Controlling wind turbine power output below rated wind speed

Abstract

A method (70) for controlling the power output of a wind turbine (10) at wind speeds below rated wind speed is provided, the method (70) comprising: determining (710) a blade torque indication for one or more rotor blades (18) of the wind turbine (10) depending on wind speed and / or rotor velocity; determining (720) a torque-corrected blade pitch based on the blade torque indication; and controlling the pitch of the one or more rotor blades (18) using (730) the torque-corrected blade pitch.

Description

Technical Field

[0001] The present invention generally relates to controlling the power output of a wind turbine under below rated wind speed conditions. Background Art

[0002] Wind turbines known in the art include a tower supporting a nacelle and a rotor having a plurality of rotor blades. The rotor blades are generally pitch-adjustable; each blade is rotatable about its longitudinal axis using a pitch actuator. The pitch of each blade is adjusted to change the angle of attack of the blade—typically, the blade is tilted in a direction into the wind. Changing the angle of attack of the blade changes the aerodynamic loads experienced by the blade, thereby changing the power generated by the wind turbine.

[0003] It is known to provide wind turbine blades that deform under load in a desired manner - e.g., blade torque and / or bending - which can improve wind turbine performance and increase blade life. In particular, it is known to provide blades that exhibit so-called "bend-torsion coupled" deformation to reduce load events on the blade, i.e., when the blade bends, it also experiences torque and vice versa. Such deformation can change the angle of attack of the bend-torsion coupled blade.

[0004] It is known that above the rated wind speed of a wind turbine, the turbine is often controlled to maintain the output of the generator at a constant level equal to the limit that the generator is capable of achieving. In contrast, below the rated wind speed, the wind speed is insufficient to produce the maximum power output that the generator is capable of achieving. Therefore, when the wind speed is below the rated wind speed, the wind turbine is often operated to maximize the power output of the generator available based on the actual wind speed. Generally, this is achieved by maintaining the blade pitch at a constant angle and varying the generator torque and rotor speed to maintain the power coefficient at a maximum value.

[0005] However, because the bend-twist coupled blades bend and / or twist in response to loads, the power output of a bend-twist coupled blade maintained at a constant pitch at wind speeds below the rated wind speed is lower than the power output of a standard blade without bend-twist coupling maintained at a constant pitch at the same wind speeds below the rated wind speed. In addition, the bend-twist coupled blades become more torque compliant during their service life, so the power output varies over time.

[0006] At these wind speeds below the rated wind speed, the pitch of the bend-torsion coupled blades can be changed to increase the power output. For example, EP 2848805 teaches that the blade pitch angle of the bend-torsion coupled blade can be adjusted based on the torque deformation of the blade detected using a blade torque sensor integrated into the blade. The torque deformation measured by the torque sensor corresponds to a specific pitch angle, resulting in increased power output.

[0007] However, blade torque sensors can be unreliable and are not standard equipment on wind turbine blades. Incorporating them into wind turbine blades is also expensive or impossible in some wind turbines. Some torque sensors also fail to operate or are inaccurate in certain weather conditions.

[0008] It is in this context that the present invention has been developed. Summary of the invention

[0009] According to one aspect of the invention, a method for controlling the power output of a wind turbine at wind speeds below rated is provided. The method may include determining a blade torque indication for one or more rotor blades of the wind turbine based on the wind speed and / or the rotor velocity. The method may include determining a torque-corrected blade pitch based on the blade torque indication. The method may include using the torque-corrected blade pitch to control the pitch of the one or more rotor blades. In particular, the method may include sending a control signal to command pitch control of the one or more rotor blades to control the power output of the wind turbine.

[0010] The method may include calculating a tip speed ratio based on the rotor speed and the wind speed.A torque-corrected blade pitch may be determined based on the tip speed ratio and the blade torque indication.

[0011] Determining the torque-corrected blade pitch may include 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 blade pitch and the second blade pitch to determine the torque-corrected blade pitch.

[0012] One or both of the first blade pitch and the second blade pitch may be determined using a lookup table.

