Abstract:
In a wind turbine, an open loop control algorithm for incrementally or positively adjusting the pitch angles of individual rotor blades may be used to increase spacing between the base of the turbine tower and an approaching blade tip. As each rotating blade passes in front of the tower base, a minimum clearance distance may be assured to avoid blade tip strikes of the base. In accordance with at least one embodiment of the control algorithm, as each blade approaches the tower base, it may be feathered to reduce its power loading, and to facilitate increased clearance beyond the normal unloading or feathering produced by the so-called tower shadow effect. To offset resultant loss of torque, the remaining blades may be correspondingly pitched toward power, i.e. into the wind, to balance and/or smooth out the overall rotor torque curve, and thus to avoid torque ripples.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to control systems for managing operations of wind turbines. More particularly, the disclosure provides control algorithms for selectively varying the pitch angle of individual turbine rotor blades to increase the distance between a blade tip and the tower as the blade tip crosses in front of the tower. 
       BACKGROUND 
       [0002]    Changing the pitch of the rotor blades of a utility scale wind turbine is commonly used as a primary control mechanism. The blades are pitched toward a power pitch position (i.e., at a lower pitch angle or into greater influence of the wind) to increase the amount of wind energy captured by the rotor, in turn increasing torque on the main shaft of the wind turbine to drive electric generators. The blades are pitched toward a feather position (at a higher pitch angle or away from influence of the wind) to decrease wind energy captured by the rotor, and to decrease torque on the main shaft. 
         [0003]    The reaction torque created by the electrical generators which are driven by the main shaft (often via a gearbox) is another controlled aspect of a utility-scale wind turbine. Balancing the rotor torque against the generator torque (opposed moments) on the main shaft is one common method of controlling shaft speed. 
         [0004]    Collective blade pitching has been and remains a common method of pitching the rotor blades, although individual blade pitching strategies have been more recently developed. Collective pitching generally involves all of the blades simultaneously being pitched to the same pitch angle. Individual blade pitching provides for adjustment of individual rotor blades to customized pitch angles, independently of the other blades. 
         [0005]    Individual blade pitch control strategies have been proposed chiefly for balancing loads on the rotor blades across the swept area of the rotor and maximizing power output. In rotor swept areas in which localized or spot wind speeds may be lower than mean wind speeds, the blades may be transiently pitched toward power positions to produce amounts of power equal to that being produced over other regions of the rotor area. Conversely, where the spot wind speeds are higher than mean wind speeds across the rotor, the blades may be pitched toward feather positions. For the various individual blade pitch control strategies which have been proposed, the purpose of each and the focus has been on load balancing, smoothing out power fluctuations, and reductions of maximum loads. None of these proposed strategies have suggested an individual blade pitch control strategy for increasing the distance between a blade tip and the tower as the blade tip crosses in front of the tower. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    This disclosure proposes open loop control methods for increasing blade tip to tower clearance by pitching individual blades toward their feathered position as each blade tip passes in front of the tower. 
         [0007]    In one aspect of the disclosure, a method to enhance the tower shadow effect, i.e. the normal tendency for each blade to produce less power and to actually unload or bend away from the tower is provided. The tower shadow effect results from lower wind speeds that normally exist immediately in front of the tower. As a result, a pitch control algorithm adapted to pitch an individual blade toward its feathered position as its tip advances toward the tower will cause that blade to unload and/or to spring away from the tower by an even greater distance to thus create a greater blade tip to tower clearance. 
         [0008]    Another aspect of this disclosure is a further enhancement of overall wind turbine performance by superimposing an open loop pitch control algorithm on each individual blade, based primarily on the azimuthal position of that blade. The open loop pitch control algorithm may be added to any pre-existing individual pitch command strategies, even if closed loop, such as in situations wherein the blades might be otherwise pitched to accommodate conditions of wind shear, for example. 
         [0009]    In yet another aspect of the disclosure, an azimuthal position-based open loop individual blade pitch control algorithm may be complemented and/or otherwise supplemented by corresponding pitch angle adjustments to the other (non-tower-crossing) blades, including having such other blades being pitched toward their power positions whenever the tower-crossing blade is being moved toward its feather position. This would achieve a more balanced, continuous torque and power output of the rotor during each revolution thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an elevational view of a wind turbine that embodies the disclosed control algorithm and related methodology, displayed to show one of the spinning rotor blades approaching the tower base. 
           [0011]      FIG. 2  is an elevational view of the same wind turbine, with the tower-crossing blade instantaneously passing over the centerline of the tower base. 
