Abstract:
A wind turbine having a wind rotor, a generator powered therewith for generating electrical energy, and a connecting line for delivering the electrical energy, optionally via a system transformer. The wind turbine includes a voltage expander that expands the voltage range of the wind turbine by means of an auxiliary voltage source. The voltage expander comprises a small transformer of having a primary and a secondary winding, and a switching mechanism. The small transformer is looped into the connecting line with the secondary winding, and the switching mechanism is connected to the primary winding of the small transformer, actuating the primary winding in a switchable multi-stage manner. Thus, a multi-stage expansion of the voltage range of the wind turbine is achieved, wherein due to the arrangement of the small transformer lengthwise in the connecting line, the small transformer can have a fraction of the nominal power of the wind turbine.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage application under 35 USC 371 of International Application No. PCT/EP2014/055009, filed Mar. 13, 2014, which claims priority to German Application No. 10 2013 206 241.9, filed Apr. 9, 2013, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a wind turbine comprising a wind rotor, a generator driven thereby for generating electrical energy and a connecting line for outputting the electrical energy. 
       BACKGROUND OF THE INVENTION 
       [0003]    As they are becoming increasingly widespread and at the same time their performance is increasing, wind farms are preferably erected at locations at which there good wind conditions, in particular on coastlines, at sea or in mountainous regions. In many cases, these are areas with only insufficient infrastructure, which applies in particular to wind farms at sea (so-called offshore wind farms), at which there is generally no infrastructure at all yet. This necessitates comparatively long line paths to consumers or to reach high-capacity distribution networks. Therefore, there are often voltage deviations in the case of wind farms with relatively high power feeds. The grid operators have responded to this and demand a greater voltage strength with respect to deviations from the rated voltage to an increasing degree from the wind turbines, wherein this voltage strength should also be provided in the case of operation with a power factor deviating from cos phi=1. 
         [0004]    In order to extend the voltage range of the wind turbines, it is known to provide said wind turbines with a tap-changing transformer. In this case, the transformation ratio of the transformer can be changed by means of a switch so that the wind turbine can be matched in this way to different voltages on the upstream grid. Therefore, although a comparatively large voltage range can be covered, the switching times of these on-load tap changers are comparatively long (up to in the minutes range), which is considered to be too slow. In addition, the on-load tap changers are susceptible to wear. In order to avoid this problem, in principle a solution based on semiconductor elements can be provided. However, this has the disadvantage that the semiconductor elements need to be dimensioned so as to be markedly large and are therefore elaborate since the total power is transmitted via said semiconductor elements. For modern powerful wind turbines in the megawatts range, in practice this is barely possible. 
       SUMMARY OF THE INVENTION 
       [0005]    On the basis of the mentioned prior art with the on-load tap changers, an object of the invention is to expand the voltage range of a wind turbine, wherein quicker switching times are intended to be achieved with little complexity. 
         [0006]    This can be achieved by the features as broadly described below. Advantageous developments are described in the detailed embodiments below. 
         [0007]    In a wind turbine comprising a wind rotor, a generator driven thereby for generating electrical energy and a connecting line for outputting the electrical energy, to be precise possibly via a turbine transformer, a voltage expander is provided in accordance with the invention which expands the voltage range of the wind turbine by means of an additional voltage source, wherein the voltage expander comprises a dedicated small transformer having a primary and a secondary winding and a switching mechanism, and wherein the small transformer is looped in with the secondary winding in the connecting line, and the switching mechanism is connected to the primary winding of the small transformer and actuates said primary winding with the capacity for switchover in multi-tap fashion. 
         [0008]    The invention is based on the concept of bringing about an expansion of the voltage range in which the wind turbine can be operated by means of a voltage expander consisting of an additional small transformer with a switching mechanism. By means of the switching mechanism, the small transformer is in this case connected in multi-tap fashion, wherein, depending on the tap changing, a more or less large or directionally different voltage shift is achieved. Since the small transformer in accordance with the invention is looped into the connecting line between the wind turbine and the grid, it can in this way increase the voltage of the wind turbine by virtue of it inducing an additional voltage with the same mathematical sign as the voltage output by the wind turbine or it can decrease the voltage of the wind turbine by virtue of it inducing a voltage with the opposite mathematical sign to the wind turbine. 