[0013] The second blade pitch may be a blade pitch offset.Determining the torque-corrected blade pitch may include adjusting the first blade pitch by the blade pitch offset.

[0014] Determining the second blade pitch may include determining a wind speed blade pitch offset based on a blade torque indication determined as a function of wind speed, determining a rotor speed blade pitch offset based on a blade torque 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.

[0015] Determining the first blade pitch may include 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 a minimum of the tip speed ratio blade pitch and the determined wind speed blade pitch and / or rotor speed blade pitch.

[0016] The torque-corrected blade pitch may be determined as a minimum of the first blade pitch and the second blade pitch.

[0017] The blade torque indication of one or more rotor blades of the wind turbine is based on a predetermined relationship between the blade torque and at least one of wind speed and rotor velocity. The predetermined relationship may be a functional relationship.

[0018] The blade torque indication may be calculated based on a model of the one or more rotor blades. In this case, the model comprises a predetermined relationship representing the blade torque under operating conditions.

[0019] This method provides a reliable source of indication of blade torque and is suitable for wind turbines that do not have dedicated sensors on the blades for measuring torque.

[0020] In an embodiment, other parameters than wind speed and / or rotor speed may be used as input parameters. According to such an embodiment, the blade torque indication may be determined depending on the blade root flapping bending moment. Furthermore, the blade torque indication may be determined depending on the current operating point of the wind turbine. Additional and / or further parameters may be used as input parameters of the predetermined relationship for determining the blade torque indication of one or more rotor blades.

[0021] Determining the torque-corrected blade pitch includes calculating an average torque along the length of the one or more rotor blades based on the blade torque indication, and determining the torque-corrected blade pitch based on the average torque along the length of the one or more rotor blades.

[0022] Controlling the one or more rotor blades may include pitching the one or more rotor blades to a torque-corrected pitch angle.

[0023] According to another aspect of the present invention, there is provided a non-transitory computer-readable storage medium storing thereon instructions which, when executed by a processor, cause the processor to perform the above-described method.

[0024] According to another aspect of the invention, a controller for controlling the power output of a wind turbine at wind speeds below rated is provided. The controller may be configured to determine a blade torque indication for one or more rotor blades of the wind turbine depending on the wind speed and the rotor velocity. The controller may be configured to determine a torque-corrected blade pitch based on the blade torque indication. The controller may be configured to send a control signal to command pitch control of the one or more rotor blades using the torque-corrected blade pitch, thereby controlling the power output of the wind turbine.

[0025] According to another aspect of the invention, there is provided a wind turbine comprising a controller as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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:

[0027] Figure 1 A wind turbine according to an example of the present invention is shown;

[0028] Figure 2 Shows Figure 1 A controller for a wind turbine, and a pitch actuator system for a wind turbine controlled by the controller;

[0029] Figure 3 (a) to Figure 3 (c) Yes Figure 1 The power optimal pitch trajectory diagram of the blades of the wind turbine; specifically, Figure 3 (a) shows the comparison of the power optimum pitch trajectory and the tip speed ratio, Figure 3 (b) shows the comparison of power-optimized pitch trajectory and wind speed, Figure 3 (c) shows the power optimum pitch trajectory versus rotor speed;

[0030] Figure 4 FIG. 1 is a diagram showing a method for determining blade pitch according to an example of the present invention. Figure 2 The control strategy of the controller;

[0031] Figure 5 FIG. 1 is a diagram showing a method for determining a blade pitch according to another example of the present invention. Figure 2 The control strategy of the controller;

[0032] Figure 6 FIG. 1 is a diagram showing a method for determining a blade pitch according to another example of the present invention. Figure 2 The control strategy of the controller; and

[0033] Figure 7 The present invention is summarized as follows: Figure 2 The controller performs the steps of the method. DETAILED DESCRIPTION

[0034] Figure 1 A wind turbine 10 is shown in which an example 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. Specifically, each of the blades 18 is a bend-torsion coupled blade—that is, a blade that twists out of the wind to relieve the load on the blade. In this example, the rotor 16 includes three blades 18, but other configurations are possible.

