position - also known as rotor imbalance - can cause higher than expected loads when pitching the blades. Misalignment is when the actual pitch angle of the rotor blades differs from the intended pitch angle. Any degree of pitch misalignment changes the loads on the blades. Large misalignments of one or more blades can result in relatively high loads, increased wear on the components connecting the blades to the rotor, or even failure of the blades.

[0004] Traditionally, blade misalignment and rotor imbalance are identified using a series of complex field measurements using large and complex measuring instruments. Since engineers need to be on site and instruments need to be installed to the wind turbine, the wind turbine may need to be out of operation for several days. Across the wind farm, this can result in significant power generation losses.

[0005] US 2012 / 018339 A1 discloses a method for balancing blades of a wind turbine, wherein the blades of the wind turbine are pitched according to a modified pitch configuration.

[0006] It is in this context that the present invention has been developed. Summary of the invention

[0007] According to one aspect of the invention, a method of controlling blade pitch in a wind turbine having a tower and a plurality of rotor blades is provided. The method includes receiving sensor output data from one or more wind turbine sensors. The sensor output data includes data indicating tower excitations for a plurality of different pitch angles for a particular one of the blades. The method includes determining a corrected pitch reference for the particular blade corresponding to a minimum tower excitation based on the received sensor output data. The method includes sending the corrected pitch reference to a pitch actuator system for the particular blade.

[0008] The received sensor output data may correspond to the pitch angle of the remaining blades being constant. The remaining blades refer to those blades of the wind turbine other than the specific blade for which the corrected pitch reference is determined.

[0009] In some examples, determining the corrected pitch reference includes determining a pitch offset from a current pitch reference to a pitch angle corresponding to minimum tower excitation. In some examples, the corrected pitch reference is determined as the current pitch reference shifted by the determined pitch offset.

[0010] The sensor output data of the plurality of different pitch angles may include sensor output data of a current pitch reference.

[0011] In some examples, determining the corrected pitch reference includes determining a component of a tower excitation in a particular direction from the received sensor output data. In some examples, the corrected pitch reference is determined based on the component of the tower excitation.

[0012] In some examples, determining a corrected pitch reference based on the component of the tower excitation includes filtering the component of the tower excitation for each of the plurality of different pitch angles to determine a fatigue content of the tower excitation for each of the plurality of different pitch angles. In some examples, the corrected pitch reference is determined as the pitch angle corresponding to a minimum value of the fatigue content.

[0013] The particular direction of the component of tower excitation may be the fore-and-aft direction of the wind turbine tower.

[0014] The method includes operating the wind turbine at each of the plurality of pitch angles of a particular blade for a predetermined period of time to receive sensor output data for the plurality of different pitch angles of the particular blade.

[0015] The method includes, for each of the plurality of pitch angles, calculating an average operating condition of the wind turbine for each of a plurality of time intervals within a predetermined period using the sensor output data, and selecting a specific time interval for which the average operating condition is within a predetermined tolerance. A corrected pitch reference may be determined for each of the plurality of pitch angles based on the sensor output data for the selected specific time interval.

[0016] The operating condition may be calculated based on at least one of: average wind speed; air density; and rotor speed.

[0017] The method may be repeated to determine a corrected pitch reference for each of the plurality of rotor blades.

[0018] The method may be performed sequentially for each rotor blade until the corrected pitch reference for each blade converges to within a specified tolerance.

[0019] The one or more wind turbine sensors may include one or more accelerometers mounted to or in the wind turbine.

[0020] According to another aspect of the present invention, there is provided a non-transitory, computer-readable storage medium having instructions stored thereon, which, when executed by a processor, cause the processor to perform the above method.

[0021] According to another aspect of the invention, a controller for controlling blade pitch in a wind turbine having a tower and a plurality of rotor blades is provided. The controller may be configured to receive sensor output data from one or more wind turbine sensors. The sensor output data may include data indicating tower excitations for a plurality of different pitch angles for a particular one of the blades. The controller may be configured to determine a corrected pitch reference for the particular blade corresponding to a minimum tower excitation based on the received sensor output data. The controller may be configured to send the corrected pitch reference to a pitch actuator system for the particular blade.

