Power output control of a wind turbine at wind speeds below nominal.

Abstract

A method (70) is provided for controlling the power output of a wind turbine (10) at wind speeds lower than the rated speed, the method (70) comprising: determining (710) an indication of the blade twist of one or more rotor blades (18) of the wind turbine (10) as a function of wind speed and / or rotor speed; determining (720) a twist-corrected blade pitch based on the blade twist indication; and using (730) the twist-corrected blade pitch to control the pitch of one or more rotor blades (18).

Description

Power output control of a wind turbine at wind speeds below nominal. Technical field The present invention relates in general to the control of the power output of a wind turbine under wind speed conditions below the nominal speed. Background Wind turbines known in the art comprise a tower supporting a nacelle and a rotor with a number of rotor blades. The rotor blades are typically pitch-adjustable; each blade can be rotated around its longitudinal axis using a pitch actuator. The pitch of each blade is adjusted to change the blade's angle of attack. Typically, blades are pitched in the direction of the incoming wind. Changing the blade's angle of attack changes the aerodynamic load experienced by the blade, and therefore the power generated by the wind turbine. Wind turbine blades are known to deform in a desired manner, such as through twisting and / or bending, under load. This can improve wind turbine performance and increase blade lifespan. Specifically, blades exhibiting so-called 'flex-twist coupling' deformation are known to reduce the load on the blade. This means that when the blade flexes, it also twists, and vice versa. Such deformation can change the angle of attack of a flex-twist coupled blade. As is well known, above a wind turbine's rated wind speed, the turbine is generally controlled to maintain the generator output at a constant level equal to the limit the generator is capable of producing. Conversely, below the rated wind speed, the wind speed is not high enough to generate the maximum power output the generator is capable of producing. Therefore, when the wind speed is below the rated wind speed, a wind turbine is typically operated to maximize the available generator power output based on the actual wind speed. This is usually achieved by maintaining a constant blade pitch angle and varying the generator torque and rotor speed to maintain the power coefficient at a maximum. However, due to the flexing and / or twisting of flex-twist coupled blades in response to load, the power output of flex-twist coupled blades maintained at a constant pitch for wind speeds below the rated wind speed is lower compared to the power output of standard blades that are not flex-twist coupled when maintained at a constant pitch for the same wind speeds below the rated wind speed. Furthermore, flex-twist coupled blades become more torsional compliant over their service life, so the power output changes over time. The pitch of flex-twist coupled blades can be altered to improve power output at wind speeds below the nominal wind speed. For example, EP 2848805 shows that the blade pitch angle of flex-twist coupled blades can be adjusted based on blade torsional deformation detected using blade torsional sensors integrated into the blade. The torsional deformation measured by the torsional sensors corresponds to a particular pitch angle that results in improved power output. However, blade torque sensors can be unreliable and are not standard equipment for wind turbine blades. Their integration into wind turbine blades is also expensive or not feasible for some wind turbines. Some torque sensors are also unable to function or are inaccurate under certain weather conditions. EP3346125 A1 discloses a method for operating a wind turbine in which a controller compensates for torsion-induced blade twist. The method includes adjusting the maximum thrust value in the control program to compensate for the induced twist. EP2096300 A2 discloses aerodynamic load control of a wind turbine blade by controlling the tip speed ratio (TSR) taking into account an aeroelastic response that includes deflection and twist. The present invention has been developed in this context. Summary of the invention According to one aspect of the present invention, a method is provided for controlling the power output of a wind turbine at a wind speed below the nominal speed, as claimed in claim 1. The method comprises calculating a tip speed ratio based on the rotor speed and the wind speed. The torque-corrected blade pitch is determined based on the tip speed ratio and the blade torque indication. Determining the torque-corrected blade pitch involves determining a first blade pitch based on the tip speed ratio, determining a second blade pitch based on the blade twist indication, and comparing the first and second blade pitches to determine the torque-corrected blade pitch. One or both of the first and second shovel steps can be determined using a lookup table. The second blade pitch can be a blade pitch