[0035] The wind turbine 10 includes a rotor wind speed detector 201 - as the skilled person will appreciate, wind speed measurement can be performed in a number of ways, one being by LIDAR, as the skilled person will appreciate from the literature on wind turbine design and control. The wind turbine 10 also includes a rotation speed sensor 202 - for example, this can be in the form of a rotary encoder on the generator shaft of the turbine 10; however, the rotor speed can be determined in any suitable manner.

[0036] A blade load sensor 181 is shown within each blade 18 - in other examples there may be multiple blade load sensors, allowing blade load to be represented by more than a single variable. The sensing element may be a fiber optic strain gauge, a resistive strain gauge or any other suitable detector.

[0037] Figure 2 A wind turbine control system 22 according to an example of 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 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 may be controlled by an actuator position control unit, which provides a positioning command signal to the hydraulic actuator 28.

[0038] 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 may itself be distributed among multiple computing devices.

[0039] It should be appreciated that controller 26 and pitch actuator system 24 may be replicated for each of blades 18 of wind turbine 10 so that the position of each blade 18 may be independently controlled—in some examples, this may be done to provide independent pitch control or adjustment of each blade 18 .

[0040] As mentioned above, wind turbines, such as Figure 1 The wind turbine 10 shown in FIG. 1 has a rated wind speed above which the maximum power output of the generator can be achieved. Above the rated wind speed, the power output is maintained at a maximum value specified for the rated wind speed by adjusting the pitch of the blades 18, as well as other measures to maintain a constant power output. Below the rated wind speed, the pitch angle can also be adjusted for bending-torsion coupled blades to account for blade torque. As described above, blade torque can be directly measured using a direct blade torque sensor such as a strain gauge.

[0041] In one exemplary method of the present invention, measured or estimated wind speed and / or rotor speed may be used as an indirect measurement, i.e., a proxy, to provide a blade torque indication. Using wind speed and / or rotor speed, and / or any other indirect measurement of blade torque, a blade torque indication may thus be determined so that the blade pitch may be adjusted accordingly when wind turbine 10 is operated below rated wind speed. This approach provides a more reliable source of blade torque indication and is applicable to wind turbines that do not have dedicated sensors on the blades for measuring torque.

[0042] Figure 3 (a) to Figure 3 (c) shows Figure 1 A power optimal pitch trajectory diagram of the bending-torsion coupled blades 18 of the wind turbine 10 is shown. Figure 3 (a) shows the power optimum pitch trajectory versus the tip speed ratio (TSR). As is known in the art, TSR is the tip speed of the blade 18 - which can be determined by the ratio of the rotor speed and the rotor diameter relative to the wind speed event on the wind turbine 10. Figure 3 (a): A given TSR value or values ​​may correspond to a number of different optimum pitch angles. In particular, this may occur for pitch angles close to 0 degrees in the so-called "optimum pitch region". Figure 3 In FIG. 1 , the optimum pitch region is labeled by reference numeral 32. It can therefore be seen that an indication of the TSR may not be sufficient to determine a single optimum pitch angle. That is, the TSR values ​​in the optimum power region may correspond to a family of pitch angles, each corresponding to a different load and blade torque. This may be because the TSR is a ratio of two variables, namely wind speed and rotor speed, and therefore a particular TSR may correspond to more than one optimum pitch angle, depending on the possible combinations of wind speed and rotor speed. This may be in Figure 3 As seen in (a), for the optimum TSR value, this is indicated by reference symbol D, and a vertical line is provided over the range of pitch angles.

[0043] Additionally, Figure 3 (b) shows the power-optimal pitch trajectory versus wind speed, and Figure 3 (c) shows the power optimum pitch trajectory versus rotor speed. Figure 3 (b) and Figure 3 As can be seen in (c), various values ​​of wind speed and rotor speed correspond to specific pitch angles.

