Patent Publication Number: US-2012027589-A1

Title: Method and apparatus for control of asymmetric loading of a wind turbine

Description:
BACKGROUND OF THE INVENTION 
     The subject matter described herein relates generally to methods and systems for controlling a wind turbine, and more particularly, to methods and systems for mitigating asymmetric loading of a wind turbine. 
     Generally, wind turbines include a tower and a nacelle mounted on the tower. A rotor is rotatably mounted to the nacelle and is coupled to a generator by a main shaft. A plurality of blades extends from the rotor. The blades are oriented such that wind passing over the blades turns the rotor and rotates the shaft, thereby driving the generator to generate electricity. 
     Vertical and horizontal wind shears, yaw misalignment and/or wind turbulence may act either collectively or individually for producing an asymmetric loading of the wind turbine. In particular, such an asymmetric loading may act across the wind turbine rotor. As a result, at least some elements of the wind turbine may be deformed. For example, the main shaft of the wind turbine may be bent (e.g., radially displaced) as a result of asymmetric rotor loading. 
     In order to mitigate the effect of the asymmetric loading of a wind turbine, a set of sensors for asymmetric load control (ALC) such as, for example, an array of proximity sensors, may be provided in the wind turbine to directly measure deformation of at least some elements of the wind turbine, such as a bending of the main shaft. An ALC assembly may use signals generated by the ALC sensors for mitigating the effect of asymmetric load of the rotor by, for example, controlling blade pitch and/or yaw alignment of the wind turbine. ALC may facilitate reducing the effects of extreme loads and fatigue cycles acting on the wind turbine. 
     However, additional methods and systems for further reducing asymmetric loading and/or increasing reliability of ALC are desirable. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to an embodiment of the invention, a wind turbine is provided. The wind turbine includes a rotor, at least one rotor blade coupled to the rotor, and a yaw system. The yaw system includes at least one yaw motor for adjusting a yaw angle, of the wind turbine. The yaw system is configured for generating a yaw drive signal. The wind turbine further includes an asymmetric load control assembly configured to receive the yaw drive signal. The asymmetric load control assembly is further configured to mitigate an asymmetric load acting on the rotor using the yaw drive signal. 
     According to another embodiment of the invention, a method of operating a wind turbine is provided. The wind turbine includes a rotor, at least one rotor blade coupled to the rotor, and a yaw system including at least one yaw motor for adjusting a yaw angle of the wind turbine. The method further includes mitigating an asymmetric load acting on the rotor using the yaw drive signal. 
     In yet another embodiment of the invention, a control system for a wind turbine is provided. The wind turbine includes at least one yaw motor for adjusting a yaw angle of the wind turbine. The control system includes an asymmetric load control assembly configured to receive a yaw drive signal. The asymmetric load control assembly is further configured to mitigate an asymmetric load acting on the rotor using the yaw drive signal. 
     According to embodiments herein, a yaw drive signal typically corresponds to, at least, a property from the at least one yaw motor. Such a property may be, for example, a torque generated by the yaw motor. Alternatively or in addition thereto, the yaw drive signal may correspond to a control signal for operating the at least one yaw motor. For example, the yaw drive signal may correspond to a set point generated for operating the at least one yaw motor. 
     According to embodiments herein, a yaw system may facilitate a yaw drive signal, which may be used as reference data to directly implement control of asymmetric rotor loading or increase reliability of ALC based on the use of ALC sensors. In particular, embodiments herein facilitate mitigating asymmetric rotor loading by implementing an asymmetric load control assembly configured such that it may use the yaw drive signal. According to at least some embodiments herein, the yaw drive signal is generated by a yaw system. 
     Further aspects, advantages and features of the present invention are apparent from the dependent claims, the description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure including the best mode thereof, to one of ordinary skill in the art is set forth more particularly in the remainder of the specification, including reference to the accompanying figures wherein: 
         FIG. 1  is a perspective view of an exemplary wind turbine; 
         FIG. 2  is an enlarged sectional view of a portion of the wind turbine shown in  FIG. 1 ; 
         FIG. 3  is a schematic drawing of a yaw system of the wind turbine shown in  FIG. 1 ; 
         FIG. 4  is a block diagram of a scheme for controlling the wind turbine shown in  FIG. 1 ; 
         FIG. 5  is an enlarged perspective view of another portion of the wind turbine shown in  FIG. 1 ; 
         FIG. 6  is a flow chart illustrating a method of operating the wind turbine shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in each figure. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet further embodiments. It is intended that the present disclosure includes such modifications and variations. 
     As mentioned above, vertical and horizontal wind shears, yaw misalignment, wake flow caused by another wind turbine, and/or turbulence may act individually or together to produce asymmetric loading across a wind turbine rotor. A resultant asymmetric load produces bending moments in the blades that are reacted through the hub and subsequently to a wind turbine shaft. Such an asymmetric load may cause deformations of elements in the wind turbine, such as a bending or radial displacement of the main shaft. 
