Abstract:
The invention relates to a method for wind turbine generators for reducing electrical disturbances in the form of power variations which are caused by damping controllers arranged the compensate structural oscillations by inducing shaft torque variations. The shaft torque variations are generated by imposing corresponding variations in a generator set-point, e.g. a power or torque set-point. Variations in the generator set-point cause undesired variations in the power injected to the grid by one or more wind turbine generators. According to an embodiment of the invention the electrical disturbances may be reduced by limiting a damping controller&#39;s control action. The amount of limitation or restriction of the damping controller may be determined on basis on electrical disturbance information determined from power measured e.g. at a location on the grid.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to control of at least one wind turbine, particularly to controlling a damping controller of one or more wind turbines. 
       BACKGROUND OF THE INVENTION 
       [0002]    Structural oscillations in wind turbines caused e.g. by the wind may be damped actively by generating counteracting oscillations. Such counteracting oscillations may be generated by inducing variations in the shaft torque, e.g. by varying a power or torque set-point for the generator of the wind turbine. The variations may be generated by a damping controller. 
         [0003]    In order to improve performance of the wind turbine there is a need to improve control of damping controllers. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the invention to improve the control of a wind turbine in relation to damping of a wind turbine. 
         [0005]    It is a further object of the invention to reduce electrical disturbances in the grid which are caused by wind turbines. 
         [0006]    In a first aspect of the invention there is provided a method for controlling at least one wind turbine for reducing spectral disturbances in an electrical grid, the at least one wind turbine comprises
       a rotor adapted to drive a power generator via a shaft, wherein the generator is connectable with the electrical grid, and   at least one damping controller configured to compensate structural oscillations of the wind turbine by controlling a torque on the shaft, wherein the at least one damping controller is configured to set a limit of the control action on the shaft dependent on a restriction value, wherein the method comprises   determining disturbance information describing an electrical disturbance at a point of measurement electrically connected to the wind turbine,   determining the restriction value based on the determined disturbance information,   assigning the restriction value to the damping controller for setting the limit of the control action.       
 
         [0012]    Advantageously, the control action performed by damping controllers may be restricted dependent on measured disturbance information determined from measured power at a point of measurement. Accordingly, if the disturbance information indicates disturbances, e.g. power variation amplitudes at a given frequency, above a given threshold, a limitation of the damping control action may be invoked in order to avoid or at least reduce further increases in the disturbance information. 
         [0013]    Each wind turbine may comprise one damping controller arranged to damp a specific structural oscillation or a plurality of damping controllers arranged to damp different structural oscillations. The determined restriction value may be determined for one of the one or more damping controllers of a wind turbine. Accordingly, a plurality of restriction values, possibly different restriction values, may be determined for a plurality of damping controllers of a wind turbine. 
         [0014]    The said point of measurement for the electrical disturbance may be located anywhere on an electrical power connection between and including the output of the wind turbine and the electrical grid. In other embodiments, the said point of measurement for the electrical disturbance may be located anywhere on an electrical power connection between the wind turbine—but not including the direct output of the wind turbine—and the electrical grid. The point of measurement may comprise a power meter capable of measuring the active power. The electrical grid may be shortly defined as an interconnected electrical network for delivering electricity from one point, e.g. a power source, to another point, e.g. a consumer. The electrical grid can have different voltage levels, e.g. for transmission and distribution. In context of the present invention, the electrical grid may sometimes for short be termed ‘the grid’. In some embodiments, the said point of measurement for the electrical disturbance may be located in, or near, the electrical grid, in particular in, or near, the point of connection (PoC) between the at least one wind turbine with the electrical grid. 
         [0015]    In the case of a plurality of wind turbines, a plurality of restriction values, possibly different restriction values, may be determined for a plurality of damping controllers or the plurality of wind turbines. Accordingly, a plurality of restriction values may be assigned to the plurality of damping controllers of the wind turbines. In embodiments with a plurality of wind turbines, it may be particular advantageous that said point of measurement for the electrical disturbance may be located in, or near, the electrical grid, in particular in, or near, the point of connection (PoC) between the plurality of wind turbines and the electrical grid. 