[0022] According to another aspect of the present invention, there is provided a wind turbine comprising the above controller. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0024] Figure 1 A wind turbine according to an example of the present invention is shown;

[0025] Figure 2 Show Figure 1 A controller for a wind turbine, and a pitch actuator system for a wind turbine to be controlled by the controller;

[0026] Figure 3(a) and 3(b) Showing an example according to the present invention, Figure 1 A graphical representation of fatigue of a rotor blade of a wind turbine versus blade pitch angle; and

[0027] Figure 4 Summary According to an example of the present invention, by Figure 2 The controller performs the steps of the method. DETAILED DESCRIPTION

[0028] Figure 1A wind turbine 10 is shown in which examples of the present invention may be incorporated. The wind turbine 10 includes a tower 12 supporting a nacelle 14 to which a rotor 16 is mounted. The rotor 16 includes a plurality of wind turbine blades 18 extending radially from a hub 20. In this example, the rotor 16 includes three blades 18, although other configurations are possible.

[0029] An accelerometer 141 is shown mounted to the wind turbine 10, and in particular to the nacelle 14. In other examples, the accelerometer 141 may be mounted on or within any suitable portion of the wind turbine, as will be readily appreciated by a skilled artisan. In fact, the wind turbine 10 may include any suitable number of accelerometers. Typically, a plurality of accelerometers may be mounted or attached to one or more of the main bearings, the gearbox, and the generator of the wind turbine.

[0030] The accelerometer 141 is configured to measure changes in velocity or speed associated with the wind turbine 10. In particular, the accelerometer 141 is configured to measure excitation or vibration of the wind turbine tower 12. Imbalance of the rotor 16, i.e., a discrepancy between the pitch of the blades 18 and a reference pitch, causes vibrations of the tower 12 during operation of the wind turbine 10. Specifically, the tower excitation is proportional to the rotor imbalance, and thus measurements from the accelerometer 141 indicative of the tower excitation may be used to identify and correct for pitch misalignment, as will be described below.

[0031] The wind turbine 10 may include other sensors. For example, a rotor wind speed detector may be provided - this measurement may be performed in several ways, as will be appreciated by the skilled person, one of which is by means of a laser radar (LIDAR), as will be appreciated by the skilled person from the literature on wind turbine design and control. In addition, a rotation speed sensor may also be provided - for example, this may be in the form of a rotary encoder on the generator shaft of the wind turbine 10; however, the rotor speed may be determined in any suitable manner, as will be appreciated by the skilled person.

[0032] Figure 2 An example wind turbine control system 22 according to the present invention is shown, which may be used in Figure 1 1. Here, the control system 22 includes a pitch actuator system 24 controlled by a controller 26. The pitch actuator system 24 is or includes a system for controlling the pitch of one or more of the wind turbine blades 18, which in turn may include a hydraulic actuator 28 configured to adjust the pitch of the blades in a known manner. The actual position of the actuator 28 may be controlled by an actuator position control unit 30, which provides a positioning command signal to the hydraulic actuator 28.

[0033] One or more functional units of the controller 26 may be provided by suitable software running on any suitable computing substrate using conventional or custom processors and memory. The different functional units of the controller 26 may use a common computing substrate (e.g., they may run on the same server) or separate substrates, or one or each functional unit may itself be distributed among multiple computing devices.

[0034] It should be appreciated that controller 26 and pitch actuator system 24 may be replicated for each blade 18 of wind turbine 10 so that the position of each blade 18 may be independently controlled—this may be done to provide individual pitch control or adjustment for each blade 18 in some examples.

[0035] In an exemplary method of the present invention, errors in the reference pitch of each blade 18, i.e., pitch misalignment, are identified and corrected based on measured tower vibrations. In particular, measured data indicating tower excitations for a plurality of different pitch angles for a particular blade 18 is received. The pitch angle of the particular blade 18 corresponding to the minimum tower excitation is then determined as the correct pitch reference angle for the particular blade 18, which is sent to the pitch actuator system 24, which can use the correct pitch reference to control the pitch of the particular blade 18. This will be described in more detail below.

[0036] As described above, during normal operation of the wind turbine 10, the blades 18 are controlled to face in the direction of the incident airflow. If the blades 18 are not properly aligned with the rotor hub 20, then pitch controlling the blades 18 in this manner will result in a difference between the orientation of the blades 18 and the direction of the incident airflow, thereby causing rotor imbalance. The rotor blades 18 have a TC (duct cut) mark, which should be at 0 degrees relative to the rotor hub 20 for a properly aligned and calibrated blade. A deviation of the TC mark from 0 degrees will cause rotor imbalance - the reason for the deviation of the TC mark may be, for example, manufacturing tolerances, lack of maintenance, or blade replacement. The 0 degree mark can be considered as a reference pitch angle for the blades 18.