offset. Determining the torque-corrected blade pitch may involve adjusting the first blade pitch by means of the blade pitch offset. Determining the second blade pitch may involve determining a wind speed blade pitch offset based on a blade twist indication determined as a function of wind speed, determining a rotor speed blade pitch offset based on a blade twist indication determined as a function of rotor speed, and calculating the second blade pitch as a sum of the wind speed and rotor speed blade pitch offsets. Determining the first blade pitch may comprise determining a tip speed ratio blade pitch based on the tip speed ratio, determining a wind speed blade pitch based on the wind speed, and / or determining a rotor speed blade pitch based on the rotor speed, and calculating the first blade pitch as the minimum of the tip speed ratio blade pitch and the determined wind speed blade pitch and / or rotor speed blade pitch. It can be determined that the torsional corrected blade pitch is a minimum of the first and second blade pitches. The blade torque indication for one or more wind turbine rotor blades is based on a predetermined relationship between blade torque and at least one of the wind speed and rotor speed. This predetermined relationship may be a functional relationship. The blade torque indication can also be calculated based on a model of the one or more rotor blades. In this case, the model includes a predetermined relationship that expresses the blade torque under operating conditions. Such an approach provides a reliable source of a blade twist indication, and is suitable for use on wind turbines that do not have dedicated sensors on the blades to measure twist. In some embodiments, parameters other than wind speed and / or rotor speed may be used as input parameters. According to such embodiments, the blade twist indication can be determined based on the bending moment along the blade root fin. Furthermore, the blade twist indication can be determined based on a current operating point of the wind turbine. Other and / or additional parameters may be used as input parameters to the predetermined ratio to determine the blade twist indication of one or more rotor blades. Determining the torque-corrected blade pitch involves calculating an average twist along the length of one or more rotor blades based on the blade twist indication, and determining the torque-corrected blade pitch based on the average twist along the length of one or more rotor blades. Control of one or more rotor blades may include regulating the pitch of one or more rotor blades to the torque-corrected pitch angle. According to another aspect of the present invention, a non-transient, computer-readable storage medium is provided that stores instructions thereon which, when executed by a processor, cause the processor to perform the method described above. According to another aspect of the invention, a controller is provided for controlling the power output of a wind turbine at wind speeds below the rated speed, as claimed in claim 13. The controller can be configured to determine a blade twist indication of one or more rotor blades of the wind turbine based on the wind speed and rotor speed. The controller can be configured to determine a twist-corrected blade pitch based on the blade twist indication. The controller can be configured to send a control signal to provide pitch control instructions for one or more rotor blades using the twist-corrected blade pitch to control the power output of the wind turbine. According to another aspect of the present invention, a wind turbine comprising a controller as described above is provided. Brief description of the drawings One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a wind turbine according to an example of the invention; Figure 2 shows a wind turbine controller from Figure 1, and a wind turbine pitch actuator system to be controlled by the controller; Figures 3(a) to 3(c) are power optimum pitch path graphs for wind turbine blades of Figure 1; in particular, Figure 3(a) shows the power optimum pitch path versus tip speed ratio, Figure 3(b) shows the power optimum pitch path versus wind speed, and Figure 3(c) shows the power optimum pitch path versus rotor speed; Figure 4 illustrates a control strategy for the controller of Figure 2 to determine the blade pitch according to an example of the invention; Figure 5 illustrates a control strategy for the controller of Figure 2 to determine the blade pitch according to another example of the invention; Figure 6 illustrates a control strategy for the controller of Figure 2 to determine the blade pitch according to another example of the invention; and, Figure 7 presents the stages of a method executed by the controller of Figure 2 according to an example of the invention. Detailed description of the achievements Figure 1 shows a wind turbine 10 in which an example of the invention may be incorporated. The wind turbine 10 comprises a tower 12 supporting a nacelle 14 on which a rotor 16 is mounted. The rotor 16 comprises a plurality of wind turbine blades 18 extending radially from a hub 20. In