[0044] Accordingly, in an exemplary method of the present invention, at wind speeds below the rated wind speed, the blade pitch of one or more of the blades 18 is adjusted to result in a torque of the blades based on an indirect measurement of the blade torque. In particular, a blade torque indication of the one or more blades 18 is determined depending on the wind speed and / or the rotor speed. Based on the blade torque indication, an adjusted pitch of the blades 18, i.e. a torque-corrected blade pitch, is determined. The torque-corrected blade pitch is used to control the pitch of the one or more blades 18. This will be described in more detail below.

[0045] The present method describes utilizing an identified relationship between blade torque and wind speed and / or rotor velocity to cause self-unloading of the blades and a resulting suboptimal power output. In a specific example, as described above, the method describes determining a blade torque indication based on wind speed and / or rotor velocity, determining an adjusted pitch angle based on the indication, and then controlling the wind turbine 10 using the adjusted pitch angle.

[0046] In other examples, the method describes determining a blade torque indication based on wind speed and / or rotor speed, and determining a corrected blade pitch based on a TSR value and the blade torque indication. The determination of the corrected pitch is based on both the TSR and the blade torque indication to obtain a family of pitch angles at an optimal TSR. In particular, such a method may include determining a first pitch angle based on the TSR, and determining a second pitch angle based on the blade torque indication. The second pitch angle may be an offset pitch angle, i.e., a correction that may be applied to the first pitch angle, or may be an absolute pitch angle used instead of the first pitch angle.

[0047] In some examples, the second pitch angle itself may be considered to be indicative of blade torque.

[0048] In another example, the method describes determining a blade torque indication based on a blade model developed based on parameters such as wind speed, rotor velocity, blade root flapwise moment, and real power output.

[0049] Figure 4 An exemplary control strategy 40 for the controller 26 is illustrated. Specific examples of control strategies 50, 60 for the controller 26 will be described later with reference to FIG. Figure 5 and Figure 6 Have a discussion.

[0050] exist Figure 4In the control strategy 40, a measured or estimated value of the rotor speed - labeled "rpm" - and the wind speed - labeled "v" - are provided as inputs to block 410. Using these inputs, based on the input rotor speed and wind speed, the TSR is calculated and output at block 410. The TSR is provided as an input to block 420, where a first pitch angle θ1 is determined based on the input TSR. The first pitch angle corresponding to the TSR is determined according to a predetermined lookup table or by other means, such as a prediction model.

[0051] At least one input - labeled 'a' - is provided to block 430. Based on the input to the block, a second pitch angle θ2 is determined at block 430. The second pitch angle may be determined based on a lookup table or other means for the input, for example, using a model or a specific function representing the blade torque, such that block 430 may be represented by the formula: θ2 = f T (a), where T is the blade torque.

[0052] The first and second pitch angles θ1 and θ2 are provided as inputs to block 440, where the final pitch angle θ final The determination of is performed using a predetermined function and based on the two input pitch angles θ1 and θ2. The final pitch angle is provided as an output from controller 26 to pitch actuator system 24. Pitch actuator system 24 uses the final pitch angle to control the pitch of one or more blades 18 of wind turbine 10.

[0053] In a specific example, 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 measurements of blade torque may also be provided as inputs to block 430, such as blade root flapping bending moment.

[0054] At block 440, a function is performed on the first and second pitch angles θ1 and θ2 to determine a final pitch angle θ final The function may include addition or subtraction to apply the second pitch angle as an offset to the first pitch angle. For example, the first pitch angle is determined as an absolute pitch angle, thereby specifying the pitch angle to which the blade 18 can be actuated. The second pitch angle is determined as an offset to be applied to the absolute pitch angle in order to adjust or correct the first pitch angle according to the second pitch angle. Returning briefly to Figure 3 (a), if the TSR is determined to be the optimal TSR, then block 420 may determine the first pitch angle to be, for example, 0. At block 430, an offset is determined based on the wind speed, and the offset may be identified as, for example, -0.5. Therefore, in the case where block 440 is an additive function, the final pitch angle output to the pitch actuator system 24 will be -0.5 degrees.

