Abstract:
A fault current limiter that maximizes transient stability by minimizing the power swing experienced by the generator during a fault condition is disclosed. A superconducting fault current limiter (SCFCL) is used, whereby the impedance of the SCFCL changes in the presence of a fault. In parallel with the SCFCL is a shunt impedance, which is the impedance seen by the generator during the fault. By decreasing the ratio of the reactance of the shunt impedance to its resistance, the stability of the power system may be enhanced.

Description:
[0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 61/356,285, filed Jun. 18, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    This disclosure relates to fault current limiters, and more particularly to fault current limiters used to improve transient stability of a power system. 
       BACKGROUND 
       [0003]    A fault current limiter is a device that limits fault currents in a power system. The power system may include transmission and distribution networks to deliver power to differing loads. A fault current is an abnormal current in an electrical circuit due to a fault such as a short circuit resulting in a short circuit current. A fault current may occur due to severe weather damaging power lines and components, e.g., lighting striking the power system. When faults occur, a very small load appears instantaneously. The network, in response, delivers a large amount of current (i.e. fault current) to this load or, in this case, the faults. This surge or fault current condition is undesirable as the condition may damage the network or equipment connected to the network. In particular, the network and the equipment connected thereto may burn or, in some cases, explode. 
         [0004]    Turning to  FIG. 1 , a circuit diagram of a power system  100  having a , conventional fault current limiter (FCL)  106  is illustrated. The conventional power system includes an AC power generator  102  to provide power to a load  110 . The FCL  106  may be coupled in series with the power generator  102  and a downstream circuit breaker  108 . A fault condition may occur at location  112  illustrated as an inadvertent path to ground. 
         [0005]    Turning to  FIG. 2 , a timing diagram illustrates the general operation of the FCL  106 . During a steady state time interval, the power generator  102  supplies the load  110  and the circuit breaker  108  is in a closed position. The FCL  106  generally has little to no resistance in the steady state mode and allows AC current (i ac ) to flow to the load  110 . During a fault interval, a fault may occur at location  112 . In response, the power generator  102  attempts to deliver a large amount of fault current. The FCL  106  essentially limits the fault current by reducing the peak to peak value of the fault current provided to the load  110  and potentially other components during this fault time interval before the circuit breaker  108  can open. Since it is important to maintain very low resistance during normal operation, typically current limiting is provided by introducing a larger reactance. Typically, the magnitude of this reactance is at least thirty times larger than the resistance of the FCL  106 . During the next post fault time interval, the circuit breaker  108  opens and no current is provided. 
         [0006]    The FCL  106  may be a superconducting fault current limiter (SCFCL). In general, a SCFCL has a superconductor that is normally in a superconducting state with essentially a negligible resistance during the steady state time interval. During a fault condition, the superconductor transitions from the superconducting state to a normal conducting state (quench). This extra resistance reduces or limits the fault current during the fault condition time interval. 
         [0007]    A conventional FCL  106  is primarily dedicated to the fault current limiting function only. An equivalent impedance of the FCL  106  is given in Cartesian form by Z FCL =R FCL +jX FCL , where the real part of impedance is the resistance (R FCL ) and the imaginary part is the reactance (X FCL ). The SI unit for both the resistance (R FCL ) and reactance (X FCL ) is the ohm. As described above, the ratio of reactance to resistance or the X FCL /R FCL  ratio is greater than 30 for the conventional FCL  106 . In most instances, it is typically as high as 100-300. 
         [0008]    Turning to  FIG. 3 , a graph of plots of power versus angles are given for power transfer from the generator  102  to the load  110  during certain time intervals. These time intervals include the steady state power (Pess) during the steady state time interval, power (Pef) during the fault time interval, and power (Pepf) during the post fault interval once the circuit breaker  108  opens. The plots are for a conventional FCL  106  having an X FCL /R FCL  ratio greater than 30. The power transfer from the generator  102  to the load  110  during these time intervals are given by the below equations. 
         [0000]    
       
         
           
             
               Steady 
                
               
                   
               
                
               State 
                
               
                 : 
               
                
               
                   
               
                
               Pess 
             
             = 
             
               
                 
                   Vs 
                   · 
                   Vr 
                 
                 Xs 
               
               · 
               
                 sin 
                  
                 
                   ( 
                   δ 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               During 
                
               
                   
               
                
               Fault 
                
               
                 : 
               
                
               
                   
               
                
               Pef 
             
             = 
             
               
                 I 
                 F 
                 2 
               
                
               
                 R 
                 FCL 
               
             
           
         
       
