System and method for reducing loads acting on rotor blades of a wind turbine

Abstract

A method for reducing loads acting on a rotor blade of a wind turbine includes receiving a plurality of load signals and determining at least one load acting on the rotor blade based on the load signals. In addition, the method includes determining a type of load acting on the rotor blade. Furthermore, the method includes comparing the load to a load threshold, such as an extreme load threshold. In addition, the method includes implementing a control scheme when the load exceeds the load threshold. More particularly, the control scheme includes providing a first pitch mode for reducing a first type of load, providing a different second pitch mode for reducing a different second type of load, and coordinating the first pitch mode and the second pitch mode to reduce the load acting on the rotor blade based on the type of at least one load.

Description

Technical Field

[0001] The present disclosure relates generally to wind turbines and, more particularly, to systems and methods for mitigating loads acting on rotor blades of a wind turbine. Background Art

[0002] Wind power is considered to be one of the cleanest and most environmentally friendly energy sources currently available, and in this regard, wind turbines have received increased attention. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The rotor blades are the main elements for converting wind energy into electrical energy. The blades typically have a cross-sectional profile of an airfoil so that during operation, air flows over the blades, creating a pressure difference between their sides. As a result, a lift force directed from the pressure side toward the suction side acts on the blades. The lift force generates a torque on the main rotor shaft, which is connected to a generator for generating electricity.

[0003] The amount of power that can be generated by a wind turbine is typically limited by the structural limitations (i.e., design loads) of the various wind turbine components. For example, the blade roots of a wind turbine may experience loads associated with average loads caused by turbine operation and dynamic fluctuating loads caused by environmental conditions (e.g., such as blade tip deflection, blade bending moments, or various blade forces). Such loads may damage turbine components, ultimately causing turbine component failure. Fluctuating loads may vary from day to day or from season to season and may be based on wind speed, wind peaks, wind turbulence, wind shear, changes in wind direction, density in the air, yaw misalignment, updrafts, or the like. In particular, for example, the loads experienced by a wind turbine may vary with wind speed.

[0004] Therefore, ensuring that loads acting on wind turbines do not exceed design loads is imperative. Consequently, many wind turbines employ one or more sensors configured to measure the loads acting on various wind turbine components. While these sensors can provide desirable information, new sensor systems can be complex and expensive to install. Furthermore, sensors can provide inaccurate information and can be prone to failure.

[0005] In addition, large rotors can suffer from low blade tip-tower clearances and high blade root bending moments, especially during extreme turbulence and extreme wind shear conditions (caused by a combination of mean thrust on the rotor and unbalanced loads on the rotor). Furthermore, conventional systems rely solely on collective pitching to reduce blade root bending moments and solely on cyclic pitching to increase tip clearance. Consequently, such control actions are independently targeted only at specific imbalance sources, namely collective pitching for mean thrust control or cyclic pitching for unbalanced loads.

[0006] In view of the foregoing, the art is continually seeking new and improved systems for mitigating loads acting on wind turbine rotor blades. Accordingly, the present disclosure relates to systems and methods for mitigating loads acting on rotor blades by providing a lead time in an estimated future load signal (by using both the speed and deflection of the blade pattern) to provide a faster pitch response. Summary of the Invention

[0007] Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

[0008] In one aspect, the present disclosure relates to a method for reducing loads acting on a rotor blade of a wind turbine. The method includes receiving a plurality of load signals for the rotor blade. The method also includes determining at least one load acting on the rotor blade based on the plurality of load signals for the rotor blade. Additionally, the method includes determining a type of load acting on the rotor blade. Furthermore, the method includes comparing the load to a load threshold, such as an extreme load threshold. Additionally, the method includes implementing a control scheme when the load exceeds the load threshold. More particularly, the control scheme includes providing a first pitch mode for reducing a first type of load, providing a different second pitch mode for reducing a different second type of load, and coordinating the first pitch mode and the second pitch mode to reduce the load acting on the rotor blade based on the type of at least one load.

[0009] In an embodiment, the type of load acting on the rotor blades may include thrust of the wind turbine's rotor (such as mean thrust) or rotor imbalance.

[0010] In a particular embodiment, the first type of load may be thrust and the second type of load may be rotor imbalance.In such an embodiment, the first pitch mode may be a collective pitch mode and the second pitch mode may be a cyclic pitch mode.