[0037] The present method describes identifying pitch misalignment by identifying the degree of deviation of each blade 18 from a reference pitch, and then correcting for such deviation. In particular, the wind turbine 10 is operated for various blade pitch angles for a particular one of the blades 18 while maintaining the pitch angles of the remaining blades 18 constant. For each pitch angle, an indication of tower vibration is determined by the controller 26 from the measurements from the accelerometer 141. The pitch angle corresponding to the minimum or lowest tower vibration is identified by the controller 26, and the offset between the identified pitch angle and the pitch angle corresponding to the reference pitch (e.g., 0 degrees) is determined. The corrected pitch reference is the reference pitch angle shifted by the determined pitch offset, and the controller 26 sends one or both of the corrected pitch reference or the pitch offset to the pitch actuator system 24. This process is then repeated for each of the remaining blades 18 in sequence.

[0038] In the described example, the measured tower vibration or excitation is quantified in a particular manner. During a defined period of time, a component of the tower excitation along a particular direction is determined or streamed by the accelerometer 141. In the described example, a component of the tower excitation along the fore-aft direction of the wind turbine 10 is used. The fore-aft direction may be considered to be tower motion along the plane entering the page, such as Figure 1 . The received fore and aft tower vibration data is used to identify the number of vibrations or cycles within the rotational frequency of the wind turbine (i.e., the 1P frequency). In particular, the number of cycles may be determined using known methods such as rain flow counting. Any other suitable method of identifying the number of cycles may be used, as will be understood by the skilled person. Specifically, the number of cycles identified will be referred to as 1P fatigue, or simply fatigue, hereinafter. The fatigue, and in particular the blade pitch angle corresponding to the minimum fatigue, will be used to determine a corrected pitch reference and / or pitch offset for a particular blade, as described below.

[0039] A specific example of implementing the above method in the controller 26 is now described. A first of the blades 18 is selected. The pitch angle of this particular blade 18 is adjusted to a specific value, such as in this example, the pitch angle is set to -0.6 degrees. The pitch angles of the remaining two blades 18 are kept constant at 0 degrees. The wind turbine 10 is then operated in this configuration for a predetermined period of time - which may be any suitable time, such as a few minutes, a few hours, a few days, etc., such as 10 minutes - during which the accelerometer 141 measures the acceleration of the tower 12. The fatigue content associated with this particular blade configuration is determined by the controller 26, as described above.

[0040] The pitch angle of the first of the blades 18 is then changed or offset by a defined amount from -0.6 degrees. In the described example, the blade pitch is offset from -0.6 degrees by 0.3 degrees to -0.3 degrees. The pitches of the other two blades 18 remain unchanged. The wind turbine 10 is then operated in this configuration for a predetermined period of time as before, and fatigue is determined based on sensor output data from the accelerometer 141 received during the predetermined period of time. This process is repeated for a plurality of different pitch angles of the first blade 18 while keeping the pitch angles of the remaining blades 18 constant. In the described example, the method is performed for the following pitch angles of the first blade 18: -0.6, -0.3, 0, +0.3, +0.6 degrees. It will be readily appreciated by the skilled person that any suitable plurality of pitch angles for the first blade may be selected.

[0041] Once the fatigue associated with each of the multiple pitch angles of the first blade 18 has been determined, the controller 26 determines which of the pitch angles has the lowest or minimum fatigue. Figure 3(a) shows a graph of fatigue versus pitch angle for the first blade 18 in one example. It can be seen that the data points generally follow a parabolic trend with a single global minimum. It can be further seen that in this example, fatigue is minimized at a pitch angle of 0 degrees. Figure 3(b) shows a graph of fatigue versus pitch angle for the first blade 18 in another example. Similar to Figure 3(a), it can be seen in Figure 3(b) that the data points generally follow a parabolic trend with a single global minimum. However, unlike Figure 3(a), it can be seen in Figure 3(b) that in this example, fatigue is minimized at a pitch angle of +0.3 degrees.