       
         
           
             
               Post 
                
               
                   
               
                
               Fault 
                
               
                 : 
               
                
               
                   
               
                
               Pepf 
             
             ≈ 
             
               
                 
                   Vs 
                   · 
                   Vr 
                 
                 
                   Xs 
                   + 
                   
                     X 
                     FCL 
                   
                 
               
               · 
               
                 sin 
                  
                 
                   ( 
                   δ 
                   ) 
                 
               
             
           
         
       
     
         [0009]    As shown in  FIG. 3 , the plot shows a fault condition occurring at angle δ 0 . At this point, the power transfer  302  during the fault condition (represented as Pef) is significantly lower than the mechanical power (P M ) of the generator  102 . Since R FCL  is small, the power transfer  302  during the fault (Pef), which is defined as the fault current squared (I F   2 ) multiplied by the resistance of the fault current limiter (R FCL ), is also very small and may approach 0. The difference between the mechanical power (P M ) and the power transfer  302  during a fault (Pef) is given by ΔP 1 =(P M −Pef)˜P M  in this instance. At angle δ 1 , the circuit breaker opens. The area defined between these two angles (δ 0  and δ 1 ) and between P M  and Pef, is labeled A 1 . This area, A 1 , is the energy gained when the generator is accelerating during the fault. For stable power system operation, this area must equal the area, A 2 , which is the energy lost after the fault has been cleared. The area A 2  is defined as the region between the mechanical power (P M ) and the power transfer post fault (Pepf) and between angles δ 1  and δ 2 . The angle δ 2  is defined as the angle at which the area of A 2  is equal to the area of A 1 . For stable operation, δ 2  must be less than 180°. Therefore, the area of region A 1  is critical to the stability of the system. The speed at which the fault is detected, as measured by δ 1 -δ 0 , is one criteria to minimizing instability. The second important parameter is the power swing (ΔP 1 ) during the fault condition. Large power swings adversely impact transient stability of the power system. 
         [0010]    Accordingly, there is a need in the art for a fault current limiter that overcomes the above-described inadequacies and shortcomings. 
       SUMMARY 
       [0011]    A fault current limiter that maximizes transient stability by minimizing the power swing experienced by the generator during a fault condition is disclosed. A superconducting fault current limiter (SCFCL) is used, whereby the impedance of the SCFCL changes in the presence of a fault. In parallel with the SCFCL is a shunt impedance, which is the impedance seen by the generator during the fault. By decreasing the ratio of the reactance of the shunt impedance to its resistance, the stability of the power system may be enhanced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which like elements are referenced with like numerals, and in which: 
           [0013]      FIG. 1  is a circuit diagram of a power system including a conventional fault current limiter; 
           [0014]      FIG. 2  is a timing diagram illustrating operation of the conventional fault current limiter in the system of  FIG. 1 ; 
           [0015]      FIG. 3  are plots of power transfer from the generator to the load of  FIG. 1  with a conventional fault current limiter; 
           [0016]      FIGS. 4A-B  are circuit diagrams of a power system having a fault current limiter consistent with an embodiment of the disclosure; 
           [0017]      FIG. 5  are plots of power transfer from the generator to the load for the embodiment shown in  FIG. 4A-B ; and 
           [0018]      FIG. 6  is a schematic diagram of one embodiment of a fault current limiter consistent with the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0020]      FIG. 4A  is a circuit diagram of a power system  400  having a fault current limiter  406  consistent with an embodiment of the disclosure. Other components of  FIG. 4  similar to  FIG. 2  have like reference numerals and hence any repetitive description is omitted herein for clarity. The fault current limiter  406  is comprised of a superconducting fault current limiter  407  and a shunt reactor  408 . The superconducting fault current limiter  407  has an impedance, which includes a reactance and a resistance, which may be expressed as Z sc =R sc +jX sc . The resistance (R sc ) of the superconducting fault current limiter  407  changes based on current. Under normal conditions, the resistance (R sc ) of the SCFCL  407  is nearly zero. Under fault current conditions, the resistance (R sc ) of the SCFCL  407  may increase to a very large value. The shunt reactor  406  also has an impedance, which includes a resistance and a reactance, which may be expressed as Z sh =R sh +jX sh . 
         [0021]    Since the SCFCL  407  and shunt reactor  408  are in parallel, an equivalent circuit can be created, where the series impedance (Z FCL ) is equal to R FCL  where Z FCL  is the parallel combination of Z sh  and Z sc . 
         [0022]    As mentioned above, under normal operation, the impedance of the superconducting fault current limiter  407  is roughly equal to 0, and therefore, the series impedance (Z FCL ) during normal operation is approximately equal to 0 as well. During a fault, the resistance (R sc ) of the SCFCL  407  may increase to a very large value, such that the series impedance (Z FCL ) is roughly equal the impedance of the shunt reactor (Z sh =R sh +jX sh ). 
         [0023]      FIG. 4B  is a circuit diagram of a power system  400  having a fault current limiter  406  consistent with an embodiment of the disclosure. The shunt reactor  408  and the SCFCL  407  have been replaced with the equivalent series impedance (Z FCL ), as described above. 
         [0024]    Advantageously, the fault current limiter (FCL)  406  has an X FCL /R FCL  ratio less than or equal to 30. 
         [0025]    Turning to  FIG. 5 , a graph of plots of power versus angles are given during certain time intervals. These time intervals include the steady state power (Pess) during the steady state time interval, power (Pef) during the fault time interval, and power (Pepf) during the post fault interval once the circuit breaker  108  opens. The power transfer from the generator  102  to the load  110  during these time intervals are given by the following equations, as was presented earlier. 
         [0000]    
       