[0055] The function may alternatively be a minimum function. In this case, the first pitch angle and the second pitch angle are both provided as absolute pitch angles. The function at block 440 determines which of the first pitch angle and the second pitch angle is the minimum pitch angle, and the pitch angle is provided as the output final pitch angle. The final pitch angle is provided to the pitch actuator system 24, and the pitch actuator system 24 uses the control system 30 to operate the hydraulic actuator 28 to adjust the pitch of one or more of the blades 18 as needed.

[0056] Figure 5 A specific example of implementing the above method is shown. Figure 5 In the example, wind speed v and rotor speed rpm are provided as inputs to block 510 to calculate TSR. TSR is provided to block 511 where a preliminary TSR pitch angle θ is performed. 1.1 The TSR pitch angle is determined based on a lookup table.

[0057] The wind speed is provided as an input to block 512 to identify the wind speed pitch angle θ 1.2 . Determine the wind speed pitch angle by referring to the lookup table.

[0058] Absolute TSR and wind speed pitch angle θ 1.1 and θ 1.2 are provided as inputs to block 513. The first pitch angle θ1 is determined as the two input pitch angles θ 1.1 and θ 1.2 The minimum value of the TSR pitch angle and the wind speed pitch angle is used as the first pitch angle using the minimum function.

[0059] The second pitch angle θ2 is an offset pitch angle, which is determined based on the two preliminary offset pitch angles. Wind speed pitch angle offset θ 2.1 is determined at block 521 based on a lookup table using wind speed v as input. Rotor speed Pitch angle offset θ 2.2 is determined at block 522 based on a lookup table using the rotor speed rpm as input. Rotor Speed ​​Pitch Angle Offset and Wind Speed ​​Pitch Angle Offset θ 2.1 and θ 2.2 Provided as input to block 523 , where the rotor velocity and wind speed pitch angle offset are added together to provide a second pitch angle θ2 as output.

[0060] The output first and second pitch angles θ1 and θ2 are provided to block 540, where Figure 4 As in block 440 in FIG. 1 , the offset second pitch angle θ2 is applied to the absolute first pitch angle θ1 to output a final pitch angle θ finalThe resulting pitch angle is provided to pitch actuator system 24 to change the pitch of one or more of blades 18. Pitch actuator system 24 operates hydraulic actuator 28 using control system 30 in response.

[0061] Figure 6 Another specific embodiment of the present invention is shown in FIG. Figure 6 In the embodiment of the present invention, the first pitch angle θ1 is determined at block 620 based on the TSR value calculated at block 610 and with reference to a lookup table.

[0062] In this example, the second pitch angle θ2 is determined at block 630. Block 630 represents an observer that receives at least one input based on an operating parameter indicating the current operating point of the wind turbine and determines the second pitch angle based on the parameter value using the at least one parameter. The observer determines the second pitch angle by inputting the one or more parameter values ​​into a model of the wind turbine or a model of a single wind turbine blade and receiving an estimate or indication of the blade torque, generally an average or averaged blade torque experienced by the blade. The observer determines the pitch angle corresponding to the estimated blade torque, and this value is output from block 630 as the second pitch angle. The output second pitch angle may be an offset value or an absolute value. In this example, the observer receives inputs of wind speed v, rotor speed rpm, blade root flapping bending moment M, and active power P. In other examples, the observer may receive more or fewer inputs and may receive values ​​for various different input parameters.

[0063] The first pitch angle and the second pitch angle are provided to block 640. Figure 4 As in block 440, a function is performed at block 640 using both the first pitch angle and the second pitch angle. If the second pitch angle is an offset pitch angle, the first pitch angle is adjusted by the amount specified by the second pitch angle to produce a final pitch angle. If the second pitch angle is an absolute pitch angle, a comparison is performed and, for example, the minimum value is selected as the final pitch angle.