[0011] In a further embodiment, coordinating the first pitch mode and the second pitch mode based on the first type and the second type may include coordinating the first pitch mode and the second pitch mode based on the type of load acting on the rotor blade using a hierarchical approach. For example, in an embodiment, the method may include first activating the cyclic pitch mode to reduce the load acting on the rotor blade to a pitch angle setpoint that reduces the load acting on the rotor blade to a reduced load amount, then receiving one or more residual load signals for the rotor blade, and comparing the residual load signals for the rotor blade to a second load threshold. If the residual load signals for the rotor blade exceed the second load threshold, the method may further include subsequently activating the collective pitch mode to further reduce the reduced load amount.

[0012] In several embodiments, the cyclic pitch mode is configured to reduce the majority of loads acting on the rotor blades, while the collective pitch mode addresses pitch system saturation and reduces average thrust. In embodiments, the cyclic pitch mode may include cyclically pitching each of the rotor blades along their respective pitch axes, and the collective pitch mode may include pitching each of the rotor blades together along their respective pitch axes.

[0013] In another aspect, the present disclosure relates to a system for reducing loads acting on rotor blades of a wind turbine. The system includes: one or more sensors configured to generate a plurality of load signals associated with the rotor blades; and a controller communicatively coupled to the sensors. The controller includes a processor configured to perform a plurality of operations, including but not limited to: receiving the plurality of load signals from the rotor blades, determining at least one load acting on the rotor blades based on the plurality of load signals from the rotor blades, determining a type of load acting on the rotor blades, comparing the load to a load threshold, and implementing a control scheme when the load exceeds the load threshold. More particularly, the control scheme includes: providing a first pitch mode for reducing a first type of load; providing a different second pitch mode for reducing a different second type of load; and coordinating the first pitch mode and the second pitch mode based on the type of load to reduce the load acting on the rotor blades. It should be understood that the system may also include any of the additional features described herein.

[0014] These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims.The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] A full and complete disclosure of the invention including the best mode thereof to those skilled in the art is set forth in the specification with reference to the accompanying drawings, in which:

[0016] Figure 1 A perspective view illustrating one embodiment of a wind turbine according to the present disclosure;

[0017] Figure 2 shows a simplified interior view of one embodiment of a nacelle of a wind turbine according to the present disclosure;

[0018] Figure 3 A schematic diagram illustrating one embodiment of a controller according to the present disclosure;

[0019] Figure 4 a flow chart illustrating one embodiment of a method for mitigating loads acting on a rotor blade of a wind turbine according to the present disclosure; and

[0020] Figure 5 A schematic diagram illustrating one embodiment of a system for mitigating extreme loads acting on a rotor blade of a wind turbine according to the present disclosure is shown. DETAILED DESCRIPTION

[0021] Reference will now be made in detail to embodiments of the present invention, one or more examples of which are illustrated in the drawings. Each example is provided as an explanation of the present invention and is not a limitation of the present invention. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present invention without departing from the scope or spirit of the present invention. For example, features shown or described as part of one embodiment may be used with another embodiment to produce further embodiments. Therefore, it is intended that the present invention encompasses such modifications and variations as fall within the scope of the appended claims and their equivalents.

[0022] In general, the present disclosure relates to improved systems and methods for mitigating rotor blade loads (such as blade root bending moments) in wind turbines. In one embodiment, for example, the present disclosure coordinates common and cyclic pitch modes during extreme unbalanced load conditions through a hierarchical approach. More particularly, when a high unbalanced load is detected by comparing an estimated / predicted blade root bending moment / tip clearance against a threshold, cyclic pitch is activated with an aggressive set point. After cyclic pitch is activated, the remaining load is again compared against a second threshold. Based on the difference between the reduced load and the threshold, an amount of common pitch is used to further reduce the unbalanced load. Thus, the cyclic pitch mode serves as the primary means of maximizing the ability of the pitch system to reduce the unbalanced load, with common pitch being used only to cover pitch system saturation and average thrust loads. As a result, the present disclosure is more effective in reducing unbalanced loads and more efficient in terms of annual energy production (AEP) losses associated with such losses.

