Abstract:
A wind turbine comprises a wind rotor, a generator driven by the wind rotor, a converter, wherein the generator and the converter generate electrical energy output via a connecting line with an inductively acting line reactor to a grid, and an overvoltage protection device comprising a plurality of different active modules, which are designed in such a way that they each effect, in different ways, a reduction in the voltage at the output of the converter, a switching matrix, which connects and disconnects the different active modules, and a selector comprising an overvoltage classifier, which is designed to select a predetermined stage depending on the overvoltage and to actuate the switching matrix in such a way that successive ones of the active modules are disconnected, wherein the overvoltage classifier defines a plurality of overvoltage ranges by virtue of in each case the selector setting different switching groups.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to German application no. 10 2014 209 332.5, filed May 16, 2014, the entire contents of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    Wind turbine comprising a wind rotor, a generator which is driven by the wind rotor, and a converter for generating electrical energy, which is output via a connecting line with an inductively acting line reactor to a grid, wherein an overvoltage protection device is provided. 
       BACKGROUND OF THE INVENTION 
       [0003]    As wind turbines become increasingly widespread, increased demands are also placed on their behavior on the grid. This also applies to the behavior of the wind turbine in the case of line faults, in particular in the case of the occurrence of overvoltages or peak voltages. Until now, wind turbines have often been disconnected in the event of the occurrence of peak voltages owing to intrinsic protection. However, such a behavior in respect of grid compatibility will no longer be fully accepted in future. It is already demanded that wind turbines tolerate overvoltages up to a value of 130% of the rated voltage. In future, increased demands are also to be expected, at least in some countries. 
         [0004]    Conventional wind turbines do not have sufficient reserves for correspondingly increased demands in respect of the overvoltage. Although it is possible in principle to give the current-conducting components and in particular the converter higher power ratings since in this case excess powers of approximately 25% need to be taken into consideration, this results in considerable increases in respect of physical size and installation costs of the converter. Moreover, such an enlargement of the converter often makes a new design of the wind turbine and its electrical system as a whole necessary. The complexity is thus further increased. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of the invention is to provide an improved overvoltage protection device which can withstand higher overvoltages in a more efficient manner. 
         [0006]    In the case of a wind turbine comprising a wind rotor, a generator which is driven by the wind rotor, and a converter, wherein the generator and the converter are designed to generate electrical energy, which is output via a connecting line with an inductively acting line reactor to a grid, wherein an overvoltage protection device is provided, in accordance with the invention the overvoltage protection device is designed in such a way that it comprises a plurality of different active modules, which are designed in such a way that they each effect, in different ways, a reduction in the voltage at the output of the converter, a switching matrix, which connects and disconnects the different active modules in the overvoltage range, and a selector comprising an overvoltage classifier, which is designed to determine an overvoltage range depending on the overvoltage and selects a predetermined stage for this purpose in order to actuate the switching matrix in such a way that successive ones of the active modules are disconnected, wherein the overvoltage classifier defines a plurality of overvoltage ranges by virtue of in each case the selector setting different switching groups. 
         [0007]    Overvoltage ranges are understood to mean voltage ranges above the rated voltage which extend over in each case predetermined sections. Thus, for example, a first overvoltage range can be between 116 and 130% of the mains voltage, a second overvoltage range can be between 130 and 140%, and finally a third overvoltage range can be between 145 and 170%. 
         [0008]    Active modules are understood to mean modular units which each per se effect an increase in the overvoltage strength of the wind turbine. 
         [0009]    By virtue of selective switching of different active modules, the invention makes it possible for different approaches for increasing the overvoltage strength to be combined with one another. As a result, a combinatorial effect occurs, which makes it possible to manage even increased demands on the overvoltage strength in the case of voltage peaks, up to, for example, 170% of the mains voltage, on this basis without the converter needing to have a higher power rating for this purpose. Therefore, the previously used converters can still be used. As a result, considerable complexity savings are made. The invention can achieve this by the innovative interaction of different measures, some of which are known per se. No example can be found of this in the prior art. 
