Abstract:
A device for increasing fault clearing time is provided having a component part designed to identify a short circuit event and load resistors connectable in the event of a fault such that the turbine power transmitted to the shaft is electrically absorbed by the generator and converted into heat until the grid comes back online.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2013/068385 filed Sep. 5, 2013, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 102012221989.7 filed Nov. 30, 2012. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to an apparatus for extending the fault clearing time and to a method for extending the fault clearing time. 
       BACKGROUND OF INVENTION 
       [0003]    Owing to a significant growth in regenerative energy generating units in the supply grids, there are consistently increasing minimum requirements that the grid operators set for all feed-in units as regards stability and supply safety. In this regard, there are grid codes which demand a uniform course of action. Thus, for example, in the Finnish grid code “Fin-Grid”, fault clearing times of 250 ms at 0 p.u. residual voltages are required, for example, which for some generators, in particular synchronous generators, results in a fall into asynchronicity, without additional measures, and therefore in a certain resynchronization after recovery of the mains voltage.  FIG. 1  shows several grid code requirements. The abovementioned “Fin-Grid” is provided with the reference symbol  1 . Further grid code requirements are illustrated here by way of example for E.on  2 , REE Spain  3  and WECC North America  4 . 
         [0004]    The course of action for resynchronization after recovery of mains voltage can take a few minutes, during which the power plant power is not available to the grid. This can primarily result in grid instability and in the worst case scenario in a large-area voltage drop in the case of failure of relatively large power plants. 
         [0005]    During the short circuit, the mechanical power impressed onto the shaft assembly by the turbine is no longer taken off at the generator, and therefore results in acceleration of the turbo set. 
         [0006]    If the rotor angle of the synchronous generator exceeds a critical transient value, said synchronous generator falls into asynchronicity and needs to be resynchronized. It is demanded in the grid codes that a power plant needs to be able to run through a predefined fault clearing time at a certain residual voltage on the transformer high-voltage side without grid isolation. If this required fault clearing time is above the fault clearing time which can be achieved for the turbo set, additional precautions need to be taken. 
         [0007]    Several possibilities for taking into consideration this circumstance are known from the prior art. Thus, for example, in EP 1 805 887 B1, an active boost circuit is connected in series with the generator field winding via slip rings on a charged capacitor in the event of a fault, as a result of which the field voltage is raised suddenly. Thus, the generator is in the over excited range in the event of grid recovery, as a result of which the stability of the turbo set/grid system is increased. 
         [0008]    A further possibility for extending the critical fault clearing time includes increasing the moment of inertia of the assembly in order to reduce the shaft acceleration in the event of a short circuit. 
         [0009]    Furthermore, for some types of turbine there is the possibility of making changes to the steam turbine in order to clear the steam away from the turbine blades even more quickly, which is referred to as fast valving. Thus, a quicker reduction in the turbine power impressed on the shaft assembly is intended to be achieved. 
         [0010]    Likewise, it is known to form similar concepts with loading resistors in the case of offshore wind turbines. However, wind farms are connected to an onshore converter plant via high-voltage DC connections, for example, said onshore converter plant diverting the excess energy from the wind farm into loading resistors in the event of a short circuit on the land. 
         [0011]    It would be desirable to have a simple possibility for a turbo set for extending the fault clearing times in the event of a short circuit. 
       SUMMARY OF INVENTION 
       [0012]    This is where the invention comes in, an objective of which includes specifying an apparatus and a method for extending the fault clearing time. 
         [0013]    This is achieved by an apparatus for extending the fault clearing time having an electrical generator, in particular synchronous generator and electrical loads and a component part designed to identify a short-circuit event, wherein the apparatus is designed in such a way that, in the event of a short circuit, the electrical loads are connected to the electrical generator. 
         [0014]    Advantageously, this is achieved according to aspects of the invention without mechanical intervention for making changes to the given shaft assembly or components thereof. 
         [0015]    In a first advantageous development, the electrical loads are in the form of resistors. Thus, the concept is followed of arranging resistors which are connected to the electrical generator as electrical loads in the event of a short circuit and thus, as connectable loading resistors, dissipate the turbine power contributing to the shaft acceleration in the event of a fault without disconnecting the generator from the grid. Thus, the critical fault clearing time is considerably extended. 
         [0016]    Advantageous developments are specified in the dependent claims. Thus, in an advantageous development, the apparatus is formed with a transformer, which is connected to the electrical generator, wherein the electrical loads are arranged in parallel with the transformer during the short circuit. 
         [0017]    In an alternative embodiment, the apparatus is formed with electrical loads, wherein the electrical loads are arranged in series with the short-circuit path at the transformer neutral point on the high-voltage side. 
         [0018]    Likewise, considerable load relief of the generator circuit breaker can be realized as a further potential application of the switchable resistors previously mentioned. 
         [0019]    For the design of a generator circuit breaker, the requirement often includes disconnecting said generator circuit breaker as soon as possible in the event of a short circuit. 
         [0020]    The sudden generator short circuit which comes about comprises an AC and a DC current component, which decay at different speeds to the steady-state short-circuit current corresponding to their time constants. In particular, the DC component of the short-circuit current is responsible for the current profile experiencing a current zero crossing after only a few milliseconds. Once a circuit breaker has opened, the switching arc burns until this first current zero crossing occurs and the arc can be quenched. In this time, considerable contact loading and thermal heat development arise in the switch owing to the extremely hot arc plasma. It is therefore desirable for the DC component of the short-circuit current to decay as quickly as possible. 
         [0021]    The time constant (T) is described in principle by virtue of the ratio of the inductance (L) in the short-circuit path to the resistance (R) effective in the short-circuit path. From the formula T=L/R it becomes clear that as the effective resistance increases, the time constant can be reduced. This can be accelerated effectively by the load resistors described here being switched on once the fault has occurred. 
         [0022]    The object is likewise achieved by a method for extending the fault clearing time, wherein the electrical generator connected to an electrical consumer grid is interconnected with additional electrical loads in the event of a short circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention will now be explained in more detail with reference to an exemplary embodiment.  FIGS. 2 and 3  show, schematically: 
           [0024]      FIG. 2  a first embodiment of the apparatus according to the invention; 
           [0025]      FIG. 3  a second embodiment of the apparatus according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0026]      FIG. 2  shows a three-phase electrical generator  5 , in particular synchronous generator, wherein a first phase  6 , a second phase  7  and a third phase  8  are formed at the output. The first phase  6 , the second phase  7  and the third phase  8  are connected to a transformer  9 . The secondary side  10  of the transformer  9  is connected to an electrical grid  11 . In the first phase  6 , a first outgoing line  12  is provided, by means of which a first switch  13  and electrical loads  14  are connected to ground  15 . The second phase  7  comprises a second outgoing line  16  and a second switch  17 , which is connected to the second outgoing line  16 , and a load  18 , which is connected to ground  15 . The third phase  8  comprises a third outgoing line  19  and correspondingly a third switch  20  and a load  21 , which in turn is connected to ground  15 . 
         [0027]    The phases  6 ,  7  and  8  are in this case connected to the transformer  9  via the generator switch  25 . 
         [0028]      FIG. 3  shows an alternative embodiment of the invention. The difference over  FIG. 2  is that the loads  14 ,  18  and  21  are in series with the short-circuit path at the transformer neutral point on the high-voltage side. In each case one switch  22 ,  23  and  24  is arranged in parallel with the loads  14 ,  18  and  21 , respectively. 
         [0029]    The electrical generator  5  is driven via a turbine (not illustrated). In the event of a fault, the turbine power impressed onto the shaft is connected by the generator  5  via connectable loads  14 ,  18 ,  21  until grid recovery and is converted into heat. In other words: in the event of a fault, the turbine power impressed onto the shaft is taken off electrically from the generator  5  and converted into heat via connectable loads  14 ,  18 ,  21  until grid recovery. During the fault time, the electrical generator  5  remains connected to the electrical grid  11 . Grid resynchronization is therefore not required and a higher degree of power station availability can be achieved. The critical fault clearing time T Ku  for the respective assembly without additional loads can generally be determined analytically corresponding to the following formula: 
         [0000]    
       
