Abstract:
A method is proposed for operating at least one wind turbine of a wind farm including determining wind turbine individual offset information based on reactive power provided by the respective wind turbine and reactive power provided by at least one further wind turbine of the wind farm, determining wind turbine individual control information based on the wind turbine individual offset information and wind farm specific control information, operating the at least one wind turbine according to the wind turbine individual control information. Further, a controller and a device as well as a computer program product and a computer readable medium are also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European application No. EP 16153397.1 having a filing date of Jan. 29, 2016, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following relates to a method, a controller and to a device for operating at least one wind turbine of a wind farm. In addition, an according computer program product and a computer readable medium are suggested. 
       BACKGROUND 
       [0003]    One goal of a wind farm controller is to control centrally an active and reactive power injected by a whole wind farm (also referred to as “wind park”) into a grid. This provides the possibility to participate actively in control tasks on the grid for the wind farms in the same way as conventional power plants do. Thereby, a wind farm control level behaves as a single centralized unit (“central wind farm control level”) which has as input, e.g., system operator orders, measurements from a power common coupling (“PCC”) and available powers from the wind turbines of the wind farm and as outputs elaborated reference information or signals for each individual wind turbine, i.e. each individual wind turbine control (“local wind turbine control level”). 
         [0004]    Active power control functions of the wind farm controller may comprise, e.g., an automatic frequency control wherein the frequency measured in the wind farm point of common coupling (PCC) is controlled. The wind farm must thus be able to produce more or less active power in order to compensate for a deviant behavior in the frequency. 
         [0005]    Reactive power control functions of the wind farm controller may comprise, e.g., an automatic voltage control wherein the voltage in the wind farm point of common coupling (PCC) is controlled. This implies that the wind farm can be ordered to produce or absorb an amount of reactive power to the grid in order to compensate for the deviations in the voltage in the grid. 
         [0006]    One reason for placing automatic frequency control in the wind farm control level is to avoid that the wind farm controller can counteract the frequency control implemented in the individual wind turbine. Automatic voltage control is placed in the wind farm control level in order to avoid a risk of instability and a high flow of reactive power between the wind turbines. Usually, the implementation of both frequency and voltage control is going to be done as a combined droop and dead band control. 
         [0007]    The central wind farm control level may comprise two separated control loops, one for the active power control and the other for the reactive power control. 
         [0008]    One possible implementation of the active and reactive control loop may be as follows: 
         [0009]    First, an active and reactive power reference signal, respectively, are derived in a control function block, based on one or several control functions required by the system operator. These reference signals may be, if necessary, adjusted further with some corrections from subordinated control loops (e.g. focusing on frequency and voltage) respectively, in order to assure that the frequency and voltage limits in the PCC are not violated. Each loop consists of a PI controller ensuring a correct power production from the wind farm. The controller computes a power error and sets up the power reference for the whole wind farm. These power references are further converted into power reference signals for each individual wind turbine of the wind farm. 
         [0010]    Controlling reactive power in a wind farm may be based on, e.g., a common voltage reference provided to the wind turbines. 
         [0011]    When controlling reactive power in a wind farm using such a common voltage reference, the individual wind turbines may produce/consume different amounts of reactive power, depending on their location in the wind farm, measurement tolerances and other factors like, e.g., converter control strategy. This unbalance may lead to a loss of energy in the wind farm and unnecessary wear on components shorting their operational lifetime. 
         [0012]    One possible solution for controlling reactive power within a wind farm may be based on distributing a reactive power reference to the wind turbines instead of a voltage reference. This solution may reduce the performance of responses to power grid events and the response times to changes in grid voltage in wind farms under voltage control. 
       SUMMARY 
       [0013]    An aspect relates to optimizing the controlling of reactive power in a wind farm. 
         [0014]    In order to overcome this problem, a method is provided for operating at least one wind turbine of a wind farm, comprising the following steps,
       determining wind turbine individual offset information based on
           reactive power provided by the respective wind turbine and   reactive power provided by at least one further wind turbine of the wind farm,   
           determining wind turbine individual control information based on
           the wind turbine individual offset information and   wind farm specific control information,   
           operating the at least one wind turbine according to the wind turbine individual control information.       
 