     The embodiments described herein facilitate reducing asymmetric loading acting on the rotor of a wind turbine system. Further, embodiments herein may increase reliability of asymmetric load control (ALC) of a wind turbine. In particular, the wind turbine includes a yaw system for adjusting a yaw angle of the wind turbine. Typically, the yaw angle is adjusted by at least one yaw motor operated by a yaw control assembly. According to at least some embodiments herein, the yaw system is a soft yaw system. In particular, the yaw system may be a soft yaw system configured to actively restrict rotation of the nacelle about a yaw angle by continuously operating the yaw motor. 
     An asymmetric load control assembly (hereinafter referred to as ALC assembly) according to embodiments herein is typically configured for receiving a yaw drive signal generated by a yaw system. The yaw drive signal may then be used to determine the magnitude and/or the orientation of the resultant rotor load. Thereby, the ALC assembly may use the yaw drive signal for mitigating an asymmetric load. 
     The yaw drive signal may correspond to one or more properties from the at least one yaw motor such as, but not limited to, a generated torque. Exemplarily, but not limited to, the yaw drive signal may correspond to an electrical current applied to the yaw motor, which current corresponds to a torque applied by the yaw motor. 
     Alternatively or in addition thereto, the yaw drive signal may correspond to a control signal for operating the at least one yaw motor. In particular, the yaw system may receive a reference signal for re-aligning or maintaining a yaw angle. For example, this reference signal may include information about oncoming wind such as a wind direction measured by a wind vane. Typically, the yaw system is configured to use this reference signal for generating a control signal for operating the at least one yaw motor, such as a yaw motor set point. Typically, a yaw motor set point is a motor torque, a motor speed, direction, and/or nacelle position that the yaw system strives to set through actuation of the at least one yaw motor. 
     According to embodiments herein, mitigating asymmetric loads may include reducing or countering asymmetric rotor loading. Thereby, an ALC assembly is typically configured for causing a more symmetric load on the rotor. The ALC assembly may mitigate the asymmetric load by adequately pitching the blades of the wind turbine. 
     The ALC assembly may mitigate the asymmetric loads directly based on the yaw drive signal. For example, the ALC assembly may implement a control scheme configured to produce a control signal based on the yaw drive signal for reducing the asymmetric loads. Alternatively, or in addition thereto, the wind turbine may implement an ALC sensor system for directly sensing asymmetric loads acting on the rotor. In such embodiments, the ALC assembly may mitigate the asymmetric loads directly based on the measurements of the ALC sensor system and use the yaw drive signal for validating the measurements. Thereby, embodiments herein may facilitate increasing reliability of ALC of the wind turbine. 
     In a wind turbine implementing an ALC sensor system, the yaw drive signal may also be used for redundancy purposes in the case that the ALC sensor system fails. Further, the yaw drive signal may also be used in combination with the measurements of the sensor system for generating an ALC signal. 
     As used herein, the term “blade” is intended to be representative of any device that provides a reactive force when in motion relative to a surrounding fluid. As used herein, the term “wind turbine” is intended to be representative of any device that generates rotational energy from wind energy, and more specifically, converts kinetic energy of wind into mechanical energy. As used herein, the term “wind generator” is intended to be representative of any wind turbine that generates electrical power from rotational energy generated from wind energy, and more specifically, converts mechanical energy converted from kinetic energy of wind to electrical power. 
       FIG. 1  is a perspective view of an exemplary wind turbine  10 . In the exemplary embodiment, wind turbine  10  is a horizontal-axis wind turbine. Alternatively, wind turbine  10  may be a vertical-axis wind turbine. In the exemplary embodiment, wind turbine  10  includes a tower  12  that extends from a support system  14 , a nacelle  16  mounted on tower  12 , and a rotor  18  that is coupled to nacelle  16 . Rotor  18  includes a rotatable hub  20  and at least one rotor blade  22  coupled to and extending outward from hub  20 . In the exemplary embodiment, rotor  18  has three rotor blades  22 . In an alternative embodiment, rotor  18  includes more or less than three rotor blades  22 . In the exemplary embodiment, tower  12  is fabricated from tubular steel to define a cavity (not shown in  FIG. 1 ) between support system  14  and nacelle  16 . In an alternative embodiment, tower  12  is any suitable type of tower having any suitable height. 
     Rotor blades  22  are spaced about hub  20  to facilitate rotating rotor  18  to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. Rotor blades  22  are mated to hub  20  by coupling a blade root portion  24  to hub  20  at a plurality of load transfer regions  26 . Load transfer regions  26  have a hub load transfer region and a blade load transfer region (both not shown in  FIG. 1 ). Loads induced to rotor blades  22  are transferred to hub  20  via load transfer regions  26 . 
     In one embodiment, rotor blades  22  have a length ranging from about 15 meters (m) to about 91 m. Alternatively, rotor blades  22  may have any suitable length that enables wind turbine  10  to function as described herein. For example, other non-limiting examples of blade lengths include 10 m or less, 20 m, 37 m, or a length that is greater than 91 m. As wind strikes rotor blades  22  from a direction  28 , rotor  18  is rotated about an axis of rotation  30 . As rotor blades  22  are rotated and subjected to centrifugal forces, rotor blades  22  are also subjected to various forces and moments. As such, rotor blades  22  may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position. 