particular, each of the blades 18 is a coupled flex-twist blade, i.e., a blade that twists outward relative to the wind to relieve the load on the blade. In this example, the rotor 16 comprises three blades 18, although other configurations are possible. The wind turbine 10 includes a rotor wind speed detector 201. Wind speed measurement can be performed in several ways, as those skilled in the art will appreciate, one being through LiDAR, as they will appreciate from the literature on wind turbine design and control. The wind turbine 10 also includes a rotational speed sensor 202. This could be, for example, in the form of a rotary encoder on a generator shaft of the turbine 10; however, the rotor speed can be determined by any suitable method. Within each blade 18 is shown a blade load sensor 181; in other examples there may be multiple blade load sensors allowing blade loads to be represented by more than one variable. The sensing element can be a fiber optic strain gauge, a resistive strain gauge, or any other suitable detector. Figure 2 shows a wind turbine control system 22 according to an example of the invention that can be implemented in the wind turbine 10 of Figure 1. In this case, the control system 22 includes a pitch actuator system 24 that is 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 rotor blades 18, which in turn may include a hydraulic actuator 28 arranged to adjust the blade pitch in a known manner. The actual position of the actuator 28 can be controlled by an actuator position control unit 30 that provides a positioning instruction signal to the hydraulic actuator 28. One or more functional units of controller 26 can be provided by suitable software running on any suitable computing substrate using conventional or client processors and memory. Different functional units of controller 26 can use a common computing substrate (for example, they can run on the same server) or separate substrates, or one or each can be distributed among multiple computing devices. It should be noted that the controller 26 and the pitch actuator system 24 can be replicated for each of the blades 18 of the wind turbine 10 so that the position of each blade 18 can be controlled independently; in some examples, this can be done to provide individual pitch control or adjustment for each blade 18. As described earlier, wind turbines, such as the wind turbine 10 shown in Figure 1, have a rated wind speed above which maximum generator power output can be achieved. Above the rated wind speed, the power output is maintained at the prescribed maximum for the rated wind speed by adjusting the blade pitch 18, among other measures to maintain constant power output. Below the rated wind speed, the pitch angle can also be adjusted for flex-twist coupled blades to account for blade twist. As described earlier, blade twist can be measured directly using direct blade twist sensors, such as strain gauges. In an exemplary method of the invention, the measured or estimated wind speed and / or rotor speed can be used as indirect, i.e., approximate, measurements to provide an indication of blade torque. Using the wind speed and / or rotor speed, and / or any other indirect measurement of blade torque, a blade torque indication can therefore be determined to adjust the blade pitch accordingly when the wind turbine 10 is being operated at wind speeds below the rated speed. Such an approach provides a more reliable source of a blade torque indication and is suitable for use in wind turbines that do not have dedicated sensors on the blades for measuring torque. Figures 3(a) to 3(c) illustrate power optimum pitch path plots for the flex-twist coupled blades 18 of the wind turbine 10 in Figure 1. Figure 3(a) shows the power optimum pitch path versus the tip speed ratio (TSR). TSR, as is well understood in the art, is the ratio of the tip speed of the blades 18, which can be determined from the rotor speed and rotor diameter, to the wind speed incident on the wind turbine 10. It can be seen in Figure 3(a) that a given value or values ​​of TSR can correspond to a number of different optimum pitch angles. In particular, this can occur for pitch angles close to 0 degrees in a so-called 'optimum pitch region'. In Figure 3, the optimum pitch region is indicated by the reference sign 32.Therefore, it is clear that a single TSR reading may not be sufficient to determine a single optimum pitch angle. That is, TSR values ​​in the optimum power region can correspond to a set of pitch angles, each corresponding to a different blade load and torque. This can be explained by the fact that TSR is a ratio of two variables, namely wind speed and rotor speed, and therefore a particular TSR can correspond to more than one optimum pitch angle, depending on the possible combinations of wind speed and rotor speed. This can be seen in Figure 3(a) for the optimum TSR value, indicated by the reference sign D, and where a vertical line is provided over a range of pitch angles. Additionally, Figure 3(b) shows the optimal power pitch path versus wind speed, and Figure 3(c) shows the optimal power pitch path versus