         [0009]    A particular highlight of the invention consists in that, thanks to its arrangement longitudinally in the connecting line, a transformer whose power can be dimensioned so as to be markedly less than the power of the wind turbine is sufficient. The transformer is therefore referred to as “small transformer”. The rated size of this small transformer is merely dependent on the desired voltage step of the voltage expander. If, for example, an expansion of 10% in relation to the rated value is sufficient, the small transformer also only needs to have a power of the order of approximately 10% of the rated power of the wind turbine. With the arrangement according to the invention, therefore, not only a multi-tap and therefore matched expansion of the voltage range of the wind turbine results but this is also successful in an astonishingly undemanding manner, namely using a voltage expander whose small transformer only needs to have a fraction of the rated power of the wind turbine. Thus, a considerable expansion of the voltage range of the wind turbine can be achieved with very little additional complexity. 
         [0010]    Some terminology will be explained below: 
         [0011]    A small transformer is understood to mean an additional transformer with an internal power supply which is looped into the connecting line between the wind turbine and the turbine transformer thereof, on the one hand, and a grid connection point in the wind turbine, on the other hand. 
         [0012]    The grid connection point may be a connection point of the wind turbine to a farm-internal grid or a transfer point to a superordinate energy transmission grid. 
         [0013]    Multi-tap is understood to mean both unipolar and bipolar tap changing. This means that multi-tap can include both tap changing in the sense of 1, 2, 3, . . . and tap changing in the sense of −1, 0, +1. 
         [0014]    It is particularly preferred if the multi-tap actuation takes place with different polarization. This provides the possibility of expanding not only the voltage range of the wind turbine by means of the voltage expander upwards to relatively high voltages, but also downwards to relatively low voltages. Expediently, a zero tap is also provided. 
         [0015]    In order to achieve a relatively large adjustment range for the voltage expander, preferably a plurality of small transformers are arranged in series in the connecting line. Therefore, corresponding to the number of small transformers, a relatively large number of taps can be provided, with the result that matching of the voltage range by means of the voltage expander can take place over a relatively large range, on the one hand, but also finer graduations and therefore matching to the respective conditions are possible, on the other hand. In this case, the small transformers can each be configured identically, with the result that they each generate the same additional voltage. Then, only one type of small transformers and possibly switching mechanisms is required, which simplifies the storage and production. In order that said small transformers can be switched randomly, in each case a dedicated switching mechanism is preferably provided for the small transformers. Particularly advantageously, the switching mechanisms are embodied in such a way that they have power contactors and/or thyristors as switching elements. Robust switching can thus be achieved which is also quick at the same time, preferably within a switching time range of at most 0.1 s. However, the possibility of the small transformers being configured with different sizes so that they generate different levels of voltages should not be ruled out. In this case, an even greater voltage range can be covered by a combination; for example, in the case of the arrangement of three small transformers in the sense of 1:2:4, eight different voltage steps of 0-7 can be provided for the voltage expander. 
         [0016]    A small transformer in accordance with the present invention is characterized by the fact that it is merely dimensioned for a power which is much lower than the rated power of the wind turbine. For the reasons already mentioned, this is entirely sufficient for the voltage expander to be able to achieve a corresponding extension of the voltage range of the wind turbine. High costs or unnecessary requirements in respect of installation space owing to overdimensioning are thus avoided. Preferably, the small transformer has a power which is at most ¼ of the rated power of the wind turbine. With such a small design for the small transformer in accordance with the invention, not only is a considerable cost saving made, but also the requirement for installation space and cooling of the small transformer is thus minimized. The voltage expander according to the invention therefore manages with a minimum of additional complexity, to be precise both in respect of costs and requirements in terms of installation space. Therefore, the voltage expander according to the invention is particularly also suitable for use in retrofitting existing wind turbines or wind farms, and this is of significant importance in respect of the numerous wind farms already in existence. In the invention, a design for the power of the small transformer depending on the additional voltage to be provided by the voltage expander has proven successful. Preferably, the power of the small transformer is dimensioned proportionally to the additional voltage, wherein the additional voltage is based on the rated voltage. Thus, for example, in order to achieve an additional voltage of the order of one tenth of the rated voltage, expediently the power of the small transformer is designed for at most a tenth of the rated power of the wind turbine. This means that, for a wind turbine with a rated power of 3 MW, only one transformer which is designed for no more than a power of 0.3 MW is required. 