     Moreover, a pitch angle or blade pitch of rotor blades  22 , i.e., an angle that determines a perspective of rotor blades  22  with respect to direction  28  of the wind, may be changed by a pitch adjustment system  32  to control the load and power generated by wind turbine  10  by adjusting an angular position of at least one rotor blade  22  relative to wind vectors. Pitch axes  34  for rotor blades  22  are shown. During operation of wind turbine  10 , pitch adjustment system  32  may change a blade pitch of rotor blades  22  such that rotor blades  22  are moved to a feathered position, such that the perspective of at least one rotor blade  22  relative to wind vectors provides a minimal surface area of rotor blade  22  to be oriented towards the wind vectors, which facilitates reducing a rotational speed of rotor  18  and/or facilitates a stall of rotor  18 . 
     In the exemplary embodiment, a blade pitch of each rotor blade  22  is controlled individually by a control system  36 . Alternatively, the blade pitch for all rotor blades  22  may be controlled simultaneously by control system  36 . Further, in the exemplary embodiment, as direction  28  changes, a yaw direction of nacelle  16  may be controlled about a yaw axis  38  to position rotor blades  22  with respect to direction  28 . 
       FIG. 2  is an enlarged sectional view of a portion of wind turbine  10 . In the exemplary embodiment, wind turbine  10  includes nacelle  16  and hub  20  that is rotatably coupled to nacelle  16 . More specifically, hub  20  is rotatably coupled to an electric generator  42  positioned within nacelle  16  by rotor shaft  44  (sometimes referred to as either a main shaft or a low speed shaft), a gearbox  46 , a high speed shaft  48 , and a coupling  50 . In the exemplary embodiment, rotor shaft  44  is disposed coaxial to longitudinal axis  116 . Rotation of rotor shaft  44  rotatably drives gearbox  46  that subsequently drives high speed shaft  48 . High speed shaft  48  rotatably drives generator  42  with coupling  50  and rotation of high speed shaft  48  facilitates production of electrical power by generator  42 . Gearbox  46  and generator  42  are supported by a support  52  and a support  54 . In the exemplary embodiment, gearbox  46  utilizes a dual path geometry to drive high speed shaft  48 . Alternatively, rotor shaft  44  is coupled directly to generator  42  with coupling  50 . For example, wind turbine  10  may be based on a direct-drive design. 
     In the exemplary embodiment, hub  20  includes a pitch assembly  66 . Pitch assembly  66  may include a pitch controller  73  (shown in Figure) operatively coupled to one or more pitch drive systems  68 . Each pitch drive system  68  is coupled to a respective rotor blade  22  (shown in  FIG. 1 ) for modulating the blade pitch of associated rotor blade  22  along pitch axis  34 . Only one of three pitch drive systems  68  is shown in  FIG. 2 . Please note that pitch controller  73  may be a centralized controller associated to a plurality of pitch drive  68 , such as exemplarily shown in  FIG. 4 . Alternatively, wind turbine  10  may include a distributed pitch controller including, for example, a plurality of pitch controllers, each of the pitch controllers being associated to a respective pitch drive  68 . 
     In the exemplary embodiment, pitch assembly  66  includes at least one pitch bearing  72  coupled to hub  20  and to respective rotor blade  22  (shown in  FIG. 1 ) for rotating respective rotor blade  22  about pitch axis  34 . Pitch drive system  68  includes a pitch drive motor  74 , pitch drive gearbox  76 , and pitch drive pinion  78 . Pitch drive motor  74  is coupled to pitch drive gearbox  76  such that pitch drive motor  74  imparts mechanical force to pitch drive gearbox  76 . Pitch drive gearbox  76  is coupled to pitch drive pinion  78  such that pitch drive pinion  78  is rotated by pitch drive gearbox  76 . Pitch bearing  72  is coupled to pitch drive pinion  78  such that the rotation of pitch drive pinion  78  causes rotation of pitch bearing  72 . More specifically, in the exemplary embodiment, pitch drive pinion  78  is coupled to pitch bearing  72  such that rotation of pitch drive gearbox  76  rotates pitch bearing  72  and rotor blade  22  about pitch axis  34  to change the blade pitch of blade  22 . 
     Pitch drive system  68  is coupled to control system  36  for adjusting the blade pitch of rotor blade  22  upon receipt of one or more signals from control system  36 . In the exemplary embodiment, pitch drive motor  74  is any suitable motor driven by electrical power, pneumatic system and/or a hydraulic system that enables pitch assembly  66  to function as described herein. Alternatively, pitch assembly  66  may include any suitable structure, configuration, arrangement, and/or components such as, but not limited to, hydraulic cylinders, springs, and/or servo-mechanisms. Moreover, pitch assembly  66  may be driven by any suitable means such as, but not limited to, hydraulic fluid, and/or mechanical power, such as, but not limited to, induced spring forces and/or electromagnetic forces. In certain embodiments, pitch drive motor  74  is driven by energy extracted from a rotational inertia of hub  20  and/or a stored energy source (not shown) that supplies energy to components of wind turbine  10 . 