[0023] Various embodiments of the systems and methods described herein provide numerous advantages. For example, the present disclosure helps reduce extreme loads, such as tip clearance loads and blade root bending moments, with minimal impact on AEP. The ability to control extreme loads can increase AEP in the knee region of the power curve. Furthermore, the present disclosure can be implemented using existing components of a wind turbine and does not require additional sensors. Thus, users do not need to purchase, install, and maintain new equipment. Furthermore, the system can be integrated with a wider range of control systems, such as, but not limited to, a wind turbine control system, a plant control system, a remote monitoring system, or a combination thereof.

[0024] Now refer to Figure 1, shows a perspective view of one embodiment of a wind turbine 10 that may implement control techniques according to the present disclosure. As shown, the wind turbine 10 generally includes a tower 12 extending from a support surface 14, a nacelle 16 mounted on the tower 12, and a rotor 18 coupled to the nacelle 16. The rotor 18 includes a rotatable hub 20 and at least one rotor blade 22 coupled to the hub 20 and extending outwardly from the hub 20. For example, in the illustrated embodiment, the rotor 18 includes three rotor blades 22. However, in alternative embodiments, the rotor 18 may include more or less than three rotor blades 22. Each rotor blade 22 may be spaced about the hub 20 to facilitate rotating the rotor 18 so that kinetic energy can be converted from the wind into usable mechanical energy and subsequently into electrical energy. For example, the hub 20 may be rotatably coupled to a generator 24 ( Figure 2 ), to allow the generation of electrical energy.

[0025] The wind turbine 10 may also include a wind turbine controller 26 centralized within the nacelle 16. However, in other embodiments, the controller 26 may be located within any other component of the wind turbine 10 or at a location external to the wind turbine. Furthermore, the controller 26 may be communicatively coupled to any number of components of the wind turbine 10 in order to control the operation of such components and / or implement corrective actions. Thus, the controller 26 may include a computer or other suitable processing unit. Thus, in several embodiments, the controller 26 may include suitable computer-readable instructions that, when implemented, configure the controller 26 to perform various functions, such as receiving, transmitting, and / or executing wind turbine control signals. Thus, as will be discussed in greater detail below, the controller 26 may generally be configured to control various operating modes (e.g., startup or shutdown sequences), derate the wind turbine, and / or control various components of the wind turbine 10.

[0026] Now refer to Figure 2 , showing Figure 1 , a simplified internal view of one embodiment of the nacelle 16 of the wind turbine 10 is shown in FIG. As shown, the generator 24 may be coupled to the rotor 18 for generating electrical power from the rotational energy generated by the rotor 18. For example, as shown in the illustrated embodiment, the rotor 18 may include a rotor shaft 34 that is coupled to the hub 20 for rotation therewith. The rotor shaft 34 may, in turn, be rotatably coupled to a generator shaft 36 of the generator 24 via a gearbox 38. As generally understood, the rotor shaft 34 may provide a low-speed, high-torque input to the gearbox 38 in response to the rotation of the rotor blades 22 and the hub 20. The gearbox 38 may then be configured to convert the low-speed, high-torque input into a high-speed, low-torque output to drive the generator shaft 36 and, therefore, the generator 24.

[0027] Each rotor blade 22 may also include a pitch adjustment mechanism 32 configured to rotate each rotor blade 22 about its pitch axis 28. In addition, each pitch adjustment mechanism 32 may include a pitch drive motor 40 (e.g., any suitable electric, hydraulic, or pneumatic motor), a pitch drive gearbox 42, and a pitch drive pinion 44. In such an embodiment, the pitch drive motor 40 may be coupled to the pitch drive gearbox 42 such that the pitch drive motor 40 imparts mechanical force to the pitch drive gearbox 42. Similarly, the pitch drive gearbox 42 may be coupled to the pitch drive pinion 44 for rotation therewith. The pitch drive pinion 44 may, in turn, be rotationally engaged with a pitch bearing 46 coupled between the hub 20 and the corresponding rotor blade 22 such that rotation of the pitch drive pinion 44 causes rotation of the pitch bearing 46. Thus, in such embodiments, rotation of pitch drive motor 40 drives pitch drive gearbox 42 and pitch drive pinion 44, thereby rotating pitch bearing 46 and rotor blades 22 about pitch axis 28. Similarly, wind turbine 10 may include one or more yaw drive mechanisms 66 communicatively coupled to controller 26, with each yaw drive mechanism 66 configured to change the angle of nacelle 16 relative to the wind (e.g., by engaging a yaw bearing 68 of wind turbine 10).

