Abstract:
Method of operating a variable speed wind turbine as a function of wind speed are disclosed, the wind turbine having a rotor with a plurality of blades, a generator, pitch mechanisms for rotating the blades around their longitudinal axes, and a system for varying a torque of the generator. The method comprises at a first moment in time estimating representative future wind speed values from the first moment in time up to a second moment in time, the second moment in time being equal to the first moment in time plus a predetermined finite period of time, and using a control strategy to optimize a cost function indicative of an energy output of the wind turbine based on the estimated representative future wind speed values by controlling the torque of the generator and the pitch angles of the blades. Wind turbines suitable for such methods are also disclosed.

Description:
[0001]    This application claims the benefit of European Patent Application EP13382485.4 filed on Nov. 29, 2013, the entire contents of which are hereby incorporated by reference. 
         [0002]    The present disclosure relates to methods of operating a wind turbine, and wind turbines suitable for such methods. 
       BACKGROUND ART 
       [0003]    Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox. 
         [0004]    A variable speed wind turbine may typically be controlled by varying the generator torque and the pitch angle of the blades. As a result, aerodynamic torque, rotor speed and electrical power generated will vary. 
         [0005]    A common prior art control strategy of a variable speed wind turbine may be described with reference to  FIG. 1 . In  FIG. 1 , the operation of a typical variable speed wind turbine is illustrated in terms of the pitch angle (β), the electrical power generated (P), the generator torque (M) and the rotational velocity of the rotor (ω), as a function of the wind speed. 
         [0006]    In a first operational range, from the cut-in wind speed to a first wind speed (e.g. approximately 5 or 6 m/s), the rotor may be controlled to rotate at a substantially constant speed that is just high enough to be able to accurately control it. The cut-in wind speed may be e.g. approximately 3 m/s. 
         [0007]    In a second operational range, from the first wind speed (e.g. approximately 5 or 6 m/s) to a second wind speed (e.g. approximately 8.5 m/s), the objective is generally to maximize power output while maintaining the pitch angle of the blades so as to capture maximum energy. In general, in the second operational range, the pitch angle of the blades may be substantially constant, although the optimal blade setting may theoretically depend on the instantaneous wind speed. In order to achieve this objective, the generator torque and rotor speed may be varied so as to keep the tip speed ratio λ (tangential velocity of the tip of the rotor blades divided by the prevailing wind speed) constant so as to maximize the power coefficient C p . 
         [0008]    In order to maximize power output and keep C p  constant at its maximum value, the rotor torque may be set in accordance with the following equation: 
         [0000]        T=k·ω   2  , 
         [0000]    wherein
 
k is a constant, and w is the rotational speed of the generator. In a direct drive wind turbine, the generator speed substantially equals the rotor speed. In a wind turbine comprising a gearbox, normally, a substantially constant ratio exists between the rotor speed and the generator speed.
 
         [0009]    In a third operational range, which starts at reaching nominal rotor rotational speed and extends until reaching nominal power, the rotor speed may be kept constant, and the generator torque may be varied to such effect. In terms of wind speeds, this third operational range extends substantially from the second wind speed to the nominal wind speed e.g. from approximately 8.5 m/s to approximately 11 m/s. 
         [0010]    In a fourth operational range, which may extend from the nominal wind speed to the cut-out wind speed (for example from approximately 11 m/s to 25 m/s), the blades may be rotated (“pitched”) to maintain the aerodynamic torque delivered by the rotor substantially constant. In practice, the pitch may be actuated such as to maintain the rotor speed substantially constant. At the cut-out wind speed, the wind turbine&#39;s operation is interrupted. 
         [0011]    In the first, second and third operational ranges, i.e. at wind speeds below the nominal wind speed (the sub-nominal zone of operation), the blades are normally kept in a constant pitch position, namely the “below rated pitch position”. Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions however depends on the complete design of the wind turbine. 
         [0012]    The before described operation may be translated into a so-called power curve, such as the one shown in  FIG. 1 . Such a power curve may reflect the optimum operation of the wind turbine under steady-state conditions and under conditions of uniform wind speed over the rotor swept area (the area swept by the blades of the wind turbine). 