     In the exemplary embodiment, pitch drive system  68  is positioned in a cavity  86  defined by an inner surface  88  of hub  20 . In a particular embodiment, pitch drive system  68 , is coupled, directly or indirectly, to inner surface  88 . In an alternative embodiment, pitch drive system  68  is positioned with respect to an outer surface  90  of hub  20  and may be coupled, directly or indirectly, to outer surface  90 . 
     Nacelle  16  also includes a yaw drive mechanism  56  that may be used to rotate nacelle  16  and hub  20  on yaw axis  38  (shown in  FIG. 1 ) to control the perspective of rotor blades  22  with respect to direction  28  of the wind. The perspective of rotor blades  22  with respect to direction  28  of the wind is also referred to as yaw angle. As shown schematically shown in  FIG. 3 , and further detailed below, yaw drive mechanism  56  forms part of a yaw system  92 . Yaw drive mechanism  56  may be placed at the join between tower  12  and nacelle  16 . Yaw drive mechanism  56  may collaborate with a bearing system for rotating nacelle  16 . For example, in the exemplary embodiment, nacelle  16  also includes a main forward support bearing  60  and a main aft support bearing  62  arranged to interact with respective bearings mounted at tower  12  for enabling rotation of nacelle  16 . 
     Forward support bearing  60  and aft support bearing  62  facilitate radial support and alignment of nacelle  16  and rotor shaft  44 . Forward support bearing  60  is coupled to rotor shaft  44  near hub  20 . Aft support bearing  62  is positioned on rotor shaft  44  near gearbox  46  and/or generator  42 . Nacelle  16  may include any number of support bearings that enable wind turbine  10  to function as disclosed herein. Rotor shaft  44 , generator  42 , gearbox  46 , high speed shaft  48 , coupling  50 , and any associated fastening, support, and/or securing device including, but not limited to, support  52  and/or support  54 , and forward support bearing  60  and aft support bearing  62 , are sometimes referred to as a drive train  64 . 
     As schematically shown in  FIG. 3 , yaw system  92  includes at least one yaw motor  94  configured to adjust a yaw angle of wind turbine  10 . In particular, yaw motor  94  may form part of, or be coupled to, yaw drive mechanism  56  for effecting rotation of nacelle  16  about yaw axis  38 . Yaw system  92  may include more than one yaw motor. For example, the exemplary embodiment depicted in  FIG. 3  includes two yaw motors  94 . Yaw system  92  may include any suitable number of yaw motors that enable yaw system  92  to conveniently control yaw of wind turbine  10 . For example, yaw system  92  may include between two and six yaw motors. 
     The at least one yaw motor  94  can generate a torque M for rotating nacelle  16 , torque M being smaller than or equal to a maximum torque Mmax. Torque M of the at least one yaw motor may be positive or negative (i.e., torque M may be effected in counter clockwise or clockwise direction) depending on the direction of rotation required in order to align, or maintain aligned, rotor  18  to the desired yaw direction. For example, but not limited to, during operation of wind turbine  10 , torque M may include torque values between 3000 and −3000 kNm or, more specifically, between 1500 and −1500 kNm. 
     Typically, yaw system  92  includes a yaw control assembly  96  for operating yaw motors  94 . Yaw control assembly  96  may be operatively coupled to the at least one yaw motor  94  through cables  102 . Yaw control assembly  96  typically forms part of control system  36 . Alternatively, yaw control assembly  96  may be provided separated from control system  36 . 
     As depicted in  FIG. 3 , yaw control assembly  96  is typically configured to receive a yaw reference signal based on a signal from one or more yaw sensor(s)  104  configured to sense at least one of position, velocity or acceleration of at least one reference point that is affected by the operation of the yaw system  92 . In particular, yaw control assembly  96  may directly receive a signal from yaw sensor(s)  104  or may receive that signal after being processed by other elements of wind turbine  10 . The reference point may be placed on the circumference of forward support bearing  60  and/or aft support bearing  62 , adjacent to a yaw motor  94 , or on another suitable location such as inside nacelle  16 . Yaw sensor(s)  104  are typically communicatively coupled to yaw control assembly  96  through one or more cables  106 , or other elements processing the signal generated by yaw sensor(s)  104 , in order to provide yaw control assembly  96  with a yaw reference signal. 
     Alternatively, or in addition thereto, yaw control assembly  96  is further configured to receive a wind reference signal from sensor(s) provided in a meteorological mast  58 . The wind reference signal typically includes strength and direction W of oncoming wind. More specifically, meteorological mast  58  (shown in  FIG. 2 ) may include a wind vane and anemometer (neither shown in  FIG. 2 ) for generating data included in the wind reference signal. A sensor in meteorological mast  58  is typically communicatively coupled to yaw control assembly  96  through one or more cables  107 , or other elements processing the signal generated by the sensor, in order to provide yaw control assembly  96  with a yaw reference signal. 