         [0016]    It should be noted that the present invention is particularly advantageous when a plurality of wind turbines, e.g. a so-called ‘wind park’ or ‘wind farm’, is controlled according to the invention because combined oscillations of the wind turbines are normally not in phase, and therefore the combined contribution of these oscillation to the electrical disturbance in the grid could be lower than the simple sum, but nevertheless there may be a significant electrical disturbance worth reducing with the present invention by measuring in, or near, the electrical grid, particular in, or near, the point of connection (PoC) between the plurality of wind turbines and the electrical grid. 
         [0017]    According to an embodiment the method further comprises selecting at least one of the damping controllers based on the disturbance information, and assigning the restriction value to the selected damping controller. 
         [0018]    The disturbance information may contain information such an spectral information which may be used to identify a particular damping controller and disturbance levels which may be used to determine if the disturbance is significant enough to invoke a restriction in the damping activity of the particular damping controller by selecting that particular controller. 
         [0019]    According to an embodiment the method further comprises selecting at least one of the damping controllers based on damping compensation values describing levels of damping compensation performed by the damping controllers, and assigning the restriction value to the selected damping controller. 
         [0020]    The damping compensation values may be used to identify which damping controller induces the largest shaft torque variations. These controllers may be selected for restricting the damping activity of the most active damping controllers. According to an embodiment the damping compensation values are determined based on output values from the damping controllers. 
         [0021]    A damping controller may further be selected based both on damping compensation values and disturbance information so that the restriction value can be assigned to the selected damping controller. 
         [0022]    According to the embodiments which comprise selection of at least one of the damping controllers based on disturbance information and/or damping compensation, the plurality of damping controllers may by comprised by a single wind turbine or a plurality of wind turbines. In the case were each of a plurality of wind turbines comprises one or more damping controllers, the selection of the at least one damping controller, and assignment of one or more restriction values, may be performed among the damping controllers of the plurality of wind turbines. This may involve an initial selection of one or more wind turbines which contains damping controllers to be restricted. However, selection of damping controllers could also be performed directly when the control system for controlling the at least one wind turbine is able to receive damping compensation values from damping controllers from different wind turbines and/or contains information, e.g. information about spectral location of power variation peaks, which enables selection of the damping controllers from different wind turbines based on the disturbance information. 
         [0023]    According to an embodiment the method further comprises determining wind turbine restriction values based on the restriction value for a plurality of the wind turbines, assigning the turbine restriction values to the wind turbines, and assigning the turbine restriction value for one of the wind turbines to the damping controller of the wind turbine for setting the limit of the control action. 
         [0024]    Advantageously, in a wind turbine park comprising a plurality of wind turbines supplying power to the grid, the restriction value, i.e. the total restriction value, is divided into wind turbine restriction values for one or more of the wind turbines. 
         [0025]    The wind turbine restriction values may be determined simply by dividing the total restriction value equally between the wind turbines, or by other methods, e.g. dependent on the damping activity performed by individual wind turbines or dependent on the disturbance information, e.g. spectral locations of disturbance peaks in an amplitude spectrum. The determined individual wind turbine restriction values are assigned to the one or more damping controllers of each of the wind turbines which have been provided with a wind turbine restriction value. 
         [0026]    According to an embodiment the method further comprises selecting at least one of the wind turbines based on the disturbance information, and assigning the wind turbine restriction values to the selected wind turbines. Alternatively or additionally, the method may further comprise selecting at least one of the wind turbines based on damping compensation values describing levels of damping compensation performed by the damping controllers of the wind turbines, and assigning the turbine restriction values to the selected wind turbines. Accordingly, wind turbine may also be selected based on a combination of disturbance information and damping compensation values. 