         [0013]    If the wind is not uniform over the swept area and/or if the wind is variable, a steady state power such as the one depicted in  FIG. 1  does not necessarily lead to maximum energy generation of the wind turbine. Let&#39;s suppose a situation in which a wind speed is given and the wind speed changes relatively rapidly. The new optimum operating point at the new wind speed may be known from the power curve, but how to transition from the current operating point to the new operating point is not given by the power curve. This transition may thus not be optimal. 
         [0014]    Since the wind is inherently variable, the situation of transition is more the rule than the exception and in many situations wind turbine operation may thus be far from optimum. 
         [0015]    The present disclosure relates to various methods of avoiding or at least partly reducing any of the aforementioned problems. 
       SUMMARY 
       [0016]    In a first aspect, a method of operating a variable speed wind turbine as a function of a wind speed is disclosed. The wind turbine has a rotor with a plurality of blades, a generator, one or more pitch mechanisms for rotating the blades around their longitudinal axes and determining pitch angles for the blades, and a system for varying a torque of the generator. The method comprises at a first moment in time estimating representative future wind speed values from the first moment in time up to a second moment in time, the second moment in time being equal to the first moment in time plus a predetermined finite period of time. And the method further comprises using a control strategy to optimize a cost function indicative of an energy output of the wind turbine up to the second moment in time based on the estimated representative future wind speed values by controlling (trajectories of) the torque of the generator and the pitch angles of the blades (and, optionally, of any additional controllable parameter) over the period ranging from the first moment in time to the second moment in time. The aforementioned steps are substantially continuously repeated. 
         [0017]    According to this aspect, a method of operating a wind turbine is provided which provides a control that also covers transitions for variable wind speeds. The wind turbine may select current operating points based on the current and future estimated wind speeds and thus on knowledge of future operating points. As such, the electrical energy produced may be optimized. 
         [0018]    The control does not abide by a power curve designed for steady state conditions. Instead, the control assumes that the wind speed will vary in accordance with the estimated wind speed values and optimizes control over a finite period of time, i.e. up to the second moment in time, rather than continuously adapting to a new given situation. 
         [0019]    The results of the optimization may be that in the sub-nominal zone of operation, the blades are pitched and/or that the rotor speed is not constant in the third operational range, and/or the generator torque may not abide to the quadratic law for the generator torque described before. As such, the distinction between the first, second and third operational ranges may disappear or, at least these ranges may be redefined according to different constraints. 
         [0020]    In some embodiments, the predetermined finite period of time may be between 2 seconds and 1 minute, preferably between 5 or 10 or 15 seconds and 30 seconds. In some experiments, between 7 and 12 seconds has been used satisfactorily. A balance needs to be made between the need for computational power and the optimization that may be achieved. On the one hand, an infinite horizon would be ideal if sufficiently reliable wind speeds would be known. However, the reliability of the estimated/predicted wind speeds reduces as the horizon for optimization is moved further away and additionally, the computational power needed to use this information to determine operating points (in terms of e.g. generator torque, pitch angles) would be too high. The calculation would take too long for it to be implemented. On the other hand, choosing the horizon too close would not give a lot of information for optimization, thus limiting any significant improvement on the performance of the wind turbine. 
         [0021]    In some embodiments, estimating representative future wind speed values may comprise measuring or estimating instantaneous representative wind speed values and determining future wind speed values based on the measured or estimated wind speed values and empirical statistical information of the wind speed to determine likely future wind speed values. 
         [0022]    In prior art control strategies, the wind is usually not measured in a direct manner. 
         [0023]    Rather, the rotational speed of the generator is used to determine the operational situation. In this aspect of the invention, wind speed values may be determined by using a nacelle mounted anemometer and determining e.g. 3 second-5 second averages. In one alternative example, wind speed values may be determined by measuring the loads on the blades. In other alternative examples, wind speed values may be estimated based on other measurements, like e.g. generator speed, rotor speed, pitch angles, generator torque. Depending on the system used, a wind field at any given moment may be represented by a single wind speed value (e.g. assuming uniform speed over the rotor swept area) or by a plurality of wind speed values representative for different sections of the rotor swept area. 
