CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a divisional of U.S. application Ser. No. 10/475,531, filed May 7, 2004, which is the U.S. National Phase of PCT/EP02/04109, filed Apr. 12, 2002, which claims priority to German Application Nos. 10119624.5, filed Apr. 20, 2001, and 10138399.1, filed Aug. 4, 2001, all of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method for operating a wind turbine with an electrical generator, drivable by a rotor, for supplying electrical power to an electrical load, in particular an electric grid.  
         [0004]     2. Description of the Related Art  
         [0005]     In known wind turbines for generating electrical energy from wind energy, the generator with the electrical load, often an electric grid, is operated in a grid-parallel mode. In other words, as soon as the wind supply is sufficient, the wind turbine will generate electrical energy and deliver it to the grid.  
         [0006]     However, if a failure occurs in the grid, for example as a result of a short circuit in the grid, wind turbines have hitherto been disconnected from the grid and not reconnected to the grid until normal operating conditions have been restored.  
         [0007]     This means that, following such a grid failure, is no longer possible to provide the rapid support for the grid that is particularly needed when there are large fluctuations in the voltage and/or power that is required.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The object of the present invention is therefore to provide a control system for one or more wind turbines that compensate as far as possible for fluctuations in the grid.  
         [0009]     This object is achieved with a method of the kind initially specified, in which the power delivered to the load by the generator is regulated in response to a current that is outputted to the load.  
         [0010]     In a device of the kind initially specified, the object is achieved by a control device comprising a current sensor for measuring an electrical current delivered to the load, such that the power delivered by the generator to the load can be controlled in response to the current that is received by the current sensor.  
         [0011]     In this way, the required power can be generated and delivered when there are fluctuations in the power requirements from the grid.  
         [0012]     In order to avoid overload of parts of the wind turbine and/or the grid in the event of a grid failure, for example as a result of a short circuit in the grid, the wind turbine is controlled in such a way that the current delivered to the grid does not exceed a predefined value.  
         [0013]     In a particularly preferred embodiment of the invention, the maximum current level that can be delivered is regulated for each grid phase, in order to support the grid as far as possible, on the one hand, without exposing components to the risk of damage, on the other hand.  
         [0014]     A particularly preferred embodiment is one in which the wind turbine can be operated by an external input that corresponds to the stipulations made by a distant control station. In this way, a power supply company for example can request the wind turbine to deliver the amount of current which is needed at that moment to support the grid.  
         [0015]     The electric power, current, phase position, proportion of reactive current, or the like, output to the electric grid does not exceed a predetermined amount during the disruption. In one embodiment, the predetermined amount is less than an amount associated with the electrical power output to the electric grid before the disruption occurs.  
         [0016]     Other advantageous embodiments of the invention are described in the claims. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0017]     One embodiment of the invention shall now be described in detail with reference to the figures. These show:  
         [0018]      FIG. 1  illustrates a wind turbine that feeds power to a grid, in a simplified view;  
         [0019]      FIG. 2  illustrates a control device according to the invention for operating a wind turbine; and  
         [0020]      FIG. 3  illustrates a block diagram of the main components in the control and regulation arrangement. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     A wind turbine  2 , shown in simplified form in  FIG. 1 , comprising a rotor  4  is connected to an electric grid  6  that may be a public grid, for example. Several electrical loads  8  are connected to the grid. The electrical generator  12  ( FIG. 2 ) of wind turbine  2  is coupled to an electrical control and regulation arrangement  10  that firstly rectifies the alternating current generated in the generator and subsequently converts the current into an alternating current with a frequency corresponding to the grid frequency. Instead of a grid  6 , a single load could also be supplied with electrical energy by the wind turbine  2 . The control and regulation arrangement  10  has a regulating device  14  according to the invention.  
         [0022]      FIG. 2  illustrates the regulating device  14  according to the invention. The rotor  4 , shown in simplified form, is coupled to a generator  12  that provides an amount of electrical power that depends on the wind speed and hence on the wind power. The alternating current produced in the generator  12  is initially rectified and subsequently converted into an alternating current that has a frequency corresponding to the grid frequency.  