         
           
             
               Steady 
                
               
                   
               
                
               State 
                
               
                 : 
               
                
               
                   
               
                
               Pess 
             
             = 
             
               
                 
                   Vs 
                   · 
                   Vr 
                 
                 Xs 
               
               · 
               
                 sin 
                  
                 
                   ( 
                   δ 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               During 
                
               
                   
               
                
               Fault 
                
               
                 : 
               
                
               
                   
               
                
               Pef 
             
             = 
             
               
                 I 
                 F 
                 2 
               
                
               
                 R 
                 FCL 
               
             
           
         
       
       
         
           
             
               Post 
                
               
                   
               
                
               Fault 
                
               
                 : 
               
                
               
                   
               
                
               Pepf 
             
             ≈ 
             
               
                 
                   Vs 
                   · 
                   Vr 
                 
                 
                   Xs 
                   + 
                   
                     X 
                     FCL 
                   
                 
               
               · 
               
                 sin 
                  
                 
                   ( 
                   δ 
                   ) 
                 
               
             
           
         
       
     
         [0026]    The plots are with the FCL  406  having an X FCL /R FCL  ratio less than or equal to 30. In this case, the value of R FCL  was much larger than that used in  FIG. 3 . As a result, the plot  502  of power (Pef) during the fault is advantageously only slightly less than the mechanical power (P M ) of the generator  102 . In this instance, the difference between the mechanical power (P M ) and the power transfer during a fault (Pef) is given by ΔP 2 =(P M −Pef) which is trending towards 0. In contrast to  FIG. 3 , ΔP 2  is much less than ΔP 1 . Therefore, the disturbance to the generator  102  is minimized compared to the greater disturbance shown in  FIG. 3 . Accordingly, transient stability is improved with the FCL  406  having an X FCL /R FCL  ratio less than or equal to  30 . In general, the FCL  406  with such a X FCL /R FCL  ratio reduces power swings (ΔP 2  is much less than ΔP 1 ) as the I 2 R losses in the FCL  406  provides electrical power output during the fault as illustrated by plot  502 . In other words, the lower X FCL /R FCL  ratio of the FCL  406  instantaneously inserts a load that sinks active power. In this way, the generator  102  “sees” a minimum loss of load which promotes a more stable operation. 
         [0027]    Turning to  FIG. 6 , a schematic diagram of a fault current limiter  600  consistent with the disclosure that can provide an X FCL /R FCL  ratio less than or equal to 30 is illustrated. To accomplish this, one or more of the reactance and resistance of the fault current limiter may be varied to lower the X FCL /R FCL  ratio from its conventionally higher value to a value less than or equal to 30. 
         [0028]    In the embodiment of  FIG. 6 , the FCL  600  may be a superconducting fault current limiter (SCFCL) and may be described as such herein. An internal shunt reactor  618  and/or an external shunt reactor  648  may be connected as illustrated to the electrical bushings  616 . Each shunt reactor  618  and  648  may be a winding fabricated of material such as copper or aluminum. The cross sectional area of one or both shunt reactors  618  and  648  may be selected so that the effective X FCL /R FCL  ratio of the SCFCL  600  is less than or equal to 30. 
         [0029]    The SCFL  600  may include other components such as an enclosure or tank  602  defining a chamber therein. In one embodiment, the tank  602  may be thermally and/or electrically insulating tank  602  such as those made with fiberglass or other dielectric material. In another embodiment, the tank  602  may be a metallic tank comprising inner and outer layers  602   a  and  602   b , and a thermally and/or electrically insulating medium interposed there between. Within the tank  602 , there may be one or more fault current limiting units  620  which, for the purpose of clarity and simplicity, are shown as a block. One or more superconducting circuits may be disposed in the fault current limiting units  620 . 