         [0010]    The invention can therefore not only achieve a high degree of overvoltage tolerance, but also makes it possible for the essential core components of the wind turbine, in particular the converter thereof, to be left unchanged. Furthermore, it enables a graduated response depending on the severity of the overvoltage situation (overvoltage range). Moreover, the behavior of the wind turbine with respect to the grid is transparent, i.e. from the point of view of the grid the wind turbine has a behavior which is completely normal as though it has the required voltage strength owing to a converter with a higher rating. Overall, therefore, much more effective protection of the wind turbine from overvoltages results, in comparison with the previously either insufficient or very expensive measures, such as a considerable overrating of the converter. 
         [0011]    Preferably, the selector is designed in such a way that at least two, preferably three, different overvoltage stages are switchable. Therefore, the already previously used overvoltage range of 116% (end of the tolerance band around the rated voltage) up to 130% can be provided as the first overvoltage range. This is followed by an extended overvoltage range of 130% to 145%. This is in turn followed by a survival range in which the intrinsic protection of the wind turbine has priority, up to approximately 170%. Thus, the demands of practically all relevant countries are covered and sufficient intrinsic safety of the wind turbine is also achieved, in the case of still minimal additional complexity. Expediently, the selector and the switching matrix are combined. This enables a particularly compact design. One example of this is an embodiment as an on-load tap changer. 
         [0012]    Advantageously, one of the active modules is in the form of a reduction element for reducing the output active power of the wind turbine, to be precise only after at least one second overvoltage range. This means that, in the case of relatively high overvoltages (in the extended overvoltage range or even in the survival range), the active power output by the wind turbine is reduced. The converter which has thus been relieved of the output of active power can then output correspondingly more reactive power. Preferably, this takes place gradually. For this purpose, expediently a current droop is implemented, wherein the output active current is reduced increasingly as the voltage increases. 
         [0013]    Another of the active modules is preferably designed to act as a variable limiter for a capacitive reactive-power output. A variable limiter is in this case understood to mean that a reliable upper limit for the capacitive reactive-power output is defined depending on the overvoltage range and is decreased as the voltage increases further. Preferably, the limiter has a limitation droop for this purpose. Thus, the output of capacitive reactive power can be reduced to zero (or another settable lower value) in the event of the occurrence of overvoltage. This measure has a voltage-reducing effect on the grid-side inverter. 
         [0014]    Advantageously, another of the active modules has an overcurrent controller, which sets an increased inductive reactive-power output, which is preferably above the rated reactive-power output. Thus, the active module makes use of the knowledge that a decrease in voltage can be achieved by the output of inductive reactive power. The extent achieved thereby is dependent on the dimensioning of the reactor and the transformer in the connecting line. Preferably, the reactor is dimensioned such that the current flow through said reactor is at most 1.6 times, preferably at most 1.1 times the rated current of the grid-side inverter at the upper end of the extended overvoltage range. In this case, in a second stage for the survival voltage range, the increased inductive reactive power can be dimensioned such that it is at least twice, preferably at least four times, the rated reactive-power output. 
         [0015]    One of the active modules is preferably in the form of a voltage droop for a setpoint voltage slide in a DC link of the converter. Therefore, the selector can, in the desired overvoltage range, methodically increase the DC-link voltage in the converter. Preferably, the increase is overproportional, i.e. when the increase is implemented, it is implemented to a greater extent than the increase in the mains voltage. Owing to the increase in the DC-link voltage, the voltage output to the grid by the grid-side inverter of the converter can likewise increase. The wind turbine can therefore be matched to the increased voltage on the grid. 
         [0016]    Preferably, one of the active modules is in the form of an overmodulation element, which increases the voltage output by the converter, more precisely the grid-side inverter thereof. By means of the overmodulation element, short-term overmodulation can be used to further increase the output voltage of the grid-side inverter, in particular in high voltage ranges (survival ranges), and therefore the output voltage can be matched to the increased mains voltage. Overmodulation degrees which can effect a voltage increase of at least 5%, preferably up to 7%, have proven successful for this purpose. Thus, firstly a considerable increase in the output voltage is achieved, and secondly an excessive increase in the total harmonic distortion and therefore an excessive current loading are still prevented. Preferably, the total harmonic distortion is set to a value of at most 20%. 