         [0024]    Measuring wind speeds directly is advantageous in particular in wind turbines with especially high rotor inertia. If the inertia of the rotor is high, this inherently means that the rotors are slow to react to changing wind fields. So if the control is based on the rotor speed (or the generator speed directly linked to the rotor speed), the control will be far from optimum. 
         [0025]    By measuring the wind speed directly or estimating the wind speed and including empirical statistical information (of e.g. the wind at the specific site) for predicting future wind speeds, optimizing a cost function may be used to increase the power output. 
         [0026]    In other embodiments, estimating representative future wind speed values may comprise measuring representative wind speeds upstream of the wind turbine. Wind speeds upstream of the wind turbine may be measured using Doppler effect instruments, such as e.g. a LIDAR or a SODAR. Depending on the circumstances, wind measurements upstream from the wind turbine may also be made using a met tower, or wind measurement devices located on other nearby wind turbines. In this respect, the information from e.g. the LIDAR is not only used to detect e.g. a potentially dangerous wind gust and adapt the wind turbine operation to such a wind gust, but rather to use the information to estimate a wind field for a finite period of time (e.g. 5-20 seconds) and based on this wind field, optimize the operation of the wind turbine within certain constrains (or “boundary conditions”) with respect to energy generation. 
         [0027]    In some embodiments, estimating representative future wind speed values comprises assuming the wind speed value to be constant over a rotor swept area of the wind turbine. In alternative embodiments, future wind speed values for different sections of the rotor swept area may be taken into account. This may be useful particularly, if the rotor blades comprises actuators that can adapt to these non-uniformities, e.g. flaps. 
         [0028]    In some embodiments, using a control strategy may comprise setting a boundary condition for a minimum rotor speed of the wind turbine at the second moment in time, optionally for the rotor speed of the wind turbine at the second moment in time to be equal to the rotor speed at the first moment in time or, alternatively, equal to the rotor speed corresponding to the steady state according to  FIG. 1  for the estimated wind speed at said second moment in time. Because a finite time period is taking into account, the control strategy may try to implement a deceleration in order to convert the kinetic energy of the rotor into electrical energy. By setting a boundary condition for the rotor speed (which may be that the future rotor speed is equal to the current rotor speed or to the theoretically “ideal” steady state rotor speed), such a deceleration of the rotor may be avoided. In these cases, the cost function to be optimized may be the electrical energy generated. 
         [0029]    In some embodiments, the cost function to be optimized may be the electrical energy generated plus the kinetic energy of the rotor at the second moment in time. Alternatively, the cost function to be optimized may be the converted aerodynamic energy from the wind. These are two alternative methods for avoiding excessive deceleration of the rotor. 
         [0030]    In some embodiments, the control strategy may be a Model Predictive Control (MPC) strategy, and optionally a non-linear Model Predictive Control. MPC aims at effectively solving problems of control and automation of processes that are characterized by having a complicated, multivariate and/or unstable dynamic behaviour. The control strategy underlying this type of control uses a mathematical model of the process to be controlled to predict the future behaviour of that system and, based on this future behaviour, it can predict future control signals. 
         [0031]    MPC is part of the so-called optimal controllers, i.e. those in which actuations correspond to an optimization of a criterion. The criterion to be optimized, or the “cost function”, is related to the future behaviour of the system, which is predicted by considering a dynamic model thereof, which is called the prediction model. 
         [0032]    MPC is a flexible, open and intuitive technique, which permits dealing with linear and nonlinear, multi-variable and mono-variable systems by using the same formulation for the algorithms of the controller. Moreover, the MPC control laws respond to optimization criteria, and allow incorporating constraints in the synthesis or implementation of the controller. MPC also provides the ability of incorporating constraints in the calculations of the actuations. These constraints may be in terms of e.g. maximum allowable loads and/or maximum rotor speed etc. 
