Abstract:
A wind turbine includes a plurality of wind turbine blades attached to a rotor positioned atop a tower affixed to a tower foundation. At least one blade pitch sensor is configured to measure blade pitch angles for one or more of the wind turbine blades. A rotor/generator speed sensor is configured to measure the rotational speed of the wind turbine rotor, a corresponding wind turbine generator, or both. A wind turbine nacelle yaw sensor is configured to measure the nacelle yaw, while at least two tower-base bending sensors are configured without use of adhesives, cements or bonding agents to provide large-area measurement of tower deflection. A controller is configured to adjust the pitch angle of one or more of the wind turbine blades in response to the measured one or more blade pitch angles, the measured rotational speed, the measured nacelle yaw and measured tower longitudinal deflection.

Description:
BACKGROUND 
     This invention relates generally to wind turbines, and more particularly to methods and apparatus for controlling wind turbine actuation to compensate for variations in wind speed. 
     Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient. 
     Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility scale grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 70 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid in some known embodiments. 
     The most significant source of variations in the operation of a wind turbine is the variation in the wind speed that acts on the rotor. An efficient utilization of controlled actuation to compensate for such variations in wind speed depends on prompt measurement or estimation of such variations. Lags and delays on the estimation of the effective wind speed lead to reduced efficacy in energy capture and tower fatigue reduction. 
     Some known techniques for estimating wind speed are based on measurements of rotor speed, blade pitch angle and generator torque/electrical power. Other known techniques for estimating wind speed use tower position information that offers improvements in response time over generator torque measurements techniques. Yet other known techniques employ measurement systems such as Lidar to obtain the effective average wind speed ahead of the turbine. Still another known technique uses strain measurements of piezoresistive cement-based materials in a 4-pole electrode arrangement. While the foregoing solutions are very effective, the cost of those measurement systems is still much too high and economically disadvantageous. Further, the use of adhesives, cements and/or bonding agents is generally highly sensitive to the electrode-tower contact and is therefore not optimal for large numbers of measurement points over a large area of a wind tower. 
     In view of the foregoing, it would be advantageous to provide methods and apparatus for controlling wind turbine actuation faster, more reliably, and in a more cost effective manner than that which is achievable using known techniques that compensate for variations in wind speed. 
     BRIEF DESCRIPTION 
     According to one embodiment, a wind turbine comprises: 
     a plurality of wind turbine blades attached to a rotor positioned atop a tower affixed to a tower foundation; 
     at least one blade pitch sensor configured to measure blade pitch angles for one or more wind turbine blades; 
     a rotor/generator speed sensor configured to measure the rotational speed of the wind turbine rotor, a corresponding wind turbine generator, or both; 
     a nacelle yaw sensor configured to measure nacelle yaw; 
     at least two tower-base bending sensor elements configured without use of adhesives, cements or bonding agents, to measure over a large area, tower deflection in real-time; and 
     a controller configured to adjust the pitch angle of one or more wind turbine blades in response to the measured one or more blade pitch angles, the measured rotational speed, the measured nacelle yaw, and measured large area tower longitudinal deflection. 
     According to another embodiment, a wind turbine control system comprises: 
     at least one sensor configured to measure blade pitch angles for one or more wind turbine blades; 
     a rotor/generator speed sensor configured to measure the rotational speed of a wind turbine rotor, a wind turbine generator, or both; 
     a nacelle yaw sensor configured to measure a wind turbine nacelle yaw; 
     at least two sensor elements configured without use of adhesives, cements or bonding agents, to measure over a large area, tower deflection in real-time; and 
     a controller configured to adjust the pitch angle of one or more wind turbine blades in response to the measured one or more blade pitch angles, the measured rotational speed, the measured nacelle yaw, and measured tower longitudinal deflection. 
     According to yet another embodiment, a method of actuating a wind turbine positioned atop a tower affixed to a tower foundation comprises: 
     providing at least one blade pitch sensor configured to measure blade pitch angles for one or more wind turbine blades; 
     providing a rotor/generator speed sensor configured to measure the rotational speed of a wind turbine rotor, a wind turbine generator, or both; 
     providing a wind turbine nacelle yaw sensor configured to measure nacelle yaw; 
     providing at least two tower-base bending sensor elements configured without use of adhesives, cements or bonding agents, to measure over a large area, tower deflection in real-time; 
     measuring blade pitch angles for one or more wind turbine blades; 
     measuring the rotational speed of a wind turbine rotor, a wind turbine generator, or both; 
     measuring nacelle yaw; 
     measuring tower deflection in real-time for a large surface area of the tower; and 
     adjusting the pitch angle of one or more wind turbine blades in response to the measured one or more blade pitch angles, the measured rotation speed, the measured nacelle yaw, and measured tower longitudinal deflection. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates a wind turbine system in which embodiments of the invention are integrated therein; 
         FIG. 2  is a block diagram illustrating a turbine control system according to one embodiment; 
         FIG. 3  is flow chart illustrating a method of controlling a wind turbine according to one embodiment; and 
         FIG. 4  illustrates a wind turbine tower in which a pair of tower-base bending sensors is installed according to one embodiment. 
     
    
    
     While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
     DETAILED DESCRIPTION 
     In some configurations and referring to  FIG. 1 , a wind turbine  10  comprises a nacelle  11  housing a generator (e.g.  26  in  FIG. 2 ). Nacelle  11  is mounted atop a tall tower  12 . Wind turbine  10  also comprises a rotor (e.g.  22   FIG. 2 ) that includes one or more rotor blades  14 ,  15 ,  16  attached to a rotating hub. Although wind turbine  10  illustrated in  FIG. 1  includes three rotor blades  14 ,  15 ,  16 , there are no specific limits on the number of rotor blades required by the embodiments described herein. 
     In some configurations and referring to  FIGS. 1 and 2 , various components are housed in nacelle  11  atop tower  12  of wind turbine  10 . The height of tower  12  is selected based upon factors and conditions known in the art. In some configurations, one or more controllers  32  including algorithmic software are used for wind-speed monitoring and turbine control and may be based on distributed or centralized control architectures. 
     In some configurations, one or more variable blade pitch drive actuators  38  are provided to control the pitch of blades  14 ,  15 ,  16 . In some configurations, the pitches of blades  14 ,  15 ,  16  are individually controlled by the blade pitch actuators  38 . 
     The drive train  24  of the wind turbine includes a main rotor shaft (also referred to as a “low speed shaft”) connected to a hub via a main bearing and (in some configurations), at an opposite end of the rotor shaft to a gear box. The gear box, in some configurations, utilizes dual path geometry to drive an enclosed high speed shaft. In other configurations, the main rotor shaft is coupled directly to a generator  26 . The high speed shaft is used to drive generator  26 . 
     With continued reference to  FIG. 2 , a wind turbine control system  20  according to one embodiment includes two or more tower-base bending sensors  34  configured to measure tower-base loads. This architecture provides advantages over known tower position measurement systems that require costly GPS-based measurement devices. The use of tower-base loads, for example, requires less expensive sensors that may include, without limitation, load cells, strain gages, fiber-optic, piezoresistive, and/or capacitive sensors. Further, the use of tower-base loads provides faster estimation of wind speed variations and gusts than that achievable when using GPS techniques. 
     Optionally combining tower-base load sensing with generator torque sensing advantageously provides a wind-speed estimator with built-in redundancy. This redundancy potentially enables monitoring of wind turbine blade condition. 
     The embodiments described herein provide a means to achieve faster tracking of optimal tip-speed ratio in the variable-speed region (low wind speeds), thus leading to increased energy capture. The embodiments described herein further provide a means for reducing tower loading in the constant speed region (medium and high wind speeds). According to one embodiment, a method provides for large-area measurement of tower deflection in real-time and continuously uses the measurement data to very quickly estimate the wind speed. This fast estimation of wind speed is transmitted to a controller  32  that operates to reduce tower loading and increase energy capture, as stated above. 
     According to one embodiment, piezoresistive material is integrated with, bonded to, or otherwise attached to the tower  12  at or near the base  13 . The piezoresistive material can, for example, be added to the cement in concrete towers or applied as surface patches (or paint) in concrete or steel towers. Electrical conductivity measurements can then be made to estimate the tower deflection. According to one aspect, deflection measurements are made over a large area of the tower in order to increase the accuracy of the wind speed estimation. This is done, for example, and without limitation, by using permanently installed sensor arrays. These sensor arrays may be implemented using, without limitation, eddy current, AC, and/or DC potential drop sensors. Measurements are then calibrated to correlate the conductivity measurement values to deflection values. 