rotor speed. As can be seen in Figures 3(b) and 3(c), the individual wind speed and rotor speed values ​​correspond to a particular pitch angle. Accordingly, in an exemplary method of the invention, at wind speeds below the nominal wind speed, the blade pitch of one or more of the blades 18 is adjusted to account for blade twist, based on indirect blade twist measurements. In particular, a blade twist indication for one or more blades 18 is determined as a function of the wind speed and / or rotor speed. An adjusted pitch, i.e., a twist-corrected blade pitch, of the blade 18 is determined based on the blade twist indication. The twist-corrected blade pitch is used to control the pitch of one or more blades 18. This is described in further detail below. The present method describes the use of the identified relationship between blade torque and wind speed and / or rotor speed to account for blade self-unloading and the resulting suboptimal power output. In a specific example, as described above, the method involves determining a blade torque indication based on wind speed and / or rotor speed, determining an adjusted pitch angle based on that indication, and then using the adjusted pitch angle to control the wind turbine 10. In other examples, the method describes determining a blade twist indication based on wind speed and / or rotor speed, and determining a corrected blade pitch based on a TSR value and the blade twist indication. Determining a corrected pitch relies on both the TSR and the blade twist indication to account for the range of pitch angles at the optimum TSR. Specifically, such a method might involve determining a first pitch angle based on the TSR and a second pitch angle based on the blade twist indication. The second pitch angle could be an offset pitch angle—a correction—that can be applied to the first pitch angle, or it could be an absolute pitch angle used instead of the first pitch angle. In some examples, the second pitch angle itself can be considered as the indication of blade twist. In other additional examples, the method describes determining a blade torque indication based on a blade model formulated based on parameters such as wind speed, rotor speed, blade root fin moment, and active power output. Figure 4 illustrates an example control strategy 40 for controller 26. Specific examples of control strategies 50, 60 for controller 26 are discussed later in relation to Figures 5 and 6. In control strategy 40 of Figure 4, measured or estimated rotor speed values, labeled 'rpm', and wind speed, labeled 'v', are provided as inputs to block 410. Using these inputs, a TSR is calculated and output in block 410 based on the input rotor speed and wind speed. The TSR is then provided as an input to block 420, where a first pitch angle, 01, is determined based on the input TSR. The first pitch angle corresponding to the TSR is determined from a predetermined lookup table or by other means, such as a predictive model. At least one input, labeled 'a', is provided in block 430. A second pitch angle, θ2, is determined in block 430 based on the input to the block. The second pitch angle can be determined based on a lookup table for the input or otherwise, for example, by using a particular model or function that represents blade torque so that block 430 can be represented by a formula: θ2 = fr(a) where T is blade torque. The first and second pitch angles, 0i and 02, are provided as inputs to block 440, where a final pitch angle, 0final, is determined using a predefined function based on the two input pitch angles, 0i and 02. The final pitch angle is provided as an output from controller 26 to the pitch actuator system 24. The pitch actuator system 24 uses the final pitch angle to control the pitch of the blade(s) 18 of the wind turbine 10. In specific examples, the input, a, provided to block 430 to determine the second pitch angle, θ2, is the wind speed, v. In other examples, the input, a, provided to block 430 to determine the second pitch angle, θ2, is the rotor speed, rpm. Other indirect blade torque measurements, such as the blade root-sense bending moment, can also be provided as inputs to block 430. In block 440, a function is performed on the first and second pitch angles, 01 and 02, to determine the final pitch angle 0final. The function may involve addition or subtraction to apply the second pitch angle as an offset relative to the first pitch angle. For example, the first pitch angle is determined as an absolute pitch angle, specifying a pitch angle at which blade 18 can be driven. The second pitch angle is determined as an offset to be applied to the absolute pitch angle, so the first pitch angle is adjusted or corrected according to the second pitch angle. Returning briefly to Figure 3(a), if the TSR is determined to be the optimum TSR, block 420 may then determine that the first pitch angle is 0, for example. In block 430, the offset is determined based on the wind speed, and the offset may be identified as -0.5, for example.Therefore, where block 440 is an addition function, the final step angle output for the 24 step actuator system