         [0017]    Expediently, the voltage expander is designed in such a way that it and its small transformer are disconnectable from the connecting line via switching contactors. It is therefore possible to achieve a situation whereby when the voltage expander is not required at voltages in the region around the rated voltage during normal operation, the voltage expander is inactive and therefore losses are reduced, in particular transmission losses owing to the small transformer. Although these losses are in any case only low owing to the already small dimensioning of the small transformer in accordance with the invention, an avoidance of these low losses also means an increase in efficiency of the wind turbine. 
         [0018]    In order to avoid, in the case of multi-tap actuation of the small transformer of the voltage expander in accordance with the invention, a situation in which the small transformer becomes load-free during switchover, whether this be only instantaneously, a separate load tap is preferably provided. It ensures that, even during switchover, a load is always connected to the small transformer. Therefore, the risk of impermissibly high voltage peaks during switchover is counteracted. 
         [0019]    The voltage expander is expediently configured in such a way that the small transformer is installed in such a way that it is combined with the turbine transformer. Preferably, in this case the small transformer is still in the form of a dedicated (possibly replaceable) component, whose electrical connection is looped into the turbine transformer. Particularly preferred is a design in which the additional voltage source formed by the small transformer is connected to the turbine transformer on the high-voltage side. Preferably, this takes place at the medium-voltage level, to be precise in such a way that the small transformer is connected within a medium-voltage winding of the turbine transformer. As an alternative, provision can also be made for the small transformer to be connected to a neutral point of the turbine transformer on the medium-voltage side. Both embodiments provide the advantage that the small transformer is combined with the medium-voltage side. This not only saves on installation space, but also means effective short-circuit protection of the small transformer without any further structural measures since the short-circuit protection is already effected by the medium-voltage winding of the turbine transformer. 
         [0020]    A preferred embodiment of the small transformer is a high-leakage-reactance transformer, which has a short-circuit voltage uk of at least 0.10, preferably of at least 0.15. In this case, it is advantageous if the primary winding of the small transformer is wound over the secondary winding. It has proven to be particularly successful to provide the high-leakage-reactance transformer with pancake coils, which are preferably arranged on remote limbs of a transformer core in particular of the turbine transformer and/or are in the form of webs in the transformer core. Preferably, magnetic steel sheets with a hard saturation characteristic are used for the core. In addition, the coils are expediently arranged in such a way that they do not form an air-core transformer. Undesired magnetic coupling is thus avoided. 
         [0021]    Expediently provision is made for the switching mechanism of the voltage expander to be actuated depending on the voltage and/or wattless power. It has proven to be particularly successful if the switching mechanism is actuated depending on both parameters. Preferably a regulator for the voltage and/or wattless power is provided for this purpose, which regulator detects, as controlled variable, the voltage and/or the wattless power in the connecting line. Advantageously, this detection takes place on a side remote from the wind turbine, i.e. a grid-side side, in relation to the voltage expander according to the invention. Thus, a high degree of accuracy in the setting of the desired expanded voltage range is achieved. In order to avoid undesired interactions between the switching mechanism of the voltage expander according to the invention and existing voltage regulation devices, in particular of the wind turbine as such or of a wind farm, provision is expediently made for a switching state of the switching mechanism of the voltage expander to be applied to such an existing regulation, to be precise in particular to disturbance variable feedforward control. This means that the regulator is already preset in the case of the actuation of the switching mechanism to a thus changed voltage level and therefore no longer needs to be adjusted. The risk of the already existing regulator operating in opposition to the voltage range expansion by means of the voltage expander according to the invention is thus counteracted. 
