Patent Publication Number: US-10326269-B2

Title: Fault current limiter system with current splitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application claiming priority of U.S. patent application Ser. No. 13/444,379 filed Apr. 11, 2012, which claims priority of U.S. provisional patent application Ser. No. 61/475,976, filed Apr. 15, 2011, the entirety of which applications are incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates to fault current limiters, and more particularly to a fault current limiter system having a fault current limiter and a variable shunt current splitting device configured to reduce the steady state current flowing through the fault current limiter. 
     BACKGROUND OF THE DISCLOSURE 
     A fault current limiter (FCL) is a device that limits fault currents, typically in a power system. Various types of FCLs have been developed over the last several decades, including superconducting fault current limiters (SCFCLs), solid state fault current limiters, inductive fault current limiters, as well as other varieties that are well known in the art. A power system in which a FCL is implemented may include generation, transmission, and distribution networks that generate and deliver power to various industrial, commercial, and/or residential electrical loads. 
     A fault current is an abnormal current in an electrical system that may result from a fault in the system, such as a short circuit. A fault current may arise in a system due to any number of events or failures, such as power lines or other system components being damaged by severe weather (e.g. lightning strikes). When such a fault occurs, a large load can instantaneously appear in the circuit. In response, the network delivers a large amount of current (i.e. fault current) to the fault load. This surge of current is undesirable because it can damage the load which may be, for example, the network itself or equipment connected to the network. 
       FIG. 1A  depicts a circuit diagram of an exemplary prior art power system  100  having a FCL with a conventional fixed shunt  114  illustrated in a steady state condition. The exemplary power system  100  includes an AC power source  102 , a circuit breaker  108  which is normally closed, and various loads  110 . Under steady state conditions, the AC power source  102  provides power to the loads  110 . The circuit breaker  108  is closed and 100% of the current from the AC power source flows through conductor  103 , the FCL  106 , and the conductor  105  to the loads  110 , as illustrated by arrow  150 . 
       FIG. 1B  depicts the circuit diagram of  FIG. 1A  illustrated in a fault condition before the circuit breaker  108  has opened. For example, a fault condition may occur at location  112  represented by the inadvertent path to ground. In response to the fault, the AC power source  102  attempts to deliver a large amount of fault current to the fault load and the FCL  106  exhibits a resistance much larger than the fixed shunt  114 . For example, if the FCL is a superconducting FCL (“SCFCL”) having a superconductor that exhibits essentially zero resistance in a superconducting, steady state condition, the fault current causes the superconductor to quench and thereby exhibit a resistance much larger than that of the fixed shunt  114 . Since the resistance of the FCL is much larger, the fault current represented by arrow  152  is commutated into the fixed shunt  114 . The fixed shunt  114  limits the fault current to an acceptable level by reducing the peak-to-peak amplitude of the fault current before the circuit breaker  108  can open. A conventional circuit breaker  108  typically takes 2 to 5 cycles of a conventional 60 Hz frequency before opening. During a post fault time interval, the circuit breaker  108  opens and no current is provided to the loads  110  either through the FCL or the fixed shunt  114 . 
     Although fixed shunt FCL systems such as the one described above can be very effective for limiting fault currents, a significant drawback of such systems is that the FCL must be configured to carry all of the anticipated steady state current of the circuit during normal operation, as described above with reference to  FIG. 1A . In high current applications this generally requires a FCL having a large physical footprint and high energy consumption. For example, in the case of a SCFCL, the FCL will include a superconductor housed in a cryogenic tank (cryostat). To operate at a nearly zero impedance, superconducting state, the superconductor must be operated below its critical temperature, critical current density, and critical magnetic field. If any one of these three levels is exceeded, the superconductor quenches from its superconducting state to a normal state and exhibits resistance much larger than the resistance of the fixed shunt  114 . To maintain the superconductor at a temperature below its critical temperature, a refrigeration system delivers a cryogenic cooling fluid to the cryostat. Accordingly, the quantity of the superconductor material, as well as the capacity of the associated cooling system to maintain the superconductor below its critical temperature, must be sufficient to accommodate all of the steady state current in the system. This may require significant equipment and energy costs. In addition, the physical size of a SCFCL required for a particular application can make installation at the application site difficult or impractical. Similar challenges exist in systems that employ solid state fault current limiters, which employ large numbers of parallel components, exhibit high power losses, and require large and costly cooling systems for handling high system load currents. It is with respect to these and other considerations that the present improvements have been needed. 