would be -0.5 degrees. The function can alternatively be a minimum function. In this case, both the first and second pitch angles are provided as absolute pitch angles. The function in block 440 determines which of the first and second pitch angles is the minimum pitch angle, and that pitch angle is provided as the final output pitch angle. The final pitch angle is provided to the pitch actuator system 24, and the pitch actuator system 24 operates the hydraulic actuator 28 using the control system 30 to adjust the pitch of one or more of the blades 18 as required. A specific example implementing the approach presented above is shown in Figure 5. In Figure 5, the wind speed, v, and the rotor speed, rpm, are provided as inputs to block 510 to calculate the TSR. The TSR is then provided to block 511, where a preliminary TSR pitch angle, 01.1, is determined. The TSR pitch angle is determined based on a lookup table. Wind speed is provided as an input to block 512 to identify a wind speed step angle, 01.2. The wind speed step angle is determined by reference to a lookup table. Both the absolute TSR and the wind speed step angles, 01.1 and 01.2, are provided as inputs to block 513. The first step angle, 01, is determined as a minimum of the two input step angles, 01.1 and 01.2, using a minimum function so that the minimum of the TSR step angle and the wind speed step angle are used as the first step angle. The second pitch angle, 02, which is an offset pitch angle, is determined based on two preliminary offset pitch angles. A wind speed pitch angle offset, 02.1, is determined in block 521 based on a lookup table that uses wind speed, v, as an input. A rotor speed pitch angle offset, 02.2, is determined in block 522 based on a lookup table that uses rotor speed, rpm, as an input. The rotor speed pitch angle offset and the wind speed pitch angle offset, 02.1 and 02.2, are provided as inputs to block 523, where the wind speed and rotor speed pitch angle offsets are summed together to provide the second pitch angle 02 as output. The first and second pitch angles, 01 and 02, are provided to block 540, where, as with block 440 in Figure 4, the second pitch angle, 02, is applied to the first absolute pitch angle, 01, to produce a final pitch angle, 0final. The final pitch angle is provided to the pitch actuator system 24 to alter the pitch of one or more of the blades 18. The pitch actuator system 24 operates the hydraulic actuator 28 using the corresponding control system 30. Figure 6 illustrates another specific embodiment of the invention. In Figure 6, the first pitch angle, 01, is determined in block 620 based on a TSR value calculated in block 610 and with reference to a lookup table. In this example, the second pitch angle, 02, is determined in block 630. Block 630 represents an observer that receives at least one input based on an operating parameter indicative of a current operating point of the wind turbine and uses that at least one parameter to determine the second pitch angle based on the parameter value. The observer determines the second pitch angle by inputting the value for the parameter(s) into a model of the wind turbine or a model of an individual wind turbine blade and receiving an estimate or indication of blade twist, typically an average or mean blade twist, experienced by the blade. The observer determines a pitch angle corresponding to the estimated blade twist, and this value is output from block 630 as the second pitch angle. The output second pitch angle can be either a displacement or an absolute value.In this example, the observer receives inputs of wind speed v, rotor speed rpm, blade root bending moment M, and active power P. In other examples, the observer may receive more or fewer inputs and may receive values ​​for a variety of different input parameters. The first step angle and the second step angle are provided to block 640. As in Figure 4 and block 440, a function is performed in block 640 using both the first and second step angles. If the second step angle is an offset step angle, the first step angle is adjusted by the amount specified by the second step angle to produce the final step angle. If the second step angle is an absolute step angle, a comparison is made, and, for example, the minimum value is selected as the final step angle. Figure 7 summarizes the steps of a method 70 performed by controller 26 to determine a torque-corrected pitch reference for each of the blades 18 of wind turbine 10. The method 70 in figure 7 consists of steps performed in each of the control strategies in figures 4 to 6. In step 710, a blade torque indication is determined based on wind speed and rotor speed. The blade torque indication is not a measured blade torque, but rather an approximation of it. For example, the indication might include an estimated blade torque value obtained from a model, or an indication based on a lookup table. The indication might include several values ​​that are indirect measurements of blade torque. In the examples described above, the wind speed is received from wind speed detector 201 and the rotor speed is received from