Abstract:
A method of controlling a wind turbine is provided. The method includes the steps of obtaining a number of operating values relevant to a wind turbine, calculating a thrust setpoint on the basis of the operating values, and controlling a wind turbine according to a thrust setpoint. A wind park controller, a wind turbine controller, and a wind park are also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 11193824.7 EP filed Dec. 15, 2011. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    A method of controlling a wind turbine, wind park controller for controlling a number of wind turbines of a wind park, wind turbine controller, and a wind park are provided. 
       BACKGROUND OF INVENTION 
       [0003]    Wind turbines convert wind energy into electricity. A wind turbine is generally controlled by a wind turbine controller that regulates the power output and voltage output according to momentary requirements at a collector grid. For example, to increase the output of the wind turbine, a voltage or power setpoint is issued to the wind turbine, which then reacts by adjusting a pitch angle, the rotor torque, etc. 
         [0004]    When multiple wind turbines feed into a grid, such setpoints are generally determined by a wind park controller with the aim of limiting active output power from the wind farm, to maintain a certain level of reactive power, to sustain grid voltage, etc. In a wind park comprising many wind turbines arranged in a group, some wind turbines will be situated downwind of others. Wind turbines at the front of such a group will be subject to the full force of the wind, while downwind turbines are effectively “shadowed” by the turbines at the front. Furthermore, shadowed wind turbines may be subject to turbulence in the wake of the wind turbines at the fore. The known methods of controlling wind turbines of a wind park are generally focussed on controlling the power output, on the basis of the available power, i.e. the power that a wind turbine could deliver under the momentary conditions, and a wind park controller can deliver power and/or voltage setpoints accordingly. However, even if the power output of the wind park can be optimised fairly well using the known methods, other important factors relating to the effects of turbulence, vibration, wear etc cannot be taken into consideration. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object to provide an improved way of controlling a wind turbine or the wind turbines of a wind park. 
         [0006]    This object is achieved by the method of the claims of controlling a wind turbine, by the wind park controller of the claims, by the wind turbine controller of the claims, and by the wind park of the claims. 
         [0007]    The method of controlling a wind turbine comprises the steps of obtaining a number of operating values relevant to a wind turbine; calculating a thrust setpoint for control of a wind turbine on the basis of the operating values; and controlling a wind turbine according to a thrust setpoint. 
         [0008]    An operating value can comprise any measurable quantity of any turbine that is related to the thrust acting on the wind turbine. For example, an operating value can comprise a thrust force, wind direction, a unit lifetime consumption of the wind turbine, a measure of the vibration experienced by the wind turbine, turbine component loading, etc. Turbine component loading can manifest as the loading of any structural component of the turbine, for example as main shaft deflections, tower oscillations, etc., depending on the turbine type. Severe loading due to a turbulent wake, resulting in a high consumption rate, is generally caused by variations in thrust, or thrust fluctuations. Turbine lifetime consumption and turbine lifetime consumption rate are terms that take into account the structural wear and tear on the turbine as a fraction of the typical lifetime for that turbine type. The construction date of the turbine can be used as a reference, and the fatigue damage on the structural components is compared to the expected fatigue damage for that wind turbine type. Turbine component loading and fatigue damage result largely as a result of the wind thrust force acting on the wind turbine. 
         [0009]    One or more such operating values are then used to compute a thrust setpoint. A thrust setpoint for a wind turbine is to be understood as a maximum thrust force that should be endured by that turbine. The thrust setpoint can be issued as a “real time” control input to the wind turbine, which reacts by operating such that this maximum thrust force level is not exceeded. An advantage of the method is that a wind turbine can then be controlled according to such a thrust setpoint, in addition to the usual setpoints such as a rotational speed, power or voltage setpoint. 
         [0010]    The method allows the operation of a wind turbine to be favourably optimised with respect to thrust. For example, a thrust setpoint for a wind turbine can be computed to have the effect of decreasing the absolute thrust experienced by that wind turbine, and can result, for example, in a reduction in the thrust fluctuations experienced by that wind turbine or another downwind wind turbine. 