[0028] Still refer to Figure 2 The wind turbine 10 may also include one or more sensors 48, 50 for generating measurement data related to the rotor blades and / or determining any other operating parameters that may be required for the various load conditions described herein. For example, in various embodiments, the sensors may include: one or more blade sensors 48 for measuring the load acting on one of the rotor blades 22; a generator sensor (not shown) for monitoring the generator 24 (e.g., torque, rotational speed, acceleration, and / or power output); a sensor for measuring unbalanced loads in the rotor (e.g., a main shaft bending sensor); and / or various wind sensors 50 for measuring various wind parameters (such as wind speed, wind peaks, wind turbulence, wind shear, changes in wind direction, air density, or the like). Furthermore, the sensors may be located near the ground of the wind turbine, on the nacelle, or on a meteorological mast of the wind turbine.

[0029] It should also be understood that any other number or type of sensors may be employed and in any location. For example, the sensors may include, for example, a micro inertial measurement unit (MIMU), a strain gauge, an accelerometer, a pressure sensor, a vibration sensor, a proximity sensor, or a camera sensor. It should be understood that, as used herein, the term "monitoring" and variations thereof indicate that the various sensors may be configured to provide direct measurements of monitored parameters or indirect measurements of such parameters. Thus, the sensors may, for example, be used to generate a signal related to the monitored parameter, which signal may then be used by the controller 26 to determine the actual parameter.

[0030] Now refer to Figure 3 , a block diagram of one embodiment of various components of the controller 26 according to the present disclosure is shown. As shown, the controller 26 may include one or more processors 58 and associated memory devices 60 that are configured to perform various computer-implemented functions (e.g., perform methods, steps, calculations, etc. and store relevant data as disclosed herein). In addition, the controller 26 may also include a communication module 62 to facilitate communication between the controller 26 and the various components of the wind turbine 10. Furthermore, the communication module 62 may include a sensor interface 64 (e.g., one or more analog-to-digital converters) to allow signals transmitted from the sensors 48, 50 to be converted into signals that can be understood and processed by the processor 58. It should be appreciated that the sensors 48, 50 may be communicatively coupled to the communication module 62 using any suitable method. For example, Figure 3 As shown in FIG, the sensors 48, 50 are coupled to the sensor interface 64 via a wired connection. However, in other embodiments, the sensors 48, 50 may be coupled to the sensor interface 64 via a wireless connection, such as by using any suitable wireless communication protocol known in the art.

[0031] As used herein, the term "processor" refers not only to integrated circuits that are considered in the art to be included in computers, but also to controllers, microcontrollers, microcomputers, programmable logic controllers (PLCs), application-specific integrated circuits, and other programmable circuits. Additionally, the memory device 60 may generally include memory elements including, but not limited to, computer-readable media (e.g., random access memory (RAM)), computer-readable non-volatile media (e.g., flash memory), floppy disks, compact disk-read only memory (CD-ROM), magneto-optical disks (MODs), digital versatile disks (DVDs), and / or other suitable memory elements. Such memory device 60 may generally be configured to store suitable computer-readable instructions that, when executed by the processor 58, configure the controller 26 to perform various functions, including, but not limited to, determining one or more current wind turbine parameters of the wind turbine 10 based on a plurality of operational data, determining a maximum wind turbine parameter, transmitting suitable control signals to implement a control action to reduce a load acting on the wind turbine, and various other suitable computer-implemented functions.

[0032] Processor 58 is configured to estimate rotor blade loads (e.g., tip deflection, blade root net moment, and / or force) of wind turbine 10 using the operating parameters measured from sensors 48, 50. For example, sensors 48, 50 are configured to measure various wind turbine and / or environmental conditions to directly or indirectly provide information regarding one or more of the following parameters: rotor thrust, mechanical torque, forces acting on rotor blades 22, rotor imbalance measurement, rotor azimuth, rotor speed, gearbox ratio, jog moment, overhang moment, yaw moment, gravity, hub radius, blade radius, taper angle, blade mass, blade weight, the location of the center of gravity for each of rotor blades 22, or any other operating parameter of wind turbine 10.