         [0033]    In some examples, as mentioned before, the cost function to be optimized may be the electrical power generated over the finite period of time. 
         [0034]    In any of these embodiments of this aspect, the boundary conditions may be “soft” boundary conditions or “hard” boundary conditions. Hard boundary conditions are those conditions that may never be violated and soft boundary conditions are those boundary conditions that are preferably not violated, but may occasionally be violated to a limited extent. Violation of such a soft constraint may be suitable when the expected gain in the cost function to be optimized is relatively or disproportionally high. 
         [0035]    In some embodiments, the blades of the wind turbine may comprise one or more deformable trailing edge sections and one or more systems for deforming the deformable trailing edge sections, and wherein using a control strategy to optimize a cost function indicative of an energy output of the wind turbine based on the estimated representative future wind speed values may include determining deformations of the deformable trailing edge sections. The deformable trailing edge sections may be Continuously Deformable Trailing Edge (CDTE) and may be trailing edge flaps (such as e.g. plain flaps, slotted flaps, Gurney flaps or Fowler flaps). In these cases, extra control parameters are provided. 
         [0036]    In another aspect, a wind turbine is provided having a rotor with a plurality of blades, a generator, one or more pitch mechanisms for rotating the blades around their longitudinal axis and determining pitch angles for the blades, and a system for varying a torque of the generator. The wind turbine furthermore comprises a wind turbine controller adapted to carry out any of the aforementioned methods. 
         [0037]    In any of the embodiments, pitch angles may be determined for each blade individually or they may be determined common for all blades (either dependent on the azimuthal position of the blades or not). 
         [0038]    Additional objects, advantages and features of embodiments of the invention will become apparent to those skilled in the art upon examination of the description, or may be learned by practice of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Particular embodiments of the present invention will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which: 
           [0040]      FIG. 1  illustrates a typical power curve of a wind turbine; and 
           [0041]      FIGS. 2   a - 2   e  illustrate respectively a wind speed profile, and a variation of the blade pitch angles, generator torque, generator speed, and electrical power generated in response to this wind speed profile, both for a “classic” control strategy of a variable speed wind turbine and a control strategy according to an example of the present invention; and 
           [0042]      FIGS. 3   a - 3   e  illustrate respectively another example of a wind speed profile, and a variation of the blade pitch angles, generator torque, generator speed, and electrical power generated in response to this wind speed profile, both for a “classic” control strategy of a variable speed wind turbine and a control strategy according to an example of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0043]    The power curve of  FIG. 1  has been discussed before. 
         [0044]      FIG. 2   a  illustrates a wind speed profile in which a relatively sudden change in wind speed from 5 m/s to 8 m/s occurs.  FIGS. 2   b  and  2   c  illustrate how, the pitch angle and generator torque may be varied in order to maximize energy production both in a “classic” control strategy and in an example of a method according to the present invention.  FIGS. 2   d  and  2   e  show the resulting generator speed and electrical power production based on the pitch angle and generator torque variation according to  FIGS. 2   b  and  2   c , both according to a classic control strategy and the same method according to the present invention.  FIGS. 2   b - 2   e  are based on a simulation using a commercial aeroelastic code using the wind speed profile according to  FIG. 2   a  as input. 
         [0045]    According to the classic control strategy, a typical quadratic curve which is based on the second operational range as illustrated in  FIG. 1  is used. In the example according to the present invention, trajectories for the different operational parameters are chosen such as to optimize electrical power production. 
         [0046]    In this example, a (non-linear) MPC strategy is implemented to optimize the cost function: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    wherein P E  is the electrical power produced, t=0 is the first moment in time and t F  is the second moment in time (a finite period of time further). The finite period of time may be e.g. 15 seconds, 20 seconds, 25 seconds or 30 seconds. 
         [0047]    The electrical power produced at any moment in time is given by: 
         [0000]        P   E   =Q   G   ·ω   G ,   (Eq. 2)
 
         [0000]    wherein Q G  is the generator torque demand and W G  is the rotational speed of the generator. 