         [0022]    In addition, the invention extends to a wind farm comprising a plurality of wind turbines, which are connected to a farm grid, which in turn is connected to a transmission grid via a coupling line, wherein in accordance with the invention, a central voltage expander is provided, which ensures the expansion of the voltage range of the wind farm as a whole by means of an additional voltage source and a switching mechanism, as described above. Expediently, a farm regulator for the voltage and/or wattless power is provided for this purpose, which farm regulator detects as controlled variable, the voltage or wattless power in the coupling line, and adjusts said voltage or wattless power by means of the central voltage expander. Preferably, this detection is performed on that side of the voltage expander which is remote from the farm, i.e. on the side toward the transmission grid. For a more detailed description and for the configuration of advantageous developments, reference is made to the above description, which applies accordingly. 
         [0023]    Preferably, a switching state of the switching mechanism of the central voltage expander is applied to disturbance variable feedforward control of the farm regulator. Therefore, a smooth switching behavior is achieved, with the result that already existing voltage regulation of the wind farm interacts in harmony with the central voltage expander according to the invention. 
         [0024]    Preferably, the wind farm has one or more wind turbines, which have a voltage expander, as described above. Thus, the voltage range and the adjustment range are further optimized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will be explained in more detail below with reference to the attached drawing and illustrated in the advantageous exemplary embodiments of the invention. In the drawing: 
           [0026]      FIG. 1  shows an exemplary embodiment of a wind turbine in accordance with the invention; 
           [0027]      FIG. 2  shows a circuit view for the wind turbine illustrated in  FIG. 1  with a voltage expander; 
           [0028]      FIG. 3  shows a detail illustration of a switching mechanism of the voltage expander; 
           [0029]      FIG. 4  shows a circuit in accordance with a second exemplary embodiment of the invention with a two-tap voltage expander; 
           [0030]      FIG. 5  shows an exemplary embodiment of a turbine transformer comprising a small transformer of the voltage expander in a combined design; 
           [0031]      FIG. 6  shows a further exemplary embodiment of the turbine transformer with another arrangement of the small transformer at the medium-voltage level; 
           [0032]      FIG. 7  shows a further exemplary embodiment with an arrangement of the small transformer on a low-voltage level of the turbine transformer; 
           [0033]      FIG. 8  shows a further exemplary embodiment with another arrangement of the small transformer on the low-voltage side of the turbine transformer; 
           [0034]      FIG. 9  shows a diagram showing the regulation structure in a wind farm comprising wind turbines in accordance with the invention; 
           [0035]      FIG. 10  shows details relating to the control for a facility as shown in  FIG. 9 ; 
           [0036]      FIG. 11  shows a characteristic curve graph for the operation of the wind turbine using the voltage modulator according to the invention; and 
           [0037]      FIG. 12  shows a detail illustration of a high-leakage-reactance transformer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The wind turbine illustrated in  FIG. 1  and provided overall with the reference numeral  1  comprises a nacelle  11  arranged on a tower  10  pivotably in the azimuthal direction at the upper end of the tower  10 . The nacelle  11  has a rotatably mounted wind rotor  12  having rotor blades  13  on one of the end sides of said nacelle. Said wind rotor drives, via a shaft (not illustrated), a generator  14  comprising a converter  15  for generating electrical energy, which is output via a line  17  with a turbine transformer  2  of the wind turbine to a farm-internal grid. The operation of the wind turbine  1  is monitored by a control system  8 , which is arranged in the nacelle  11 . Said control system is connected to a farm master  7  and/or to superordinate control devices (not illustrated), in particular the grid operator, via communications lines (not illustrated). 
         [0039]    The wind turbine  1  outputs the electrical energy on a low-voltage level, which is typically in the range of 600-1000 V. For the transmission, and this also applies already for the transmission over the farm-internal grid  9 , relatively high voltages are generally required, however, namely those in the medium-voltage range, for example 20 kV. 