[0033] Now refer to Figure 4 , a flow chart showing one embodiment of a method 100 for alleviating loads acting on a rotor blade of a wind turbine. The method 100 may be performed using, for example, the method described above with reference to Figure 1-3 The turbine controller 26 discussed above is implemented. Figure 4 The steps are depicted as being performed in a particular order for purposes of illustration and discussion. Using the disclosure provided herein, one of ordinary skill in the art will appreciate that the various steps of method 100, or any of the methods disclosed herein, may be adjusted, modified, rearranged, performed simultaneously, or modified in various ways without departing from the scope of the present disclosure.

[0034] As shown at ( 102 ), method 100 includes receiving a plurality of load signals of rotor blades 22 .

[0035] As shown at ( 104 ), method 100 includes determining at least one load acting on rotor blade 22 based on a plurality of load signals of rotor blade 22 .

[0036] As shown at (106), the method 100 includes determining a type of load acting on the rotor blade 22. For example, in an embodiment, the type of load acting on the rotor blade 22 may include rotor imbalance or thrust of the rotor 18, such as mean thrust. As shown at (108), the method 100 includes comparing the load to a load threshold, such as an extreme load threshold.

[0037] As shown at (110), the method 100 includes implementing a control scheme when the load exceeds a load threshold. More particularly, as shown at (112), the control scheme 110 includes providing a first pitch mode for reducing a first type of load. As shown at (114), the control scheme 110 includes providing a different second pitch mode for reducing a different second type of load. In a particular embodiment, for example, the first type of load may be average thrust and the second type of load may be rotor imbalance. In such an embodiment, the first pitch mode may be a cyclic pitch mode and the second pitch mode may be a collective pitch mode. In such an embodiment, the cyclic pitch mode may include cyclically pitching each of the rotor blades along their respective pitch axes, and the collective pitch mode may include pitching each of the rotor blades together along their respective pitch axes. Thus, the cyclic pitch mode is configured to reduce a majority of the loads acting on the rotor blades, while the collective pitch mode addresses pitch system saturation as well as average thrust loads.

[0038] Thus, as shown at (116), the control scheme 110 includes coordinating the first pitch mode and the second pitch mode based on the type of load to reduce the load acting on the rotor blade 22. In a further embodiment, coordinating the first pitch mode and the second pitch mode based on the first type and the second type may include coordinating the first pitch mode and the second pitch mode based on the type of load acting on the rotor blade using a hierarchical approach. For example, in an embodiment, when a high unbalanced load is detected by comparing the estimated / predicted blade root bending moment / tip clearance relative to a threshold, the controller 26 may first activate the cyclic pitch mode by reducing the load acting on the rotor blade 22 to a pitch angle set point of a reduced load amount. Subsequently, the method 100 may include receiving one or more residual load signals of the rotor blade 22, and comparing the residual load signals of the rotor blade 22 to a second load threshold. If the residual load signal of the rotor blade 22 exceeds the second load threshold, the controller 26 may then activate the collective pitch mode to further reduce the reduced load amount.

[0039] Therefore, the systems and methods of the present disclosure coordinate common and cyclic pitch modes through a hierarchical approach during extreme unbalanced load conditions. Cyclic pitch is thus used as the primary means of minimizing the pitch system's ability to minimize the presence of unbalanced loads, while the common pitch mode is used to address pitch system saturation and mean thrust loads. Consequently, this approach is more effective in reducing unbalanced loads and more efficient in terms of the AEP losses associated with them.

[0040] Additionally, in certain embodiments, controller 26 is configured to implement any other suitable control actions based on the comparison of the load to the load threshold to reduce blade tip deflection and blade root bending moment. For example, in several embodiments, the control actions may include temporarily de-rating or up-rating the wind turbine to prevent excessive loads on one or more of the wind turbine components. Up-rating the wind turbine (such as by increasing the torque) may temporarily slow the wind turbine and act as a brake to help reduce or prevent loads.

[0041] De-rating the wind turbine may include speed de-rating, torque de-rating, or a combination of both. Furthermore, as mentioned, the wind turbine 10 may be de-rated by pitching one or more of the rotor blades 22 about their pitch axis 28. More specifically, the controller 26 may generally control each pitch adjustment mechanism 32 so as to vary the pitch angle of each rotor blade 22 between 0 degrees (i.e., the power position of the rotor blade 22) and 90 degrees (i.e., the feathered position of the rotor blade 22). Thus, in one embodiment, the controller 26 may command a new pitch setpoint (e.g., from 0 degrees to 5 degrees), while in another embodiment, the controller 26 may specify a new pitch constraint (e.g., a constraint that ensures that subsequent pitch commands are at least 5 degrees).