[0064] Figure 7 The steps of method 70 performed by controller 26 to determine a torque-corrected pitch reference for each of blades 18 of wind turbine 10 are summarized. Figure 7 Method 70 is in Figures 4 to 6 The steps performed in each of the control strategies.

[0065] In step 710, a blade torque indication is determined based on the wind speed and the rotor speed. The blade torque indication is not a measured blade torque, but a representation of it. For example, the indication may include an estimated blade torque value obtained from a model, or an indication based on a lookup table. The indication may include several values ​​that are indirect measurements of the blade torque. In the above example, the wind speed is received from the wind speed detector 201 and the rotor speed is received from the rotor speed detector 202. In other examples, other detectors or calculators may be used to determine the wind speed and / or the rotor speed. In the described example, the method may include calculating the tip speed ratio based on the rotor speed and the wind speed.

[0066] 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 above example, the torque-corrected blade pitch is the minimum of the two absolute blade pitches or the first blade pitch corrected by the second blade pitch. Any suitable function may be used to determine the torque-corrected blade pitch.

[0067] The torque-corrected blade pitch is determined to obtain a self-unloading of the blade at a specific load that results in a reduction in power output potential. As the blade angle relative to the wind direction changes, the torque-corrected blade pitch results in a torque experienced by the blade in response to the load, thereby increasing the maximum power output achievable using the blade.

[0068] The torque-corrected blade pitch is used to control the pitch of the rotor blades at step 730. In the above example, using the torque-corrected blade pitch includes outputting the torque-corrected blade pitch to the pitch actuator system and, under the control of the control system 30, adjusting the pitch of one or more of the blades 18 using the hydraulic actuator 28. In the above example, the pitch of the blades 18 is adjusted to the torque-corrected blade pitch.

[0069] Step 710 , step 720 , and step 730 may be repeated as often as necessary to account for changing wind speeds and / or rotor speeds.

[0070] An advantage of examples of the invention is that blade pitch is corrected to account for blade torque without the need for a dedicated torque sensor or detector.By using an indication of blade torque, ie an indirect measurement rather than a direct measurement, power output can be accurately and reliably controlled through blade pitch adjustment.

[0071] Furthermore, the blade torque indication of the one or more blades is based on at least one or both of wind speed and rotor speed. Both of these parameters are already measured or calculated during normal operation of conventional wind turbines, and the sensors used to measure or calculate these parameters are accurate in their measurements. It is also important to note that wind speed and rotor speed sensors are common in all wind turbines, including older models, while torque sensors are a relatively new development. Therefore, the above method is also applicable to existing wind turbines without requiring any modifications to the wind turbine.

[0072] Importantly, the sensors used to measure wind speed and rotor velocity are also reliable, and are generally more reliable than the torque sensors on the blades. As these sensors are often separate from the wind turbine blades, the blades are not affected by their use in the method.

[0073] This approach is particularly useful in bend-twist coupled blades, which function to unload the aerodynamic loads acting thereon by deforming, because the optimum or ideal pitch angle for bend-twist coupled blades may depend on values ​​other than blade torque for specific operating conditions of the wind turbine. Taking into account at least the wind speed and rotor velocity, and using them to determine the blade pitch, may improve the power output of a wind turbine with bend-twist coupled blades.

[0074] It should be understood that various changes and modifications can be made to the present invention without departing from the scope of the application.

[0075] In some examples, a torque correction or final blade pitch may be determined using an appropriate function. For example, in the case of finding a minimum or performing a subtraction or addition in the above control strategies, a median or average pitch angle may be identified, or a more complex calculation may be performed to identify the appropriate pitch angle.

[0076] In some examples, the method may include a verification step whereby the measured blade torque received from the blade torque detector is used to determine a corrected pitch angle which may be compared to a torque corrected pitch angle obtained using wind speed and rotor velocity. The blade torque detector may be periodically mounted to the blade or may be integrated therewith, such as Figure 1 In the event of a discrepancy in the determined pitch angle, the controller 26 iteratively improves the model and / or lookup table used to make decisions to improve system accuracy. Such a system may employ machine learning to identify changes in blade torque over time so that further adjustments can be made in the future.