rotor speed detector 202. In other examples, the wind speed and / or rotor speed might be determined using other detectors or calculators. In the examples described, the method might involve calculating a tip speed ratio based on the rotor speed and the wind speed. In step 720, a torque-corrected blade pitch is determined using the blade torque indication determined in step 710. As described above, the final pitch angle, referred to herein as the torque-corrected blade pitch, is generally a function of at least a first blade pitch angle and a second blade pitch angle. In the examples described above, the torque-corrected blade pitch is the minimum of two absolute blade pitches or a first blade pitch corrected by a second blade pitch. Any appropriate function may be used to determine a torque-corrected blade pitch. Torsion-corrected blade pitch is determined to account for the self-unloading of a blade under a particular load, which leads to a reduction in power output potential. Torsion-corrected blade pitch takes into account the twist experienced by the blade in response to the load as the blade's angle of attack changes relative to the wind direction, thereby improving the maximum power output achievable with the blade. In stage 730, the torque-corrected blade pitch is used to control the pitch of the rotor blades. In the examples described above, using the torque-corrected blade pitch involves sending the torque-corrected blade pitch to the pitch actuator system 26 and adjusting the pitch of one or more of the blades 18 using the hydraulic actuator 28 under the control of the control system 30. The pitch of the blades 18 is adjusted to the torque-corrected blade pitch in the examples above. Steps 710, 720 and 730 can be repeated as often as necessary to account for changes in wind speed and / or rotor speed. Examples of the invention are advantageous because the blade pitch is corrected to account for blade twist without the need for dedicated twist sensors or detectors. By using a blade twist indication—that is, an indirect rather than a direct measurement—power output can be precisely and reliably controlled through blade pitch adjustments. Furthermore, the blade torque indication for one or more blades is based on at least one or both of the wind speed and rotor speed. Both of these parameters are already measured or calculated during the normal operation of conventional wind turbines, and the sensors used to measure or calculate these parameters are accurate. It is also important to note that wind speed and rotor speed sensors are common to all wind turbines, including older models, while torque sensors are a relatively recent development. As such, the method described above is also suitable for application to existing wind turbines without requiring any modifications. Importantly, the sensors for measuring wind speed and rotor speed are also reliable, and typically more reliable than the torque sensors on the blades. Because these sensors are generally separate from the wind turbine blades, the blades are not affected by their use in this method. Such a method is particularly useful for flex-twist coupled blades, which act to unload incident aerodynamic loads through deformation. The optimal or desirable pitch angle for flex-twist coupled blades can depend on a specific wind turbine operating condition other than blade twist. Considering at least wind speed and rotor speed, and using these to determine blade pitch, allows for improved power output with wind turbines using flex-twist coupled blades. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of this application. In some examples, the final or torque-corrected blade pitch can be determined using an appropriate function. For example, where a minimum is found, or where a subtraction or addition is performed in the previous control strategies, a mean or median pitch angle can be identified, or a more complex calculation can be performed to determine an appropriate pitch angle. In some examples, the method may include a validation stage, whereby measured blade torque received from blade torque detectors is used to determine a corrected pitch angle that can be compared to the torque-corrected pitch angle obtained using wind speed and rotor speed. The blade torque detectors can be mounted periodically on the blades or be integral with them, such as the strain gauges 181 in Figure 1. When differences in the determined pitch angles are present, the controller 26 iteratively improves the models and / or lookup tables used to make its determinations to enhance the system's accuracy. Such a system can employ machine learning to identify changes in blade torque over time so that further adjustments can be made in the future. In some examples, a centralized wind farm controller performs the above control methods and strategies instead of an individual wind turbine controller. In some examples, one or more switches are included to allow different combinations of step angles for use in determining the torque-corrected step angle. The switches can respond to operating conditions or centralized control instructions.