         [0017]    In order to control the overmodulation element, preferably a voltage droop is provided. Said voltage droop is preferably divided into a plurality of ranges. Therefore, a different behavior can be established depending on the overvoltage range. Advantageously, this already takes place in the first overvoltage range, to be precise to an overproportional extent. For this purpose, a characteristic gradient greater than one (overproportional) in the voltage droop is implemented. Preferably, it is part of a ramp function, which has a then increased constant value in another overvoltage range. 
         [0018]    Expediently, the reduction element also has a control output for activating a chopper at the DC link. The activation can take place directly or indirectly. “Indirectly” is understood to mean that the reduction element does not activate the chopper directly, but switching thresholds or limit values of the chopper control provided per se are modified and therefore the use behavior of said chopper is changed in a desirable manner. Therefore, the reduction element can additionally also at least temporarily dissipate power into the chopper in order thus to relieve the load on the DC link. The current loading of the grid-side inverter is thus further reduced and said inverter can correspondingly output more reactive current and therefore reactive power. 
         [0019]    Furthermore, provision can be made for a maximum voltage to be reduced, preferably in the case of a line frequency below a rated line frequency. Therefore, a more favorable magnetic rating of the transformer of the wind turbine is made possible. Furthermore, an interaction with the overvoltage classifier is preferably provided such that, at a reduced maximum voltage, the classification is performed with correspondingly reduced values. Therefore, the classification is tracked in respect of the reduced maximum voltage, and thus a finely graduated response is furthermore ensured. 
         [0020]    Preferably, the turbine transformer in the connecting line of the wind turbine is dimensioned such that it reaches magnetic saturation in the overvoltage range in the case of line frequencies below a rated line frequency. With a transformer with such a rating, said transformer behaves in the same way as an inductive reactor in the event of the occurrence of overvoltages as a result of additional saturation current, and therefore acts automatically, so to speak, so as to passively reduce the voltage. The transformer therefore intervenes in self-stabilizing fashion. Owing to its considerable inductance, therefore, a considerable contribution to the voltage reduction is achieved without any additional complexity. A certain disadvantage does consist in that the starting point for its voltage-reducing effect owing to the frequency dependence cannot be established very precisely, but this is not serious in view of the advantages achieved thereby. 
         [0021]    Further preferably, the turbine transformer has a reduced main-field voltage rating. Therefore, the turbine transformer can be given smaller dimensions so that ultimately it is even less expensive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will be explained in more detail below with reference to the attached drawing, in which an advantageous exemplary embodiment is illustrated and in which: 
           [0023]      FIG. 1  shows a wind farm as an overview comprising a wind turbine in accordance with one exemplary embodiment of the invention; 
           [0024]      FIG. 2  shows a block circuit diagram illustrating the wind turbine; 
           [0025]      FIG. 3  shows a detail view of a selector comprising an overvoltage classifier; 
           [0026]      FIG. 4  shows an example of overvoltage ranges; 
           [0027]      FIG. 5  shows switching stages of the selector for active modules; 
           [0028]      FIGS. 6   a  and  6   b  show graphs for an active module for active power reduction; 
           [0029]      FIG. 7  shows graphs for a limiter for capacitive reactive power; 
           [0030]      FIGS. 8   a  and  8   b  show graphs for an active module for feeding in additional reactive power; 
           [0031]      FIG. 9  shows graphs for an active module for increasing the DC-link voltage and the output voltage of a grid-side inverter; 
           [0032]      FIGS. 10   a  and  10   b  show graphs for the active module for overmodulation; 
           [0033]      FIGS. 11   a  and  11   b  show graphs for effective voltage characteristics without and with overmodulation; 
           [0034]      FIGS. 12   a  and  12   b  show graphs for an active voltage output to the grid by the converter and for current harmonics; 
           [0035]      FIGS. 13   a - c  show graphs showing the rating of a line reactor; and 
           [0036]      FIGS. 14   a  and  14   b  show graphs of the mains current and of the current output by the wind turbine for the overvoltage ranges. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    A wind farm illustrated in the exemplary embodiment shown in  FIG. 1  comprises a plurality of wind turbines  1 , which are connected in each case via a connecting line  19  comprising a turbine transformer  8  to a farm-internal busbar grid  90 . The busbar grid  90  is connected to a high-voltage energy transmission grid  99  via a farm transformer  92  by means of a main line  91 . The wind farm in its entirety is controlled by a farm master  9 . 