     According to one embodiment, a pair of tower deflection measurement sensors  34  are placed within a range about one meter to about 1.5 meters above of the tower/foundation joint  13  or base of the tower  12  such as depicted in  FIG. 4 . Other embodiments include two or more sensors placed more than about 1.5 meters above the tower/foundation joint  13  or base of the tower  12 . The sensors  34  are radially displaced  72  from one another by about 90° according to one aspect as depicted in  FIG. 4 . It can be appreciated that the nacelle  11  will be moving as the wind changes direction, but the tower  12  is stationary. Therefore, at least two tower-base bending sensors  34  in conjunction with an accurate measurement of the nacelle  11  yawing direction provided via a nacelle yaw sensor  37  together operate to separate the tower  12  longitudinal bending moment (good for wind speed estimation) from the tower lateral bending moment. 
     The embodiments described herein advantageously provide an ability to measure wind tower deflection quickly in real-time, and at large numbers of points over a large area of the tower  12 . These embodiments further overcome inaccuracy of deflection measurements due imperfect contact between electrodes and the tower  12  such as generally associated with use of strain measurements of piezoresistive cement-based materials using a 4-pole electrode arrangement. 
       FIG. 3  is flow chart illustrating a method  50  of controlling a wind turbine  10  according to one embodiment. According to one embodiment, method  50  commences by measuring rotor/generator speed, blade pitch angle and tower-base bending moment as represented in block  52 . 
     The rotor/generator speed measurement data is obtained via one or more rotor/generator speed sensors  30  such as depicted in  FIG. 2 . According to one aspect, a rotor/generator converter  28 , also depicted in  FIG. 2 , generates the electrical signals transmitted to the rotor/generator speed sensors  30 . 
     The blade pitch angle measurement data is obtained via one or more blade pitch sensors  40  such as depicted in  FIG. 2 . According to one embodiment, each turbine blade  14 ,  15 ,  16  has one or more corresponding pitch sensors  40 . 
     The tower-base longitudinal bending moment measurement data is obtain via two or more tower-base bending sensors  34  such as depicted in  FIGS. 2 and 4 , that can be piezoresistive material that is integrated with, bonded to, or otherwise attached to the tower  12  at or near the base  13 , as stated above. According to one embodiment, the piezoresistive material is added to the cement in concrete towers or applied as surface patches (or paint) in concrete or steel towers. Electrical conductivity measurements can then be made to estimate the tower deflection. According to one aspect, deflection measurements are made over a large area of the tower in order to increase the accuracy of the wind speed estimation by using permanently installed sensor arrays. These sensor arrays may be implemented using, without limitation, eddy current, AC, and/or DC potential drop sensors. Measurements are then calibrated to correlate the conductivity measurement values to deflection values. 
     It can be appreciated that hybrid towers may require more than one set or pair of bending moment sensors  34 . A hybrid tower that includes a steel top section and a bottom concrete section, for example, may employ one set or pair of sensors  34  at the base or bottom portion of the steel top section and a second set or pair of sensors  34  at the base or bottom portion of the concrete section. 
     A nacelle yaw sensor  37  provides for the measurement of the wind turbine nacelle  11  yaw such that longitudinal moments can be distinguished from lateral bending moments. 
     Effective wind-speed estimates are then determined using the rotor/generator speed measurement data, the blade pitch angle measurement data, the nacelle  11  measurement data, the tower-base bending moment measurement data, or combinations thereof along with prior estimates of effective wind speed as represented in block  54 . The effective wind-speed estimates are determined via an algorithmic effective wind-speed estimator  42  within a controller  32 . 
     A desired rotor/generator speed and a desired generator torque are next determined using the estimated effective wind-speed as represented in block  56 . The desired rotor/generator speed and desired generator torque operate to reduce tower loading and increase energy capture, as stated above. 
     The foregoing measured rotor/generator speed, measured blade pitch angle, estimated effective wind-speed, desired generator torque, and desired rotor/generator speed are then used by the effective wind-speed estimator algorithm  42  to determine one or more desired blade pitch angles, as represented in block  58 . Desired blade pitch angle signals are then communicated to the blade pitch actuator(s)  38 , and desired generator torque signals are communicated to the generator converter  28  as represented in block  60 , via a turbine control unit  36  that forms one portion of wind turbine controller  32  as shown in  FIG. 2 . 
     The blade pitch angle(s) and the rotor/generator speed are adjusted in response to the desired blade pitch angle signals and the desired generator torque signals to reduce tower loading and increase wind turbine energy capture. 
     Providing fast, real-time, large-area measurements of the wind turbine tower deflection allows for accurate and effective measurement of wind speeds at the wind turbine rotor. This information enables faster tracking of optimal tip-speed ratio to increase energy capture in low-wind speed regions where the wind speed variation is great. This information further enables faster tracking of optimal tip-speed ratio to increase energy capture in medium and high wind speed regions to reduce tower loading. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.