         [0027]    According to an embodiment the restriction value, i.e. the total restriction value, is determined as a function of a difference between a disturbance value determined from the disturbance information and a desired disturbance value. Accordingly, the restriction value may be determined so that the damping controllers are continuously restricted, if required, to keep the disturbance value at an acceptable level. 
         [0028]    According to an alternative embodiment the restriction value is determined by comparing a disturbance value determined from the disturbance information with predetermined one or more predetermined threshold values. 
         [0029]    It is understood that the disturbance information may comprise spectrally resolved disturbance information, e.g. an amplitude spectrum containing the spectral distribution of power variation amplitudes within a frequency range. 
         [0030]    A second aspect of the invention relates to a control system for controlling at least one wind turbine for reducing spectral disturbances in an electrical grid, the at least one wind turbine comprises
       a rotor adapted to drive a power generator via a shaft, wherein the generator is connectable with the electrical grid, and   at least one damping controller configured to compensate structural oscillations of the wind turbine by controlling a torque on the shaft, wherein the at least one damping controller is configured to set a limit of the control action on the shaft dependent on a restriction value, wherein the control system comprises   a detector configured to determine disturbance information describing an electrical disturbance at a point of measurement electrically connected to the wind turbine,   a compensation controller configured to determine the restriction value based on the determined disturbance information, and to assign the restriction value to the damping controller for setting the limit of the control action.       
 
         [0035]    According to an embodiment the control system further comprises a distributor configured to assign the restriction value to at least one of the damping controllers. 
         [0036]    According to an embodiment the control system further comprises a wind turbine distributor configured to determine wind turbine restriction values based on the restriction value for a plurality of the wind turbines, and to assign the turbine restriction values to the wind turbines. 
         [0037]    A third aspect of the invention relates to a wind turbine comprising a control system according to the second aspect. 
         [0038]    A fourth aspect of the invention relates to a power plant controller adapted to control a plurality of wind turbines in a wind park, the power plant controller comprises a control system according to the second aspect. 
         [0039]    In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which 
           [0041]      FIG. 1  shows a wind turbine, 
           [0042]      FIG. 2  illustrates an example of structural oscillations and variations in shaft torque and generator power, 
           [0043]      FIG. 3  illustrates an amplitude spectrum of power variations from a generator 
           [0044]      FIG. 4  shows a control system for controlling a one wind turbine for reducing spectral disturbances in an electrical grid, and 
           [0045]      FIG. 5  shows a control system for controlling a plurality of wind turbines for reducing spectral disturbances in an electrical grid. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0046]      FIG. 1  shows a wind turbine  100  comprising a tower  101  and a rotor  102  with at least one rotor blade  103 , such as three blades. The rotor is connected to a nacelle  104  which is mounted on top of the tower  101  and being adapted to drive a generator situated inside the nacelle. The rotor  102  is rotatable by action of the wind. The wind induced rotational energy of the rotor blades  103  is transferred via a shaft to the generator. Thus, the wind turbine  100  is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the generator. In this document the wind turbine  100  may also be referred to with the common abbreviation WTG (Wind Turbine Generator). 
         [0047]    According to an embodiment, the wind turbine  100  may be configured so that the torque on the shaft and/or the electric power produced by the generator can be controlled via a control input to the generator, e.g. via a control input to a power invertor or power converter connected electrically connected to the generator. The control input may be in the form of a power reference or a torque reference. The control input may be supplied directly to the generator, invertor or converter, or indirectly via a power controller or a torque controller which controls the generator. In addition, also the pitching of the blades are controlled in order to control the rotation speed of the shaft. 
         [0048]    Structural components of the wind turbine such as the tower may be influenced by the wind and other effects to oscillate. Such structural oscillations affects the structural components with fatigue loads and, therefore, it may be desired to damp such oscillations. 
         [0049]    In an embodiment the wind turbine comprises a damping controller (see damping controller  414 - 416  in  FIG. 4 ) configured to damp structural oscillations by inducing variations in the shaft torque. The torque variations cause structural oscillations in structural components. It is possible to damp the undesired oscillations by inducing torque variations which generate structural oscillations which counteracts the undesired oscillations. The torque variations can be created in response to variations in the control input for the generator (i.e. the control input in the form of a power reference or a torque reference). 