         [0011]    The wind park controller for controlling a number of wind turbines of a wind park comprises an input for obtaining operating values for a number of wind turbine of the wind park; and a thrust setpoint generating unit for generating a number of thrust setpoints for a number of wind turbines of the wind park on the basis of the operating values. 
         [0012]    An advantage of such a wind park controller is that it can issue one or more thrust setpoints to the various wind turbines of the wind park with the effect of increasing the efficiency of the wind turbine array. 
         [0013]    The wind turbine controller for a wind turbine comprises a parameter computation unit for computing operational parameters on the basis of a thrust setpoint (in addition to the usual power reference and a voltage reference). 
         [0014]    The wind park comprises a number of wind turbines, wherein the wind turbines are controlled using the method. 
         [0015]    In a wind park, thrust setpoints can be dispatched to any or all of the wind turbines in the wind park, and the relevant wind turbine controllers will ensure that their maximum thrust levels are not exceeded, so that the power production of the wind park can be favourably optimized in accordance with this constraint also. 
         [0016]    Particularly advantageous embodiments and features are given by the dependent claims, as revealed in the following description. Features of different claim categories may be combined as appropriate to give further embodiments not described herein. 
         [0017]    As mentioned in the introduction, the wind turbines of a wind park are exposed by different amounts to the wind power, depending on their positions in the wind turbine array. Therefore, in a particularly preferred embodiment, the operating values determined for a wind turbine are used to calculate a thrust setpoint for another wind turbine. For example, the thrust force exerted on a downwind wind turbine as a result of wake turbulence caused by an upwind wind turbine can be used to derive a new thrust setpoint for the upwind wind turbine, so that the wake turbulence experienced by the downwind wind turbine is decreased. 
         [0018]    The thrust force essentially acts on the rotor blades of a wind turbine, and can be computed by taking into account certain factors such as the relevant dimensions of the wind turbine. For example, the thrust force can be expressed by the following equation: 
         [0000]        F   THRUST =1/2 ·ρ·A·C   T (λ,β) 20 ·ν 2    (1)
 
         [0000]    where F THRUST  is the thrust (in Newtons) exerted by the wind on the rotor; ρ is the air density (kg/m 3 ); A is the rotor disc area or “rotor swept area” (m 2 ); λ is the blade tip speed ratio; β is the pitch angle (°); C T  is a thrust coefficient and a function of λ and β; and ν is the rotor effective wind speed (m/s). 
         [0019]    The momentary thrust force can be estimated based on known or measurable information such as blade pitch angle, power output, rotor speed, etc. The rotor effective wind speed is the average wind speed across the rotor disc of a wind turbine. The rotor effective wind speed can be estimated by means of instruments such as an anemometer arranged on the canopy or nacelle of the wind turbine, or can be estimated using parameters monitored by the wind turbine controller, for example rotor rotational speed, pitch angle, power, etc. 
         [0020]    In certain cases, the thrust force may be measured indirectly, for example by measuring the tower acceleration or tower loads using a sensor such as a strain gauge. In this way, the momentary thrust force acting on a wind turbine can be obtained and analysed to determine whether it is acceptable, or whether the thrust force acting on a turbine can be increased, or whether it should be decreased. 
         [0021]    A wind turbine generates electrical energy by extracting kinetic energy from the wind. As a result, when wind passes through the rotor disc of a wind turbine, it will lose a portion of its energy, and thereby lose velocity as well. Furthermore, the turbulence in the wake of a wind turbine can be increased by that wind turbine. 
         [0022]    The power captured by the rotor disc will depend on the power available from the wind, and also on the capability of the wind turbine. This “available power” can be expressed as 
         [0000]        P=P   WIND   ·C   P (λ, β  (2)
 
         [0000]    where P WIND  is the power available from the wind (Watts), and C P  is the power coefficient for that wind turbine. 