         [0040]    For this purpose, the turbine transformer  2  is provided on or in the wind turbine  1 . The voltage on the farm-internal grid  9  can fluctuate, and the wind turbine  1  has to follow the voltage fluctuation correspondingly via its transformer  2 . Furthermore, it is necessary for the wind turbine to provide wattless power on request, whether this be inductive or capacitive wattless power. In principle, known wind turbines  1  can provide this, but not always over the entire required voltage range. In order to expand the voltage range which is useable for the wind turbine  1 , a voltage expander  3  is looped into the connecting line  17  connecting the wind turbine  1  to the grid  9  in accordance with the invention. In the exemplary embodiment illustrated in  FIG. 1 , said voltage expander is located on the medium-voltage side of the transformer  2 , i.e. between the transformer  2  and the grid  9 . However, it could just as well also be arranged on the low-voltage side of the transformer  2 , i.e. between the wind turbine  1  and the transformer  2 . The voltage range of the expander  3  serves the purpose of expanding the voltage range of the wind turbine  1  on the grid  9  in order thus to also meet more stringent requirements from the grid operators in respect of the ability of the wind turbines to withstand voltage fluctuations whilst at the same time provide wattless power. 
         [0041]    The voltage expander  3  comprises a small transformer  30  having a primary winding  31  and a secondary winding  32  and a switching mechanism  33 . The secondary winding  32  is looped into the connecting line  17  between the wind turbine  1  and the grid  9 , to be precise on the grid side, i.e. on the medium-voltage level, in the exemplary embodiment illustrated in  FIGS. 1 and 2 , as already mentioned above. The primary winding  31  is connected to the switching mechanism  33 , and electrical energy is supplied to said primary winding by the switching mechanism, which electrical energy is supplied to the switching mechanism  33  via a connection  34 . The power supply to the connection  34  can in turn take place from the grid  9 , from the wind turbine  1  itself or else from any other desired source, wherein this source then needs to be matched with respect to its frequency and phase angle with respect to the voltage on the grid. 
         [0042]    The design and mode of operation of the switching mechanism will be explained in more detail with reference to the exemplary embodiment illustrated in  FIG. 3 . Said figure shows a switching mechanism  33  which switches using three taps, namely the switching taps MINUS, ZERO and PLUS. At the switching tap PLUS, an additional voltage for the small transformer  3  is connected by the voltage expander  3  so that the total voltage of the wind turbine  1  is increased. Conversely, in the MINUS position, the voltage output by the wind turbine  1  is reduced by a corresponding magnitude. In the ZERO setting, there is no change. The magnitude of the voltage U 2  additionally applied by the small transformer  30  is determined by the transformation ratio between the primary coil  31  and the secondary coil  32 . A voltage U 1  is applied to the primary coil  31  by the switching mechanism  33 , which in turn draws its electrical energy from a supply transformer  4  which is connected to its supply connection  34 , with a voltage U 3  being fed in turn to said supply transformer  4  by the wind turbine  1  itself via the supply line  17  (the feed is not illustrated in  FIG. 3  for reasons of clarity). 
         [0043]    In order to actuate the primary winding  31  at the three different taps MINUS, ZERO and PLUS, pairs of switches  35 ,  36  and  35 ′,  36 ′ and  37  are provided. For the tap PLUS, the pair of switches  35 ,  36  turns on, while the pair of switches  35 ′,  36 ′ and the switch  37  are open. In this case, the voltage U 3  applied to the supply connection  34  is output from the switching mechanism  33  to the primary winding  31  with the same polarity. Depending on the transformation ratio between the primary winding  31  and the secondary winding  32 , the small transformer  30  feeds an additional voltage U 2  into the connecting line  17  correspondingly. At the tap for MINUS, the pair of switches  35 ′,  36 ′ is closed, while the pair of switches  35 ,  36  and the switch  37  are open. In this case, the supply voltage U 3  applied unchanged to the supply connection  34  is output to the primary winding  31  with the reverse polarity (−U 1 ). Accordingly, the small transformer feeds a voltage −U 2  into the connecting line  17  at the secondary winding  32  of said small transformer. In the ZERO position, the switch  37  is closed, while the pairs of switches  35 ,  36  and  35 ′,  36 ′ are open. The primary winding  31  is thus short-circuited via the switch  37 . This means that the switching mechanism  33  does not apply any voltage to the primary winding  31 . Accordingly, the small transformer  30  also does not feed any voltage additionally to the supply line  17 . Thus, depending on the position of the switches in the switching mechanism  33  and the small transformer  30 , either a positive voltage +U 2 , a negative voltage −U 2  or even no voltage (U=0) is fed into the connecting line  17  of the wind turbine additionally from the small transformer  30 . Correspondingly, depending on the switch position, the voltage range can be extended, decreased or kept constant. 
