Abstract:
Operating a wind farm in which the power generated by the wind energy installations is fed to a power supply system via a system internal to the wind farm and via a substation is disclosed. The farm director of the wind farm can ascertain a standard target voltage value and transmit it to the control units of the individual wind energy installations on the wind farm, which can use an overall factor to regulate the reactive power generated by a wind energy installation. The overall factor can be calculated from the difference between the actual voltage across the wind energy installation and the target voltage value, multiplied by a gain factor. On account of the impedance in the internal system of the wind farm, wind energy installations situated far away from the substation thus generate less reactive power than the wind energy installations which are situated close to the substation.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage application under 35 USC 371 of International Application No. PCT/EP2011/073981, filed Dec. 23, 2011, which claims the priority of German Application No. 10 2010 056 456.7, filed Dec. 29, 2010, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for operating a wind farm, and also to a wind farm. 
       BACKGROUND OF THE INVENTION 
       [0003]    On wind farms, the power that is generated by a plurality of wind energy installations is fed into an internal wind farm network. The power that is fed into the internal wind farm network is fed into a power supply network at a substation after undergoing transformation as required. In order to maintain the quality of the power supply in the power supply network, the power that is fed into the power supply network by the wind farm must meet certain specifications, for example, with respect to the power factor, that is, the ratio between the real power and the reactive power. This is monitored by a farm control system known as a “farm master,” which is able to influence individual control devices of the wind energy installations in such a way that the sum of the power generated by the wind energy installations fulfills the requirements resulting from the power supply network. 
         [0004]    The real power generated on a wind farm cannot be freely determined by the farm master, but rather is dependent on the wind speed and the design output of the individual wind energy installations. However, the farm master can be used to determine the amount of reactive power generated by the wind farm. 
         [0005]    Power losses occur in the lines of the internal wind farm network. These losses can be reduced by varying the reactive power. Prior art describes measures for doing this, some of which work in contrary directions. 
         [0006]    For example, if the wind energy installations are at a standstill, but there is a requirement for reactive power, passive components can be used to meet the reactive power requirement, instead of the converters in the wind energy installations, which are subject to operating losses. 
         [0007]    Furthermore, at times when the wind energy installations are operating at high capacity, it is possible to select the maximum possible voltage in the internal wind farm network at the substation. This results in a reduction of the losses in the cables in the internal wind farm network. This is particularly advisable when the wind energy installations are operating at high capacity, since cable losses dominate in this case. In contrast, when the wind energy installations are operating at low capacity, the minimum possible voltage should be selected in the internal wind farm network, since transformer losses dominate in this case. These losses can be reduced by selecting a low voltage in the internal wind farm network. DE 10 2008 048 258 A1 describes a method for changing the voltage in the internal wind farm network depending on the amount of power generated by the wind energy installations. 
         [0008]    Furthermore, it is possible to minimize the distance over which the power in the internal wind farm network must be transported. This allows the distance over which the reactive power must be transported to be minimized, thus reducing cable losses. However, the potential for reducing losses in the internal wind farm network in this way is limited. 
         [0009]    European patent application EP 2 108 828 A2 describes a process for operating a wind farm in which a farm control system is designed such that the reactive power required to meet the requirements of the power supply network is generated primarily in the wind energy installations that are near the node, whereas the wind energy installations that are more distant make a smaller contribution to the generation of reactive power. This ensures that a large portion of the reactive power must only be transported over a short distance between the wind energy installation and the node in the internal wind farm network, and that a smaller portion of the reactive power must be transported over a longer transport route. Overall, there is a reduction of loss due to reactive power transport in the internal wind farm network. According to this prior art, the farm control system transmits individual target values for real and reactive power to every wind energy installation. The farm control system must therefore be designed to be able to solve a complex system of equations. Solving the system of equations takes considerable time, causing a delay in the adjustment of the real and reactive power generated on the wind farm when changes occur in the power supply network. During this delay, it cannot be ensured that the power that is fed into the power supply network by the wind farm meets the requirements of the power supply network. 
       SUMMARY OF THE INVENTION 
       [0010]    An object of the current invention is to create a method for operating a wind farm, as well as a wind farm, in which the losses in the internal wind farm network due to reactive power are reduced, and the real and reactive power of the wind farm can be quickly adjusted. 
         [0011]    This object is achieved through the method for operating a wind farm and a wind farm as broadly described herein. Advantageous developments arise from the detailed embodiments described below. 
