Abstract:
An isolation transformer is introduced into a matrix fault current limiter (MFCL) and is used to couple elements in trigger matrices and current limiting matrices. The isolation transformer can either be a voltage step-up or step-down configuration. In step-up configurations, the increased voltage supplied to the current limiting elements improves the quenching of the superconductor. In step-down configurations, current limiting elements are subject to lower voltage potentials thereby reducing the electrical insulation requirement between the trigger matrix and the current limiting matrix. In addition, the voltage amplification coefficient of each isolation transformer can vary for different columns of the current limiting matrix to maximize the current limiting performance.

Description:
BACKGROUND 
   The invention relates generally to a current limiter and more particularly to a superconducting current limiter with a transformer coupled trigger mechanism. 
   Current limiting devices are critical in electric power transmission and distribution systems. For various reasons such as lightening strikes, short circuit conditions can develop in various sections of a power grid causing sharp surge in current. If this surge of current, which is often referred to as fault current, exceeds the protective capabilities of the switchgear equipment deployed throughout the grid system, it could cause catastrophic damage to the grid equipment and customer loads that are connected to the system. 
   Superconductors, especially high-temperature superconducting (HTS) materials, are well suited for use in a current limiting device because of their intrinsic properties that can be manipulated to achieve the effect of a “variable impedance” under certain operating conditions. A superconductor, when operated within a certain temperature and external magnetic field range (i.e., the “critical temperature” (T c ,) and “critical magnetic field” (H c ,) range), exhibits no electrical resistance if the current flowing through it is below a certain threshold (i.e., the “critical current level” (J c ,)), and is therefore said to be in a “superconducting state.” 
   However, if the current exceeds this critical current level the superconductor will undergo a transition from its superconducting state to a “normal resistive state.” This transition of a superconductor from a superconducting state to a normal resistive state is termed “quenching.” Quenching can occur if any one or any combination of the three factors, namely the operating temperature, external magnetic field or current level, exceeds their corresponding critical level. 
   The surface plot shown in  FIG. 1  illustrates the inter-dependency among these three factors (T c , H c , and J c ,) for a typical superconducting material type. As shown in  FIG. 1 , the surface plot includes three axes T, H, and J, where T c  is the critical temperature below which the superconducting material must be cooled to remain in the superconducting state, where H c  is the critical magnetic field above which the superconducting material cannot be exposed in order to remain in a superconducting state, and where J c  is the critical current density in the superconducting material that cannot be exceeded for the superconductor to remain in a superconducting state. The “critical J-H-T surface” represents the outer boundary outside of which the material is not in a superconducting state. Consequently, the volume enclosed by the critical J-H-T surface represents the superconducting region for the superconducting material. 
   A superconductor, once quenched, can be brought back to its superconducting state by changing the operating environment to within the boundary of its critical current, critical temperature and critical magnetic field range, provided that no thermal or structural damage was done during the quenching of the superconductor. An HTS material can operate near the liquid nitrogen temperature 77 degrees Kelvin (77 K) as compared with a low-temperature superconducting (LTS) material that operates near liquid helium temperature (4 K). Manipulating properties of a HTS material is much easier because of its higher and broader operating temperature range. 
   The quenching of a superconductor to the normal resistive state and subsequent recovery to the superconducting state corresponds to a “variable impedance” effect. A superconducting device with such characteristics is ideal for a current limiting application. Such a device can be designed so that under normal operating conditions, the operating current level is always below the critical current level of the superconductors, therefore no power loss (I 2 R loss) will result during the process. When the fault conditions occurs the fault current level exceeds the critical current level of the superconducting device, thus creating a quenching condition. By the same token, mechanisms altering the device operating temperature and/or magnetic field level can be put in place either as a catalyst or an assistant to achieving faster quenching and recovery of such a superconducting device. 
   For some HTS materials such as the bulk BSCCO elements, there often exist, within the volume of the superconductor, non-uniform regions resulted from manufacturing process. Such non-uniformed regions can develop into the so-called “hot spots” during the surge of current that exceeds the critical current level of the superconductor. Essentially, at the initial stage of the quenching by the current, some regions of the superconductor volume become resistive before others do due to the non-uniformity. A resistive region will generate heat from its associated i 2 r loss. If the heat generated could not be propagated to its surrounding regions and environment quickly enough, the local heating will damage the superconductor and could lead to the breakdown (burn-out) of the entire superconductor element. 