         [0044]    In order to ensure that the primary winding  31  is always provided with a load even during switchover of the taps, a load tap  5  is actuated. The load tap  5  comprises an actuating switch  50  and a load resistor  51 . During switchover of the taps, before the switch pairs  35 ,  36  and  35 ′,  36 ′ or the switch  37  are actuated, the actuating switch  50  is closed and therefore the resistor  51  is connected to the primary winding  31  as load. The pairs of switches  35 ,  36  and  35 ′,  36 ′ or the switch  37  can then be actuated without the primary winding  31  becoming load-free or the circuit thereof being interrupted. Once the new switching state of the switching mechanism  33  has been reached, the actuating switch  50  is opened again and therefore the load resistor  51  is decoupled. 
         [0045]      FIG. 4  shows a further exemplary embodiment which differs substantially from that illustrated in  FIG. 3  in that a small transformer  30 ′ is provided with a second pair of primary and secondary coils  31 ′,  32 ′. The switching mechanism  33 ′ is likewise modified correspondingly. Thus, the additional voltage added by the voltage expander  3  can be varied in 5 steps. This enables a relatively coarse or relatively fine adjustment. 
         [0046]    In the exemplary embodiments previously illustrated, the small transformer  30 ,  30 ′ of the voltage expander is in the form of a separate element. However, this is not necessary. It may be expedient to integrate the small transformer in the turbine transformer  2  of the wind turbine  1 . A first exemplary embodiment of this is illustrated in  FIG. 5  using a three-phase diagram. The transformer  2  has the switching group Dyn5. The primary windings  21  to which the wind turbine  1  is connected are arranged in the left-hand half of the diagram. The secondary windings are arranged in the center or in the right-hand region, wherein the secondary windings are formed in two parts, namely the two parts  22 ,  22 ′. In each phase, two secondary coils  32 ,  32 ′ of the small transformer  30 ′ of the voltage expander are arranged in the region between the two parts  22 ,  22 ′ of the secondary windings. This results not only in a more compact design, but at the same time also a higher level of intrinsic safety of the small transformer is achieved. This is because short-circuit currents in the case of external faults are limited by the impedances of the secondary windings  22 ,  22 ′ present on both sides. 
         [0047]    A further alternative exemplary embodiment is illustrated in  FIG. 6  using a transformer in the switching group YNd7yn0. The transformer has a primary winding  21  for each phase on the low-voltage side illustrated on the left-hand side. On the medium-voltage side illustrated on the right in the center, the transformer likewise has a secondary winding  22  for each phase, which secondary windings are combined via a neutral point  24 . In each phase, two secondary coils  32 ,  32 ′ of the small transformer  30 ′ are arranged between the secondary coils  22  and the neutral point  24 . In addition, the transformer has a compensating winding  25 . In the same way as in the exemplary embodiment illustrated in  FIG. 5 , in this exemplary embodiment integration of the secondary coils  22 ,  22 ′ of the small transformer  30 ′ in the turbine transformer  2  is achieved. Short-circuit protection for the small transformer thanks to the impedances of the turbine transformer  2  is likewise provided. 