[0077] In some examples, a centralized wind farm controller rather than individual wind turbine controllers performs the above methods and control strategies.

[0078] In some examples, one or more switches are included to allow different combinations of pitch angles for determining the torque-corrected pitch angle. The switches may be responsive to operating conditions or centralized control commands.

Claims

1. A method (70) for controlling the power output of a wind turbine (10) below rated wind speed, the method (70) comprising: determining a blade torque indication for one or more rotor blades (18) of the wind turbine (10) based on a predetermined relationship between the blade torque and at least one of wind speed and rotor velocity; Based on the blade torque indication, determining a torque-corrected blade pitch, wherein determining the torque-corrected blade pitch comprises: calculating an average torque along the length of the one or more rotor blades (18) based on the blade torque indication; and, determining a torque-corrected blade pitch based on an average torque along the length of the one or more rotor blades (18); and Torque-corrected blade pitch is used to control the pitch of the one or more rotor blades (18).

2. The method (70) of claim 1, further comprising: Based on the rotor speed and the wind speed, a tip speed ratio is calculated, and wherein a torque-corrected blade pitch is determined based on the tip speed ratio and the blade torque indication.

3. The method (70) of claim 2, wherein: Determining the torque-corrected blade pitch includes 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 blade pitch and the second blade pitch to determine the torque-corrected blade pitch.

4. The method (70) of claim 3, wherein: Using a lookup table, one or both of the first blade pitch and the second blade pitch are determined.

5. The method (70) according to claim 3 or claim 4, wherein: The second blade pitch is a blade pitch offset, and wherein determining the torque-corrected blade pitch includes adjusting the first blade pitch by the blade pitch offset.

6. The method (70) of claim 5, wherein: Determining the second blade pitch includes: determining a wind speed blade pitch offset based on an indication of blade torque determined as a function of wind speed; determining a rotor rate blade pitch offset based on a blade torque indication determined as a function of the rotor rate; and The second blade pitch is calculated as the sum of the wind speed blade pitch offset and the rotor speed blade pitch offset.

7. The method (70) of claim 5, wherein: Determining the first blade pitch includes: Based on the tip speed ratio, determining a tip speed ratio blade pitch; Based on the wind speed, determining a wind speed blade pitch; and / or based on the rotor speed, determining a rotor speed blade pitch; and The first blade pitch is calculated as the minimum of the tip speed ratio blade pitch and the determined wind speed blade pitch and / or the rotor speed blade pitch.

8. The method (70) according to claim 3 or claim 4, wherein: The torque-corrected blade pitch is determined to be a minimum of the first blade pitch and the second blade pitch.

9. The method (70) according to any one of claims 1 to 4, wherein: Depending on the blade root flapping bending moment, the blade torque indication is determined.

10. The method (70) according to any one of claims 1 to 4, wherein: Depending on the current operating point of the wind turbine (10), a blade torque indication is determined.

11. The method (70) according to any one of claims 1 to 4, wherein: Based on the model of the one or more rotor blades (18), a blade torque indication is calculated.

12. The method (70) according to any one of claims 1 to 4, wherein: Controlling the one or more rotor blades (18) includes pitching the one or more rotor blades (18) to a torque-corrected pitch angle.

13. A controller (26) for controlling the power output of a wind turbine (10) at wind speeds below rated, the controller (26) being configured to: determining a blade torque indication of one or more rotor blades (18) of a wind turbine (10) based on wind speed and rotor velocity; Based on the blade torque indication, a torque-corrected blade pitch is determined, wherein: Determining the torque-corrected blade pitch involves: calculating an average torque along the length of the one or more rotor blades (18) based on the blade torque indication; and, determining a torque-corrected blade pitch based on an average torque along the length of the one or more rotor blades (18); and A control signal is sent to command pitch control of the one or more rotor blades (18) using torque-corrected blade pitch.

14. A wind turbine (10) comprising: A controller (26) according to claim 13.

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