   U.S. Patent Publication Ser. No. 2003/0021074A1, Ser. No. 10/051,671, published Jan. 30, 2003, entitled, “Matrix-type Superconducting Fault Current Limiter” assigned to the assignee of the present invention, incorporated by reference in its entirety, uses a mechanism that combines all three of the quenching factors of the superconductor, namely current, magnetic field and temperature, to achieve a more uniformed quenching of superconductor during current limiting. This so-called MFCL concept can dramatically reduce the burnout risks in bulk superconducting materials due to the non-uniformity existed in the superconductor volume. In addition, the detection of a fault and subsequent activation of the current-limiting impedance of the MFCL are done passively by built-in matrix design, without assistance of active control mechanism. This makes a fault current limiter based on the MFCL concept more easily designed, built and operated for a wide range of potential current-limiting applications. 
   The MFCL concept utilizes the voltage generated by the quenching of superconducting elements in the so-called trigger matrix and the magnetic field generated in parallel-connected trigger inductors ( ) from that voltage, to quench the superconducting elements in the so-called current-limiting matrix. The magnetic coupling is achieved by physically wind the parallel-connected coils of the trigger matrix, directly around the superconducting elements in the current-limiting matrix. Because of this intricate relationship between the elements of the two matrices, the design of the MFCL requires careful consideration of voltage, magnetic field strength, coil design and various other factors. 
   BRIEF DESCRIPTION 
   It is an object of this invention to introduce a MFCL where the voltage used to generate magnetic field to quench superconducting elements in the current-limiting matrix can be controlled and isolated. To that end an isolation transformer is used to couple the voltage generated by the quenching of superconducting elements in the so-called trigger matrix, and the voltage used to generate magnetic field to quench superconducting elements in the current-limiting matrix. The transforming ratio of this transformer is then be used to control the latter voltage to accomplish any design requirement. This configuration also reduces design dependency of the trigger matrix and the fault current matrix. 
   Briefly, in accordance with one embodiment of the present invention, a current limiting device incorporates components made of superconducting and non-superconducting electrically conductive materials. This so-called Matrix Fault Current Limiter (MFCL) device includes a trigger matrix having “1×n” (column×row) number of trigger elements electrically connected in series with a current limiting matrix containing “m×n” number of current-limiting elements. Each trigger element within the trigger matrix includes one non-inductively arranged superconducting component electrically connected in parallel with a non-superconducting inductor as well as the primary winding of an isolation transformer. Each current limiting element within the current limiting matrix includes one non-inductively arranged superconducting component electrically connected in parallel with one non-superconducting inductor as well as another inductor that is electrically connected in parallel with the secondary winding of the isolation transformer. The inductor that is electrically connected in parallel with the secondary winding of the isolation transformer, is physically wound around the superconducting component of the current-limiting element to achieve the magnetic coupling. 

   
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
       FIG. 1  shows the inter-dependency of critical current level, critical temperature and critical magnetic field of a typical superconducting material. 
       FIG. 2  shows an example of an MFCL device being used in an AC circuit that is representative of a single-phase electric power transmission and distribution system in its simplest form. 
       FIG. 3  shows a block diagram of the matrix type current limiter MFCL. 
       FIG. 4  shows a preferred embodiment of the trigger matrix incorporating isolation transformers and associated current limiting matrix. 
       FIG. 5  illustrates a preferred embodiment of the physical relationship between the inductors and superconductors for the MFCL configured as a single trigger matrix element having one isolation transformer, coupled to corresponding current limiting elements of its respective row in the MFCL matrix and an electrical representation thereof. 
       FIG. 6  illustrates an alternative embodiment of the physical relationship between the inductors and superconductors for the MFCL configured such that for respective rows of the MFCL matrix there is one isolation transformer for the trigger matrix element and for each current limiting element. 
       FIG. 7  illustrates a further alternative embodiment of the physical relationship between the inductors and superconductors for the MFCL,configured as a single isolation transformer (for each trigger matrix element) having multiple secondary windings, one secondary winding for at least one current limiting element in the respective row of the current limiting matrix. 
       FIG. 8  illustrates a further alternative embodiment of the physical relationship between the inductors and superconductors of a dual current limiting element. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows an AC circuit  200  that is representative of a single-phase electrical power transmission and distribution system in its simplest form. The AC circuit  200  includes an AC source  210  that is a single-phase power source in a three-phased electricity transmission or distribution network with associated impedance Z source  and overall line impedance Z Line . The AC source  210  supplies a load  216  that has associated impedance Z Load . Electrically connected in series between the AC source  210  and the load  216  is a matrix fault current limiter (MFCL)  212  device having an associated impedance Z MFCL  when a fault condition occurs, and a conventional circuit breaker  214 . Absent the MFCL  212  in the AC circuit  200 , the fault current level when the load  216  is electrically shorted to ground is determined by i 1 =V Source /(Z Source +Z Line ). However, the inclusion of the MFCL  212  in the AC circuit  200  limits the fault current level to a value i 2 =V Source /(Z Source +Z Line +Z MFCL ). As long as Z MFCL  is non-zero when the fault occurs, the i 2  level is lower than i 1 , thus achieving the fault current limiting function. If i 2  is limited to within the designed fault current interrupting capability of the circuit breaker  214 , the circuit breaker  214  will be able to function normally, thereby providing protection to the power grid and customer equipment. 