[0042] In yet another embodiment, wind turbine 10 can be temporarily derated by modifying the torque demand on generator 24. Generally, the torque demand can be modified using any suitable method, process, structure, and / or approach known in the art. For example, in one embodiment, the torque demand on generator 24 can be controlled using controller 26 by transmitting suitable control signals / commands to generator 24 to modulate the magnetic flux generated within generator 24.

[0043] Wind turbine 10 can also be temporarily derated by yaw-ing nacelle 22 to change the angle of nacelle 16 relative to the wind direction. In further embodiments, controller 26 can be configured to actuate one or more mechanical brakes to reduce the rotational speed of rotor blades 22, thereby reducing component loads. In still further embodiments, controller 26 can be configured to perform any suitable control action known in the art. Furthermore, controller 26 can implement a combination of two or more control actions.

[0044] Now refer to Figure 5 , a schematic diagram of one embodiment of a system 200 for implementing extreme unbalanced load control is shown to further explain the method 100 described herein. As shown at (202), the system 200 estimates the loads to be compensated by the control action. As shown at (204), the system 200 estimates or measures the tip clearance and blade root load. As shown at (206), the system 200 adds the estimates / measurements from (202) and (204) to obtain the tip clearance and blade root estimate without any control action (208). As shown at (210), a maximum value between the tip clearance and blade root load estimate / measurement (204) and the tip clearance and blade root estimate without any control action (208) is determined. As shown at (212), the maximum value 210 is compared to a cycle threshold value. As shown at (214), if the maximum value 210 is less than the cycle threshold value, no further action is taken. Conversely, as shown at (216), if the maximum value 210 is greater than the cyclic threshold value, the system 200 activates the cyclic pitch mode described herein. Additionally, as shown at (218), the tip clearance and blade root load estimate / measurement (204) may be compared to a common threshold value. As shown at (220), if the tip clearance and blade root load estimate / measurement (204) are greater than the common threshold value and the cyclic pitch mode is activated, the system 200 may then activate the common pitch mode (222) described herein.

[0045] Various aspects and embodiments of the invention are defined by the following numbered clauses:

[0046] Clause 1. A method for reducing a load acting on a rotor blade of a wind turbine, the method comprising:

[0047] receiving a plurality of load signals of the rotor blades;

[0048] determining at least one load acting on the rotor blade based on a plurality of load signals of the rotor blade;

[0049] determining a type of at least one load acting on the rotor blade;

[0050] comparing the at least one load to a load threshold; and

[0051] When the load exceeds the load threshold, a control scheme is implemented, which includes:

[0052] providing a first pitch mode for reducing loads of a first type;

[0053] providing a different second pitch pattern for reducing a different second type of load; and

[0054] The first pitch pattern and the second pitch pattern are coordinated based on at least one type of load to reduce the load acting on the rotor blade.

[0055] Clause 2. The method of Clause 1, wherein the type of at least one load acting on the rotor blade comprises at least one of thrust of a rotor of the wind turbine or rotor imbalance.

[0056] Clause 3. The method of any preceding clause, wherein the first type of load comprises rotor imbalance and the second type of load comprises thrust.

[0057] Clause 4. The method of any preceding clause, wherein the first pitch pattern comprises a cyclic pitch pattern, and wherein the second pitch pattern comprises a collective pitch pattern.

[0058] Clause 5. The method of any preceding clause, wherein coordinating the first pitch pattern and the second pitch pattern based on the first type and the second type further comprises:

[0059] A hierarchical approach is used to coordinate the first pitch pattern and the second pitch pattern based on a type of at least one load acting on the rotor blade.

[0060] Clause 6. The method of any preceding clause, wherein coordinating the first pitch mode and the second pitch mode based on a type of at least one load acting on the rotor blade using a hierarchical approach further comprises:

[0061] first activating the cyclic pitch mode by reducing the load acting on the rotor blades to a pitch angle set point of a reduced load amount; then receiving one or more residual load signals of the rotor blades; and

[0062] One or more residual load signals of the rotor blade are compared to a second load threshold value.