         [0050]    For example, the damping controller may be tower damping controller configured to damp tower oscillations. The tower damping controller may configured to modify a power control input, i.e. a power set-point, to the generator so that the control input comprises sinusoidal variations. The variations in the control input generate variations in the shaft torque which generate tower oscillations having a frequency and phase which counteracts the wind-induced tower oscillations. 
         [0051]    Since the control input for the generator contains variations, the power generated by the generator varies in response the control input variations. The power from the generator and, therefore, also the power variations, are supplied to the grid. Certain amplitudes of power variations may be acceptable, but generally variations in the power output due to torque damping activities from a wind turbine are undesired. 
         [0052]      FIG. 2  illustrates an example of structural oscillations  201  (Osc) as a function of time t, such as tower top lateral vibrations. The oscillations  201  are damped by inducing torque variations  202  in the shaft from time t 1 . The torque variation may be set-point variations (ΔPset) super-imposed onto the power set-point signal. 
         [0053]    The signal  202  may reflect the power set-point variation (APset) to be super-imposed onto the power set-point to counter the tower vibrations of  201 . The torque variations will in itself induce structural oscillations, which when properly timed will counteract the structural oscillations  201 , via structural oscillations induced by the torque variations and, therefore, the structural oscillations start to decay after time t 1 . Since the torque variations in the shaft are generated by the generator, e.g. by varying the power set-point, the amplitude of the power from the generator Pout varies correspondingly with the torque variations and with the same or substantially the same frequency. The variations in the power output  203  is illustrated with an exaggerated amplitude of the variations. 
         [0054]    The power variations  203  caused by a damping controller may have a particular frequency or may have a spectral range corresponding to the spectral range of the control signal generated by the damping controller. A wind turbine  100  may have more than one damping controller where each damping controller may be configured to damp different structural oscillations. Accordingly, different damping controllers of a wind turbine generator may generate power variations  203  having different spectral ranges. 
         [0055]    Herein the term “spectral range” or “spectrum” refer to a range of frequencies describing frequencies of the control signal, generator power, power variations or other parameters relating to physical quantities. Accordingly, a “spectral range” may also be referred to as a frequency range and a spectrum may also be referred to as a frequency spectrum. 
         [0056]    The power variations  203  caused by damping controllers, i.e. power variations in a spectral range, are also referred to as spectral disturbances  203  or power disturbances  203  since they appear as disturbing power variations in particular spectral ranges, typically at frequencies below 2 Hz. The spectral disturbances may be present in the electrical grid or other power lines connected with a wind turbine, e.g. power lines connecting wind turbines with the grid. 
         [0057]      FIG. 3  shows a schematic example of an amplitude spectrum  301  of amplitudes Pout of power variations  202  within a frequency spectrum or frequency band between 0 and 2 Hz. The different amplitude peaks  302 - 305  may correspond to spectral disturbances due to damping actions from different damping controllers.  FIG. 3  also shows a threshold line  305  indicating acceptable amplitudes of the spectral disturbances. 
         [0058]      FIG. 4  shows a control system  401  according to an embodiment of the invention for at least one wind turbine  100  capable of delivering electrical power to an electrical grid  430 . The control system is configured for reducing spectral disturbances  203  in the electrical grid  430 . 
         [0059]    The generator of the wind turbine  100  is connectable with the electrical grid  430 , for example via a power line  420 . The wind turbine  100  may be configured in different ways, for example as described in connection with  FIG. 1 . 
         [0060]    According to this embodiment, the wind turbine comprises at least one damping controller  414 - 416  (Damp 1 -Damp 3 ) configured to compensate structural oscillations of the wind turbine by controlling a torque on the shaft via a control input to the generator. Each of the damping controllers comprises an output for supplying a control signal, e.g. to a torque controller, for generating shaft torque variations by means of the power generator. 
         [0061]    The damping control signals from different damping controllers may be combined into a single control signal, e.g. a combined control signal intended for generating shaft torque variations by means of the power generator, e.g. via the torque controller. 
         [0062]    Wind turbine component  417  (Generator) principally illustrates systems of the wind turbine, e.g. the torque controller, which receives the damping control signals or the combined control signal and systems which generate electric power and inject the power to the grid  430 . Accordingly, wind turbine component  417  includes the coupling between the shaft torque variations induced in response to the damping control signal and the power variations  203  in the electrical power supplied to the electrical grid. 
         [0063]    One or more of the damping controllers  414 - 416  are configured to set a limit of the control action on the shaft dependent on a restriction value or a value derived from a restriction value supplied via an input to each of the damping controllers. By control action is meant e.g. the amplitude of variations in the damping control signal. By limiting the control action the amplitude of torque variations  202  generated in response to the damping control signal and, therefore, the amplitude of the power variations  203  is limited correspondingly. 
         [0064]    For example, the damping controller  414  may be configured with an adjustable limit function arranged to set a limit on the output from the damping controller, e.g. the output from a PI control algorithm, so that the control action can be reduced if the output from the control algorithm exceeds the limit. The limit is adjusted in response to the restriction value or in response to a value derived from the restriction value. The damping controller with an integral control-function, e.g. a PI control-function, may be provided with an anti-windup function in order to handle the limit function. 
         [0065]    The control system comprises a detector  411  (Detector) configured to determine disturbance information describing an electrical disturbance at a point of measurement  431  electrically connected to the wind turbine. The point of measurement  431  may be located anywhere on an electrical power connection between and including the output of the wind turbine and the grid  430 . The point of measurement  431  may alternatively be located anywhere on an electrical power connection between the wind turbine and the grid  430 . The point of measurement  431  may comprise a power meter capable of measuring the active power. The disturbance information may be in the form of a value describing variations in the power. 
         [0066]    For example, the detector may be configured to determine a low pass filtered power signal (in order to remove the normal AC component, e.g. the 50 Hz component) and to determine the disturbance information as variations in the low pass filtered signal. 
         [0067]    In another example, the detector may be configured to determine disturbance information by determining amplitudes of power variations  203  within a frequency spectrum, possibly in the form of an amplitude spectrum  301 . For that purpose the detector  411  may be configured with a spectrum analyzer such as a FFT analyzer. Accordingly, the disturbance information may be in the form of a value describing an average amplitude value of the amplitude peaks  302 - 305  or values describing the amplitudes of different amplitude peaks  302 - 305  within a frequency spectrum. 
         [0068]    The detector may be configured with a threshold function so that that disturbance information is set to e.g. zero in case the determined disturbance information is below a given threshold. For example, if a amplitude peak  302 - 305  is below the threshold line  305  the disturbance information may be set to a zero or other suitable value. 
         [0069]    The control system further comprises a compensation controller  412  (Comp ctrl) configured to determine a restriction value for one of the damping controllers  414 - 416 , and/or configured to determine restriction values for two or more damping controllers. The determined restriction value is outputted by the compensation controller  412  as an output value. The restriction value is determined based on the determined disturbance information from the detector  411 . 
         [0070]    The compensation controller  412  may be configured to determine the restriction value by comparing a disturbance value determined from the disturbance information with predetermined one or more predetermined threshold values. For example, the compensation controller may be configured to determine the restriction value as a simple binary output value, e.g. 0 and 1, or an ON and OFF value, dependent on a magnitude of the disturbance information, e.g. in comparison with a threshold  305 . The compensation controller could also be configured to generate a predetermined number of output values, e.g. three or more values in the interval 0 to 1, e.g. in comparison with a corresponding number of predetermined threshold values. 
         [0071]    Alternatively, the compensation controller may comprise a feedback controller, e.g. a PI controller, configured to determine the restriction value dependent on a difference between a disturbance value determined from the disturbance information and a desired disturbance value, i.e. a reference disturbance value provided via input  451 . In this way, the restriction value may be determined so that it is proportional to the difference and possibly dependent on a time integrated value of the difference between the disturbance information and the reference. 