[0042] The first current pitch reference in the blade can be at 0 degrees (as described above). Therefore, in the example shown in Figure 3(a), the minimum fatigue corresponds to the current reference pitch angle of blade 18. Therefore, no correction or offset is required, so the current pitch reference is the correct pitch reference. That is, there is a zero pitch error, so the current pitch reference needs to be zero offset. However, in the example shown in Figure 3(b), the minimum fatigue corresponds to the current reference pitch angle of the first blade 18. That is, the minimum fatigue is not at 0 degrees. Instead, the minimum fatigue occurs at +0.3 degrees. Therefore, the controller 26 determines that the corrected reference pitch angle is at +0.3 degrees, that is, the pitch reference is to be offset or shifted from the current pitch reference (0 degrees) by 0.3 degrees to +0.3 degrees.

[0043] Once the corrected pitch reference has been determined, controller 26 is configured to send the corrected pitch reference for the first of the blades and / or an offset to the current pitch reference to pitch actuator system 24, which accordingly adjusts or offsets the pitch of the first of blades 18. The corrected pitch reference then becomes the (updated or new) current pitch reference for the first of blades 18.

[0044] The second of the blades 18 is then selected, and the process outlined above for the first of the blades 18 is repeated for the second of the blades 18. That is, the wind turbine 10 is operated for a predetermined period of time for each of the plurality of pitch angles, and the fatigue associated with each of the plurality of pitch angles is determined. During this process, the pitches of the other two blades 18 are held constant. In particular, it is noted that the first of the blades 18 is held constant at its corrected blade reference pitch angle. The corrected blade pitch reference for the second of the blades 18 is determined in the same manner as described above, and the pitch actuator system 24 offsets the pitch of the second blade 18 by an appropriate amount in order to update the second blade 18 to its corrected pitch reference.

[0045] The third and final one of the blades 18 is then selected and the process is repeated again to determine and implement the corrected pitch reference for this blade 18 .

[0046] Since the pitch references of the second and third of the blades 18 may have changed after determining the corrected pitch reference of the first of the blades 18, a further correction of the pitch reference of the first of the blades 18 may be required. This is because the pitch angle of the first blade 18 with the lowest associated fatigue may be different when the wind turbine 10 is operated with the second and third blades 18 keeping their corrected pitch references (rather than their previous pitch references) constant. Therefore, the above process is repeated for the first of the blades to determine a (further) corrected pitch reference. In fact, this process may be repeated until the corrected pitch references of each of the plurality of blades 18 have converged to within a predetermined tolerance.

[0047] Some further details on how to use the streamed or received accelerometer data are now described. Since the wind tower excitation is a function of the wind turbine operating conditions, the operating conditions should be the same or substantially similar for all different configurations to ensure a consistent or meaningful comparison, i.e., the operating conditions should be the same when measuring the accelerometer data for all pitch angles in the plurality of different pitch angles of one of the blades. Examples of parameters that are part of or a function of the operating conditions and that may affect the tower excitation are average wind speed, air density, and rotor speed.

[0048] However, it is very difficult or even impossible to maintain constant operating conditions during the predetermined time period for collecting accelerometer data for a particular configuration. Therefore, some steps can be taken to alleviate this problem. In one example, accelerometer data is streamed over an extended period of time in each configuration, for example, the predetermined period may be a period of several hours or even days. During this extended predetermined period of time, the received accelerometer data is binned or divided into individual time intervals within the predetermined period. As an illustrative example, each individual time interval may be 10 minutes; however, any suitable time interval may be used. This will result in N bins of received sensor data for each specific configuration.

[0049] The controller 26 is configured to calculate an average operating condition for each of the N bins based on the respective received sensor output data. In order for the received accelerometer data associated with a particular bin to be suitable or valid for use in determining a corrected pitch reference, the average operating condition for the particular bin needs to be within a certain or predetermined tolerance. The controller 26 is configured to select a specific time interval that causes the average operating condition to be within a predetermined tolerance. The controller 26 determines the fatigue of the specific configuration based on the sensor output data for the selected specific time interval. The controller 26 repeats this process for each different configuration of one of the blades, i.e., a different pitch angle, to determine a corrected pitch reference based on the sensor output data for the selected specific time interval for each configuration.