         [0048]      FIG. 7  shows an alternative circuit concept, again using the example of a transformer of the switching group Dyn5. In this case, the small transformer  30 ′ is connected on the low-voltage side (illustrated on the left-hand side in the figure). The transformer has primary windings  21  on the low-voltage side and secondary windings  22  on the medium-voltage side. In each phase, two secondary windings  32 ,  32 ′ of the small transformer  30 ′ are arranged between the primary windings  21  and the wind turbine  1  feeding said primary windings. This exemplary embodiment comprising a two-winding transformer having a primary winding and a secondary winding for each phase can be extended to give a three-winding transformer having two primary windings and one secondary winding for each phase. Such an exemplary embodiment comprising a transformer of the switching group Dyn5yn5 is illustrated in  FIG. 8 . In this case, two sets of primary windings  21 ,  23  are provided, which are at different voltage levels of 660V and 950V, for example. This makes it possible to operate the stator of the generator  14  of the wind turbine  1 , which stator transmits a large portion of the power, at a higher voltage level than the rotor of the generator  14 , which transmits only a relatively small proportion of the power. Since a dedicated set of secondary coils  32 ,  32 ′ and  34 ,  34 ′ of the small transformer  30 ′ are provided both for the primary winding  21  and for the additional primary winding  23 , in this embodiment the expansion of the voltage range in accordance with the invention can therefore also be achieved. 
         [0049]    The interaction of the voltage expander  3 , on the one hand, and voltage regulation in a wind farm comprising a plurality of wind turbines  1 , on the other hand, is illustrated in  FIG. 9 . The figure shows, on the left-hand side, the wind turbine  1 , which outputs electric power via its turbine transformer  2  with the voltage expander  3  arranged thereon onto a farm-internal grid  9 , to which further wind turbines (not illustrated in  FIG. 9 ) are connected. The farm-internal grid  9  is furthermore connected to a high-voltage transformer  2 * for outputting electric power to a wide-area transmission grid  99 . A voltage expander  3 * is likewise provided for the high-voltage transformer  2 *. 
         [0050]    Voltage regulation which is part of the control system  8  of the wind turbine can be provided for each of the wind turbines. This voltage regulation comprises an input  80  for actual values of the voltage and an input  81  for corresponding setpoint values. The actual values are detected by means of sensors  83  for voltage and current. They are arranged on the low-voltage side of the turbine transformer  2  in the exemplary embodiment illustrated. As an alternative, provision can also be made, however, for said sensors to be arranged on the medium-voltage side of the turbine transformer  2  as sensors  83 ′. 
         [0051]    In addition, the wind farm has voltage regulation for the entire wind farm in the farm master  7  of said wind farm. This voltage regulation likewise has two inputs, one input  70  for actual values and one input  71  for setpoint values. The actual values for voltage and current in the wind farm are detected by means of sensors for voltage and current  73 , which are arranged on the medium-voltage side of the high-voltage transformer  2 *. As an alternative, provision can also be made for sensors  73 ′ to be arranged on the high-voltage side of the high-voltage transformer  2 * instead. 
         [0052]    In the event of fluctuations in the voltage on the transmission grid  99  or on the farm-internal grid  9 , corresponding fluctuations on the respective other side of the transformers  2 ,  2 * result. Since a voltage expander  3  is arranged on the turbine transformer  2  and a further, central voltage expander  3 * is arranged on the high-voltage transformer  2 * as well, the voltage on the respective primary side of the transformers  2 ,  2 * additionally also changes when the voltage expander  3  connected to the turbine transformer  2  or the central voltage expander  3 * connected to the farm transformer  2 * are actuated. This is because in this case the voltage additionally applied by said voltage expanders is changed stepwise. This results in sudden changes in the case of the sensors  73 ,  83  arranged on the primary side. These changes would be identified by the voltage regulation  7 ,  8  and attempts would be made to correct said changes. However, this is undesirable since the connection of the voltage took place by means of the voltage expanders  3 ,  3 * in order to expand the voltage range. This should not be counteracted by a corresponding counter measure formed by the integrated voltage regulation  7  of the wind turbine or the wind farm. 