         [0012]    The invention thus relates to a method for operating a wind farm having a plurality of wind energy installations, each having a control unit, which are connected to an internal wind farm network, a substation at which electrical power that is generated on the wind farm is delivered by the internal wind farm network to a power supply network, and a farm master that is connected to the control units of the individual wind energy installations, wherein the farm master determines a uniform target voltage value (Utarget) and transmits the target voltage value (Utarget) to the control units of the individual wind energy installations, and the reactive power that is generated by a wind energy installation is adjusted via the control unit of the wind energy installation by an overall factor (Ib), wherein the overall factor (Ib) comprises a first factor (Ib 1 ) that is calculated from the difference between the actual voltage (Uactual) present at the wind energy installation and the target voltage value (Utarget), multiplied by a gain factor (k_Ib). 
         [0013]    Furthermore, the invention relates to a wind farm having a plurality of wind energy installations, each having a control unit, which are connected to an internal wind farm network, a substation at which the electrical power that is generated on the wind farm is delivered by the internal wind farm network to a power supply network, and a farm master that is connected to the control units of the wind energy installations, wherein the farm master is configured to determine a uniform target voltage value (Utarget) and to transmit the target voltage value (Utarget) to the control units of the individual wind energy installations, and the reactive power that is generated by a wind energy installation can be adjusted via the control unit of the wind energy installation by an overall factor (Ib), wherein the overall factor (Ib) comprises a first factor (Ib 1 ) that can be calculated from the difference between the actual voltage (Uactual) present at the wind energy installation and the target voltage value (Utarget), multiplied by a gain factor (k_Ib). 
         [0014]    The invention is based on the knowledge that controlling reactive power by means of a target voltage results in an optimization of the reactive power distribution on the wind farm due to impedance in the internal wind farm network. The wind energy installations receive a uniform value for the target voltage, so that it is not necessary to perform a complex calculation of individual target values. Instantaneous reactive power control is thus possible. 
         [0015]    If, for example, a capacitive (overexcited) feed-in of reactive power should occur, the farm master sets the target voltage value to a somewhat higher level than the voltage value measured at the substation. This determined value is transmitted to the control units of the wind energy installations. There, the factor Ib 1  is calculated from the difference of the actual voltage Uactual present at each wind energy installation and the target voltage value Utarget, multiplied by a gain factor kIb: 
         [0000]        Ib   1 =( U   actual   −U   target )× k   Ib  
 
         [0016]    The factor Ib 1  is incorporated into the overall factor Ib, the following being true in the simplest case: 
         [0000]      Ib=Ib 1    
         [0017]    Based on the overall factor Ib, the control unit can adjust the reactive power generation of the wind energy installation, with a higher overall factor Ib resulting in a higher level of reactive power generation. 
         [0018]    As a consequence of the impedance in the internal wind farm network, different actual voltages arise at the individual wind energy installations. The voltages at wind energy installations near the substation are lower than at wind energy installations located farther away from the substation. By basing the generation of reactive power at each wind energy installation on the difference between the uniform target voltage value and the respective actual voltages, the wind energy installations near the substation can feed in more reactive power into the internal wind farm network than the wind energy installations located farther away from the substation. 
         [0019]    According to the invention, if the wind energy installations that are arranged more closely to the substation feed a higher portion of reactive power into the internal wind farm network than wind energy installations that are located farther away from the substation, the higher portion of reactive power only has to be transported over a shorter distance within the internal wind farm network. In this way, the transport losses are reduced relative to a wind farm in which the reactive power generation is equally distributed across all wind energy installations. Unlike in prior art, the invention does not require a complex farm master to calculate reactive power target values for each individual wind energy installation. In fact, according to the invention, it is sufficient merely to specify a reactive power target value in the form of a uniform target voltage value that is transmitted to all wind energy installations. The reactive power control thus takes place practically instantaneously. By utilizing effects according to the invention in the internal wind farm network, a loss-optimized distribution of the generated reactive power to the individual wind energy installations on the wind farm is achieved. 
         [0020]    To avoid corrections being made too rapidly when changes occur in the power supply network, another factor Ib 2  can be incorporated into the overall factor Ib, in addition to the factor Ib 1 . The factors Ib 1  and Ib 2  can preferably be added to the overall factor Ib: 
         [0000]    
       
      
       Ib=Ib 
       1 
       +Ib 
       2  
      
     
         [0021]    Whereas the factor Ib 1  can be determined as described above, the factor Ib 2  can preferably be calculated from the difference between the actual voltage present at the wind energy installation averaged over a specified period of time Uactual;averaged and the target voltage value Utarget, multiplied by a gain factor kIb: 
         [0000]        Ib   2 =( U   actual;averaged   −U   target )× k   Ib  
 
         [0022]    This factor Ib 2  can be used to ensure that if a network fault or a similar event occurs, a portion of the generated reactive power is instantaneously adjusted to the new circumstances (via the factor Ib 1 ), whereas another portion is adjusted with a certain delay (via the factor Ib 2 ). In this way, it is possible to avoid unnecessary oscillation excitement in the internal wind farm network and/or power supply network. 