           [0012]      FIG. 3  is an elevational view of the same wind turbine, with the same blade having just passed by the tower base. 
           [0013]      FIG. 4  is a graph displaying variation of blade pitch angle as a function of the azimuthal position of a tower-crossing blade, as may be provided by an exemplary control algorithm. 
           [0014]      FIG. 5  is an elevational view of the same wind turbine, but embodying an alternate control algorithm and methodology. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Referring initially to  FIG. 1 , an exemplary wind turbine  10  is schematically shown in accordance with at least one embodiment of the present disclosure. While all components of the wind turbine are not shown or described herein, the wind turbine  10  may include a vertically standing tower  12  having an axis “a-a”, and supporting a rotor  14 . The rotor is defined by a collective plurality of equally spaced rotating blades  16 ,  18 , and  20 , each connected to and radially extending from a hub  22 , as shown. The blades  16 ,  18 ,  20  may be rotated by wind energy such that the rotor  14  may transfer such energy via a main shaft (not shown) to one or more generators (not shown). Those skilled in the art will appreciate that such wind-power driven generators may produce commercial electric power for transmission to an electric grid (not shown). Those skilled in the art will appreciate that a plurality of such wind turbines may be effectively employed on a so-called wind turbine farm to generate a significant amount of electric power. Although the disclosed embodiments focus on wind only, this disclosure is pertinent to fluids generally, including other gases and even liquids such as water, that may be used to drive similar turbine structures. 
         [0016]    In the embodiments described herein, each of the blades  16 ,  18 ,  20  is individually adjustable, i.e. it can be pitched about its radial axis “b-b” (shown only with respect to blade  16  for simplicity), independently of the pitch angle of any other blade. Generally, blades  16 ,  18 , and  20  can be individually pitched toward a feathered position in which the blade produces little or no torque about the hub  22 , or toward a power position in which the blade produces a maximum amount of torque about the hub. 
         [0017]    A prime motivation of this disclosure relates to the avoidance of blade tip strikes against abase section  24  of tower  12 , as such strikes can result in complete destruction of the wind turbine structure. It will be appreciated by those skilled in the art that during operation of the wind turbine, and because the blades  16 ,  18 ,  20  are long and flexible and positioned to capture energy from wind to convert same to rotor torque, the blade tips  16 A,  18 A, and  20 A may on occasion be deflected toward the base  24 . It will further be appreciated by those skilled in the art that various wind and air movements including wind gusts may impart transient forces on the blade tips  16 A,  18 A, and  20 A, producing higher than normal tip deflections. As a consequence of such transient tip deflections, wind turbines are designed to ensure that adequate margins of safety exist to reduce actual amounts of tip deflection that might cause the tips  16 A,  18 A, and  20 A to strike the base  24 . Such margins of safety are normally ensured by designing the blades to be stiff, so as to avoid excessive deflections, or to control operation of the wind turbine so that operational conditions which might result in a blade tip to tower strike are avoided. Also, by slightly tilting the rotational axis of the rotor  14  from a true horizontal orientation to an orientation that is slightly inclined, the blade tips  16 A,  18 A, and  20 A can be spaced a greater distance away from the tower base  24  when any given blade is positioned at the six o&#39;clock position, or in alignment with axis a-a. 
         [0018]    In accordance with this disclosure, to further counteract such strikes, a control system  30  (shown schematically at the upper portion of the tower  12 ) may be employed to feather in real-time each individual blade approaching the base  24 , to reduce its deflection in the direction of the tower  12 , thus providing an additional margin of safety against blade tip to tower strikes. 
         [0019]    Continuing reference to  FIG. 1 , the hub  22  is attached through a main shaft (not shown) to a nacelle  26 , as shown. The nacelle  26  is adapted to revolve about axis a-a, at the top of the tower  12  at the interface  28  of the tower  12  and nacelle  26 . Such turntable like nacelle movement is within a generally horizontal plane (not shown) that passes through the interface  28 , and is managed by a yaw control system (not shown). The rotatable nacelle  26  may be adapted to freely turn, so as to be able to position the rotor directly perpendicularly to any prevailing winds, and to thereby optimize power generation under conditions of shifting winds. 