[0063] Clause 7. The method of any preceding clause, wherein if the one or more residual load signals of the rotor blades exceed a second load threshold, the method further comprises subsequently activating the collective pitch mode to further reduce the reduced load amount.

[0064] Clause 8. The method of any preceding clause, wherein the cyclic pitch mode reduces a majority of the loads acting on the rotor blades, and the collective pitch mode addresses pitch system saturation and reduces thrust.

[0065] Clause 9. The method of any preceding clause, wherein the cyclic pitch mode comprises pitching each of the rotor blades cyclically along their respective pitch axes, and the collective pitch mode comprises pitching each of the rotor blades together along their respective pitch axes.

[0066] Clause 10. The method of any preceding clause, wherein the load threshold corresponds to a limit load threshold.

[0067] Clause 11. A system for mitigating loads acting on a rotor blade of a wind turbine, the system comprising: one or more sensors configured to generate a plurality of load signals associated with the rotor blade; and a controller communicatively coupled to the one or more sensors, the controller comprising a processor configured to perform a plurality of operations, the plurality of operations comprising:

[0068] receiving a plurality of load signals of the rotor blades;

[0069] determining at least one load acting on the rotor blade based on a plurality of load signals of the rotor blade;

[0070] determining a type of at least one load acting on the rotor blade;

[0071] comparing the at least one load to a load threshold; and

[0072] When the load exceeds the load threshold, a control scheme is implemented, which includes:

[0073] providing a first pitch mode for reducing loads of a first type;

[0074] providing a different second pitch pattern for reducing a different second type of load; and

[0075] The first pitch pattern and the second pitch pattern are coordinated based on at least one type of load to reduce the load acting on the rotor blade.

[0076] Clause 12. The system of Clause 11, wherein the type of at least one load acting on the rotor blade comprises at least one of thrust of a rotor of the wind turbine or rotor imbalance.

[0077] Clause 13. The system of clauses 11-12, wherein the first type of load comprises rotor imbalance and the second type of load comprises thrust.

[0078] Clause 14. The system of Clauses 11-13, wherein the first pitch pattern comprises a cyclic pitch pattern, and wherein the second pitch pattern comprises a collective pitch pattern.

[0079] Clause 15. The system of clauses 11-14, wherein coordinating the first pitch mode and the second pitch mode based on the first type and the second type further comprises:

[0080] A hierarchical approach is used to coordinate the first pitch pattern and the second pitch pattern based on a type of at least one load acting on the rotor blade.

[0081] Clause 16. The system of clauses 11-15, wherein coordinating the first pitch mode and the second pitch mode based on a type of at least one load acting on the rotor blade using a hierarchical approach further comprises:

[0082] first activating the cyclic pitch mode by reducing the load acting on the rotor blades to a pitch angle set point of a reduced load amount; then receiving one or more residual load signals of the rotor blades; and

[0083] One or more residual load signals of the rotor blade are compared to a second load threshold value.

[0084] Clause 17. The system of Clauses 11-16, wherein if the one or more residual load signals of the rotor blades exceed a second load threshold, the plurality of operations further comprises subsequently activating the collective pitch mode to further reduce the amount of the reduced load.

[0085] Clause 18. The system of Clauses 11-17, wherein the cyclic pitch mode reduces a majority of the loads acting on the rotor blades, and the collective pitch mode addresses pitch system saturation and reduces thrust.

[0086] Clause 19. The system of Clauses 11-18, wherein the cyclic pitch mode comprises cyclically pitching each of the rotor blades along their respective pitch axes, and the collective pitch mode comprises pitching each of the rotor blades together along their respective pitch axes.

[0087] Clause 20. The system of clauses 11-19, wherein the load threshold corresponds to a limit load threshold.

[0088] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

1. A method for reducing loads acting on a rotor blade of a wind turbine, the method comprising: receiving a plurality of load signals of the rotor blades; determining at least one load acting on the rotor blade based on a plurality of load signals of the rotor blade; determining a type of the at least one load acting on the rotor blade, the type of the at least one load acting on the rotor blade comprising at least one of thrust or rotor imbalance of a rotor of the wind turbine; comparing the at least one load to a load threshold; as well as Implementing a control scheme when the load exceeds the load threshold, the control scheme comprising: providing a first pitch pattern for reducing a first type of load, the first type of load comprising the rotor imbalance, the first pitch pattern comprising a cyclic pitch pattern; providing a different second pitch pattern for reducing a different second type of load, the second type of load comprising the thrust, the second pitch pattern comprising a common pitch pattern; and The first pitch pattern and the second pitch pattern are coordinated based on the type of the at least one load to reduce loads acting on the rotor blade.