[0050] Figure 4 Summarizing the steps of a method 40 performed by the controller 26 to determine a corrected pitch reference for each blade 18 of the wind turbine 10. This may also be considered a dynamic pitch calibration method or procedure for the blades 18 of the wind turbine 10. At step 410, sensor output data is received from one or more wind turbine sensors 141. In the described example, the wind turbine sensors 141 include one or more accelerometers 141. The sensor output data includes data indicative of wind turbine tower 12 excitations for a plurality of different pitch angles for a particular one of the blades 18. In the described example, at each different pitch angle, the wind turbine 10 is operated with the particular blade 18 for a predetermined period of time while keeping the pitch of the other blades 18 constant.

[0051] At step 420, a corrected pitch reference for the particular blade is determined. The corrected pitch reference corresponds to a minimum tower excitation based on the received sensor output data. In the described example, a fore-aft component of the tower excitation is filtered out from the received sensor output data - the fore-aft signal may be one of a plurality of signals in the sensor output data - for determining the corrected pitch reference. The fore-aft direction of the wind turbine 10 is the dominant direction of additional loads (i.e., additional tower excitation) caused by blade misalignment. In particular, in the described embodiment, the fore-aft component is used to determine fatigue, and the corrected pitch reference is determined as the pitch angle corresponding to the minimum fatigue.

[0052] At step 430, the revised pitch reference and / or the offset to the current pitch reference is sent or transmitted to the pitch actuator system 24 for the particular blade 18. The pitch actuator system 24 then corrects the blade pitch of the first blade as appropriate.

[0053] Steps 410, 420, and 430 are repeated for each of the remaining blades 18 except the first blade 18. Furthermore, once the method steps have been performed successively for each blade 18, the method loops back to the first of the blades 18 and repeats the method steps again. This process continues until the corrected pitch reference for each blade 18 converges to within a specified tolerance.

[0054] An advantage of examples of the present invention is that excitation or vibration of a wind turbine tower may be reduced or minimized, which may significantly increase the service life of the wind turbine. In particular, an advantage of the present invention is that rotor imbalances that lead to tower excitations may be remotely identified and corrected. In fact, the present invention may be used as part of an automatic procedure that does not require manual input, for example automatically correcting rotor imbalances every year. This is in contrast to known methods in which a skilled technician is required to correct rotor imbalances on site, resulting in expensive downtime of power generation from the wind turbine. Furthermore, tower excitation is a continuously measurable and robust wind turbine parameter, so using tower excitation in a method for correcting pitch misalignment results in a reliable such method.

[0055] An advantage of examples of the present invention is that relatively complex sensors such as blade flap strain gauge sensors, which may only be provided on more modern wind turbines, are not required. In fact, only sensors that are common to almost all wind turbines, such as accelerometers, are required to implement the present invention. This means that the present invention can be implemented on more turbines, including relatively older wind turbines where service life may be a more immediate issue. In addition, because the present invention utilizes existing wind turbine hardware, its implementation is relatively cost-effective.

[0056] Examples of the present invention are advantageous because having properly pitched or aligned rotor blades may increase the power output of a wind turbine and may reduce the frequency with which maintenance on the wind turbine is required.

[0057] It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

[0058] In the examples described above, the corrected pitch reference angle is determined as the pitch angle at which the wind turbine has been operating, which corresponds to the minimum tower excitation. However, in a different example, data points of measured tower excitation (particularly fatigue) versus pitch angle may be plotted and a curve fit performed to determine the corrected reference pitch. In particular, the minimum turning point of the fitted curve may be used to define the corrected reference pitch. Any suitable curve or spline fitting technique may be used, as will be clear to the skilled person.

[0059] In the example described above, the wind turbine is operated with one blade for a predetermined period at each of a plurality of defined blade pitch angles (while keeping the pitch angles of the remaining blades constant) to determine a corrected reference pitch for that particular blade. This process is then repeated for the other blades. In a different example, rather than operating the wind turbine in such a (controlled) manner to obtain the required tower vibration data, the blade pitch angles and the associated tower vibrations are monitored during normal operation of the wind turbine over a period of time. During this period, the required tower vibration data for various pitch angles of a particular blade can be obtained to allow determination of a corrected reference pitch.

[0060] In the above example, the steps of adding the offset to the pitch angle are constant values, for example, steps of 0.3 degrees. That is, the tower vibration is measured for a plurality of equally spaced pitch angles. However, in a different example, the tower vibration may be measured for any suitable pitch angle range with any suitable minimum and maximum values ​​of the range. When controlling a wind turbine to operate at a plurality of different pitch angles to obtain corresponding tower vibration data, it may be natural to operate the wind turbine at a plurality of equally spaced pitch angles, although this is not necessary. However, if the required tower vibration data is collected over time during normal operation of the wind turbine, the tower vibration data obtained may be unlikely to be for equally spaced pitch angles.