         [0020]    An azimuthal encoder  32  (shown schematically on the base  24 ) may be adapted to be in electronic communication with the control system  30 . The azimuthal encoder  32  may sense the approach and proximity of any given blade tip  16 A,  18 A,  20 A to the base  24 , and respond by sending an appropriate signal to the control system  30 . The control system  30  may respond in turn by feathering the single approaching blade  16 A (in  FIG. 1 ), at least in part as a function of the encoder-sensed azimuthal position of that blade. A blade torque arrow  34  about the blade tip  16 A indicates that the blade  16  is being rotated about axis “b-b” toward a feather position by the control system  30  as the blade approaches the tower base  24 . Although proximity sensing via an encoder  32  mounted at the bottom of the base  24  is suggested above, this is merely exemplary. The encoder  32  may also be positioned between the rotor and the nacelle, rather than on the base  24 , in which case the encoder  32  may be calibrated to specific azimuthal positions of each blade. The encoder  32  may be any sensor capable of sensing the current or real-time azimuthal positions of the blades  16 ,  18 , and  20 , or sensing the approach of blade tips  16 A,  18 A, and  20 A to the base  24 . 
         [0021]    Referring now to  FIG. 2 , the blade  16  and hence the blade tip  16 A is at a traditionally referenced six o′clock position, or instantaneously positioned in-line with axis a-a, which coincides with the vertical centerline of the tower  12 . It will be noted that in such position the blade  16  is feathered, i.e. edged into the wind. As earlier stated, the six o′clock position is the position of the so-called tower shadow effect, in which the effect of the wind on the tower-crossing blade  16  is already minimized As such, the feathering of the blade  16  works with, rather than against, the tower shadow effect to further increase the clearance between the blade tip and the tower. Finally, and just as a point of azimuthal reference, the six o&#39;clock position of the blade represents 180° of clockwise rotation from the top or 12 o&#39;clock position, which represents the 0° and 360° blade position. 
         [0022]    The magnitude of the blade feathering or pitch angle adjustment can be selected according to the amount of additional tower clearance or reduced deflection desired, and may also depend upon the particular design of the wind turbine, particularly the blades. The amount of blade feathering or pitch angle adjustment may also depend upon wind speed, rotor speed, nominal pitch angle or demand, wind turbine power output, and other operational factors. For example, for a certain turbine and during certain operation conditions, the peak magnitude of the pitch angle adjustment could be around 3° . If the wind turbine is normally operated at a 5° pitch angle at these operational conditions (the 5° pitch angle is referred to as the nominal pitch demand or command), then the control system  30  may cause an incremental, or positive, pitch angle adjustment of 3°, resulting in an actual pitch angle for the tower crossing blade of 8°. At 8° of pitch angle compared to 5°, the blade will be unloaded and produce less power and torque, but will also experience less deflection in the direction of tower  12 . 
         [0023]    The peak magnitude of the pitch angle adjustment could be higher or lower than 3° at different operation conditions. Likewise, the pitch rate (how quickly the blade pitches), and the azimuthal positions at which the pitch adjustment toward feather begins and the pitch adjustment back toward power ends, may all be adjusted by the control system  30 , as a function of operational factors such as wind speed, rotor speed, nominal pitch angle or demand, wind turbine power output, etc. 
         [0024]    In  FIG. 3 , the blade  16  is instantaneously shown to have advanced past the axis a-a, wherein communication of the azimuthal encoder  32  with the control system  30  may be effective to return the blade  16  toward its power pitch position, as indicated by the blade torque arrow  35 . It will be appreciated that the same control system  30  may be adapted to initiate both the feathering and the return to power pitch responses at predetermined azimuthal positions. The control system  30  may function as a simple open loop algorithm, to the extent that no feedback of any other real-time variable is required for its effectiveness. 
         [0025]      FIG. 4  is a graph depicting the positive pitch angle adjustment relative to the azimuthal position of the blade, with an exemplary positive pitch angle adjustment profile having a peak of 3°. As the three blades  16 ,  18 , and  20  of the wind turbine  10  are rotating and producing power, a nominal pitch demand is generated by the control system  30  to pitch the blades to an appropriate power pitch angle, for example between 5° and 10°. In a typical wind turbine control system, the nominal power pitch angle demand is provided to the pitch control system, which may further process the demand to smooth transitions and achieve other beneficial effects before actually controlling pitch angle actuation. The azimuthal encoder  32  communicates an azimuthal position signal for each blade to the control system  30 , which uses this signal to generate additional commands of incremental pitch, as appropriate. Such commands of incremental pitch may be achieved by consulting a look-up table, by using a mathematical function, or any other appropriate methodology. 