         [0038]    The wind turbine  1  in accordance with the exemplary embodiment of the invention comprises a tower  10 , with a nacelle  11  being arranged pivotably in the azimuthal direction on the upper end of said tower. A wind rotor  12  is mounted rotationally movably on one end side of said nacelle, said wind rotor driving a generator  2  for generating electrical energy via a rotor shaft (not illustrated). Said generator is connected to a converter  3  for outputting the generated electrical energy via a low-voltage connecting line  18 , which is connected to the turbine transformer  8  and comprises a line reactor  4 . The line reactor  4  is preferably in the form of an LCL filter. 
         [0039]    The block circuit diagram shown in  FIG. 2  visualizes the electrical function blocks of the wind turbine  1 . Electrical energy is generated by the generator  2  and fed into a DC link  31  via a grid-side inverter  30  of the converter  3 . The DC link  31  has a capacitor (not illustrated) as energy store. In addition, a chopper  32  is arranged on the DC link  31  in a manner known per se, wherein said chopper can be actuated optionally and can be used for reducing the electrical energy supplied in the DC link  31 . The voltage in the DC link  31  is denoted by U DC . In order to dissipate the energy stored in the DC link  31 , a grid-side inverter  33  is provided, to which the reactor  4  is connected. The voltage at the output of the grid-side inverter  33  of the converter  3  is denoted by U LSI , and the current flowing out of the grid-side inverter  33  into the line reactor  4  is denoted by I′ LSI . 
         [0040]    The line reactor  4  is in the form of an LCL line filter and comprises two inductances, a converter-side inductance  41  and a grid-side inductance  42 . A filter capacitor  43  is connected between said inductances. A low-voltage switch disconnector  40  is arranged at the output of the line reactor  4 . The current flowing through said low-voltage switch disconnector in the closed state is denoted by I LSI . On the grid side upstream thereof is a connection point  44 , which is used for supplying power to the electrical components of the wind turbine  1  including the controller  14  thereof (so-called auxiliaries service). On the grid side of this connection point  44 , the machine transformer  8  of the wind turbine  1  is connected. The current flowing through a low-voltage connecting line  18  between the connection point  44  for the auxiliaries service and the machine transformer  8  is denoted by I LV , and the voltage prevailing there is denoted by U LV . This is the voltage on the low-voltage side of the machine transformer  8 . A medium-voltage switch  80  is arranged on the medium-voltage side of said machine transformer, with the connecting line  19  leading from said medium-voltage switch to the farm-internal busbar grid  90 . The current flowing through the connecting line  19  is denoted by I MV , and the voltage prevailing there is denoted by U MV . 
         [0041]    The controller  14  of the wind turbine comprises an overvoltage protection device  5 , which serves the purpose of protecting the wind turbine  1  from damagingly high voltages and the negative effects thereof in the case of voltage faults on the grid, in particular in the case of the occurrence of overvoltages. 
         [0042]    The overvoltage protection device  5  in accordance with the exemplary embodiment of the invention comprises a plurality of active modules  61 ,  62 ,  63 ,  64 ,  65 ,  66 , which contribute to a reduction in a voltage increase in in each case different ways. The active modules  61 - 66  are designed such that, in the case of a grid-side increase in the medium voltage U MV , and correspondingly also in the voltage U LV  present on the low-voltage connecting line  18 , a small voltage increase is set in the region of the converter  3  of the wind turbine, so that said converter is protected from excessively high voltages. Critical for this is the voltage at the output of the inverter  33 , i.e. U LSI . 
         [0043]    The overvoltage protection device  5  comprises a switching matrix  6 , to which the active modules  61 - 66  are linked. The switching matrix  6  connects or disconnects the different active modules depending on the switching state. For this purpose, apart from the switching matrix  6 , a selector  7  comprising an overvoltage classifier  70  is provided. The overvoltage classifier  70  is connected to a voltage sensor  17 , which measures the voltage on the connecting line  19  of the wind turbine  1  (a measurement on the low-voltage connecting line  18  is also possible). Values for overvoltage ranges I, II, III are set using the overvoltage classifier. The overvoltage classifier is designed to determine, depending on the voltage measured by the sensor  17 , whether there is an overvoltage, i.e. a voltage above the rated voltage of the grid plus a predetermined tolerance range. The following values relate to the medium voltage (in the case of an alternatively possible reference to the low voltage, the percentage limit values would be slightly different). In this case, the predetermined tolerance range is intended to be around a value of up to +16% of the mains voltage. Therefore, the overvoltage begins above a value of 116% of the rated voltage. The overvoltage classifier  70  now determines the stage of the selector  7  that it actuates, depending on whether the mains voltage is between 116 and 130% of the rated voltage, between 130 and 145% of the rated voltage or between 145 and 170% of the rated voltage. The selector now actuates switching groups  71 ,  72 ,  73 , which output corresponding connection or disconnection signals via the switching matrix  6  to the active modules  61 - 66 , depending on which of its stages have been actuated by the overvoltage classifier  70 . Therefore, it is possible to establish via the selector and the switching matrix  6  which of the active modules in which overvoltage range have been activated or deactivated. This can be set freely via the switching matrix  6 . In this case, the switching group is actuated in a first overvoltage range (between 116% and 130% of the rated voltage). This case is referred to below as overvoltage range I. For the case of an overvoltage in the range between 130 and 145%, the switching group  72  is actuated; this case is referred to as overvoltage range II. If the voltage is in the range between 145% and 170%, the switching group is actuated, and this case is referred to as overvoltage range III. The overvoltage ranges are illustrated in  FIG. 4  and the switching states for the active modules  61 - 66  which result in the respective overvoltage ranges taking into consideration the set switching groups  71 - 73  are illustrated in  FIG. 5 . 
         [0044]    The active modules will be described in more detail below in respect of their design and their mode of operation as well as their effect on the electrical parameters, in particular the voltage and currents. 
         [0045]    The active module  61  is in the form of a reduction element and is designed to reduce the active power output by the converter  3  in the case of a critical overvoltage (i.e. in the overvoltage range III). For this purpose, a voltage droop  61 ′ is implemented in the active module  61 , said voltage droop being in the form of a negative proportional control. This means that, above a settable threshold value, the active power output by the converter  3  is reduced as the voltage increases. This is dependent on the voltage of the medium-voltage level U MV . The proportional control in the voltage droop  61 ′ is set such that, when the upper end of the overvoltage range III is reached, the active power feed is reduced to zero in the case of a value of 170% of the rated voltage (see point X in  FIG. 6   a ). The reduction begins in the case of a voltage value U Z  which is greater than the lower end of the overvoltage range III (see point Y in  FIG. 6   a ). In this case, the voltage U Z  and point Y are selected such that, in the case of an extension of the characteristic implemented in the voltage droop  61 ′ downwards, i.e. towards the lower voltage, when the lower end in the overvoltage range III is reached (the voltage is then 145% of the rated voltage), an active power value is reached which corresponds to the dynamic active-power output, which can be provided by the converter  3  for a short period of time (see point Z). This connection means that the reduction in the active-power feed which is not desired per se for the operation of the wind turbine  1  only takes place when the voltage is already at a critical level. Owing to the delayed onset only in the case of the voltage U Z  (point Y), a further switch-on delay is achieved. Nevertheless, by extending the characteristic (at point Z), safe support is also ensured in those situations in which the converter outputs dynamic (excess) active power owing to the operating situation. In order in this case to ensure sufficient dissipation of excess power out of the DC link  31 , the active module  61  preferably furthermore has a control output  61 ″ for activating another active module  66 , which activates the chopper  32  of the converter  3 . Then, in particular in the case of a high active power, in particular in the dynamic range (see range between points Z and Y in  FIG. 6   a ), the excess power can be effectively reduced. The scope of the reduction is illustrated in  FIG. 6   b . By virtue of this combination of the active module  61  with the control output  61 ″ for the chopper  32 , therefore, a dynamic overload strength is also achieved. 
