Abstract:
An apparatus for limiting fault current in a power distribution system. Such an apparatus includes a tank for containing a cryogenic fluid; and a winding supported in the tank, the winding being electrically connected to the tank. The winding and the tank are at the same electrical potential.

Description:
FIELD OF DISCLOSURE 
       [0001]    This disclosure relates to power distribution systems, and in particular, to devices for addressing current faults in such systems. 
       BACKGROUND 
       [0002]    Power distribution systems occasionally experience sudden surges of current. These surges are often referred to as “current faults.” Because the current surge is often quite large, it is important that they be controlled or limited in some way. 
         [0003]    It is known to provide current fault limiters. In some cases, current fault limiters take the form of windings suspended in tanks filled with cryogenic fluid. The cryogenic fluid serves to cool the windings, thus causing the windings to become superconducting. 
         [0004]    A risk associated with the known current fault limiters arises from the possibility of arcing between the windings and the walls of the tank. This arcing tends to heat, and possibly boil, the cryogenic fluid. Rapid boiling of cryogenic fluid can suddenly increase gas pressure within the tank and cause a catastrophic rupture or explosion. 
         [0005]    To some extent, the choice of a cryogenic liquid having suitable insulating properties can reduce this risk. In addition, one can reduce the risk of arcing by providing sufficient clearance between the walls of the tank and the winding. 
       SUMMARY 
       [0006]    In one aspect, the invention features an apparatus for limiting fault current in a power distribution system. Such an apparatus includes a tank for containing a cryogenic fluid; and a winding supported in the tank, the winding being electrically connected to the tank. The winding and the tank are at the same electrical potential. 
         [0007]    In some embodiments, the tank includes walls forming an aperture in the tank through which a portion of the winding in the tank extends. Among these embodiments are those that include thermally insulating bushings configured to receive the portion of the winding. 
         [0008]    Among the embodiments of the invention are those that also include an electrical insulator electrically insulating the tank from ground. In some of these embodiments, the insulator includes a support stand. In others of these embodiments, the insulator includes an exterior tank surrounding the tank and separated from the tank by an insulator. 
         [0009]    Some embodiments also include a switch for selectively connecting the winding to a load. Among these are embodiments that also include a controller for controlling the switch, and those that further include a sensor for providing, to the controller, data for determining whether a current fault has occurred. 
         [0010]    In some embodiments, the apparatus also includes an electrical connector for providing an electrical connection between the winding and the tank. Among these are those in which the connector is integrated into a bushing, and those in which the connector includes a conductor extending from the winding to the tank. 
         [0011]    Other embodiments include those having means for thermally insulating the tank, means for electrically insulating the tank, means for electrically connecting the tank and the winding, means for selectively disconnecting the winding from a load, or any combination of the foregoing. 
         [0012]    A variety of windings can be used in the apparatus. However, in some embodiments, the winding includes a winding that transitions between a superconducting state and a non-superconducting state. 
         [0013]    In another aspect, the invention features an apparatus for use in a power distribution system. Such an apparatus includes a live tank; and a fault current limiter contained in the tank, the fault current limiter transforming from a superconducting state into a non-superconducting state in response to fault current. 
         [0014]    In some embodiments, the live tank is configured to contain a cryogenic fluid. 
         [0015]    In another aspect, the invention features an electric power plant for providing electric power to a power grid. Such a power plant includes a generator for generating electric power; a variable resistance path extending between the generator and the power grid; means for increasing resistance on the second path in response to a current fault; and a tank enclosing the means for increasing resistance, the tank being maintained at line potential. 
         [0016]    These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which: 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0017]      FIG. 1  shows a power distribution system that includes a current fault limiter in a tank; 
           [0018]      FIG. 2  shows a more detailed view of the current fault limiter and tank shown in  FIG. 1 ; and 
           [0019]      FIG. 3  shows an alternative embodiment of the tank in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    A power distribution system  10  incorporating a fault current limiter  12  includes a power generator  14  connected to a load  16 , such as an electric power utility, via one of two alternative paths  18   a ,  18   b  selected by a switch  20 . The first path  18   a  passes through a reactor  22  and on to the load  16 , whereas the second path  18   b  passes through the fault current limiter  12 . A controller  24  opens or closes the switch  20 , and hence selects the path  18   a ,  18   b , on the basis of information provided to it by a current sensor  26 . 
         [0021]    The fault current limiter  12 , shown in more detail in  FIG. 2 , features a winding  28  suspended within a tank  30  The winding  28  is made of a material that, when sufficiently cooled, becomes a superconductor. 
         [0022]    A tank connector  32  electrically connects the winding  28  and the tank  30 . As a result, the tank  30  and the winding  28  are at the same potential. When the winding  28  is at line potential, the tank  30  is also at line potential. For this reason, the tank  30  is said to be a “live” tank, as distinguished from a “dead” tank in which the tank is maintained at ground potential. 
         [0023]    A refrigeration system  34  circulates cryogenic fluid into and out of the tank  30  via first and second conduits  36   a ,  36   b . A pair of bushings  38   a ,  38   b  located at apertures  40   a ,  40   b  through which electric current flows into and out of the tank  30  provides thermal insulation to suppress entry of heat into the tank  30 . However, because the tank  30  is electrically connected to, and hence at the same potential as, the superconducting winding  28 , there is no need for the bushings  38   a ,  38   b  to also provide electrical insulation between the tank  30  and the winding  28 . This greatly reduces the size and cost of the bushings  38   a ,  38   b.    
         [0024]    In the particular embodiment shown in  FIG. 2 , the tank connector  32  is at one end of the winding  28 . However, the tank connector  32  can also extend from the middle of the winding  28  to the tank  30 . In addition, the tank connector  32  can be integrated into one or both bushings  38   a ,  38   b.    
         [0025]    Since the tank  30  is at the same potential as the winding  28 , it is preferable that the tank itself be electrically insulated from ground  42 . This is achieved by providing an insulating stand  44  between the tank  30  and ground  42 . Such stands  44  are commercially available for a variety of voltage levels. 
         [0026]    In operation, the winding  28  is normally superconducting, and hence presents virtually no resistance to current flowing toward the load  16 . Upon occurrence of a current fault, current in the winding  28  exceeds the critical current. As a result, the winding  28  loses its superconducting properties and begins to present considerable resistance to current flow along the second path  18   b . This, in turn, tends to quench the excess current. Meanwhile, shortly after detecting the occurrence of a current fault, the controller  24  sets the switch  20 . This diverts the current from the second path  18   b  to the first path  18   a , where it encounters the reactor  22 . This reactor  22  then develops a voltage tending to resist the current. 
         [0027]    The use of a live tank  30 , instead of a dead tank essentially eliminates the risk of arcing between the winding  28  and the tank  30 . This essentially eliminates the risk of boiling the cryogenic fluid, and thus eliminates the pressure spikes that may result in rupture or catastrophic failure of the tank  30 . The reduced likelihood of arcing also reduces the requirement for mechanical strength sufficient to accommodate pressure spikes, and thus reduces the cost of the tank  30 . 
         [0028]    In addition, because of the significant reduction in the risk of arcing between the winding  28  and tank  30 , clearance between the tank  30  and the winding  28  can be reduced. This means that the tank  30  can be made significantly smaller. In addition, the insulating properties of the cryogenic fluid filling the tank  30  become less important. For example, instead of liquid nitrogen, liquid helium, liquid neon, or liquid air can be used. 
         [0029]    In an alternative embodiment, the tank  30  can be located within an outer tank  46 , as shown in  FIG. 3 , which is filled with an insulating liquid. In such a case, the outer tank  46  serves as an insulator and thus eliminates the need for a stand  44 . In addition, because the outer tank  46  is not at line potential, there is no longer a need to provide a safety barrier to prevent for plant workers from inadvertently being electrocuted by inadvertently touching the live tank  30 .