         [0023]    The power captured by the wind turbine can also be expressed as: 
         [0000]        P= 1/2 ·ρ·A·C   P (λ, β)·ν 3    (3)
 
         [0024]    The coefficients C T  and C P  are turbine-specific and can be computed with regard to rotor characteristics such as airfoil geometry, rotor properties, and turbine operational trajectories. For example, the computation of the power coefficient as a function of blade-tip speed ratio and pitch angle can be based on assumptions regarding the operational characteristics and blade profile information, and can be computed using a blade element momentum (BEM) code. These coefficients C T , C P  may be stored as lookup tables in a memory, for example in a memory of the wind park controller and/or a memory of the wind turbine controller. 
         [0025]    In a particularly preferred embodiment, the thrust setpoint for a wind turbine is computed to optimize the power that can be output by a wind turbine versus the thrust that will be exerted on that turbine. In other words, a wind turbine will be driven to output a maximum of power while not being subject to a thrust exceeding that governed by its thrust setpoint. In a preferred embodiment, a thrust setpoint for a wind turbine is calculated to specify the maximum thrust force that should be experienced by that wind turbine. For example, the thrust setpoint can be calculated to alter or to decrease, the thrust exerted on that wind turbine. A thrust setpoint can be expressed, for example, as a value in newtons. 
         [0026]    A thrust setpoint calculated for a wind turbine can be used by that wind turbine in a number of ways. To this end, using equation (1) above with the thrust setpoint as F THRUST , the wind turbine controller can determine a new C T  value, which in turn can be converted into relevant control signals for the various controllable parts of the wind turbine, for example to adjust the blade pitch angle or the yaw angle so that the thrust force acting on the rotor disc is reduced. To this end, the wind turbine controller can consult predefined operating trajectories and look-up tables, from which it can obtain appropriate control parameters for the relevant controllable elements of the wind turbine. In this way, the wind turbine controller can convert the new thrust setpoint into control signals that not only ensure that the maximum thrust level should not be exceeded, but also honour the momentary voltage and/or power references or setpoints that apply. 
         [0027]    In another preferred embodiment, the thrust setpoint for a first wind turbine is calculated to decrease the loading of a second wind turbine shadowed by the first wind turbine. For example, the thrust setpoint for an upwind turbine can be calculated so that, when converted into control signals for that upwind turbine, the wake turbulence downwind of that wind turbine is reduced. In this way, the shadowed wind turbine(s) will experience a smoother airflow and be subject to less wear. The wind direction is also used as a basis for determining the thrust setpoints issued to one or more wind turbines, to obtain a desired loading distribution over the wind park. 
         [0028]    The method can make use of the fact that the wake downwind of a wind turbine is altered as a result of the thrust acting on that wind turbine, and can therefore be controlled to some extent by the thrust setpoint issued to that wind turbine. Therefore, in a preferred embodiment, a plurality of thrust setpoints is issued to wind turbines of a wind park, wherein the thrust setpoints are calculated to adjust an overall wake distribution in the wind park. In this way, for example, a less turbulent wake distribution can be achieved for a plurality of downwind turbines. 
         [0029]    The wind park controller is able to control the wind turbines in a specific manner, such that at any given time, the wind turbines receive thrust setpoints for optimising their performance. Since the effect of thrust will generally be different across the wind park, the wind park controller comprises a thrust setpoint distribution unit for distributing thrust setpoints to specific wind turbines of the wind park. In this way, each wind turbine can receive a thrust setpoint in response to the thrust exerted on that wind turbine. For example, a group of wind turbines facing into the wind at the fore of the wind park can be collectively given a certain thrust setpoint to optimise a wake distribution in the wind park. Equally, the wind turbines arranged in the “middle” of the wind park can be given a collective thrust setpoint for that region, while wind turbines at the rear of the wind park can be given another collective thrust setpoint. Wind turbines far removed from the front of the wind park may not receive any thrust setpoints at all. Of course, certain wind turbines can be given tailored thrust setpoints. For example, an upwind wind turbine can receive a thrust setpoint that reduces the turbulence for a wind turbine in its shadow. Equally, a shadowed wind turbine can be given a thrust setpoint that takes into account the wake turbulence caused by another wind turbine upwind of it. 
