Abstract:
A wind energy installation including a wind rotor, a doubly-fed asynchronous generator driven by the wind rotor, a converter, and a controller configured to determine an operating torque for an operating rotation-speed, the operating torque not exceeding a maximum torque. The installation also includes a frequency-adaptive torque limiter having a classifier—for an overfrequency or underfrequency, a torque shifter configured to reduce the maximum torque in the event of frequency discrepancies, and an inhibitor configured to block the torque shift at an underfrequency. The installation further includes a frequency-dependent rotation-speed limiter configured to interact with the classifier such that a lower limit rotation-speed is increased only at an overfrequency and an upper limit rotation-speed is reduced only at an underfrequency. Accordingly, the operating torque can be adapted with respect to the permissible limit values rather than over the entire operating range, thus minimizing yield losses when frequency discrepancies occur.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application No. 10 2009 037 238.5, filed Aug. 12, 2009, the contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a wind energy installation having a wind rotor, a doubly-fed asynchronous generator which is driven by the wind rotor and, having a converter, which comprises a generator-side inverter and a grid-side inverter, as well as a controller which is designed to determine an operating torque for an operating rotation-speed, with the operating torque not exceeding a maximum torque, and which furthermore has a grid frequency detector. 
     BACKGROUND OF THE INVENTION 
     Modern wind energy installations are designed for variable rotation speeds and, for this purpose, are provided with a converter. Normally, they have a doubly-fed asynchronous generator, whose stator is permanently connected to the grid, and the rotor is connected to the grid via the converter. This allows the generator to be operated not only at a rotation-speed which corresponds to the grid frequency but also at a lower rotation-speed than that corresponding to the grid frequency (or the synchronous rotation-speed that is governed by it), or operation at a higher rotation-speed than that corresponding to the synchronous rotation-speed. The respective discrepancy between the actual rotation-speed and the synchronous rotation-speed, which is governed by the grid frequency, is referred to as slip. Slip values of ±30%, or in some cases even more, are normal for modern wind energy installations. The wind energy installation can therefore be operated over a wide rotation-speed range. 
     The synchronous rotation-speed, which is critical for determining the slip, is not constant during practical operation on the actual grid, but is subject to the same fluctuations as the grid frequency. For example, when an overfrequency occurs in the grid, then the synchronous rotation-speed is correspondingly shifted upwards, and vice versa. If the slip values in consequence become too high or too low, then there is a risk of overloading and of damage to components of the wind energy installation. Various remedial areas to avoid this are known from the prior art. 
     A first measure is to base the design of the components of the wind energy installation on the respectively worst extreme case, that is to say to take account of the maximum permissible grid frequency discrepancy. In the end, this leads to planned overengineering of the components, and is therefore correspondingly expensive, in terms of production costs. Furthermore, when the characteristics are matched to the worst case, this leads to sub-optimum operation at the nominal frequency, resulting in yield losses. In addition, this design based on the extreme case is still completely inadequate and, furthermore, the steady-state rotation-speed range must be matched to the grid frequency in order to prevent overloading in the event of excessive rotor slip, in particular in the event of a relatively heavy load, when an actual underfrequency occurs. 
     US 2007/069522 A1 discloses a different approach for the adaptation of the characteristics. The grid frequency is measured in order to determine whether an overfrequency or underfrequency situation exists, in order to shift the rotation-speed/torque characteristics toward the synchronous point, as a function of this. This reduces the slip that actually occurs. The entire characteristic is therefore adapted as a function of the actual grid frequency. This has the disadvantage that this adaptation acts over the entire operating range, which can likewise result in yield losses as a result of unnecessary reduction, as already described above for the static design based on the extreme case. Furthermore, this results only in inadequate protection for certain operating states, for example for the combination of overfrequency and high load. 
     Adequate protection cannot be ensured in particular for this critical operating situation. 
     SUMMARY OF THE INVENTION 
     Against the background of the prior art cited immediately above, the invention is based on the object of specifying an improved wind energy installation and an operating method which reduces the disadvantages mentioned above. 