2. The method according to claim 1, characterized in that Coordinating the first pitch pattern and the second pitch pattern based on the first type and the second type further includes: The first pitch pattern and the second pitch pattern are coordinated based on a type of the at least one load acting on the rotor blade using a graded approach.

3. The method according to claim 2, characterized in that Coordinating the first pitch pattern and the second pitch pattern based on the type of the at least one load acting on the rotor blade using the staged method further includes: The cyclic pitch mode is first activated by reducing the load acting on the rotor blades to a pitch angle set point of a reduced load amount; subsequently receiving one or more residual load signals of the rotor blades; and One or more residual load signals of the rotor blade are compared to a second load threshold value.

4. The method according to claim 3, characterized in that If one or more residual load signals of the rotor blades exceeds the second load threshold, the method further includes subsequently activating the collective pitch mode to further reduce the reduced load amount.

5. The method according to claim 4, characterized in that The cyclic pitch mode reduces most of the loads acting on the rotor blades, and the collective pitch mode addresses pitch system saturation and reduces the thrust.

6. The method according to claim 3, characterized in that The cyclic pitch mode includes cyclically pitching each of the rotor blades along their respective pitch axes, and the collective pitch mode includes pitching each of the rotor blades together along their respective pitch axes.

7. The method according to claim 1, characterized in that The load threshold corresponds to a limit load threshold.

8. A system for alleviating loads acting on a rotor blade of a wind turbine, the system comprising: one or more sensors configured to generate a plurality of load signals associated with the rotor blades; as well as a controller communicatively coupled to the one or more sensors, the controller comprising a processor configured to perform a plurality of operations, the plurality of operations comprising: receiving a plurality of load signals of the rotor blades; determining at least one load acting on the rotor blade based on a plurality of load signals of the rotor blade; determining a type of the at least one load acting on the rotor blade, the type of the at least one load acting on the rotor blade comprising at least one of thrust or rotor imbalance of a rotor of the wind turbine; comparing the at least one load to a load threshold; and Implementing a control scheme when the load exceeds the load threshold, the control scheme comprising: providing a first pitch pattern for reducing a first type of load, the first type of load comprising the rotor imbalance, the first pitch pattern comprising a cyclic pitch pattern; providing a different second pitch pattern for reducing a different second type of load, the second type of load comprising the thrust, the second pitch pattern comprising a common pitch pattern; and The first pitch pattern and the second pitch pattern are coordinated based on the type of the at least one load to reduce loads acting on the rotor blade.

9. The system according to claim 8, characterized in that Coordinating the first pitch pattern and the second pitch pattern based on the first type and the second type further includes: The first pitch pattern and the second pitch pattern are coordinated based on a type of the at least one load acting on the rotor blade using a graded approach.

10. The system according to claim 9, characterized in that Coordinating the first pitch pattern and the second pitch pattern based on the type of the at least one load acting on the rotor blade using the staged method further includes: The cyclic pitch mode is first activated by reducing the load acting on the rotor blades to a pitch angle set point of a reduced load amount; subsequently receiving one or more residual load signals of the rotor blades; and One or more residual load signals of the rotor blade are compared to a second load threshold value.

11. The system according to claim 10, wherein: If one or more residual load signals of the rotor blades exceed the second load threshold, the plurality of operations further include subsequently activating the collective pitch mode to further reduce the reduced load amount.

12. The system according to claim 11, wherein: The cyclic pitch mode reduces most of the loads acting on the rotor blades, and the collective pitch mode addresses pitch system saturation and reduces the thrust.

13. The system according to claim 10, wherein: The cyclic pitch mode includes cyclically pitching each of the rotor blades along their respective pitch axes, and the collective pitch mode includes pitching each of the rotor blades together along their respective pitch axes.

14. The system according to claim 8, wherein: The load threshold corresponds to a limit load threshold.

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