         [0026]    With reference to the exemplary pitch angle adjustment profile in  FIG. 4 , as the azimuthal position of the tower-crossing blade approaches the base  24 , for example within a range of 90 to 180°, or more particularly within a range of 100 to 160°, or more particularly at about 120° in the example provided in  FIG. 4 , a small, incremental pitch angle adjustment command is first provided. Those skilled in the art will appreciate that even though the tower-crossing blade actually passes the tower base  24  precisely at the 180° position, some amount of lead time, or advance, such as the 10° advance angle of the disclosed example, may be provided to accommodate lag time between change in blade pitch angle and reduced blade deflection. Thus, as the blade pitches out of the wind and unloads, a small amount of time is required for the blade deflection to decrease so that the blade moves away from the tower. As such, in the disclosed example the full amount of pitch angle adjustment will be completed by the time the blade has reached the 170° position. As the blade passes the azimuthal position where the peak pitch angle adjustment occurs, the pitch angle adjustment is then reduced, i.e. the blade is powered back up, starting at a point beginning at 170° in the example of  FIG. 4 , until a pre-determined termination point is reached, for example at a point within the azimuthal range of 180 to 270°, or more particularly within a range of 200 to 260°, or more particularly at about 220° in the disclosed example. 
         [0027]    The azimuthal position and the shape of the incremental pitch angle adjustment profile may be changeable according to various operating conditions. For example, the azimuthal position of peak incremental pitch angle adjustment may shift depending upon current rotor speed and/or other factors. A faster rotor speed may dictate a phase shift greater than the exemplary 10° such that the peak pitch command occurs in advance of 170°, or a slower rotor speed may dictate a phase shift less than the exemplary 10° such that the peak pitch command occurs closer to 180°. The starting and termination points for the incremental pitch angle adjustments may also shift as a function of factors such as the rotor speed, wind speed, or power output. In addition, the maximum or peak incremental pitch angle adjustment may vary depending upon any of the aforementioned factors or other factors. For example, at lower power outputs and greater nominal pitch angles, the maximum or peak incremental pitch command may be decreased because less unloading of the tower-crossing blade to promote enhanced tip to tower clearance is necessary. For example, if the nominal pitch angle is, say 15 to 20°, instead of 5 to 10° which was assumed in the example of  FIG. 4 , the maximum or peak amount of pitch angle adjustment provided to feather the tower-crossing blade could be 1° rather than a maximum of about 3°. 
         [0028]    The control algorithm outlined above may be utilized in addition to and/or may be superimposed upon any other existing or in-place control function for determining pitch angles. For example there may be a basic closed loop control function already in place that addresses wind velocity and direction, including feedback calling for pitch changes to avoid overloading. The disclosed control algorithm may thus be adapted to work in concert with such pre-existing closed loop control systems. 
         [0029]      FIG. 5  depicts a different blade algorithm for an alternative control system  30 ′. As already described with respect to the wind turbine  10 , the wind turbine  10 ′ of  FIG. 5  includes the same capability for individually feathering any given tower-crossing blade. However, the control system  30 ′ is additionally adapted to cause each of the non-tower-crossing blades to over-pitch to compensate for the loss of torque and power associated with the feathering of the tower-crossing blade. The alternative control system  30 ′ may be a separate control function, or may be an added-on part of the above-described control system  30 . The torque arrows  36  and  38 , displayed around the tips  18 A and  20 A of the non-tower-crossing blades  18  and  20 , respectively, indicate that each of those blades is moving into an over-pitch position to compensate for the torque loss of the feathering blade  16 . 
       INDUSTRIAL APPLICABILITY 
       [0030]    The present disclosure generally sets forth a control methodology for modifying the pitch angles of rotor blades of utility scale wind turbines to achieve or enhance desired margins of safety for blade tip to tower clearance. The control methodology may offer a wind turbine designer additional methods or tools to achieve required margins of safety for blade tip to tower clearance. The control methodology may further result in designs of lighter blades, or longer and more flexible blades, and/or other beneficial outcomes. 
         [0031]    Individual feathering of the tower-crossing blade may be combined with other pitch adjustments to the non-tower-crossing blades to achieve different effects. In one example, the non-tower-crossing blades may be pitched toward power while the tower-crossing blade is being feathered. In this manner, the total amount of torque on the rotor generated collectively by the blades may thus be maintained more evenly, and the power output may experience less fluctuation. 
         [0032]    Finally, the feathering of the tower-crossing blade may be controlled in large part as an azimuthal function of the position of the rotor. As such, the control algorithm may be an open loop function, rather than a feedback or closed loop function, utilizing encoder generated signals that reflect real-time positions of the tower-crossing blade with respect to the tower base.