         [0053]    In order to avoid this, expediently feedforward control is implemented, to be precise in the form of a disturbance variable compensation unit  6 . Signals for the actual values and for the setpoint values are applied both to the disturbance variable compensation unit  6  and to the voltage regulation  8  to corresponding inputs  60 ,  61 . From these signals, a correction value is determined by means of the disturbance variable compensation unit  6 , said correction value being applied to an input for disturbance variable compensation  86  of the regulator  8 . The disturbance variable compensation unit  6  preferably has a differentiating element  62 , which determines a voltage deviation between the actual voltage at the input  60  and the setpoint voltage at the input  61 . The resultant differential voltage value ΔU is applied to a characteristic element  63 , which determines a value for a reactive current to be set depending on a provided operating range  18  of the wind turbine depending on the voltage deviation. This value is output by the characteristic module  63  and applied to the input for disturbance variable compensation  86  of the voltage regulator  8  of the wind turbine  1 . A corresponding circuit can be provided for the voltage regulator  7  of the farm master. 
         [0054]    The effect of the voltage expander according to the invention is illustrated in  FIG. 11 . Said figure shows an example of a requirement spectrum defined by a grid operator, namely for the output of for what capacitive or inductive wattless power in what voltage range the wind turbine needs to be competent (illustrated by a thick dashed line  98 ). The wattless power is plotted on the x axis and the voltage range required for the respective wattless power is plotted on the y axis. In comparison with this requirement by the grid operator, an operating range  18  of the wind turbine  1  in accordance with the exemplary embodiment of the invention is illustrated (with a continuous line). It can be seen that the operating range  18  bounded by the continuous line does not completely cover the requirement range defined by the dashed line  98 . This means that the wind turbine  1  is insufficient in its initial form for meeting the requirements as are established by the dashed region  98 . Thus, the low-voltage range in the left-hand lower quadrant cannot be sufficiently adhered to and in addition the high-voltage range is not sufficiently covered both for capacitive and for inductive wattless power. 
         [0055]    If the wind turbine  1  is operated in the left-hand lower quadrant, for example, i.e. at a low voltage and at the requirement for capacitive wattless power, the voltage expander  3  is connected by means of the MINUS tap-changing operation. Thus, the voltage of the wind turbine is reduced by the (for this case negative) voltage U 2  applied by the voltage expander, as a result of which the lower voltage limit is shifted downwards by the hatched region. It can be seen that the wind turbine  1  can therefore maintain the required low-voltage range. 
         [0056]    This likewise applies to the capacitive range in the high-voltage range (see left-hand upper quadrant). In this case too, the wind turbine  1  cannot generate the required overvoltage in the case of a request for capacitive wattless power (the dashed line in accordance with the requirements  98  is above the native operating range  18  of the wind turbine  1 ). In accordance with the invention, if required the voltage expander  3  is switched by means of the switching mechanism  33  into the PLUS position in this quadrant, with the result that the additional voltage U 2  (in this case positive), is added. The voltage range therefore shifts correspondingly upwards, as is demonstrated by the hatched region in the left-hand upper quadrant. 
         [0057]    A corresponding procedure is followed in the right-hand upper quadrant. It can be seen, however, that the addition of a tap, as is illustrated by the region with angled hatching, is insufficient for meeting the requirements illustrated by the thick dashed line  98 . In this case, the multi-tap nature in accordance with the invention comes into play in the sense that additional taps are enabled. In this case, the additional voltage increases correspondingly, with the result that the voltage range of the wind turbine is expanded upwards to such an extent that the requirements are finally met (see the cross-hatched region) by virtue of the connection of two or, in the case of a greater requirement for inductive wattless power, even by the addition of three taps. 
         [0058]      FIG. 12  shows a detailed illustration of a high-leakage-reactance transformer as a possible embodiment of the small transformer  30 . The high-leakage-reactance transformer comprises a core  39  composed of magnetic steel sheets with a hard saturation characteristic, with first a secondary winding  32  being wound onto said core, and in turn then a primary winding  31  being wound onto said secondary winding. The magnetization profile resulting in the process and the energy stored in the magnetic field between the secondary coil  32  and the primary coil  31  are illustrated schematically in the graph in the lower half of the figure. Therefore, current limitation is effected in the event of a short circuit, to be precise in particular in respect of the switching mechanism. It goes without saying that this can also be achieved with other designs.