         [0072]    The damping controller  414  may be configured so that the adjustable limit of the damping controller is set according to a predetermined look-up table or a function defining a relation between the restriction value and the limit of the damping controller, i.e. dependent on a value derived from the restriction value. For example, the damping controller may contain a look-up table defining limits in percentages relative to a nominal limit or absolute limits. As an example, the limit may be set to 100% or zero in case the restriction value is equal to one implying that the damping control action of the affected damping controller  414  is reduced to zero. When the control action is reduced to zero any power variations  203  potentially caused by the affected damping controller are removed. 
         [0073]    Generally, the limit of the damping controller may be set as an absolute value, an absolute reduction (e.g. reduce limit by a certain percentage), or a relative value or reduction (e.g. relative to an actual or recent activity level of the damping controller). The absolute value, the absolute reduction or relative change is determined dependent on the restriction value, e.g. by means of a look-up table. 
         [0074]    Alternatively, the adjustable limit value of the controller  414 - 116  may be identical to the restriction value so that a conversion via e.g. a look-up table is not required. 
         [0075]    The control system may further comprise a distributor  413  (Distributor) configured to assign the restriction value to one or more of the damping controllers  414  for setting the limit of the control action. Clearly, for wind turbines with only one damping controller  414  a distributor is not required since the assignment of the restriction values may be a fixed assignment or the assignment could be performed e.g. by the compensation controller  412 . 
         [0076]    For example, the distributor  413  may be configured to assign the restriction value, i.e. the same restriction value, to all damping controllers  414 - 416  of a wind turbine. 
         [0077]    The distributor  413  may also be configured to distribute the restriction value among a plurality of damping controllers. For example, the restriction value may be distributed by assigning a fraction of the restriction value to each of the damping controller. For example, if three damping controllers should be limited, derived restriction values equal to one third of the original restriction value may be distributed among the damping controllers. 
         [0078]    The distributor  413  may be configured to select at least one of the damping controllers  414 - 416  based on the disturbance information and to assign the restriction value (original or derived restriction value) to the selected damping controller. 
         [0079]    For example, the disturbance information may be in the form of values describing the amplitudes of different amplitude peaks  302 - 305  within a frequency spectrum of the power variations  203 . Since particular amplitude peaks may be associated with particular damping controllers  414 - 416 , one or more damping controllers may be selected according the levels of the amplitude peaks  302 - 305 . 
         [0080]    The control system  401  may further comprise an activity monitor  418  (Monitor) configured to determine compensation values describing levels of damping compensation performed by the damping controllers  414 - 416 . The compensation values may be determined based on output values from the damping controllers  414 - 416 , e.g. from individual damping control signals from different damping controllers  414 - 416  or from a combined damping control signal, e.g. by determining root-mean-square value(s) of the damping control signals. 
         [0081]    The distributor  413  may be configured to select at least one of the damping controllers  414 - 416  based on damping compensation values from different damping controllers  414 - 416  and to assign the restriction value to the selected  30  damping controller(s). For example, the restriction value may be a assigned to the damping controller having the largest damping compensation values, or plurality of restriction values may be a assigned or distributed to the damping controllers having the largest damping compensation values. 
         [0082]      FIG. 5  illustrates an embodiment of the invention wherein a control system  501  is arranged for controlling a plurality of wind turbines  100  for reducing spectral disturbances in the electrical grid  430 . The control system  501  is equivalent to the control system  401  meaning that elements in  FIGS. 4 and 5  having the same reference signs perform the same or equivalent functions. The control system  501  mainly differs from control system  401  by having a plurality of distributors  413  (Distributor) and by having an additional wind turbine distributor  502  (WTG distributor).  FIG. 5  indicates first and second wind turbines WTG 1  and WTG 2  which each comprises components  413 - 417 . Alternatively, the distributors  413  could be located externally to the wind turbines, e.g. they could be comprised by a central power plant controller. As seen in  FIG. 5 , the point of measurement  431  for the electrical disturbance is located in the electrical grid  430 . The point of measurement  431  could alternatively be in, or near, the point of connection (PoC) between the two wind turbines WTG 1  and WTG 2  with the electrical grid (not shown here). 