         [0046]    The active module  62  is in the form of a limiter and is designed to act on the grid-side inverter  33  of the converter  3  in such a way that the overexcited reactive power thereof is reduced to zero in the overvoltage range I as the mains voltage increases. Again the reference point is the medium voltage U. For this, a corresponding reactive-power droop  62 ′ is implemented in the active module  62 . This is illustrated in  FIG. 7  by point B, which, at the upper end of the overvoltage range I, goes along with a reduction in the overexcited reactive power to zero. The reactive-power droop  62 ′ is, for this purpose, matched to the line reactor  4  in such a way that the voltage drop across the line reactor  4  is so great that still the entire, unreduced reactive power (see the value Q cap =1 in  FIG. 7 ) can be fed in at the lower end of the overvoltage range I (corresponding to the voltage of 116%) (see point A in  FIG. 7 ). However, provision can also be made for a smaller line reactor  4  to be used, which is dimensioned such that the entire feed of overexcited reactive power Q crp  is still possible beyond a voltage value of 116% (up to point A 2  in  FIG. 7 ) and only then does a reduction take place (see dashed line in  FIG. 7 ). The latter provides the advantage that, owing to the relatively small dimensions of the reactor  4 , the feed of overexcited reactive power can remain maintained for even longer. 
         [0047]    The active module  62  is preferably no longer active in overvoltage ranges II and III since the feed of overexcited reactive power has then already reached the value zero in any case (see point B in  FIG. 7 ). 
         [0048]    In addition, an active module  63  is provided as an inductive overcurrent controller, which likewise acts on the reactive power. It is designed to interact with the limiter of the active module  62  in such a way that the active module  63  is only used when the active module  62  has reduced the output of overexcited reactive power to the value zero. The active module  63  has two reactive-power droops  63 ′,  63 ″, one for the overvoltage range II and a further one for the overvoltage range III. Reference is made to  FIGS. 8   a  and  8   b . Owing to the interaction with the active module  62 , the active module  63  takes over at point B (cf.  FIGS. 7 and 8   a ). As the voltage U MV  increases, the reactive-power droop  63 ′ for the overvoltage range II is designed in such a way that visibly increases an inductive (underexcited) reactive power to be output. For the case of a large line reactor  4 , the reactive-power droop  63 ′ is embodied in such a way that, when the upper end of the overvoltage range II is reached (in the case of a voltage value of 145%), the output underexcited reactive power Q IND  precisely reaches the rated value for the underexcited reactive power (see point C in  FIG. 8   a ). Alternatively, provision can also be made for a line reactor  4  with a small inductance to be provided. In this case, a characteristic profile for the voltage droop results, as is characterized by the points B and C in  FIG. 8   a . Accordingly, when the upper end of the overvoltage range II is reached, an inductive reactive power of 1.6 times the rated value is set. It goes without saying that, for this purpose, the converter  3 , more precisely the grid-side inverter  33  thereof, needs to have a corresponding current loading rating. 
         [0049]    In the overvoltage range III, the active module  63  switches over to the voltage droop  63 ″. Reference is made to  FIG. 8   b . For the case of a large line reactor  4 , the reactive-power droop  63 ″ is designed in such a way that, when the upper end of the overvoltage range III is reached (when U MV =170% of the rated value), an underexcited reactive power of approximately 2.5 times the rated value is reached (see X-axis value −2.5 for Q IND ), visualized by point D in  FIG. 8   b . It is further illustrated how the reactive-power droop  63 ″ is designed for the alternative case of a small line reactor  4 , namely in such a way that it ends at point D 2 . It can be seen that a marked increase in the reactive-power output of the grid-side inverter  33  is therefore required since said inverter needs to be able to provide a reactive power of approximately 4.5 times the underexcited reactive-power rated value. It is therefore clear that the case of the selection of a small reactor  4 , which is favorable for the case of the low overvoltage ranges I, in the overvoltage value III goes along with a very high reactive-power loading of the inverter  33 . 