         [0023]    Alternatively, it is possible to use the difference between the actual voltage Uactual and the target voltage value Utarget, rather than the actual voltage Uactual present at the wind energy installation, for determining the factor Ib 2 . In this case, the factor Ib 2  is calculated from the difference averaged over a specified period of time between the actual voltage Uactual present at the wind energy installation and the target voltage value Utarget, multiplied by a gain factor kIb: 
         [0000]        Ib   2 =( U   actual   −U   target ) averaged   ×k   Ib    
         [0024]    Alternatively, it is possible to use a time average of the target voltage Utarget to obtain the factor Ib 2 . In such a case, the factor Ib 2  is thus calculated from the difference between the actual voltage Uactual present at the wind energy installation and the target voltage value averaged over a specified period of time Utarget, multiplied by a gain factor kIb: 
         [0000]        Ib   2 =( U   actual   −U   target;averaged)   ×k   Ib    
         [0025]    It is also possible to calculate the factor Ib 2  from a time average of the factor Ib 1 : 
         [0000]      Ib 2 =Ib 1;averaged    
         [0026]    It is preferred if the gain factor kIb is determined from a global gain factor kIb;global and a gain factor that can be individually specified for each wind energy installation kIb;WEI: 
         [0000]    
       
      
       kIb=k 
       Ib;glaobal 
       +k 
       Ib;WEI  
      
     
         [0027]    The individual factor kIb;WEI can be used to take into account characteristics of the individual wind energy installations and/or the topography of the wind farm when generating reactive power. The factor kIb;WEI can be stored manually in the control unit of each wind energy installation. It is also possible for the farm master to determine the factor kIb;WEI for each wind energy installation and then transmit it to the individual control units. However, since the factor kIb;WEI is not a value that would have to be calculated in real time, the farm master does not have to have unusually high computing power. 
         [0028]    Alternatively or additionally, it is possible for the control unit to add an adjustment factor deltaUtarget;WEI to the global target voltage value Utarget, in which case the adjusted target voltage value Utarget’ thus calculated is used to calculate the factors kIb 1  and kIb 2 , instead of the target value Utarget. The adjusted target voltage value Utarget’ is calculated as follows: 
         [0000]    
       
      
       U′ 
       target 
       =U 
       target 
       +ΔU 
       target;WEI  
      
     
         [0029]    The adjustment factor deltaUtarget;WEI can be determined by the farm master. However, since this is not a value that would have to be calculated in real time, the farm master does not have to have unusually high computing power. 
         [0030]    It is preferred if a plurality of wind energy installations on the wind farm is combined into groups, wherein the wind energy installations in a group are operated using identical values for kIb;WEI and/or deltaUtarget;WEI. In this way, it is possible to reduce the computing effort required for calculating the aforementioned factors. 
         [0031]    The wind farm according to the invention is configured to carry out the method according to the invention. In describing the wind farm, reference will be made to the above embodiments of the method according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The invention will now be described using advantageous embodiments as examples and making reference to the drawings. The following are shown: 
           [0033]      FIG. 1 : An embodiment of a wind farm according to the invention; and 
           [0034]      FIGS. 2   a - e : Partial representations of a control unit for wind energy installations. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]      FIG. 1  shows a representation of a wind farm  1  according to the invention that is operated using the method according to the invention. 
         [0036]    The wind farm  1  comprises a plurality of wind energy installations  10 . 1 - 10 . 4 , of which the wind energy installation  10 . 1  is represented in greater detail as an example. A wind energy installation  10  comprises a rotor  11  that is rotatably arranged on a nacelle  12  at the top of a tower  13 . The rotor  11  drives a generator  14 , which can be a doubly fed asynchronous generator having a stator and a rotor. A converter  15  is connected to the rotor of the generator  14 . In addition, a transformer  16  is provided, which transforms the voltage output by the converter  15  and the stator of the generator  14 . The operation of the wind energy installation  10  is controlled by a control device  17 , which acts on the individual components of the wind energy installation  10  via control lines  18 . In addition to the illustrated components, the wind energy installation  10  can of course comprise additional components, such as a pitch adjustment system for the rotor blades or positioning motors with which the nacelle  12  can be pivoted relative to the tower  13 . 