         [0048]    The rotational speed of the generator may be determined in accordance with: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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         [0000]    wherein Q R  is the aerodynamic torque of the rotor and N GBX  is the transmission ratio of the gearbox. In direct drive wind turbines N GBX =1. 
         [0000]        Q   R ( t )=1/2· ρ·A·C   Q (λ( t ), θ( t )) ̂2   u   x ( t ),   (Eq. 4)
 
         [0000]    wherein ρ is the air density, C Q  is the torque coefficient, (which is a function of λ, the tip speed ratio, and θ, the pitch angle of the blades) and û x  is the axial free wind speed. The pitch angle may be directly set by a wind turbine control system. The tip speed ratio may be determined as 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    wherein R is the blade radius. 
         [0049]    In the MPC strategy, this cost function may be optimized within certain constraints. These constraints may include maximum and minimum generator torque, generator rotor speed, maximum and minimum pitch angles and maximum and minimum generator output power: 
         [0000]      Q g   MIN ≦Q G ≦Q G   max    (Eq. 6a)
 
         [0000]      ω g   MIN ≦ω G ≦ω G   max    (Eq. 6b)
 
         [0000]      θ MIN ≦θ G ≦θ max    (Eq. 6c)
 
         [0000]      P E   MIN ≦P E ≦P E   max    (Eq. 6d)
 
         [0050]    Further possible constraints may include e.g. limit loads, conditions on accumulated (fatigue) loads and also pitch rates. In any of these embodiments of this aspect, the boundary conditions may be “soft” constraints or “hard” boundary conditions. Hard constraints are those conditions that may never be violated (e.g. maximum pitch rate is prescribed by the pitch drives employed) and soft constraints (e.g. maximum output power, or maximum load) are those constraints that are preferably not violated, but may occasionally be violated to a limited extent. Violation of such a soft constraint may be suitable when the expected gain in the cost function to be optimized is relatively or disproportionally high. 
         [0051]    The control strategy underlying this type of control uses a mathematical model of the process to be controlled to predict the future behaviour of that system and, based on this future behaviour, it can predict future control signals. 
         [0052]    Based on estimated future wind speed values, the cost function may be optimized by varying the generator torque and the pitch angles of the blades. The estimated future wind speed values may in reality be based on LIDAR measurements. A suitable time period for the optimization function may correspond to the prediction range (in time) of the LIDAR system employed. 
         [0053]    With reference to  FIG. 2   b , even before the increase in wind speed occurs, the blades are pitched. This is not intuitive because the wind speed is clearly below the nominal wind speed. In a classic control strategy for variable speed wind turbines, no pitching would take place. Furthermore, the effect of knowing or estimating the future wind speed values may be seen from the fact that pitching takes place even before a change in wind speed occurs. 
         [0054]    With reference to  FIG. 2   c , it may be seen that the generator torque also behaves in a rather unusual manner. After a slight increase, the generator torque is decreased and maintained at a relatively low level, before the wind speed increases and when the wind speed increases. Again this is counterintuitive because in a classic control strategy the generator torque should be increased to maintain a certain tip speed ratio in order to optimize C p . 
         [0055]    The results may be seen in  FIGS. 2   d  and  2   e . In  FIG. 2   d , it may be seen that because of the chosen control strategy, the generator speed (and thus the rotor speed) is increased beyond normal values. In a first instance, the generator speed is too high, so that electrical power production is momentarily below the electrical power production as compared to the classic control strategy. However, since the algorithm takes advantage of knowing or reliably estimating future wind speed values, it is able to adapt to the future steady state condition much quicker and electrical power production is actually higher at a later stage. The example of the present invention thus consciously reduces the near future energy to gain more energy at the end of the simulation. This is not an obvious behaviour. In this case, the energy gain was about 1.5%. 