Claims

1. A method (40) of modifying blade pitch in a wind turbine (10) having a tower (12) and a plurality of rotor blades (18), the method (40) include: receiving (410) sensor output data from one or more wind turbine sensors (141), the sensor output data including data indicative of tower (12) excitation for a plurality of pitch angles that differ from one another for a particular one of the blades (18); determining (420) a corrected pitch reference for a particular blade (18), the corrected pitch reference corresponding to a minimum tower excitation based on the received sensor output data; as well as sending (430) the corrected pitch reference to the pitch actuator system (24) of the particular blade (18); wherein the wind turbine (10) is operated for a predetermined period at each of the plurality of pitch angles of a particular blade (18), to receive sensor output data for the plurality of pitch angles of the particular blade (18), and Wherein, for each of the plurality of pitch angles: calculating an average operating condition of the wind turbine for each of a plurality of time intervals within a predetermined period using the sensor output data; and selecting a particular time interval among the time intervals that results in the average operating condition being within a predetermined tolerance, Wherein, for each of the plurality of pitch angles, a corrected pitch reference is determined based on sensor output data at a selected specific time interval.

2. The method (40) according to claim 1, in, The received sensor output data corresponds to the pitch angle of the remaining blades (18) being constant.

3. The method (40) according to claim 1, in, Determining (420) a corrected pitch reference includes: determining a pitch offset from a current pitch reference to a pitch angle corresponding to a minimum tower excitation; and A corrected pitch reference is determined as the current pitch reference shifted by the determined pitch offset.

4. The method (40) according to claim 3, in, The sensor output data of the plurality of pitch angles includes sensor output data of a current pitch reference.

5. A method (40) according to any one of claims 1 to 4, in, Determining (420) a corrected pitch reference includes: determining a component of tower excitation in a particular direction from the received sensor output data; and A corrected pitch reference is determined based on the components of the tower excitation.

6. The method (40) according to claim 5, in, Determining (420) a corrected pitch reference based on a component of the tower excitation includes: filtering a component of the tower excitation for each of the plurality of pitch angles to determine a fatigue level of the tower excitation for each of the plurality of pitch angles; and A corrected pitch reference is determined as the pitch angle corresponding to the fatigue minimum.

7. The method (40) according to claim 5, in, The specific direction of the component of tower excitation is the fore-aft direction of the wind turbine tower (12).

8. A method (40) according to any one of claims 1 to 4, in, The operating condition is calculated based on at least one of: average wind speed; air density; and rotor speed.

9. The method (40) according to any one of claims 1 to 4, in, The method (40) is repeated to determine a corrected pitch reference for each of the plurality of rotor blades (18).

10. The method (40) of claim 9, comprising sequentially performing the method (40) for each rotor blade (18) until the corrected pitch reference for each blade (18) converges to within a specified tolerance.

11. The method (40) according to any one of claims 1-4, wherein the one or more wind turbine sensors (141) comprise one or more accelerometers mounted to or in the wind turbine (10).

12. A controller (26) for modifying blade pitch in a wind turbine (10) having a tower (12) and a plurality of rotor blades (18), the controller (26) being configured to: receiving sensor output data from one or more wind turbine sensors (141), the sensor output data including data indicative of tower (12) excitation for a plurality of pitch angles that differ from one another for a particular one of the blades (18); determining a corrected pitch reference for a particular blade (18) corresponding to a minimum tower excitation based on the received sensor output data; as well as sending the corrected pitch reference to the pitch actuator system (24) of the specific blade (18); wherein the wind turbine (10) is operated for a predetermined period at each of the plurality of pitch angles of a particular blade (18), to receive sensor output data for the plurality of pitch angles of the particular blade (18), and Wherein, for each of the plurality of pitch angles: calculating an average operating condition of the wind turbine for each of a plurality of time intervals within a predetermined period using the sensor output data; and selecting a particular time interval among the time intervals such that the average operating condition is within a predetermined tolerance, Wherein, for each of the plurality of pitch angles, a corrected pitch reference is determined based on sensor output data at a selected specific time interval.

13. A wind turbine (10) comprising the controller (26) of claim 12.

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