         [0023]     With the help of a current sensor  15 , the amount of current being fed into the grid  6  ( FIG. 1 ) is detected. Said current is compared at the same time with a predefined value l(max). In alternative embodiments, the sensor  15  may comprise a device for detecting a short circuit in the grid, a voltage sensing device, and/or a phase monitor.  
         [0024]     If the current fed into the grid  6  now exceeds the predefined maximum current l(max), the power generated by the entire wind turbine (and/or its generator) is adjusted by the regulating device in such a way that the current delivered to the grid does not exceed the predefined threshold value l(max). In alternative embodiments, the power and/or current delivered to the grid does not exceed a predefinable phase position and/or a predefinable proportion of reactive current. In a multiphase system, the amount, phase position and/or the proportion of reactive current for each phase may be independently predefined, and for each phase, the predefinable value for that phase is not exceeded. In embodiments wherein reactive power is controlled, real power may also be controlled.  
         [0025]     In the event of a short circuit, said current regulation can be accomplished, for example, by the wind turbine delivering a significantly lower level of power output to the grid than previously, while using elsewhere outside the grid the power that consequently is not fed to the grid, for example for a dump load (resistance), or by feeding the power which is not fed to the grid to capacitors or other interim storage devices. As soon as full availability of the grid is restored, delivery of the stored energy to the grid can be resumed.  
         [0026]     In this way, even when there is a short circuit in the grid, the wind turbine can continue to deliver power to the grid and support the grid without the current exceeding the predefined threshold value as a result of the short circuit.  
         [0027]      FIG. 3  shows constituent parts of the control and regulation arrangement  10  in  FIG. 1 . The control and regulation arrangement  10  includes a rectifier  16 , in which the alternating current produced by the generator is rectified. An inverter  18  connected to the rectifier  16  converts the direct current back into an alternating current with a frequency corresponding to the grid frequency. This current is fed into the grid  6  in three phases L 1 , L 2  and L 3 . The inverter  18  is controlled with the help of a microcontroller  20  that forms part of the regulating device. The microprocessor  20  is coupled for this purpose to the inverter  18 . The input variables for regulating the current with which the electrical power provided by the wind turbine  2  is fed into the grid  6  are the momentary current and/or the momentary currents, the grid frequency, the electrical power output P of the generator, the power factor cos φ and the power gradient dP/dt. Regulation, pursuant to the invention, of the current to be delivered to the grid is implemented in microprocessor  20 . The current in each of phases L 1 , L 2  and L 3  is separately detected and the respective levels are taken into account in the regulation arrangement pursuant to the invention.  
         [0028]     If the measured current (level) l(actual) of a phase rises above a predetermined maximum current, the inverter  18  is controlled in such a way that the current (level) falls below the predefined maximum current l(max), with the electrical energy generated from wind energy and not delivered to the grid being used elsewhere, for example by being outputted to a resistance (dump load) or stored in an interim storage device (e.g., a capacitor or Ultracap).  
         [0029]     The control system for the wind turbine can operate independently. The wind turbine then detects a short circuit in the grid, for example by monitoring the voltages of the separate grid phases and/or their phase position. If predefinable threshold values for voltages and/or phase differences are reached, the wind turbine recognizes a short circuit and operates according to an algorithm provided for such a case.  
         [0030]     Owing to the external access  22 , it is possible, for example for the power supply company to whose grid the wind turbine is connected, to intervene in the operation of the wind turbine and, for example, to modify the amount of current to be delivered to the grid, the type of current (active current, reactive current) and/or the phase angle and/or phase position, etc. In this way, the power supply company can adjust precisely those values (current, voltage, electrical power) in respect of the power to be delivered to the grid by the wind turbine that correspond to the requirements of the network operator.
A system and method for operating a wind turbine with a generator, drivable by a rotor, for supplying electrical power to an electric load, in particular an electric grid, are disclosed. Briefly described, one embodiment generates electrical power from an electrical generator coupled to the wind turbine, outputs the generated electrical power to an electric grid, senses a disruption in the electric grid, maintains connection of the electrical generator during the disruption, and limits the generated electrical power during at least a portion of the disruption.