         [0030]    In order to be able to evaluate the effect of variables such as wind on the performance of a wind turbine or a wind turbine in its wake, for example, a wind turbine controller comprises an operating value determination unit for monitoring, collecting or computing a number of operating values relevant to the thrust force acting on or experienced by that wind turbine. As mentioned above, an operating value can be, for example, wind direction, a unit lifetime consumption of the wind turbine, a measure of the vibration experienced by the wind turbine, turbine component loading, etc. 
         [0031]    As indicated above, the thrust force exerted on a wind turbine is coupled or linked to the structural loading on that wind turbine. Therefore, in a further preferred embodiment of the method, the thrust setpoint issued to a wind turbine is evaluated in relation to a maximum thrust reference for that wind turbine. The maximum thrust reference effectively limits the level of aerodynamic thrust that will be experienced by that wind turbine. For example, if the thrust setpoint exceeds the maximum thrust reference, the wind turbine is controlled according to the maximum thrust reference. In this way, a safety measure can be included in a wind turbine controller if required, for example if a wind turbine is to be spared from loads exceeding a certain level. Such a maximum thrust reference for a wind turbine can be updated as the need arises, for example by the wind park controller. 
         [0032]    Since wind turbines are more often utilized as part of a wind park comprising many wind turbines, a wind turbine controller comprises a operating value output for sending operating values for that wind turbine to a wind park controller, which can then use these values in an appropriate manner, for example to issue a thrust setpoint back to that wind turbine on the basis of those operating values, and/or to issue a thrust setpoint to one or more other wind turbines on the basis of those operating values. 
         [0033]    The wind turbine controller is realized to act in response to a thrust setpoint received from a wind park controller in addition or as an alternative to a thrust setpoint that it calculates itself using its own operating values. Therefore, in a preferred embodiment, the parameter computation unit is realised to compute the operational parameters on the basis of a thrust setpoint provided by a wind park controller and/or on the basis of the operating values for that wind turbine. 
         [0034]    The setpoints received by a wind turbine controller are converted into control signals that are applied to the wind turbine to optimise its performance with respect to various aspects such as maximising output power, minimising noise, etc. In the wind turbine controller, such control signals are generated not only on the basis of the usual power and/or voltage setpoints, but also on the basis of a thrust setpoint. Therefore, in a preferred embodiment, the parameter computation unit is realised to compute a blade pitch reference signal and/or a yaw reference signal and/or a power/torque reference signal on the basis of the thrust setpoint in addition to or as an alternative to other setpoints. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    Other objects and features will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration. 
           [0036]      FIG. 1  shows a schematic representation of a prior art wind park and wind park controller; 
           [0037]      FIG. 2  shows a block diagram of a wind turbine controller according to an embodiment; 
           [0038]      FIG. 3  shows a schematic representation of a wind park and wind park controller according to an embodiment. 
       
    
    
       [0039]    In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale. 
       DETAILED DESCRIPTION OF INVENTION 
       [0040]      FIG. 1  shows a schematic representation of a prior art wind park  3  and wind park controller  30 . The prior art wind park controller  30  can monitor the output performance of the wind park  3  using values  31  delivered from a collector network or point of common connection of the wind turbines  1 . The wind turbines  1  are arranged in an array or farm. The rotor discs of the wind turbines  1  are generally controlled to face into the wind, so that some wind turbines  1  will be upwind of the other wind turbines  1 , of which some are in the middle of the farm and others at the rear. In the diagram, for the sake of argument, it may be assumed that the wind is travelling from left to right. Therefore, the upwind or upstream wind turbines  1  to the left in the diagram are exposed to a relatively high wind speed and have a correspondingly high available power. The wind turbines  1  behind the upwind units will experience a lower wind speed, so that their available power will be lower. The prior art wind park controller  3  can only react by issuing voltage and power setpoints  32 ,  33  for example to curtail the output power of some of the wind turbines and to allow others to operate according to their available power. However, the prior art park controller  3  can not control the wind turbines to take thrust force or wake distribution into account. Therefore, the power output of the wind park  3  may be less than optimal owing to the wake distribution over the wind park  3 , and the wind turbines  1  may be subject to avoidable wear and tear. 