         [0022]    Wind turbine individual control information may be a power reference or a power reference signal representing the power reference or a value thereof provided to an individual wind turbine for controlling reactive power production of the wind turbine according to the proposed solution. 
         [0023]    The wind turbine individual control information may be a voltage reference signal generated by a central wind farm controller. 
         [0024]    Wind farm specific control information may be control information like, e.g., active and reactive power control information provided e.g. by a system operator controlling operation of the wind farm. 
         [0025]    Wind turbine individual offset information may be information determined based on wind turbine individual and/or wind farm specific information and may be used as a correction/offset factor being applied to the wind farm specific control information. 
         [0026]    Operating several wind turbines of a wind farm individually according to the proposed solution allows a leveling of, e.g., reactive power among several individual wind turbines of the wind farm compensating for, e.g., different converter types, measurement principles and tolerances. As an advantage, a lifetime of the wind turbines may be lengthened as well as losses in the wind farm due to high reactive currents being exchanged within the wind farm may be limited. 
         [0027]    In an embodiment, the wind turbine individual offset information is determined
       based on the reactive power provided by the wind turbine and   based on a mean value of reactive power provided by
           the wind turbine and   the at least one further wind turbine of the wind farm.   
               
 
         [0032]    In another embodiment, the wind turbine individual offset information is determined based on a deviation information representing a deviation or difference between the reactive power provided by the wind turbine and the mean value of reactive power. 
         [0033]    According to an exemplary embodiment, the difference may be derived by subtracting the reactive power provided by the wind turbine from the mean value of reactive power. 
         [0034]    In a further embodiment, the deviation information is processed based on at least one out of the following functionalities:
       applying a gain factor,   applying an integration,   applying a limiting factor.       
 
         [0038]    In a next embodiment, the mean value of reactive power is weighted by a value of a wind turbine individual weight factor. Thereby, the weighting may be implemented by multiplication. 
         [0039]    It is also an embodiment that a wind turbine individual reactive power offset is added to the reactive power provided by the wind turbine. Alternatively other information processing operations may be allied like, e.g., subtraction or multiplication. 
         [0040]    Pursuant to another embodiment, the wind turbine individual control information is determined by adding
       the wind turbine individual offset information and   the wind farm specific control information.       
 
         [0043]    According to an embodiment, the wind turbine individual control information is representing a wind turbine individual voltage reference controlling the reactive power to be provided by the respective wind turbine. 
         [0044]    The problem stated above is also solved by a controller for operating at least one wind turbine of a wind farm, comprising a processing unit that is arranged for
       determining wind turbine individual offset information based on
           reactive power provided by the respective wind turbine and   reactive power provided by at least one further wind turbine of the wind farm,   
           determining wind turbine individual control information based on
           the wind turbine individual offset information and   wind farm specific control information,   
           operating the at least one wind turbine according to the wind turbine individual control information.       
 
         [0052]    According to an exemplary embodiment, the controller may be part of or may be implemented in a wind farm controller. 
         [0053]    The problem stated above is also solved by a device comprising and/or being associated with a processing unit and/or hard-wired circuit and/or a logic device that is arranged such that the method as described herein is executable thereon. 
         [0054]    Said processing unit may comprise at least one of the following: a processor, a microcontroller, a hard-wired circuit, an ASIC, an FPGA, a logic device. 
         [0055]    The solution provided herein further comprises a computer program product directly loadable into a memory of a digital computer, comprising software code portions for performing the steps of the method as described herein. 
         [0056]    In addition, the problem stated above is solved by a computer-readable medium, e.g., storage of any kind, having computer-executable instructions adapted to cause a computer system to perform the method as described herein. 
     
    
     
       BRIEF DESCRIPTION 
         [0057]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0058]      FIG. 1  shows an exemplary embodiment of a control scheme for balancing reactive power between several wind turbines of a wind farm; 
           [0059]      FIG. 2  shows an alternative embodiment of the proposed control scheme; and 
           [0060]      FIG. 3  shows a further alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0061]      FIG. 1  shows an exemplary control scheme  100  for balancing reactive power between several wind turbines of a wind farm. The control scheme  100  of  FIG. 1  may be part of a wind farm controller individually controlling operation of N wind turbines of the wind farm. 
         [0062]    According to the exemplary embodiment of  FIG. 1  information  111  representing reactive power Q[n] provided by an individual wind turbine (here the “n-th” wind turbine of the N available wind turbines of the wind farm) is passed to a negative input  116  of an adding element  114 . Further, information  110  representing reactive power individually provided by each of the N wind turbines Q[ 1  . . . N] is forwarded to an input of a calculation element  112  determining a mean value of reactive power based on the information  110 . A resulting information  113  representing the mean value of reactive power is forwarded to an adding input  115  of the adding element  114  calculating a difference or deviation between the mean value  113  and the value  111  of individual reactive power Q[n] provided by the n-th wind turbine. 
         [0063]    A deviation information  120  provided at an output  117  of the adding element  114  representing a resulting value of the calculation is provided to an input of a processing unit  160  exemplarily comprising an amplifier  121 , an integrator  123  and a limiter  125 . 
         [0064]    According to the exemplary control scheme of  FIG. 1  the information  120  is forwarded to an input of the amplifier  121  applying, e.g., a gain factor k to the information  120 . Resulting information  122  available at an output of the amplifier  121  is provided to an input of the integrator  123  processing an integration based on the provided information  122 . Resulting information  124  at the output of the integrator  123  may be forwarded to limiter  125  generating limited voltage information based on the information  124  wherein resulting wind turbine individual offset information  130  is passed to a first positive input of an adding element  135 . 
         [0065]    Further, common voltage reference information  140  controlling in general the reactive power to be provided by the wind farm is passed to a second positive input of the adding element  135 . The common voltage reference information  140  may be exemplarily provided as wind farm specific control information U RefPark  by a system operator controlling operation of the wind farm. Resulting information  150  representing a value of the sum of the common voltage reference information  140  and the wind turbine individual offset information  130  is provided to an output of the adding element  135 . 
         [0066]    According to the exemplary control scheme of  FIG. 1  the wind turbine individual control information  150  is representing a wind turbine individual voltage reference U R [n] which might be forwarded to the respective n-th wind turbine thereby controlling the reactive power Q[n] to be provided by that n-th wind turbine based on the control information  150 . 
         [0067]    The wind turbine individual voltage reference U R [n]  150  may be provided to a local wind turbine control level of the respective wind turbine being responsible for a proper production of wind turbine individual reactive power according to the control information  150 . 
         [0068]      FIG. 2  shows an alternative embodiment of the proposed control scheme allowing a faster implementation of the proposed control scheme. The proposed solution  200  as visualized in  FIG. 2  is mainly based on the control scheme of  FIG. 1 , so the same reference numbers are used and reference is made to the respective part of the description. The differing feature to the proposed solution of  FIG. 1  is the missing integrator (indicated by the reference number  123  in  FIG. 1 ) in the processing unit  160  of  FIG. 2 , wherein an output of the amplifier  121  is directly linked to an input of the limiter  125 . 
         [0069]      FIG. 3  shows a further alternative embodiment  300  of the proposed solution. 
         [0070]    Thereby, information  311  representing reactive power Q[n] provided by an individual n-th wind turbine is passed to a negative input  316  of an adding element  314 . Further, information  310  representing individual reactive power provided by each of the N wind turbines Q[ 1  . . . N] is forwarded to a an input (or several inputs according to an alternative embodiment) of an averaging (“mean value calculating”) element  312  determining a mean value of reactive power based on the information  310 . Differing to the solution as shown in  FIG. 1  a resulting information  313  representing the mean value of reactive power Q[ 1  . . . N] provided by all of the N wind turbines of the wind farm is forwarded to an input of a multiplication element  319  multiplying the mean value of reactive power  313  by a value  301  of a wind turbine individual weight factor w[n] provided to a further input of the multiplication element  319 . A resulting value  303  representing a weighted mean value is routed to a further positive input  315  of the adding element  314 . Differing to the embodiment of  FIG. 1  a value  302  representing a wind turbine individual reactive power offset Q Offset [n] is provided to a further positive input  318  of the adding element  314 . Based on the information  303 ,  302  and  311  provided to the respective inputs  315 ,  318 ,  316  a value  320  representing a sum of the provided information  303 ,  302 ,  311  is calculated by the adding element  314 . The information  320  being available at an output  317  of the calculation element  314 , representing the result of the calculation is provided to an input of a processing unit  360  which might have similar functionality like the processing unit  160  as shown in  FIG. 1  (comprising, e.g., an amplifier  321 , an integrator  323  and a voltage limiter  325 ), so reference is made to the respective description of  FIG. 1   
         [0071]    Similar to the scenario of  FIG. 1  a resulting information  330  representing wind turbine individual offset information provided via an output of the voltage limiter  325  is passed to a first positive input of an adding element  335  wherein a common voltage reference information  340  controlling the reactive power to be provided by the wind farm is routed to a second positive input of the adding element  335 . Resulting information  350  representing a value of the sum of the common voltage reference information  340  and the wind turbine individual offset information  330  is provided to an output of the adding element  335  and may be forwarded as a wind turbine individual control information U R [n] to the respective n-th wind turbine. 
         [0072]    The control scheme  300  as exemplarily shown in  FIG. 3 , in particular by applying an additional value  301  of a wind turbine individual weight factor w[n] and by applying a value  302  of a wind turbine individual offset based on reactive power Q Offset [n] allows, e.g., a proper definition of a reactive power balancing strategy like, e.g., controlling wind turbines physically located closer to the PCC to deliver more reactive power than wind turbines having a greater distance to the PCC. Further, the control scheme  300  enables an optimization of a reactive power and voltage control within the wind farm on a Medium Voltage side of a turbine transformer. 
         [0073]    Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
         [0074]    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. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.