     SUMMARY 
     In view of the forgoing, a current splitting system for facilitating reduced steady state current handling in a fault current limiter is disclosed. In particular, a FCL system that includes a current splitting device with a variable shunt is disclosed. 
     An embodiment of a FCL system in accordance with the present disclosure may include a FCL electrically coupled with a variable shunt. The FCL may be any type of FCL, such as a superconducting FCL, a solid state FCL, or an inductive FCL. The current splitting device may include first and second conductive windings wound about a core, such as in a bifilar arrangement or other configuration that facilitates a strong magnetic coupling between the windings, wherein the presence or loss of such coupling introduces a variable impedance (shunt) to be used for current limiting applications. The first conductive winding may be electrically connected in parallel with the fault current limiter and is configured to carry current in a first direction. The second conductive winding may be electrically connected in series with the fault current limiter and is configured to carry current in a second direction opposite to the first direction. 
     During steady state operation of the FCL system, the current splitting device splits current into two branches that flow through the conductive windings in opposite directions to produce a net zero or negligible magnetic field, thereby resulting in a negligible equivalent or net impedance in the circuit. The reactance of the first winding is therefore substantially negated by the oppositely-directed reactance of the second winding. Thus, by selecting first and second windings having appropriate numbers of turns, a predetermined portion of steady state current can be routed through the variable shunt. The steady state current load on the FCL is thereby reduced relative to conventional FCL systems. The cost and physical size of the FCL can therefore also be reduced. 
     Upon the occurrence of a fault condition, the FCL is driven into a fault state wherein the impedance exhibited by the FCL increases and the proportion of current through the second winding and the FCL is significantly reduced relative to the first winding compared to the proportion of the currents during steady state operation. Thus, the first and second windings will no longer produce equal and opposing magnetic fields and will lose their strong magnetic coupling. The windings will therefore exhibit a higher equivalent or net current-limiting impedance relative to steady state operation, thereby limiting the fault current in the system. In the case of a superconducting FCL, the fault state is achieved by causing the FCL to quench, whereby the proportion of the current through the FCL is reduced. 
     An embodiment of the device disclosed herein can thus include a fault current limiter system comprising a fault current limiter and a variable shunt. The variable shunt may comprise first and second conductive windings wound about a core, wherein the first conductive winding is electrically coupled in parallel with the fault current limiter and is configured to carry current in a first direction. The second conductive winding is electrically coupled in series with the fault current limiter and is configured to carry current in a second direction opposite to the first direction so that a first reactance of the first winding is at least partially cancelled by a second reactance of the second winding during steady state operation of the fault current limiter system. Thus, a first portion of a steady state current is conveyed by the fault current limiter and a second portion of the steady state current is conveyed by the variable shunt. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example, a specific embodiment of the disclosed device will now be described, with reference to the accompanying drawings, in which: 
         FIG. 1A  is a circuit diagram of a power system of the prior art having a FCL and a fixed shunt in a steady state condition; 
         FIG. 1B  is a circuit diagram of the prior art consistent with  FIG. 1A  in a fault condition; 
         FIG. 2A  is a circuit diagram of a FCL system consistent with an embodiment of the disclosure; 
         FIG. 2B  is a circuit diagram consistent with  FIG. 2A  in a steady state condition; 
         FIG. 2C  is a circuit diagram consistent with  FIG. 2A  in a fault condition; 
         FIG. 2D  is a circuit diagram consistent with an alternative embodiment of the disclosure with a voltage control shunt; 
         FIG. 2E  is a circuit diagram consistent with an alternative embodiment of the disclosure with a voltage control shunt and a tuning reactor; 
         FIG. 3  is a block diagram of one embodiment of a SCFCL for use as the FCL depicted in  FIGS. 2A and 2B ; 
         FIG. 4  is circuit diagram of a variable shunt configuration that is consistent with the present disclosure; 
         FIG. 5  is a circuit diagram of an alternative variable shunt configuration consistent with the present disclosure having concentric windings; 
         FIGS. 6 and 7  are circuit diagrams of further alternative variable shunt configurations that are consistent with the present disclosure having crisscrossed windings. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 2A-2C and 4  illustrate circuit diagrams of a fault current limiter (FCL) system  200  consistent with an embodiment of the present disclosure. In particular, the FCL system  200  may comprise a current splitting device  202  coupled with a FCL  206 .  FIG. 2B  illustrates the FCL system  200  during steady state operation and  FIG. 2C  illustrates the FCL system  200  during a fault condition as further described below. 