         [0041]      FIG. 2  shows a block diagram of a wind park controller  20  and a wind turbine controller  10  according to an embodiment. A wind turbine controller  10 , usually arranged in the tower or nacelle of a wind turbine, is generally quite a distance removed from a wind park controller  20 , as indicated by the dashed line. In this embodiment, the wind turbine controller  10  comprises a parameter computation unit  101  for generating control parameters for the wind turbine, for example a blade pitch reference signal  102 , a yaw reference signal  103 , and a power/torque reference signal  104 . The wind turbine controller  10  also comprises an operating value determination unit  11  for determining one or more operating values  110 , which are output to a wind park controller  20 , for example, as described above, such operating values can comprise strain gauge values, oscillation values, consumption values, thrust force values, etc., using measurements  112  from one or more sources. In addition to or as an alternative to these operating values  110 , the wind park controller  20  can receive meteorological readings from an external source  202 , for example from a meteorological mast  202 , and can use such readings to compute thrust setpoints based, for example, only on wind direction, wind speed, etc. The wind park controller  20  then computes a voltage setpoint  22  and a power setpoint  23 , and also a thrust setpoint  21 , for one or more wind turbines. 
         [0042]    In the wind turbine controller, the parameter computation unit  101  receives a voltage setpoint  22  and a power setpoint  23  from the wind park controller, and also a thrust setpoint  21 . The parameter computation unit  101  can therefore take all these setpoints  21 ,  22 ,  23  into consideration when generating the control parameters  102 ,  103 ,  104  for the wind turbine. 
         [0043]    The operating values  110  can also be forwarded to the parameter computation unit  101 . This allows the parameter computation unit  101  of a standalone wind turbine to use the operating value  110  to minimize the loading of that wind turbine. In such a standalone configuration, only the parts of the diagram to the right of the dashed line are relevant. 
         [0044]      FIG. 3  shows a schematic representation of a wind park  2  and wind park controller  20  according to an embodiment. Here, some or all of the wind turbines  1 ,  1 _up,  1 _down comprise a wind turbine controller  10  as described in  FIG. 2 . The wind park controller  2  obtains data  201  relating to the power output at the point of common connection, as well as operating values  110  received from the wind turbine controllers  10  of the wind turbines  1 ,  1 _up,  1 _down. The operating values  110  delivered by the upwind units  1 _up will differ from the operating values  110  delivered by the other units  1 ,  1 _down. Here, the wind park controller  20  comprises a setpoint generation unit  200  for calculating voltage and power setpoints  22 ,  23  for the wind turbines  1 ,  1 _up,  1 _down; and a thrust setpoint generation unit  210  for generating thrust setpoints  21 ,  21 _up,  21 _down for the wind turbines  1 ,  1 _up,  1 _down according to their position in the wind farm  2  for the momentary wind situation. The thrust setpoint generation unit  210  and the setpoint generation unit  200  can be realised as software modules running on a processor of the wind park controller  20 , and can exchange information as indicated here by the arrow. 
         [0045]    The wind park controller  20  can also comprise a thrust setpoint distribution unit  211  for identifying which wind turbines are to receive specific thrust setpoints. In this way, the wind park  20  controller can optimise the power output of the wind farm  2  in consideration of the thrust force exerted on the individual wind turbines  1 ,  1 _up,  1 _down, e.g. the wind park  20  controller can maximise the power output of the wind farm  2  while minimizing the thrust force exerted on the individual wind turbines  1 ,  1 _up,  1 _down. 
         [0046]    Although embodiments and variations thereon have been disclosed, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the disclosure. For example, the wind park controller could generate customized voltage and power setpoints for certain wind turbines of a wind park on the basis of the operating values delivered by wind turbines of the wind park. 
         [0047]    For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. A unit or module does not preclude the use of several units or modules.