     Typically, yaw control assembly  96  also receives input data from the at least one yaw motor  94  regarding the current motor torque M and/or other operating conditions of the at least one yaw motor  94 , and gives instructions to the at least one yaw motor  94  as output data. 
     Yaw system  92  is typically configured to achieve optimal operation of wind turbine  10 . This optimal operation may be achieved when nacelle  16  with rotor  18  are rotated towards a specific direction, herein referred to as the yaw set point. This specific direction may be determined using the wind direction or other factors that are deemed to be relevant. For example, a yaw setpoint may strive to achieve an orientation of the plane of rotor  18 , i.e. the plane comprising rotor blades  22 , perpendicular to wind direction  28 . The yaw setpoint may also be a value corresponding not to a specific alignment but to other properties of the yaw system, such as for instance the yaw speed, the yaw acceleration or the yaw torque. 
     Typically, yaw system  100  is configured to use the reference signals set forth above for generating one or more control signals for operating the at least one yaw motor  94 , so that yaw system  92  facilitates an optimal operation of wind turbine  10 . Typically, the yaw control signal may correspond to a yaw motor set point or other control signal generated by yaw control assembly  96  for operation the at least one yaw motor  94 . Further, yaw system  100  may generate output data based on one or more control parameters for effecting operation of the at least one yaw motor  94 . The output data may include an instruction regarding magnitude of the desired motor torque M and/or the desired direction and speed of movement of nacelle  16  relative to tower  12  in accordance with the set point. 
     According to at least some embodiments herein, after yaw system  92  establishes a yaw setpoint, the actual yaw angle of rotor  18  is compared with the yaw setpoint and the difference is determined by yaw system  92  as the yaw error. The yaw system  92  applies a torque M through the at least one yaw motor  94  in order to minimize this yaw error and turn nacelle  16  and rotor  18  towards the yaw setpoint. The yaw setpoint can be monitored and re-calculated at any given time, in order to keep the setpoint up to date as the wind direction or wind strength changes. Thereby, yaw system  92  may continuously strive to minimize the yaw error and reach the yaw set point. In particular, motor torque M may be controlled by a yaw control assembly as described in the International Patent Application with publication number WO 2010/100271, which is incorporated herein by reference to the extent in which the application is not inconsistent with this disclosure and in particular those parts thereof describing a yaw system for a wind turbine. 
     According to embodiments herein, and as described above, the yaw system may be a soft yaw system. In particular, yaw control assembly  96  may be configured to continuously operate the at least one yaw motor during a period of time for maintaining the wind turbine at a yaw set point. In other words, yaw system  92  may be configured for: a) re-orienting nacelle  16  and rotor  18  towards a specific direction; and b) actively maintaining nacelle  16  and rotor  18  pointing to the specific directions. The latter function could be compared to an active braking of yaw rotation of wind turbine  10 . According to at least some embodiments herein, a soft yaw system facilitates operation of an ALC system since the soft yaw system can continuously generate data related to: a) at least one property from the at least one yaw motor  94 ; and/or, b) one or more control signals for operating the at least one yaw motor  94 . These data may be used by ALC assembly  100  for mitigating an asymmetric load acting on rotor  18 , as further detailed below. 
     According to at least some embodiments herein, wind turbine  10  may further include a yaw brake system (not shown) for use in combination with, or alternatively, to yaw system  92 . For example, such yaw brake system may be a hydraulic or electric brake configured to fix the position of nacelle  16  when required in order to avoid wear and high fatigue loads on wind turbine components. Yaw brake system may be configured to operate in case of failure of yaw system  92 . The yaw brake system may be configured to operate in combination with yaw system  92  for maintaining nacelle  16  and rotor  18  pointing to a specific direction. 
       FIG. 4  is a block diagram of an exemplary scheme for controlling exemplary wind turbine  10 . In the exemplary scheme, asymmetric load control (ALC) assembly  100  is configured to receive a yaw drive signal generated by yaw system  92 . Further, ALC assembly  100  may be operatively connected to one or more ALC sensors  134  to receive signals corresponding to direct measurements of effects caused by an asymmetric rotor loading such as, but not limited to a bending or radial displacement of main shaft  44 . 
     ALC assembly  100  is typically configured to process the yaw drive signal and, optionally, the signal from ALC sensor(s)  134 . For example, ALC assembly  100  may analyze the yaw drive signal and/or the signal from ALC sensor(s)  134  to determine an asymmetric load acting on rotor  18  and generates information for mitigating the asymmetric load. Alternatively or in addition thereto, ALC assembly  100  may use one of these signals for validating a reference signal used for ALC or as a redundant data. Further, ALC assembly  100  is typically configured to generate an ALC signal based on the received signal(s) for mitigating an asymmetric loading. 
     According to the exemplary embodiment, and other embodiments herein, ALC assembly  100  is operatively connected to a pitch controller  73 . Pitch controller  73  receives the ALC signal and, based on this signal, operates at least one of pitch drive systems  68  for mitigating an asymmetric loading acting on rotor  18 . 