         [0056]    A more realistic wind speed profile variation over 100 seconds is illustrated in  FIG. 3   a .  FIGS. 3   b  and  3   c  illustrate how, the pitch angle and generator torque may be varied in order to maximize energy production both in a “classic” control strategy and in an example of a method according to the present invention.  FIGS. 3   d  and  3   e  show the resulting generator speed and electrical power production based on the pitch angle and generator torque variation according to  FIGS. 3   b  and  3   c , both according to a classic control strategy and the same method according to the present invention. The figures are based on a simulation using a commercial aeroelastic code using the wind speed profile according to  FIG. 3   a  as input. 
         [0057]    In the following, just a few aspects of this simulation are highlighted. 
         [0058]    In the first part of the simulation (up until about 50 seconds), the wind speed is above the nominal wind speed. This means that the wind turbine is operating at nominal power. It may be seen particularly in  FIG. 3   b  that the pitch actuator during this period uses information of the future wind field and acts in order to adapt to wind speed variations much quicker than in the classic control strategy. The generator torque during this same period varies significantly less in the example of the invention than in the classic control strategy. The result is that generator speed and electrical power also vary significantly less than when using a classic control strategy. Overall the electrical power production may be increased. It may be said that at least a part of the improvement is due to the simple fact that the wind field is fed forward to the control. 
         [0059]    On the other hand, an interesting portion of the simulation is from approximately 70 second-85 seconds. In this time period, the wind speed increases significantly from below the nominal wind speed to above the nominal wind speed. Based on simply feeding forward the wind field, one would have expected to see the pitch actuator adjust quicker (compared to the classic control strategy) as the wind speed increases above the nominal wind speed. However this is not the case as may be seen in  FIG. 3   b  at the time from approximately 80-85 seconds. Accordingly, during this same time period it may be seen in  FIG. 3   e  that the electrical power production is significantly lower than in the classic control strategy. 
         [0060]    However, the loss of electrical power production is more than compensated by the gain in electrical power production in the period from approximately 70 seconds-80 seconds (see  FIG. 3   e ). During this same period the generator speed (and thus the rotor speed) is decreased relatively rapidly, meaning that kinetic energy is converted into electrical energy during this period. 
         [0061]    On a smaller scale throughout the simulation more examples of such “counterintuitive” behaviour may be found. In conclusion, the resulting gains in electrical power production are not merely due to the feed forward of the wind field but are also due to the explicit goal of optimization of electrical power production (within certain constraints). 
         [0062]    In these examples, in order to avoid the control extracting kinetic energy from the rotor (thus slowing down the rotor) because of the finite time period employed in the optimization, a terminal equality constraint was set to the rotor speed, such as in Equation 6b above. 
         [0063]    In an alternative example, the cost function to be optimized may be 
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         [0000]    wherein I R  is the inertia of the rotor, ω R  is the rotational speed of the rotor, E ELEC  is the electrical energy generated in the finite time prediction horizon and E LOSS  represents the energy losses in the wind turbine. 
         [0064]    In this example, instead of optimizing the electrical energy generated, the aerodynamic energy that is converted is optimized. An inconvenient deceleration of the rotor in order to extract the kinetic energy may thus be avoided. 
         [0065]    In a further example, the cost function to be optimized may be the electrical energy plus kinetic energy, i.e. the function to be optimized is: 
         [0000]      1/2 I   R ω R   2   +E   ELEC ,   (Eq. 1c)
 
         [0000]    wherein E ELEC  may be defined in accordance with Eq. 1a). 
         [0066]    The results of the simulations illustrated in  FIGS. 2   a - 2   e  and  FIGS. 3   a - 3   e  may already be considered to be impressive. In less turbulent winds, the gains of the employed control strategies are probably less significant. On the other hand, if further actuators are included (such as e.g. CDTE or flaps) and non-uniform wind speed over the rotor swept area is taken into account, significant gains may be expected employing any of the methods of the present invention as compared to classic control strategies. 
         [0067]    Although only a number of particular embodiments and examples of the invention have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof are possible. Furthermore, the present invention covers all possible combinations of the particular embodiments described. Thus, the scope of the present invention should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.