     The solution according to the invention resides in the features broadly disclosed herein. Advantageous developments are the subject matter of the detailed disclosure. 
     In the case of a wind energy installation having a wind rotor, a doubly-fed asynchronous generator which is driven by the wind rotor and, having a converter, which comprises a generator-side inverter and a grid-side inverter, as well as a controller which is designed to determine an operating torque for an operating rotation-speed, with the operating torque not exceeding a maximum torque, the invention provides a frequency-adaptive torque limiter having a classifier for an overfrequency or underfrequency outside of a tolerance band, a torque shifter, which is designed to reduce the maximum torque in the event of frequency discrepancies, and an inhibitor, which blocks the torque shift at an underfrequency, and as well as a frequency-dependent rotation-speed limiter to whose input a signal for the frequency discrepancy is applied and which interacts with the classifier such that the lower limit rotation-speed is increased only at an overfrequency, and the upper is reduced only at an underfrequency. 
     The invention is based on the idea of not adapting the operating torque over the entire operating range, but of carrying out the adaptation only with respect to the permissible limit values, in particular the maximum value, but with this adaptation not being carried out statically, but dynamically as a function of the actually occurring frequency discrepancy. This makes it possible to link the previously contradictory advantages of the previously known solution options, and therefore to reduce their respective disadvantages. In particular, dynamic limiting avoids the disadvantage of yield losses always occurring, such as those which have previously unavoidably occurred in the past when designing for the worst case. Since only the limit value and not the entire characteristic is shifted over the entire operating range, the losses can be further reduced. 
     The invention operates in two ways. On the one hand, the permissible rotation-speed range is matched to the respective change in the grid frequency. This ensures that the permissible operating range of the converter is not departed from. When an overfrequency occurs, the lower limit of the permissible rotation-speed range is therefore increased, in order to keep the slip within the operating limits of the converter. There is no need to change the maximum limit of the permissible operating range, since lower slip values occur here in any case when an overfrequency occurs. Correspondingly, when an underfrequency occurs, the maximum limit of the permissible rotation-speed range is changed, to be precise by reducing it to such an extent that the maximum slip is not undershot. In practice, this is expediently done by implementing the reduction in proportion to the grid frequency discrepancy. The minimum rotation-speed of the permissible rotation-speed range does not need to be adapted, since lower slip values would occur here in any case in the event of an underfrequency. This restriction to the change of in each case only one maximum limit on the one hand provides protection against unacceptably high values when corresponding extreme operating conditions occur while, on the other hand, the manner of operation remains unchanged in the normal state. There is therefore no need to be concerned about yield losses during normal operation. 
     The second aspect consists in adaptation of the torque limit of the additional permissible rotation-speed range, to be more precise the maximum permissible torque, in addition to adaptation of the permissible rotation-speed range. The torque shifter is provided for this purpose. This varies the maximum permissible torque as a function of the actual rotation-speed and the determined overfrequency. This means that the slip-dependent maximum permissible torque is not overshot even when overfrequencies occur. According to the invention, when an underfrequency occurs, the adaptation is blocked by the inhibitor, in order to prevent the maximum torque being raised at an underfrequency, which could lead to overloading of other components. This means that, in this case, deliberately asymmetric adaptation is implemented, specifically only when an overfrequency occurs, but that no adaptation is implemented when an underfrequency occurs. Since, once again, only the maximum value is influenced, not operation during normal conditions, the invention means that there are no yield losses in comparison to normal operation. 
     The torque shifter preferably interacts with a rotation-speed modifier, which is designed to generate a modified rotation-speed signal, which is changed by a correction value that is dependent on the frequency discrepancy, from the actual operating rotation-speed, and which modified rotation-speed signal is applied to a rotation-speed-dependent maximum torque element which emits the maximum torque. This is particularly suitable for embodiments in which the maximum permissible torque is determined as a function of the rotation-speed, for example by means of a look-up table or a characteristic element. In order to avoid a change to the table or characteristic element, which could involve considerable cost for programming and adaptation, the intrinsically unchanged characteristic element or the table has a changed (modified) rotation-speed signal applied to it. In this case, the rotation-speed signal is modified as a function of the frequency discrepancy. When the frequency is at the nominal value, the rotation-speed signal is then not changed; in the event of a frequency discrepancy in the form of an overfrequency, the rotation-speed signal is correspondingly reduced, and when an underfrequency occurs, it is correspondingly increased. This artifice results in the modified rotation-speed signal corresponding to that rotation-speed (or frequency) which has to be compensated for by the converter as the difference between the actual rotation-speed of the rotor and the synchronous rotation-speed, as defined by the instantaneous grid frequency. Therefore, the converter is not only utilized optimally independently of the grid frequency, but is also protected against overloading. 
     The rotation-speed modifier advantageously has an input for a maximum permissible frequency, and a saturation element is provided which limits the rotation-speed modification to the maximum permissible frequency. This prevents overadaptation, which could lead to damage to other components in the installation. In this case, the saturation element is expediently designed such that a value for the rated frequency is applied as the lower limit, and a value for the maximum permissible grid frequency is applied as the upper limit. 
     The classifier is preferably designed to inhibit the rotation-speed modification at full load. At full load, there is essentially a risk of overloading, except in extraordinary operating conditions such as excessive rotation speeds in the event of gusts. Inhibiting in this way therefore makes it possible to ensure that the full yield is achieved at full load. 
     It is also possible to design the rotation-speed modification element to form the modified rotation-speed signal from the real rotation-speed, using the ratio of the nominal grid frequency to the actual grid frequency, and possibly taking into account the saturation element. The rotation-speed can therefore be modified in a particularly simple and expedient manner. 
     The invention furthermore relates to a corresponding method. Reference is made to the above statements for a more detailed explanation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail in the following text using exemplary embodiments and with reference to the drawings, in which: 
         FIG. 1  shows an overview illustration of a wind energy installation according to one exemplary embodiment of the invention; 
         FIG. 2  shows a block diagram for the control of a wind energy installation as shown in  FIG. 1 ; 
         FIG. 3  shows a rotation-speed/torque diagram of the wind energy installation as shown in  FIG. 1 ; 
         FIG. 4  shows a block diagram for the control according to a second exemplary embodiment; and 
         FIG. 5  shows an alternative rotation-speed/torque diagram. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The wind energy installation has a tower  10  at whose upper end a pod  11  is arranged such that it can swivel in the azimuth direction. A wind rotor  12  is arranged, such that it can rotate, on an end face of the pod  11  and, via a rotor shaft  13 , drives a generator which is designed to convert mechanical energy to electrical energy. In the illustrated exemplary embodiment, this is a doubly-fed asynchronous generator  14 , which interacts with a converter  15 . The electrical power produced by the generator  14  with the converter  15  is emitted via a connecting line  16  to a transformer  17 , which is arranged at the foot of the tower  10  of the wind energy installation. This is connected to a distribution grid  9 , which may be an internal grid for a wind farm, or a public power transmission grid. 
     Furthermore, a controller  2  is arranged on the pod  11 , is connected via signal lines  50  to the various components of the wind energy installation, and controls them. In turn, control signals are applied to the controller  2 , with these control signals being obtained via a cable-based or wire-free connecting link  21  from a superordinate control entity, in particular a wind farm control system or a grid control center of a power supply company. The controller  2  is designed to define the operating point of the wind energy installation, at which the wind energy installation is operated, depending on the prevailing environmental conditions, in particular the wind speed. In particular, this includes the functionality of determining the torque T as a function of the instantaneous rotation-speed and of passing this as an electrical nominal torque via the converter  15  to the generator  14 . A further functionality is to determine a maximum permissible torque T max , as a function of the rotation-speed. 
     In the case of asynchronous generators of the doubly-fed type, such as those used in the illustrated exemplary embodiment, a slip s occurs during operation. This slip depends on the actual rotation-speed n of the generator  14  and on a so-called synchronous rotation-speed n sync , which results from the speed of revolution of the field, which revolves at the grid frequency. This slip is the magnitude of the rotation-speed discrepancy from the synchronous rotation-speed, divided by the synchronous rotation-speed: 
     