     According to at least some embodiments herein, ALC assembly  100  is configured to mitigate an asymmetric load directly based on a yaw drive signal. That is, ALC assembly  100  may be configured for determining an ALC signal facilitating mitigation of an asymmetric rotor loading directly based on the reference data contained in the yaw drive signal. Thereby, ALC may be implemented using information generated by yaw system  92 . The yaw drive signal is typically suitable for directly implementing ALC since a yaw drive signal according to embodiments herein typically provides information, which can be correlated to displacement of wind turbine components (e.g., main shaft  44 ) caused by an asymmetric load of wind turbine  10 . 
     Exemplarily, ALC assembly  100  may be further configured to obtain an estimation of at least one wind turbine property associated to a bending of rotor shaft caused by an asymmetric rotor loading, such as a deflection of a main shaft flange of the wind turbine or a displacement of a gearbox of the wind turbine from one or more predetermined positions. For example, but not limited to, an ALC function implemented in ALC assembly  100  may estimate a yaw moment, which might be equivalent to the measurement of a yaw torque reported by ALC sensors. This estimation may be obtained based on, at least, the yaw drive signal. In such embodiments, ALC assembly  100  may be further configured to mitigate an asymmetric rotor loading directly based on the estimation. 
     According to embodiments herein, the yaw drive signal corresponds to at least one property from the at least one yaw motor  94 . For example, the at least one property may be dependent on a motor workload of the at least one yaw motor  94 . In particular, the at least one property from the at least one yaw motor  94  may be a yaw motor torque and the yaw drive signal may then correspond to the yaw motor torque. 
     Exemplarily, the motor torque M applied by a soft yaw system for keeping wind turbine  10  in a desired yaw angle may be recorded and transmitted to ALC assembly  100  for implementing ALC or validating measurements from ALC sensors. Typically, the magnitude of this motor torque M will be dependent on asymmetric loads acting on the rotor that cause a yaw wise rotational force to be applied to wind turbine  10  itself. 
     According to at least some embodiments herein, the yaw drive signal may correspond to a control signal for operating the at least one yaw motor such as, but not limited to, a yaw motor setpoint, a yaw error, or from any other data generated by the yaw system  92  for controlling the at least one yaw motor  94 . Further, the yaw drive signal may correspond to a plurality of properties. For example, the yaw drive signal may include data corresponding to a yaw motor setpoint and to yaw motor torque M. 
     A yaw drive signal according to embodiments herein may be generated in a number of different ways. For example, the current that is applied to the yaw motor may be measured. The control system of wind turbine  10  may estimate the yaw motor torque M based on the measured current and, optionally, on other parameters of yaw system  92 . The estimation of the yaw motor torque M may then be used for ALC. Alternatively or in addition thereto, the power of the at least one yaw motor  94  and/or a rotational speed thereof may be measured and used for generating the yaw drive signal. Any other property of yaw motor  94  or control signal for operation thereof may be used for generating a yaw drive signal such as, but not limited to, voltage or frequency applied to the at least one yaw motor  94 . 
     ALC assembly  100  may process a yaw drive signal for conveniently implement control of asymmetric loads. Alternatively or in addition thereto, yaw system  92  may generate a yaw drive signal based on already processed data, so that the yaw drive signal may be used directly by ALC assembly  100 . The yaw drive signal may be generated in analog and/or digital format. 
     A yaw system typically provides a yaw drive signal having a high quality. Thereby, reliability of ALC may be further improved by using the yaw drive signal for mitigating an asymmetric rotor loading. Furthermore, an ALC assembly  100  mitigating an asymmetric load directly based on the yaw drive signal may render unnecessary implementation of sensors for ALC thereby reducing costs. It should be further noted that ALC sensors may degrade with time or may be prone to failure. Typically, a yaw system is less prone to such degradation or failure, so that it provides a reliable signal for implementing and/or validating ALC. 
     According to at least some embodiments herein ALC yaw system  100  may generate the yaw drive signal in a continuous manner during operation of wind turbine  10 . In particular, ALC yaw system  100  may be a soft yaw system configured to: a) generate a yaw drive signal during yaw re-alignment of wind turbine  10 , and, b) generate a yaw drive signal during time periods in which yaw is semi-stationary, so that the yaw system strives to maintain wind turbine  10  at a specific yaw angle. 
     According to embodiments herein, a continuous generation of the yaw drive signal includes a discrete yaw drive signal generated during sufficiently short time intervals. For example, a soft yaw system may provide a yaw drive signal at time periods between 1 and 1000 milliseconds, such as 20 milliseconds. A soft yaw system may provide a particularly reliable signal for facilitating operation of ALC assembly  100 . 
     Optionally and as set forth above, at least some embodiments herein contemplate implementation of ALC sensors. In such embodiments, ALC assembly  100  may be configured to mitigate an asymmetric load using an asymmetric load signal generated by the ALC sensors and a yaw drive signal. Thereby, reliability of ALC may be increased. In some embodiments herein, ALC assembly  100  is configured to: a) perform ALC based on the signal provided by ALC sensors; and b) use the yaw drive signal for evaluating and/or validating performance of the ALC sensors. According to other embodiments, ALC assembly  100  is configured to use the yaw drive signal only as a redundant signal for ALC in case that the ALC sensors fail. Further, ALC assembly  100  may be configured to mitigate asymmetric load by generating an ALC control signal based on the combination of the signal from ALC sensors and the yaw drive signal. 