Claims

i. Method (70) for controlling the power output of a wind turbine (10) at a wind speed below the rated speed, the method (70) comprising: calculating a tip speed ratio based on the rotor speed and the wind speed; determining (710) a blade twist indication of one or more rotor blades (18) of the wind turbine (10) based on a predetermined relationship between the blade twist and at least one of the wind speed and the rotor speed; determining (720) a twist-corrected blade pitch based on the blade twist indication; and using (730) the twist-corrected blade pitch to control the pitch of the one or more rotor blades (18), characterized in that the twist-corrected blade pitch is determined based on the tip speed ratio and the blade twist indication,and wherein determining (720) the torque-corrected blade pitch comprises determining a first blade pitch based on the tip speed ratio, determining a second blade pitch based on the blade torque indication, and comparing the first and second blade pitches to determine the torque-corrected blade pitch.

2. Method (70) of claim 1, wherein one or both of the first and second blade pitches are determined using a lookup table.

3. Method (70) of claim 1 or claim 2, wherein the second blade pitch is a blade pitch offset, and wherein determining (720) the torque-corrected blade pitch comprises adjusting the first blade pitch by the blade pitch offset.

4. Method (70) of claim 3,wherein determining the second blade pitch comprises: determining a wind speed blade pitch offset based on a blade twist indication determined as a function of wind speed; determining a rotor speed blade pitch offset based on a blade twist indication determined as a function of rotor speed; and calculating the second blade pitch as a sum of the wind speed and rotor speed blade pitch offsets.

5. Method (70) of claim 3 or claim 4, wherein determining the first blade pitch comprises: determining a tip speed ratio blade pitch based on the tip speed ratio; determining a wind speed blade pitch based on wind speed and / or determining a rotor speed blade pitch based on rotor speed; and,Calculate the first blade pitch as the minimum of the tip speed ratio blade pitch and the wind speed blade pitch and / or the determined rotor speed blade pitch.

6. Method (70) of claim 1 or claim 2, wherein the torsion-corrected blade pitch is determined to be the minimum of the first and second blade pitches.

7. Method (70) of any of the preceding claims, wherein the blade torsion indication is determined based on the bending moment about the blade root fin.

8. Method (70) of any of the preceding claims, wherein the blade torsion indication is determined based on a current operating point of the wind turbine (10).

9. Method (70) of any of the preceding claims,wherein determining (720) the twist-corrected blade pitch comprises: calculating an average twist along the length of one or more rotor blades (18) based on the blade twist indication; and determining the twist-corrected blade pitch based on the average twist along the length of one or more rotor blades (18).

10. Method (70) of any of the preceding claims, wherein the blade twist indication is calculated based on a model of one or more rotor blades (18).

11. Method (70) of any of the preceding claims, wherein controlling one or more rotor blades (18) comprises regulating the pitch of one or more rotor blades (18) to the twist-corrected pitch angle.

12. A non-transient, computer-readable storage medium that stores instructions thereon which, when executed by a processor,cause the processor to perform the method according to claims 1 to 11.

13. Controller (26) for controlling the output power of a wind turbine (10) at a wind speed below the nominal speed, the controller (26) being configured to: calculate a tip speed ratio based on the rotor speed and the wind speed; determine (710) a blade twist indication of one or more rotor blades (18) of the wind turbine (10) as a function of the wind speed and the rotor speed; determine (720) a twist-corrected blade pitch based on the blade twist indication; and, send (730) a control signal to give pitch control instructions to the one or more rotor blades (18) using the twist-corrected blade pitch characterized in that the twist-corrected blade pitch is determined based on the tip speed ratio and the blade twist indication,and wherein determining (720) the torque-corrected blade pitch comprises determining a first blade pitch based on the tip speed ratio, determining a second blade pitch based on the blade torque indication, and comparing the first and second blade pitches to determine the torque-corrected blade pitch.

14. Wind turbine (10) comprising the controller (26) of claim 13.

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