         
           
             
               
                 T 
                 Ku 
               
               = 
               
                 
                   
                     
                       2 
                       · 
                       
                         ω 
                         0 
                       
                       · 
                       J 
                     
                     
                       P 
                       r 
                     
                   
                   · 
                   
                     ( 
                     
                       
                         δ 
                         Ku 
                         ′ 
                       
                       - 
                       
                         δ 
                         0 
                         ′ 
                       
                     
                     ) 
                   
                 
               
             
             , 
           
         
       
     
         [0030]    where 
         [0031]    ω 0  denotes the rated circuit frequency 
         [0032]    J denotes the moment of inertia of the entire assembly 
         [0033]    δ′ Ku  denotes the maximum transient voltage angle until stability of the turbo set is obtained 
         [0034]    δ′ 0  denotes the transient voltage angle prior to the onset of a short circuit 
         [0035]    P T  denotes the turbine power. 
         [0036]    The loads  14 ,  18  and  21 , which can be in the form of electrical resistors, dissipate the turbine power contributing to the shaft acceleration in the event of a fault, as a result of which the critical fault clearing time is considerably extended and, as a result, there is an increase in the transient stability of the electrical generator  5 , in particular synchronous generator, via loading resistors  14 ,  18  and  21  connectable in the event of a short circuit. The load resistors  14 ,  18  and  21  illustrated in  FIG. 2  are in parallel with the transformer  9  on the transformer low-voltage side in order to make use of the short-circuit residual voltage present in the event of a short circuit over the transformer series impedance. The additional use of adjustable reactances can improve the reactivity of the circuit even more. 
         [0037]      FIG. 2  shows the topology for this first embodiment of the invention. 
         [0038]    The topology of the second embodiment is shown in  FIG. 3 . The load resistors  14 ,  18  and  21  are in series with the short-circuit path at the transformer neutral point on the high-voltage side. They are connected into the short circuit by opening of the parallel switches  22 ,  23 ,  24 . 
         [0039]    Thus, advantageously the critical fault clearing time for electrical generators  5  in the event of a fault is increased, both on the transformer low-voltage side and on the transformer high-voltage side. An expansion of the circuit topology with switchable or adjustable reactances can increase the fault clearing time further still. 
         [0040]    In accordance with the invention, therefore, the critical fault clearing time can be considerably extended without needing to make any design changes to the turbine and generator  5 , which results in an inexpensive measure of the invention illustrated here. In addition, no grid isolation during the temporally limited short circuit is required, so that permanent availability of the electrical generator  5  without resynchronization can be achieved.