   MFCL  212  functions essentially as a “variable impedance” in an electric network. Under normal operating conditions the impedance of the MFCL  212  device is essentially zero. When there is a sudden surge of current in the network, due to presence of a fault condition, the MFCL  212  immediately introduces pre-determined impedance Z MFCL  into the network, thus achieving the current limiting function. As described earlier, superconducting materials can undergo a transition from a superconducting no-electrical-resistance state to a normal resistive state (i.e., quenching) when any one or any combination of three factors, namely the passed-through current, the external magnetic field and the operating temperature, exceeds their corresponding “critical level.” The quenching of a superconductor and subsequent recovery to its superconducting state corresponds to a “variable impedance” effect. The “variable impedance” feature of the MFCL  212  is achieved by incorporating such superconducting components into the device design and by manipulating the three factors that trigger the quenching of those superconducting components. The superconducting components are represented by variable resistance symbols in all the figures contained within. 
   As shown in  FIG. 2 , the MFCL  212  includes a trigger matrix  218  arranged between a node A and a node B in series with a current-limiting matrix  220  that is arranged between node B and a node C. The primary function of the trigger matrix  218  is, under a fault condition, to generate a voltage that can be used to create additional magnetic field that is sufficient enough to trigger the quenching of superconducting components in the current-limiting matrix  220 . The primary role of the current-limiting matrix  220  is to provide a majority of the required overall current-limiting impedance once the superconducting components in the MFCL  212  are transitioned to their resistive state during the fault. 
     FIG. 3  illustrates a high-level block diagram of the MFCL  212  that includes a “1×n” (column×row) trigger matrix  218  and an “m×n” current-limiting matrix  220 . The trigger matrix  218  includes a plurality of trigger matrix elements  310  (i.e., trigger matrix elements  310 - 1  through  310 -n) while the current-limiting matrix  220  contains a plurality of current-limiting modules  312  (i.e., modules  312 - 1  through  312 -m). Each current-limiting module  312  includes a plurality of current-limiting elements  314  (i.e., current-limiting elements  314 - 1  through  314 -n). Each trigger matrix element  310  is to trigger “m” number of current-limiting elements  314  that have the same row number. For example, trigger element  310 - 1  is to trigger all current-limiting elements  314 - 1  of modules  312 - 1  through  312 -m. 
   In the prior art the voltage of the triggering coil is supplied from the voltage of a quenched superconducting triggering element. In one instance the voltage difference between the triggering coils and the superconducting current limiting elements, which are closely adjacent the triggering coils because they are magnetically coupled together, could be large and may cause electrical insulation problems. In this invention, one stepped-down transformer  316 - 1  is installed between a quenched triggering element  310 - 1  and the current limiting element  314 - 1  to isolate the voltage between the triggering element  310 - 1  and the current limiting element  314 - 1 . As the result, the dielectric insulation incompatibility between the trigger elements and the current limiting elements is improved. Moreover, the voltage of the triggering coils can now be controlled by predetermining the isolation transformer&#39;s  316 - 1  winding ratio. In another instance, the voltage generated by the quenched superconducting trigger element could be low, a stepped-up transformer  316 - 1  can then be installed between the trigger element  310 - 1  and current limiting element  314 - 1  so that a higher voltage across triggering coils can be achieved to generate higher magnetic field to quench the superconducting component in  314 - 1 . Again, the winding ratio of the isolation transformer  316 - 1  determines the degree of such a voltage transformation. The term “magnetic coupling” is used to describe the physical arrangement between the triggering coil and the superconductor element in which the magnetic field generated in the triggering coil is used to further quench the superconducting element during the current limiting process. 