     The FCL system  200  can be electrically connected in series intermediate an AC power source  201  and one or more electrical loads  210  via conductors  203  and  205 . It will be appreciated by those of ordinary skill in the art that the FCL system  200  can be implemented in a variety of other applications and power system configurations in which fault current limiting is desirable. The particular power system depicted in  FIGS. 2A-2C  is therefore shown by way of example only and is not intended to be limiting. 
     It is contemplated that various types of FCLs can be implemented in the FCL system  200 , including, but not limited to, superconducting fault current limiters (SCFCLs), solid state fault current limiters, inductive fault current limiters, and other types of fault current limiters that are well known in the art. For purposes of illustration, the FCL system  200  will be described herein as incorporating a superconducting FCL  206 . However, it will be understood that any of the aforementioned varieties of FCLs, as well as many varieties of FCLs that are not explicitly named herein, can be substituted for the superconducting FCL  206  without departing from the present disclosure. 
     The current splitting device  202  includes first and second windings  404  and  406  that may be configured to exhibit minimal impedance during the steady state operation shown in  FIG. 2B  and a comparatively larger impedance during the fault condition shown in  FIG. 2C  as further described below, to effectively limit the fault current. Particularly, during the steady state condition shown in  FIG. 2A , the windings  404  and  406  of the current splitting device  202  may be set to distribute the steady state current along parallel paths  207  and  209  in a predefined manner. For example, if x % of the steady state current flows along path  207  then the remainder (100−x)% of the steady state current flows along path  209 . In one embodiment, the current may be distributed so that 50% flows along path  207  and 50% flows along path  209 . In other embodiments, the ratio may be set to 40% that flows along path  207  and 60% that flows along path  209 ; 30% that flows along path  207  and 70% that flows along path  209 , etc. 
     Again,  FIG. 2B  illustrates system  200  during a fault condition. During such a fault condition, the impedance of the FCL  206  is much greater than the impedance of the current splitting device  202  so the fault current is commutated into the first winding  404  of the current splitting device  202  (i.e. along path  209 ). The current splitting device  202  still exhibits an equivalent impedance that is great enough to limit the fault current to acceptable peak-to-peak amplitudes to loads  210 . 
       FIG. 3  is a block diagram of an exemplary FCL  206  for use as the FCL system  200 . FCL  206  may be an SCFCL that includes a cryogenic tank (cryostat)  302  defining an interior chamber  303 , a superconductor  307  positioned within the chamber  303 , a refrigeration system  312 , a controller  320 , a temperature sensor  308 , and a current sensor  326 . For ease of illustration and explanation, only one FCL  206  is illustrated for accommodating a single phase AC power system. Those skilled in the art will recognize that three separate FCLs can be similarly implemented for accommodating a three phase AC power system. 
     The superconductor  307  may be any type of superconducting material, such as yttrium barium copper oxide (YBCO), that exhibits suitable superconducting properties when held below its critical temperature, critical current density, and critical magnetic field. The superconductor  307  may include a plurality of modules depending on the amount of superconducting material required for a particular application (i.e. systems that convey larger electrical currents will generally require a FCL with greater amounts of superconducting material). The refrigeration system  312  is configured to maintain the temperature of the superconductor  307  below its critical temperature, which may be between about 77° K and 93° K for high temperature superconductors. This may be achieved by cycling a cryogenic cooling fluid through a cryostat  302  via a supply conduit  316  and return conduit  314  that are operatively connected to the refrigeration system  312  and the cryostat  302 . In particular, the refrigeration system  312  may include a cryogenic cooling unit to cool the input cryogenic fluid received from the return conduit  314  before cycling the cooled fluid back to the cryostat  302  via supply conduit  316 . The cryostat  302  can be fabricated from a variety of different materials, including, but not limited to, dielectric materials and/or thermally insulating materials. The cryogenic cooling fluid may be any suitable cooling fluid, including, but not limited to, liquid nitrogen, liquid helium, liquid argon, liquid neon, and various mixtures of the same. The refrigeration system  312  may further include various valves, pumps, and sensors for facilitating fluid movement and a storage tank for storing additional quantities of cryogenic cooling fluid. 
     The controller  320  may receive input signals from a variety of systems and components, such as the temperature sensor  308  and the current sensor  326  to manage the operation of the refrigeration system  312  in accordance with input signals as further described below. The controller  320  can be, or may include, a general-purpose computer or network of general-purpose computers that may be programmed to perform desired input/output functions. The controller  320  can also include other electronic circuitry or components, including, but not limited to, application specific integrated circuits, other hardwired or programmable electronic devices, and discrete element circuits. The controller  320  may further include communication devices (e.g. WiFi, Bluetooth, etc.), data storage devices, and software. 