       
         
           
             s 
             = 
             
               
                 n 
                 - 
                 
                   n 
                   sync 
                 
               
               
                 n 
                 sync 
               
             
           
         
       
     
     In this case, n is the actual (mechanical) rotation-speed of the generator  14 , and n sync , is the synchronous rotation-speed which results from the grid frequency. When the wind energy installation is being operated at a rotation-speed below the synchronous rotation-speed, then this is referred to as subsynchronous operation; in contrast, when the wind energy installation is being operated at a rotation-speed above the synchronous rotation-speed, this is referred to as supersynchronous operation. 
     The dependency of the synchronous rotation-speed n sync  on the grid frequency if means that the synchronous rotation-speed likewise changes when the grid frequency changes. A grid frequency above the rated frequency (overfrequency) therefore leads to the synchronous rotation-speed rising, and therefore to the slip decreasing when the mechanical rotation-speed remains constant in the supersynchronous range, while in contrast the slip increases during subsynchronous operation. The opposite situation occurs when the grid frequency falls to a value below the rated frequency (underfrequency). The corresponding reduction in the synchronous rotation-speed when the mechanical rotation-speed is constant during supersynchronous operation leads to an increase in the slip, while the slip decreases during subsynchronous operation. 
     The controller  2  has an element  22  for determining a nominal torque T relating to the mechanical rotation-speed n, and a limit torque determining means  23 , which determines a respective maximum permissible torque T max  relating to an applied signal for an actual rotation-speed n i . This determining means may be in the form of an algebraic function, preferably a look-up table (LUT) or a characteristic element. In the illustration shown in  FIG. 2 , a look-up table (LUT) is provided for the determining means  23 . A function as illustrated by a solid line in  FIG. 3  is implemented in it. As can be seen, one value for the maximum permissible torque T max  is in each case assigned for each rotation-speed n between the minimum operating rotation-speed n min  of the wind energy installation and the maximum operating rotation-speed n max . The profile of the maximum torque T max , which results in this case has a knee point at the synchronous rotation-speed n sync . 
     Reference will now be made to  FIG. 2  which, in addition to the maximum torque determining means  23 , which is known per se, illustrates a frequency-adaptive limiting device  3  according to the invention. This comprises a classifier  31 , to which input signals are applied for the nominal frequency f N  of the grid  9 , as well as the measurement signal from a grid frequency detector for the actually prevailing frequency f. The classifier  31  is designed to use these signals to emit a positive frequency discrepancy signal +Δf in the event of an overfrequency, and a negative discrepancy signal −Δf when a negative frequency discrepancy occurs. The positive discrepancy signal +Δf is applied to the input of a torque shift  32 , which is designed to determine a maximum torque correction value ΔT max , which is emitted as an output signal, as a function of the frequency discrepancy when an overfrequency occurs. This signal is applied to a summation element  34  with a negative mathematical sign via an inhibitor  33 , which is closed in the normal state. The maximum torque T max  determined by the maximum torque determining means  23  is therefore reduced by the torque correction value ΔT max  determined by the torque shift  32 , thus forming a modified maximum value T max *. In contrast, when the classifier  31  finds that an underfrequency has occurred, the inhibitor  33  is operated via the output signal that is correspondingly emitted by the classifier  31 . A change in the value of the maximum torque T max  is therefore blocked, such that no adaptation takes place in the event of an underfrequency. The torque of the generator/converter system  14 ,  15  is therefore restricted in the event of an overfrequency, but the maximum permissible torque remains unchanged when an underfrequency occurs. 