         [0037]    The wind energy installation  10  is connected to an internal wind farm network  20  in such a way that the power generated by the wind energy installation  10  is fed into the internal wind farm network  20 . A substation  30  is likewise connected to the internal wind farm network  20 . The electrical power generated by the wind energy installations  10  is delivered by the internal wind farm network  20  to an external power supply network  40  at this substation  30 . The internal wind farm network  20  is a medium-voltage network, whereas the power supply network  40  can be a high-voltage network. The substation  40  includes a transformer (not shown) for feeding the power from the internal wind farm network into the power supply network. 
         [0038]    A farm master  50  is also provided, which is connected via control lines  51  to the substation  30  and the control units  17  of the individual wind energy installations  10 . 
         [0039]    In order to be able to feed in the electrical power generated by the wind energy installations  10  into the power supply network  40 , it must meet certain requirements. One of these requirements particularly concerns the power factor, that is, the ratio between real and reactive power. Whereas the real power depends practically entirely on the wind that sets the rotors  11  of the wind energy installations  10  in motion, the generated reactive power can be actively controlled, making it possible to maintain the desired power factor. Every wind energy installation  10  on a wind farm does not have to meet these requirements. In fact, it is sufficient if the power that is delivered to the substation  30  by the internal wind farm network  20  meets the requirements of the power supply network  40 . 
         [0040]    The farm master  50  is configured to determine target values for the reactive power in order to deliver the desired amount of reactive power to the substation  30 . The target values for the reactive power can be determined via the power supply network  40  by means of information obtained at the substation  30 . In particular, this can be the complex power or the voltage in the power supply network  40 . The values obtained at the substation  30  are transmitted via the control lines  51  to the farm master  50 , which uses them to determine the target values for the reactive power. When determining the target values for the reactive power, received target values can also be taken into account via an external data line  52 . The operator of the power supply network  40  is thus able to specify a reactive power level to be generated by the wind farm  1 . 
         [0041]    The target values for the reactive power are converted by the farm master  50  into a uniform target voltage value Utarget and transmitted via the control lines  51  to the control devices  17  of the individual wind energy installations  10 . The control devices  17  are configured to take into account the received target voltage value Utarget when controlling the wind energy installation with respect to the generation of reactive power. By causing the wind energy installations also to feed reactive power according to the received target voltage value Utarget into the internal wind farm network  20  in addition to the real power, it is ensured that the desired reactive power is delivered to the substation  30 . 
         [0042]    Every control unit  17  of a wind energy installation  10  is configured such that it determines an overall factor Ib from the uniform target voltage value Utarget and the actual voltage Uactual of the internal wind farm network  20  that is present at the wind energy installation  10 , which is used as a basis for the generation of reactive power by the wind energy installation  10 . 
         [0043]    Impedance causes an unequal voltage distribution in the internal wind farm network  20 . The voltage in the internal wind farm network at the wind energy installation  10 . 1  that is located far away from the substation  30  is thus higher than at the wind energy installation  10 . 4  that is located close to the substation  30 . By also taking into account the actual voltage in the internal wind farm network  20  in addition to the target voltage when determining the overall factor Ib, it can be ensured that less reactive power is fed into the internal wind farm network by the wind energy installation  10 . 1  that is located far away from the substation  30  than by the wind energy installation  10 . 4  that is located close to the substation  30 . By transporting a greater portion of reactive power over a short distance to the substation  30  and a smaller portion over a longer distance, there is a lower level of loss relative to a wind farm in which the reactive power from all wind energy installations is generated at the same level. 
         [0044]      FIGS. 2   a - e  show schematic representations of various options for determining the overall factor Ib via a control unit  17 .  FIGS. 2   a - e  show only the portion of the control unit  17  that is relevant to the invention for determining the overall factor Ib. The transformation of the overall factor Ib into control signals for the components  11 - 16  of the wind energy installation  10 , so that the desired reactive power generation takes place, is not shown. In all examples, the target voltage value Utarget that is determined by the farm master  50  and the actual voltage Uactual that is present at the wind energy installation  10  are provided to the control unit  17  as input values. In all examples according to  FIGS. 2   a - e , the output value is the overall factor Ib. 