     The temperature sensor  308  is provided for measuring the temperature of the superconductor  307  and/or the cryogenic cooling fluid within the interior chamber  303  of the cryostat  302  and outputting such temperature measurement to the controller  320 . It is contemplated that any type of conventional temperature sensor that is capable of measuring low temperatures such as those achieved within the cryostat  302  can be utilized. The temperature sensor  308  is illustrated as being mounted on the outside of the cryostat  302 , but this is not critical. 
     The current sensor  326  may be operatively connected to the conductor  205  at a position intermediate the current splitting device  202  and the loads  210 . The current sensor  326  is provided for measuring the real-time current draw on conductor  205  and outputting such current draw measurement to the controller  320 . It is contemplated that any type of conventional current sensor may be utilized, such as, for example, a current transformer positioned about the conductor  205 . 
     In operation, the superconductor  307  will remain in a superconducting state until one of three parameters is exceeded, namely critical temperature, current density, and magnetic field. During steady state operation, the refrigeration system  312  may maintain the temperature of the superconductor  307  below its critical temperature which may be between about 77° K and 93° K for high temperature superconductors. The current splitting device  202  advantageously permits a portion of the steady state current to flow along this path. For example, 70% of the current may flow through the current splitting device  202  with the remaining 30% flowing through the FCL  206  in one embodiment. Therefore, the FCL  206 , which may be an SCFCL, may be sized appropriately given the expected steady state current levels. For example, compared to a SCFCL for use with a fixed shunt, the quantity of superconducting material necessary for the superconductor  307  may be reduced, the size of the cryogenic tank  302  may therefore also be smaller, and the capacity of the refrigeration system  312  may also be reduced accordingly including the necessary volume of cryogenic fluid. In addition, energy costs for the refrigeration system  312  may also be reduced. Accordingly, significant material and energy cost savings may be realized. Furthermore, the physical size of the FCL  206  can be reduced enabling it to be installed in locations that may otherwise have been difficult, if not impractical. 
       FIG. 4  illustrates a diagram of an exemplary embodiment of the current splitting device  202  consistent with the present disclosure that includes a first conductive winding  404  electrically connected in a reverse-parallel relationship with a second winding  406 . Current entering the variable shunt  402  is directed through the first winding  404  in a first direction and through the second winding  406  in a second, opposite direction. The variable shunt  402  optionally further includes a conductive tuning winding  408  that is electrically connected in series with the first winding  404  for facilitating precise tuning of the current limiting in the system in a manner that will become apparent below. The windings  404  and  406  may be wound about a core (e.g. core  603  shown in  FIG. 6 ), such as, for example, in a bifilar coil arrangement. Other winding arrangements are available and will be described below. It is contemplated that the core may be an iron core or an air core having dimensions that are dictated by the current limiting requirements of a particular application as will be appreciated by those of ordinary skill in the art. 
     The first winding  404  of the variable shunt  402  is electrically connected in parallel with the FCL  206  and the second winding  406  of the variable shunt  402  is electrically connected in series with the FCL  206 . During steady state operation of the system  200  (i.e. in the absence of a fault condition), the superconductor  307  will remain in a superconducting state and will exhibit substantially zero impedance. The current flowing through the first winding  404  will therefore be substantially equal to the current flowing through the second winding  406  and, because the windings  404  and  406  are arranged in the above-described reverse-parallel configuration, the windings will be magnetically-coupled and will exhibit a negligible net or equivalent impedance. 
     Thus, by selecting first and second windings  404  and  406  having appropriate numbers of turns, the steady state operation of the system  200  may be tailored to distribute steady state current along parallel paths  207  and  209  such that if x % of the steady state current flows along path  207  the remainder (100−x)% of the steady state current flows along path  209 . In one embodiment, for example, first and second windings  404  and  406  may be selected with appropriate numbers of turns for evenly distributing the current in the FCL system  200  between the paths  207  and  209 , with 50% of the current flowing along path  207  and 50% flowing along path  209 . In other contemplated embodiments, the ratio may be set to 40/60, 30/70, 20/80, etc., for example, along respective paths  207  and  209 . In some cases where current distribution must be set more precisely, the external tuning winding  408  can implemented as an optional device. 