     FIG. 4  illustrates one embodiment of the MFCL  212  of the present invention. Here an isolation transformer  316 - 1  is electrically coupled in parallel with the first element of trigger matrix  218 . The primary winding T p1  of isolation transformer  316 - 1  is electrically coupled in parallel to LL 1  and RR 1  of the first trigger matrix element  310 - 1 . The secondary winding T s1  of isolation transformer  316 - 1  is magnetically coupled to each of the current-limiting elements  314 - 1  of the current limiting modules  312 - 1  through  312 -m via inductor L T11  through L T1m . Each trigger inductor, from  318 - 1  through  318 -m in row one, up to and including  324 - 1  through  324 -m in row n is magnetically coupled with its corresponding superconducting element R 11  through R nm  ( 318 - 1  to R 11 ,  318 - 2  to R 12 , . . . etc.). One isolation transformer  316  is coupled to each of the elements in trigger matrix  218 , from  316 - 1  up to and including isolation transformer  316 -n, which is electrically coupled in parallel to trigger element  310 -n. The primary winding T pn  of the isolation transformer  316 -n is electrically connected in parallel to superconducting element RR n  and non-superconducting element LL n . The secondary winding T sn  of the isolation transformer  316 -n is magnetically coupled to each of the current limiting elements  314 -n via inductors L Tn1  ( 324 - 1  through L Tnm  ( 324 -m). The transforming ratio of each isolation transformer  316 - 1  through  316 -n is x:1, where “×” is the relative ratio of the primary winding to the secondary winding and “1” is the relative ratio of the secondary winding to the primary winding. 
     FIG. 5  illustrates an example of the physical relationship between the non-superconducting inductors, isolation transformer  316 - 1  and superconductors in the MFCL. In illustration  404 , Inductor LL 1 , is wound around superconductor RR 1 . The primary winding T p1  of isolation transformer  316 - 1  is electrically connected in parallel with LL 1  and RR 1 . The secondary winding T s1  of isolation transformer  316 - 1  is electrically connected in parallel to inductors L T11  ( 318 - 1 ) through L T1m  ( 318 -m). Inductor L T11  ( 318 - 1 ) is physically wound around superconductor R 11 , and each inductor L T2  through L T1m  is physically would around the respective superconducting element R 12  through R 1m , creating the magnetic coupling between the two. This electrical and magnetic relationship is repeated for each additional trigger element (2−n) and each additional current limiter matrix row (2−n) (not shown). The non superconducting inductors of the current limiters of the present invention may comprise devices selected from the group including rods, bars, tubes, bifilar-wound solenoid coils or other non-inductive devices, which devices are known in the art. The superconducting components of the current limiters of the present invention may comprise coils of electrically conductive materials selected from the group including helically-wound solenoid coils, racetrack coils, or saddle coils, which devices are known in the art. 
     FIG. 6  illustrates an alternative embodiment of the present invention, wherein a separate isolation transformer is coupled to each element in the trigger matrix  218  and each element in the current limiter matrix  220 . In illustration  500 , isolation transformer  316 - 1  for the trigger element  310 - 1  is coupled to RR 1  and LL 1  through its primary winding T p1 . Isolation transformer  316 - 1  is electrically connected in series to inductor L T11  ( 318 - 1 ) and to the primary winding T p11  of isolation transformer  502 - 1 . (L T11  is magnetically coupled to superconductor R 11 .) The secondary winding T s11  of isolation transformer  502 - 1  is electrically connected in series to inductor L TI2  ( 318 - 2 ) and to the next isolation transformer  502 - 2 . This current limiter element configuration is repeated for each current limiting matrix element in the MFCL. This electrical and magnetic relationship in illustration  500  is repeated for each additional trigger element  310 - 2  through  310 -n and their corresponding isolation transformer  316 - 1  through  316 -n, and each additional current limiting matrix row (2−n) (not shown). 
     FIGS. 7 and 8  illustrates additional alternative embodiments of the physical relationship between the inductors, isolation transformer, and superconductors of the MFCL. Illustration  600  shows that in the first trigger matrix element  310 - 1 , inductor LL 1  is wound around superconductor RR 1  The primary winding T p1  of isolation transformer  316 - 1  is electrically connected in parallel with LL 1  and RR 1 . A plurality of secondary windings T s1  through T sm  of isolation transformer  316 - 1  are magnetically coupled to current limiting elements  314 - 1  via inductors L T11  through L T1m . Inductor L T11  is physically wound around superconductor R 11  and each inductor L T12  through L T1m  is physically would around the respective superconducting element R 12  through R 1m . This electrical and magnetic relationship is repeated for each additional trigger element (2−n) and each additional current limiter matrix row (2−n) not shown. This embodiment is beneficial because the relative voltage boost or reduction from the primary winding to each respective secondary winding of isolation transformer  316 - 1  can be predetermined and/or adjusted, through utilization of different transforming ratio, to accommodate the difference between the superconducting elements in the current limiter matrix. This embodiment can also be configured so that each secondary winding of the isolation transformer can be used to be magnetically coupled to more than one or (in this example a dual) current limiting element  314 - 1 , as exemplified by the diagram in FIG.  8 . 
   While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.