         [0050]    In order to enable the provision of sometimes very high power requirements, as described above, to the inverter  33 , the active module  64  is provided. It acts on the voltage in the DC link  31  of the converter  3 . The active module  64  has a setpoint voltage slide, which acts on a control element  36  for the voltage in the DC link. It is designed to shift the setpoint value for the voltage in the DC link to a higher value when the overvoltage range I is reached, in the exemplary embodiment illustrated from a normal value for the voltage in the DC link of 1100 V to an increased voltage value of 1150 V (see  FIG. 9 , graph at the top). For this purpose, the setpoint voltage slide comprises a ramp element  64 ′ which is designed in such a way that an overproportional increase in the setpoint value takes place. This is understood to mean that, in the case of an increase in the voltage which is critical for this at point  19  by 1%, the voltage in the DC link increases by more than 1%. 
         [0051]    By virtue of the increase in the voltage in the DC link, the voltage U LSI  output by the grid-side inverter  33  can be increased correspondingly. The increase achieved hereby reproduces that in the DC link  31 . The increase in the voltage U LSI  thus achieved is illustrated in the graph at the bottom in  FIG. 9 . It has proven successful when the increase performed in the overvoltage range I is maintained in the overvoltage range II and III but not continued any further. 
         [0052]    For further support, an active module  65  is provided. It acts on a pulse-width-modulation control element  34  of the grid-side inverter  33 , to be precise in such a way that the modulation  33  performed by the control element  34  is performed as overmodulation. Overmodulation up to approximately 105% of the maximum output voltage in the overvoltage range III (see the graph at the top in  FIG. 10 ) is provided. The effect of the overmodulation is illustrated in  FIGS. 11 and 12 . On the basis of the case with normal modulation (i.e. without overmodulation), the line-to-line voltages in the three-phase AC system of the inverter  33  are illustrated in  FIG. 11   a .  FIG. 11   b  shows the case with overmodulation. It can be seen that the areas beneath the curves (in each case based on the base line at zero) in the overmodulated case are greater than in the non-overmodulated case (cf.  FIG. 11   b  with  FIG. 11   a ). This represents an increase in the rms value of the respective voltage. However, the voltage generated during overmodulation is no longer sinusoidal, as illustrated in  FIG. 11   b . Therefore, harmonics occur. 
         [0053]    The desired increase in the rms value of the output voltage is plotted in  FIG. 12   a  over the degree of overmodulation, while  FIG. 12   b  shows the increase, which is not desired per se, in the harmonics in the case of respectively achieved (excessive) voltages. It can clearly be seen that in particular the fifth (continuous line) and the seventh (dashed line) harmonics increase considerably above a value of 836 V, which is reached in the case of overmodulation of 105% of the maximum output voltage. Thus, the overmodulation is preferably limited to a value of 1.05, corresponding to 105%. Therefore, a voltage increase by approximately 5% can be achieved (see  FIG. 12   a ). Owing to the known relationship, in accordance with which the power increases linearly with the voltage given a constant current, a power increase of over 5% can therefore be achieved by this overmodulation alone. By virtue of the interaction with the active module  64 , even a total voltage increase by 10% in two 5% stages results (see  FIG. 10 , bottom graph). That is to say that an additional power of over 10% can therefore be achieved. This represents a valuable support by the active module  65 , which requires considerable additional loading in the form of reactive power in the overvoltage range III. 
         [0054]    In addition, the active module  65  interacts with the active module  61  in such a way that said active module  61  feeds back the active power to be output in the overvoltage range. In this way, the current available from the inverter  33  can be used visibly more, up to exclusively, for the generation of the reactive power required by the active module  65 . Thus, a synergistic interaction of the active modules  61 ,  63  and  64  in the overvoltage ranges results. 
         [0055]    The active module  65  contains a modulation droop  65 ′. This modulation droop is designed in such a way that it determines an overmodulation degree depending on an overvoltage at the medium voltage U MW . In this case, the droop is in the form of a proportional control element. A characteristic is implemented in this which has a gradient of greater than 1, with the result that the control element is overproportional. Therefore, a rapid response can be achieved even at the beginning of the critical overvoltage. 