     Upon the occurrence of a fault condition, such as illustrated in  FIG. 2C , the current through the system  200  suddenly increases where the increased fault current is measured by the current sensor  326  (shown in  FIG. 3 ). Upon receiving output from the current sensor  326  indicating a fault current above a predefined level, the controller  320  immediately “trips” the FCL  206 . In other words, the controller  320  causes the FCL  206  to go into a fault state wherein the FCL  206  exhibits an impedance that is much greater than that of the first winding  404 . In the particular case where FCL  206  is a SCFCL, this is achieved by causing the FCL  206  to quench, thereby driving the FCL  206  into a high impedance, non-superconducting state wherein the impedance of the FCL  206  is much greater than the impedance of the second winding  406 . Therefore, the fault current is commutated into the first winding  404 . The magnetic fields produced by the first and second windings  404  and  406  become decoupled and no longer cancel one another, and the impedance of the first winding increases to a current limiting impedance that effectively limits the fault current and acts as a shunt reactor for the FCL  206 . During recovery, the FCL  206  recovers and I W2  increases and eventually may reach the value of I W1  and hence there is reactance cancellation again. 
     Returning briefly to  FIGS. 2C and 4 , the relatively high impedance of the path  205  (which includes the tripped FCL  206  and the second winding  406 ) relative to the path  207  (which includes the first winding  404  and, optionally, the tuning winding  408 ) during a fault condition results in substantially the entire fault current being commutated to the first winding  404  of the current splitting device  202 . The current through the second winding  406  is therefore no longer sufficient to cancel the magnetic field generated by the first winding  404 , and the total impedance of the first winding  404  is thus greater than that exhibited during steady state operation. Thus, the current splitting device  202  acts as a shunt reactor for the FCL  206  and exhibits an impedance that is great enough to limit the fault current to acceptable peak-to-peak amplitudes. During recovery, as the FCL  206  returns to a steady state condition, currents through the first and second windings  404  and  406  may eventually return to their steady state values, thereby achieving the steady state reactance-cancellation described above. 
     In view of the above-described configuration of the system  200 , with the current splitting device  202  handling a significant portion of the steady state current in the system  200 , the FCL  206  may be sized to handle much less steady state current than it would otherwise be required to accommodate. Material and labor costs may therefore be reduced. The physical size of the FCL may also be reduced, enabling it to be installed in locations that may otherwise have been difficult, if not impractical. In addition, electromagnetic forces and their effects could also be reduced due to less active FCL components. The amount of energy dissipated in the FCL  206  may also be reduced. When the FCL  206  is a SCFCL, the quantity of superconducting material may be reduced by 50% in the case that 50% of the steady state current is diverted to the current splitting device  202 . Less energy is therefore required to cool the superconducting material below its critical temperature. The refrigeration system capacity may also be reduced, including the volume of cryogenic fluid required, thereby realizing further cost and space savings relative to conventional fault current limiters. 
     Alternative embodiments of the FCL system  200  are contemplated in which a fixed shunt  214  (voltage control shunt) is coupled in parallel with the FCL  206  for facilitating voltage control in a conventional manner, either without or with an external tuning reactor  208  as shown in  FIGS. 2D and 2E , respectively. The voltage control shunt may be a reactor, a resistor, a varistor (MOV) or any other electrical device that clamps or sets the voltage to a desirable value. 
     Although the first and second windings  404  and  406  are illustrated  FIG. 4  in a bifilar coil arrangement, other winding configurations are contemplated and can alternatively be implemented in the current splitting device  202 . For example,  FIG. 5  illustrates a variable shunt  502  having a first winding  504  and second winding  506  wound in an opposing concentric arrangement. The tuning winding  408  is connected in series to the first winding  504  and the second winding  506  is connected to FCL  206 . In another example,  FIG. 6  illustrates a variable shunt  602  having a first winding  604  and second winding  606  wound about a magnetic core  603  in a crisscrossed arrangement with each winding having an equal number of turns. Again, the first winding  604  is connected to tuning winding  408  and the second winding  606  is connected to FCL  206 . In another example,  FIG. 7  illustrates a variable shunt  702  having a first winding  704  and a second winding  706  wound about a magnetic core  703  in a crisscrossed arrangement with an unequal number of turns to further control and limit the distribution of steady state current flow through the FCL  706 . The first winding  704  is connected to tuning winding  408  and the second winding  706  is connected to FCL  206 . 
     While certain embodiments of the disclosure have been described herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.