     A frequency-dependent rotation-speed limiter  4  is also provided. This comprises a lower branch  41  and an upper branch  42 . Once again, the signals determined by the classifier  31  for positive frequency discrepancies and negative frequency discrepancies, +Δf and −Δf, are applied to its inputs. The signal for positive frequency discrepancies is in this case applied to the lower limiter module  41 , but not to the upper limiter  42 . This means that the lower limit for the nominal rotation-speed is increased when an overfrequency occurs, and the upper limit for the nominal rotation-speed n max  remains unchanged. In contrast, the discrepancy signal for an underfrequency is applied exclusively to the upper limiter module  42 , as a result of which the upper limit for the nominal rotation-speed n max  is reduced when an underfrequency occurs, and the lower limit for the nominal rotation-speed n min  remains unchanged when an underfrequency occurs. 
     This means that the steady-state rotation-speed range is adapted when frequency discrepancies occur. When overfrequencies occur, the lower limit n min  is therefore increased, thus preventing unacceptably high (negative) slip values from occurring. In this case, the upper rotation-speed limit remains unchanged. Conversely, when underfrequencies occur, the upper rotation-speed limit is reduced. This also means that no unacceptably high (positive) slip values occur. The limiting function which is implemented in the limiter modules  41 ,  42  is preferably designed such that the nominal rotation-speed n s  is adapted corresponding to the percentage discrepancy between the actual grid frequency and the nominal grid frequency. 
     Furthermore, the torque shifter  32  and the inhibitor  33  ensure that the permissible torque is reduced when an overfrequency occurs, while the maximum permissible torque is not changed when an underfrequency occurs. This ensures that the slip-dependent torque limit is not overshot even when an overfrequency occurs. The profile of the corresponding characteristic for the maximum torque and rotation-speed after limiting is shown by a dashed line in  FIG. 3 . 
       FIG. 4  illustrates a second exemplary embodiment with the limiting device  3 . This is provided with integrated components, to be precise a rotation-speed modification element  38  and a saturation element  37 . Input signals for the actual frequency f, as determined by the grid frequency detector, the nominal grid frequency f N —and as a special feature—the maximum permissible grid frequency f max  are applied to the input of the saturation element  37 . The nominal frequency in this case forms the lower limit value, and the maximum permissible frequency f max  forms the upper limit value, of the saturation element  37 . The applied measurement signal for the actually prevailing grid frequency f is therefore processed via the saturation element  37 , and is emitted as a saturated frequency signal f*. This is applied to a divisor input of the rotation-speed modification element  38 . The rotation-speed modification element  38  furthermore has two multiplier inputs. The signal for the nominal frequency f N  is applied to one of the multiplier inputs, and a signal for the actual rotation-speed n of the generator  14  is applied to the other multiplier input. A modified rotation-speed signal n* is now formed by means of the function: 
     
       
         
           
             
               n 
               * 
             
             = 
             
               n 
               × 
               
                 
                   f 
                   N 
                 
                 
                   f 
                   * 
                 
               
             
           
         
       
     
     This modified rotation-speed signal is applied to the input of a maximum torque element  23 ′. This can be designed as described above, or, as in the exemplary embodiment illustrated in  FIG. 4 , may be in the form of a characteristic element. This emits the associated maximum value for the permissible torque, T max  in response to the applied (modified) rotation-speed signal n*. In this embodiment, both functionalities, specifically both rotation-speed limiting and torque limiting, can be achieved in a particularly simple and elegant manner by calculating a synthetic intermediate variable, specifically the modified rotation-speed signal n*.  FIG. 5  illustrates one example of a corresponding shift for a relatively complex torque limit characteristic (solid line at the rated frequency and dashed line at overfrequency).