     An ALC sensor is typically able to detect an asymmetric load acting on rotor  18  and translating into moments acting on hub  20  and, subsequently, to rotor shaft  44 . These moments may be manifested as a bending of a shaft of wind turbine  10 , a deflection of a main shaft flange of the wind turbine, a displacement of a gearbox of the wind turbine from one or more predetermined positions as deflections, and/or strains at a main shaft flange  132  caused by an asymmetric rotor loading. More specifically, wind turbine  10  may include an ALC sensor system configured to: a) directly measure displacements or moments resulting from an asymmetric rotor loading; and b) generate an asymmetric load signal based on the direct measurement. For example, wind turbine  10  may include one or more ALC sensors  134  configured to: a) directly measure a deflection and/or displacement of an element of wind turbine  10  from a predetermined position, and b) generate an asymmetric load signal corresponding to the direct measurement. Typically, ALC sensor(s)  134  is a proximity sensor including a sensor bracket  136  configured to enable measurement of a bending or radial displacement of rotor shaft  44 . 
     In the exemplary embodiment, and other embodiments herein, wind turbine  10  includes ALC sensor  134  configured to directly measure effects of asymmetric rotor loading, such as a bending of a shaft of wind turbine  10  caused by an asymmetric rotor loading, a deflection of a main shaft flange of the wind turbine, or a displacement of a gearbox of the wind turbine from one or more predetermined positions. In particular, ALC sensor  134  may be a proximity sensor that measure displacement or strain of the shaft using sensor technologies based on acoustic, optical, magnetic, capacitive or inductive field effects. 
     An ALC sensor according to embodiments herein may be configured to sense asymmetric rotor loading acting on other elements of wind turbine  10 , such as gearbox  46 . In the exemplary embodiment, only one set of sensors  134  is illustrated. According to at least some embodiments, wind turbine  10  includes at least three set of sensors  134  to measure displacements of main shaft flange  132  or displacement of gearbox  46  caused by an asymmetric load. ALC sensor(s)  134  may be configured as described in the US Patent Applications with publication numbers US2004/0151575 and US 2006/0002792 which are incorporated herein by reference to the extent in which the application is not inconsistent with this disclosure and in particular those parts thereof describing sensors for measuring effects of asymmetric rotor loading. 
     As set forth above, ALC assembly  100  may be configured to mitigate an asymmetric rotor loading by pitching at least one of rotor blades  22 . In particular, the yaw drive signal and/or the asymmetric load signal may be used to determine a pitch for each of rotor blades  22 . For example, the yaw drive signal may be used to estimate a shaft displacement and, thereby, the magnitude and/or phase angle of asymmetric rotor loading. The estimated magnitude and/or phase angle can then be used to determine a blade pitch command for at least one of rotor blades  22  to reduce the asymmetric rotor loading. A coordinate transformation (e.g., a Parks DQ transformation), a bias estimation method and/or other control scheme may be implemented into control system  36  and used to calculate a pitch angle for each rotor blade to reduce the overall asymmetric rotor loading. 
     As another example; sensor readings from ALC sensor  134  indicating measured displacement or moments may be used by ALC assembly  100  to determine a pitch command for each rotor blade  22  to reduce or counter an asymmetric rotor loading. In this control scheme, a yaw drive signal may be used for validating the reading from ALC sensor  134 . In particular, the yaw drive signal may be used for providing an estimation of a presumably correct measurement from ALC sensor  134 . Thereby, the estimation may then be compared with actual readings from ALC sensor  134  in order to detect any abnormalities occurring in ALC sensor  134 . 
     According to at least some embodiments, the pitch command is determined by using information from both the asymmetric load signal from ALC sensor  134  and the yaw drive signal generated by yaw system  92 . ALC may also include determining a favorable yaw orientation to reduce pitch activity during mitigation of asymmetric rotor loading, as described in the US Patent Application with publication number US 2006/0002792 A1. 
     Control system  36  may implement yaw control, ALC, pitch control, and management of ALC sensors. Control system  36  may also implement a balance control of wind turbine  10  for decreasing an unbalance of rotor  18 . Such balance control may also use a yaw drive signal generated by yaw system  92  as described in the International Patent Application with publication number WO 2010/133512, which is incorporated herein by reference to the extent in which the application is not inconsistent with this disclosure and in particular those parts thereof describing a method and a system for balancing a wind turbine. 
     In the exemplary embodiment, control system  36  is shown as being centralized within nacelle  16 , however, control system  36  may be a distributed system throughout wind turbine  10 , on support system  14 , within a wind farm, and/or at a remote control center. Control system  36  typically includes a processor  40  configured to perform the methods and/or steps described herein. Further, many of the other components described herein include a processor. As used herein, the term “processor” is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), a field programmable gate array (FPGA), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. It should be understood that a processor and/or a control system can also include memory, input channels, and/or output channels. Control system  36  typically includes means for communication between the different systems such as electrical connections and/or wireless communication devices. 