         [0083]    The wind turbine distributor  502  (WTG distributor) is configured to determine wind turbine restriction values based on the restriction value determined by the compensation controller  412  (Comp ctrl). The wind turbine restriction values are determined from a plurality of the wind turbines. The wind turbine distributor  502  is further configured to assign the turbine restriction values to the wind turbines, i.e. to the distributors  413  associated with the wind turbines. Similarly to the embodiment in  FIG. 4 , each of the distributors  413  are configured to assign the received restriction value, here the received turbine restriction value for the associated of the wind turbine to the one or more damping controllers of the wind turbine for setting the limit of the control action of the one or more damping controllers. 
         [0084]    The wind turbine distributor  502  may further be configured to select at least one of the wind turbines based on the disturbance information, and to assign the wind turbine restriction values to the selected wind turbines. The wind turbine distributor  502  may be configured similarly to the distributor  413  to perform selection of wind turbines based on disturbance information. For example, the distributor  502  may be configured to select wind turbines based on the disturbance information containing information about amplitude peaks  302 - 305  which may be associated with particular damping controllers  414 - 416 . Since damping controllers of different wind turbines may have distinct frequencies of the amplitude peaks  302 - 305 , the wind turbines responsible for the highest amplitude peaks  302 - 305  may be selected by the wind turbine distributor  502 . 
         [0085]    Additionally or alternatively, the wind turbine distributor  502  may be configured to select at least one of the wind turbines based on damping compensation values describing levels of damping compensation performed by the damping controllers of the wind turbines, and to assign the wind turbine restriction values to the selected wind turbines. 
         [0086]    As shown in  FIG. 5 , the compensation values determined by the activity monitor  418  (Monitor) may be supplied to the damping controller distributors  413  and/or to the wind turbine distributor  502 . Accordingly, based on the compensation values describing levels of damping compensation performed by the damping controllers  414 - 416 , the wind turbine distributor  502  may select the wind turbines having the largest damping compensation values. 
         [0087]    The functions of the distributors  413  for selecting at least one of the damping controllers  414 - 416  of the wind turbines WTG 1 -WTG 2  based on the disturbance information and/or damping compensation values may be hosted by the wind turbines WTG 1 -WTG 2  or by a central control unit such as a power plant controller configured to control a plurality of wind turbines WTG 1 -WTG 2 . Accordingly, the wind turbine distributor  502  may additionally comprise the functions of the distributors  413 , so that distributors  413  located at each wind turbine may be dispensed with. 
         [0088]    One or more of the components  411 ,  412 ,  413 ,  418  of the control system  401  may be comprised by a wind turbine, e.g. by a control system of a wind turbine, or one or more of the components  411 ,  412 ,  413 ,  418  may be located externally to the wind turbine, e.g. one or more of the components may be comprised by a central control unit such as a power plant controller configured to control a plurality of wind turbines. Accordingly, some of the components may be comprised by a wind turbine, and other components may be comprised by a central control unit. 
         [0089]    Since the control system  501  is configured for controlling a plurality of wind turbines, at least components  411 ,  412 ,  502  may preferably be comprised by a central control unit such as a power plant controller, whereas damping controller distributors  413  may be comprised by the central control unit or by different wind turbines. 
         [0090]    Accordingly, a power plant controller configured to control plurality of wind turbines in a wind park may comprise the control system  401  where the distributor  413  is configured to assign restriction values to damping controllers  414 - 416  of a plurality of the wind turbines, or the power plant controller may comprise the control system  501  or part of the control system  501  such as components  411 ,  412 ,  502 . 
         [0091]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.