         [0056]    Furthermore, an active module  66  is provided which is in the form of an additional activation module for the chopper  32 . In this case, the chopper  32  is activated in the overvoltage range III. Therefore, the active power is reduced in comparison with the grid, while the generator  2  in the wind turbine can operate continuously and feeds the power generated thereby into the chopper. Therefore, relieving of load on the generator  2  can be avoided. Preferably, the module  66  interacts with the active module  61 , to be precise in such a way that the chopper  32  is connected by the active module  66  in particular in the event of the occurrence of additional dynamic active power in the overvoltage range. This is visualized in  FIG. 6  by the dashed lines in the overvoltage range III. 
         [0057]    In the graphs shown in  FIGS. 13   a - c , the interaction of the active modules  61  to  66  is illustrated. The continuous lines represent the resulting voltage profiles in the case of an overvoltage on the grid in each case for the case of off-load operation (central continuous characteristic), a case with 110% active power and 50% overexcited reactive power (the continuous line at the top, annotated by “CAP”), and the case of an active power of 110% and 50% underexcited reactive power, in each case based on the rated values (continuous line at the bottom, annotated by “IND”). In this case, the graph in  FIG. 10   a  shows the initial state without any activity of the active modules. It can be seen from  FIGS. 10   b  and  c  that, owing to the effect of the active modules, the voltage increase for the voltage U LSI  at the output of the converter  3  is weakened markedly in comparison with the initial situation as illustrated in  FIG. 10   a . In  FIG. 10   b , the characteristic profile is illustrated for the case where a small line reactor  4  is provided. A small line reactor is understood to mean one with a low inductance. The voltage increase in this case continues until in the higher ranges, but with a considerably weaker extent. The respectively achieved limit values for the voltage U LSI  at the converter  3  for the different cases are plotted on the right-hand side, with 840 V as the maximum. The rating-relevant point is therefore the upper end point of the overvoltage range III (labeled by a circle). The case with a rating for a large line reactor  4  is illustrated in  FIG. 10   c . It can be seen here that the voltage U LSI  at the converter even goes back in the case of relatively high overvoltages U MW  on the grid. The rating-relevant point (labeled by a circle) is now a bend at the beginning of the overvoltage range I. 
         [0058]    The resultant currents on the low-voltage side of the line transformer  8  (current I LV ) are illustrated over the medium voltage U MC , in  FIG. 14   a , and  FIG. 14   b  correspondingly illustrates the currents at the output of the inverter  33  (I LSI ). It can be seen from  FIG. 14   a  that, even in the most unfavorable case, namely when the upper end of the overvoltage range III is reached, the current is no higher than at the lower end of the voltage spectrum (when U MV =0.9). Therefore, the rating limits for the transformer  8  are adhered to. In addition, it can be seen from  FIG. 14   b  that the currents to be produced by the inverter  33  are no higher in the most unfavorable case at the upper end of the overvoltage range III than at the beginning of the permissible voltage spectrum (in turn at point 0.9). Therefore, the loading of the inverter  33  is also in the permissible range, even for the case of critically high overvoltages in the overvoltage range III. The graphs in  FIGS. 14   a  and  b  apply to small or large line reactors  4 . 
         [0059]    The line transformer  8  is preferably rated in such a way that magnetic saturation occurs above a predefined voltage-frequency combination. As a result, a plurality of advantages are achieved. Firstly, the transformer  8  can be smaller. In the case of an operating frequency range of 45 Hz to 55 Hz provided, the transformer is expediently rated in such a way that saturation occurs at the lower end of the operating range (in this case 45 Hz) above an overvoltage of 116%. By virtue of this saturation, the transformer generates additional reactive current, which is further favorable for the protection of the turbine. Preferably, the onset point is selected such that the action occurs primarily in the overvoltage ranges II and III. 
         [0060]    Since this effect is frequency-dependent, preferably the limits for the overvoltage ranges need to be carried along depending on the line frequency f. For this purpose, a frequency adaptation element  77  for the overvoltage classifier  70  is expediently provided.