     In the embodiments described herein, memory may include, without limitation, a computer-readable medium, such as a random access memory (RAM), and a computer-readable non-volatile medium, such as flash memory. Alternatively, a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, in the embodiments described herein, input channels include, without limitation, sensors and/or computer peripherals associated with an operator interface, such as a mouse and a keyboard. Further, in the exemplary embodiment, output channels may include, without limitation, a control device, an operator interface monitor and/or a display. 
     Processors described herein process information transmitted from a plurality of electrical and electronic devices that may include, without limitation, sensors, actuators, compressors, control systems, and/or monitoring devices. Such processors may be physically located in, for example, a control system, a sensor, a monitoring device, a desktop computer, a laptop computer, a programmable logic controller (PLC) cabinet, and/or a distributed control system (DCS) cabinet. RAM and storage devices store and transfer information and instructions to be executed by the processor(s). RAM and storage devices can also be used to store and provide temporary variables, static (i.e., non-changing) information and instructions, or other intermediate information to the processors during execution of instructions by the processor(s). Instructions that are executed may include, without limitation, wind turbine control system control commands. The execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions. 
     In the exemplary embodiment, control system  36  includes a real-time controller that includes any suitable processor-based or microprocessor-based system, such as a computer system, that includes microcontrollers, reduced instruction set circuits (RISC), application-specific integrated circuits (ASICs), logic circuits, and/or any other circuit or processor that is capable of executing the functions described herein. In one embodiment, the controller may be a microprocessor that includes read-only memory (ROM) and/or random access memory (RAM), such as, for example, a 32 bit microcomputer with 2 Mbit ROM, and 64 Kbit RAM. As used herein, the term “real-time” refers to outcomes occurring a substantially short period of time after a change in the inputs affect the outcome, with the time period being a design parameter that may be selected based on the importance of the outcome and/or the capability of the system processing the inputs to generate the outcome. 
       FIG. 6  is a flow chart illustrating an exemplary method  600  of operating wind turbine  10 . Method  600  may include generating  610  a signal appropriate for being used for ALC of wind turbine  10 . According to embodiments herein, the generated signal includes, at least, a yaw drive signal generated by yaw system  92 , as described above. According to at least some embodiments herein, the signal further includes an asymmetric load signal generated by ALC sensor  134 . 
     Method  600  may further include receiving  620  the signal(s) generated for ALC. Typically, these signals are received by ALC assembly  100 . Typically, the components of ALC assembly  100  receiving the signals (e.g., a processor or an analog to digital converter) are coupled to the elements of wind turbine  10  used for detecting an asymmetric load (e.g., yaw system  92  and/or ALC sensor  134 ). ALC assembly  100  may convert these signals to a usable format, if required. 
     Method  600  further includes mitigating  630  an asymmetric load acting on rotor  18  using the signals for ALC, namely using a yaw drive signal and, optionally, an asymmetric load signal generated by ALC sensor(s)  134 . Mitigating  630  may further include a step  632  for determining the effects (e.g., loads) caused on one or more components of wind turbine  10  by an asymmetric load of rotor  18  using the signals for ALC. The control system of wind turbine  10  may use any suitable mathematical equation or previously acquired semi-empirical data to convert the input data (e.g., motor torque, current, yaw setpoint, etc) to relevant asymmetric load data (e.g., a shaft bending, a deflection of main shaft flange  132 , and/or a displacement of gearbox  46 ). Step  632  may also include determining the load on rotor blades  22  as well as any properties of an asymmetric rotor loading. 
     Mitigating  630  may further include a step  634  for determining a response to reduce or counter asymmetric rotor loading. For example, in response to a particular asymmetric rotor loading, the control system of wind turbine  10  may determine that the response should be to change the pitch of one or more blades  22 . As another example, the determined response may be applying a brake to stop or slow rotation of hub  20 . As a further example, the determined response may be to exert some action such as inducing a compensatory yaw adjustment. 
     Mitigating  630  may further include a step  636  for generating a signal that enables responding to an asymmetric load. For example, a response signal may be generated in the form of, for example, a data packet or a set of control signals transmitted over individual control lines, to cause pitch controller  73  to change the pitch of one or more of blades  22 . If the selected response fails to cause the wind turbine to operate within an acceptable operating range, method  600  can be repeated as often as necessary or even discontinued, resulting in a pitch control without the benefits of the described ALC algorithm(s). 
     Exemplary embodiments of systems and methods for operating a wind turbine are described above in detail. The systems and methods are not limited to the specific embodiments described herein, but rather, components of the systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, ALC according to embodiments herein may be implemented by a remote controller communicatively coupled to wind turbine  10 . The embodiments described herein are not limited to practice with only the wind turbine systems as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other rotor blade applications. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the present disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     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. While various specific embodiments have been disclosed in the foregoing, those skilled in the art will recognize that the spirit and scope of the claims allows for equally effective modifications. Especially, mutually non-exclusive features of the embodiments described above may be combined with each other. 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 have 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.