         [0045]    In  FIG. 2   a , in the control unit  17 , a difference module  60  forms the difference from the input values comprising the target voltage value Utarget and the actual voltage Uactual, which a multiplication module  61  then multiplies by a factor kIb that is stored in a storage module  62 . After the multiplication is performed, the factor Ib 1  exists, which is equal to the overall factor Ib as an output value. 
         [0046]    Like the control unit  17  according to  FIG. 2   a , the control unit  17  according to  FIG. 2   b  includes a difference module  60 , which forms the difference from the input values comprising the target voltage value Utarget and the actual voltage Uactual, and a multiplication module  61  that is arranged after it for multiplying the output of the difference module  60  by a factor kIb that is stored in a storage module  62 . After this multiplication is performed, the factor Ib 1  exists. In addition, the control unit  17  also comprises an averaging module  63 , with which a time average of an input value can be formed. In the illustrated embodiment, the averaging module  63  is used to average the actual voltage Uactual to a value Uactual;averaged over time. Another difference module  64  then forms the difference between the target voltage value Utarget and the averaged actual voltage Uactual;averaged, which another multiplication module  65  then multiplies by a factor kIb 2  that is stored in a storage module  66 . The result is thus the factor Ib 2 . The addition module  67  then adds the factors Ib 1  and Ib 2  to form the overall factor Ib. 
         [0047]    A control unit  17  configured according to  FIG. 2   b  offers the advantage that in the event of a network fault or a similar event, a portion of the generated reactive power continues to be adjusted to the new situation instantaneously (via the factor Ib 1 ), while another portion is adjusted with a certain delay (via the factor Ib 2 ). In this way, it is possible to avoid unnecessary oscillation excitement in the internal wind farm network and/or power supply network. 
         [0048]    Alternatively, in order to determine the factor Ib 2 , it is possible to average the target voltage value Utarget to a value Utarget;averaged over time.  FIG. 2   c  shows a correspondingly configured control unit  17 . In the embodiment according to  FIG. 2   c , which is otherwise identical to  FIG. 2   a , the averaging module  63  is used for forming the value Utarget;averaged. 
         [0049]    Alternatively, in order to determine the factor Ib 2 , it is possible to average the difference between the target voltage value Utarget and the actual voltage Uactual over time, before it is multiplied by the factor kIb 2  via the multiplication module  65  to form the factor Ib 2 .  FIG. 2   d  shows a correspondingly configured control unit. 
         [0050]    Alternatively, it is possible for the factor Ib 2  to represent the factor Ib 1  averaged over time.  FIG. 2   e  shows a correspondingly configured control unit  17 . The averaging module  63  averages the factor Ib 1  over time to form the factor Ib 2 . The factors Ib 1  and Ib 2  are then added via the addition module  67  to form the overall factor Ib. 
         [0051]    The factors kIb 1  and kIb 2  can be prespecified and stored in storage modules  62 ,  66  in the control unit  17 . However, it is also possible for the farm master to determine these factors kIb 1  and kIb 2  and transmit them to the wind energy installations. Since these factors are not values that must be calculated in real time (as are, for example, the reactive power target values in prior art), the farm master does not have to have extraordinarily high computing power. 
         [0052]    The factors kIb 1  and kIb 2  can be determined separately for each wind energy installation. It is also possible to combine a plurality of wind energy installations into one or a plurality of groups and to determine the factors kIb 1  and kIb 2  for the wind energy installations in this group. The wind energy installations in a group then operate using identical factors kIb 1  and kIb 2 . It is preferred if the factors kIb 1  and kIb 2  are composed of global factors kIb 1 ;global and kIb 2 ;global, as well as individual factors kIb 1 ;WEI and kIb 2 ;WEI, for each wind energy installation or group of wind energy installations. However, it is also possible for all wind energy installations on the wind farm to be operated using the same factors kIb 1  and kIb 2 . 
         [0053]    Alternatively or additionally, the control unit  17  can individually adjust the target voltage value Utarget that is received by the farm master  50  as an input value via a value deltaUtarget;WEI. The value deltaUtarget;WEI can, for example, be added to the received target voltage value Utarget. 
         [0054]    Alternatively, it can be provided that a mixed controller is used by causing the farm master  50  to transmit an absolute or relative reactive power target value to the control units  17  of the wind energy installations  10 , in addition to the target voltage value Utarget. The control units  17  of the wind energy installations  10  can then determine a reactive current by means of the reactive power target value that is additionally modified using the deviation of the actual voltage from the target voltage value Utarget multiplied by a factor kIb 1 . It is then a mixed reactive power-voltage control.