         [0030]    The SCFL  600  may also comprise one or more electrical bushings  616 . The bushings  616  may comprise an inner conductive material (not shown) and an outer insulator. The distal end of the bushings  616  may be coupled to a respective power line  642  ( 642   a  and  642   b ) via terminals  644  and  646 . The power lines  642  may be transmission or distribution lines of a power system. The inner conductive material in the bushings  616  may connect the terminals  644  and  646  of the bushings  616  to the fault current limiting unit  620 . Meanwhile, the outer insulator is used to insulate the tank  602  from the inner conductive material, thereby allowing the tank  602  and the terminals  644  and  646  to be at different electrical potentials. 
         [0031]    The temperature of one or more fault current limiting units  620  may be maintained at a desired temperature range by coolant  614  contained in the tank  602 . In one embodiment, it may be desirable to maintain the fault current limiting units  620  at a low temperature, for example, ˜77° K. To maintain at such a low temperature range, liquid nitrogen or helium gas may be used as coolant  614 . In another embodiment, it may be desirable to maintain the temperature of the one or more fault current limiting units  620  at other temperature range, and other types of coolant, in gaseous or liquid form, may also be used. For example, it may be desirable to maintain the temperature of the fault current limiting units  620  at a room temperature. In such a case, air maintained at a room temperature may also be used as the coolant  614 . When introduced, the coolants  614  may enter the tank  602  via a feed line (not shown) and a port  615  coupled to the tank  602 . In the present disclosure, the feed line and the port  615  may preferably be made from thermally and/or electrically insulating material. If the feed line and the port  615  do not provide grounding of the tank  602  or any component contained therein, they may be made from any type of material. 
         [0032]    The tank  602  may be supported from the ground by an optional external support  634 . Meanwhile, the fault current limiting units  620  may be supported from the tank  602  by an optional internal support  632 . Those of ordinary skill in the art may recognize that both of the internal supports  632  and the external support  634  may be optional as the fault current limiting units  620  may be supported from the tank  602  by some other components. If included, each of the internal support  632  and the external support  634  may preferably be made from thermally and/or electrically insulating material. 
         [0033]    In operation, a superconductor of the fault current limiting units  620  is in a superconducting state and the SCFCL  600  provides negligible resistance to the system under normal or steady state operating conditions. During a fault condition, the superconductor transitions from the superconducting state to a normal conducting state to add resistance which limits the fault current during the fault condition. The SCFCL  600  has an X FCL /R FCL  ratio of equal to or less than 30 to assist with transient stability of the power system to which it is coupled. 
         [0034]    There has thus been provided a fault current limiter with an X FCL /R FCL  ratio of equal to or less than 30. Such a fault current limiter can improve power system transient stability significantly by damping the dynamic disturbance to the power system. In particular, such a fault current limiter may reduce power swings as the I 2 R losses of the fault current limiter provides electrical power during the fault. In other words, the lower X FCL /R FCL  ratio of the fault current limiter instantaneously inserts a load that sinks active power. In this way, generators of the power system experience a minimum loss of load which promotes a more stable operation. 
         [0035]    Furthermore, such a fault current limiter with an X FCL /R FCL  ratio of equal to or less than 30 provides additional benefits. One additional benefit is a reduction in transient overvoltage for a circuit breaker in the power system such as the circuit breaker  108  of  FIG. 4A . A transient overvoltage condition may occur for loads  110  that are more inductive in nature as the circuit breaker  108  opens to interrupt such an inductive circuit. The amplitude of the transient overvoltage may be as great as two times the rated voltage. For example, for a rated 13.8 kV circuit breaker the transient overvoltage may be as high as 27.6 kV. In contrast, with a fault current limiter with an X FCL /R FCL , ratio of equal to or less than 30 positioned as illustrated in  FIG. 4 , the circuit breaker  108  may experience transient overvoltage of only 20% higher than the rated voltage or about 16.56 kV for a 13.8 kV rating. 
         [0036]    Yet another benefit of a fault current limiter with an X FCL /R FCL  ratio of equal to or less than 30, is that is reduces the rate of rise of recovery voltage (RRRV) for the circuit breaker, which is the slope of the transient recovery voltage at the instant of current interruption. 
         [0037]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes.