Abstract:
A method and system for providing protection for a superconducting electrical cable located in a utility power network includes detecting a fault current on the superconducting electric cable, determining the cumulative total energy dissipated in the superconducting electrical cable from the fault current and at least one prior fault current over a predetermined time period, and determining whether to disconnect the superconducting electrical cable from the utility power network on the basis of the cumulative total energy dissipated.

Description:
RELATED APPLICATION  
       [0001]     Under 35 U.S.C. §119(e)(1), this application claims the benefit of prior U.S. provisional application 60/703,855, filed Jul. 29, 2005. 
     
    
     GOVERNMENT RIGHTS  
       [0002]     This invention was made with U.S. government support under Contract No.; DE-FC36-03G013032. The Government may have certain rights in the subject invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     Problems in a utility network, or “faults”, (such as network device failures) can affect how power is distributed throughout the network. In particular, faults tend to drain energy from power sources, leaving less energy for distribution throughout other areas of the network and for recovering from voltage “sags” resulting from the fault.  
         [0004]     When a fault occurs in a utility network, momentary voltage depressions are experienced, which may result in voltage collapse or voltage instability on the network.  
         [0005]     In general, such a fault appears as an extremely large load materializing instantly on the utility network. In response to the appearance of this load, the network attempts to deliver a large amount of current to the load (i.e., the fault). Detector circuits associated with circuit breakers on the network detect the over-current situation immediately (within a few milliseconds). Activation signals from the detector circuits are sent to protective relays which initiate opening of the circuit. The mechanical nature of the relays generally requires 3 to 6 cycles (i.e., up to 100 milliseconds) to open. When the breakers open, the fault is cleared.  
         [0006]     Power cables using high temperature superconductor (HTS) wire are being developed to increase the power capacity in utility power networks while maintaining a relatively small footprint. Among other advantages, the HTS power cables are much easier to site, even in dense, older urban areas. Such HTS cables also allow larger amounts of power to be pumped economically and reliably into congested areas of a utility power network and transferred precisely where it is needed to relieve congestion. An HTS power cable uses HTS wire in the core of the cable instead of copper for the transmission and distribution of electricity. The design of HTS cables results in significantly lower impedance compared to conventional lines and cables. The use of HTS wire enables a three to five times increase in current-carrying capability compared to alternating current (AC) conventional cables, and up to ten times more power flow through direct current (DC) conventional cables.  
         [0007]     HTS power cables behave differently than conventional non-superconducting cables to fault currents. First, a cold dielectric HTS power cable requires that the cooling liquid must remain in a sub-cooled state during a major fault or multiple through faults. This is necessary to maintain the dielectric strength between the high voltage cable core and the shield, which is at ground potential. Any bubble formation inside dielectric will threaten the dielectric properties of the insulation. Second, the cable must be off line following major faults in order to allow enough time for the HTS conductors to be cooled back down to the operating temperature range. As a result, conventional cable fault protection schemes are not suitable for use with HTS power cables.  
       SUMMARY OF THE INVENTION  
       [0008]     In a general aspect of the invention, a method of providing protection for a superconducting electrical cable located in a utility power network includes the following steps. A fault current is detected on the superconducting electric cable. The cumulative total energy dissipated (or stored) in the superconducting electrical cable is determined from the fault current and at least one prior fault current over a predetermined time period. A determination as to whether to disconnect the superconducting electrical cable from the utility power network on the basis of said cumulative total energy dissipated (or stored) is made.  
         [0009]     Embodiments of the aspect of the invention may include one or more of the following features. The step of detecting includes determining the magnitude of the fault current I j  and a time duration of the fault current t dj  on the superconducting electrical cable. The method further includes determining if the fault current I j  exceeds a predetermined threshold current level and, if it does, disconnecting the superconducting electrical cable from the utility power network for a time period based on the level of the fault current I j .  
         [0010]     The time period is based on the geometry of the superconducting electrical cable and the associated cooling system. For example, the geometry of the superconducting electrical cable includes a plurality of layers and the time period is based on A j  is cross section area of layer j, k j  is the conductivity of layer j, T j  is local temperature of layer j which is a function of position and time, R jj−1  is the thermal resistance between layer j−1 and layer j, R jj+1  is the thermal resistance between layer j and j+1, ρ i  is the density of the layer j, C j  is specific heat of layer j, and m is coolant flow rate.  
         [0011]     The method further includes determining the time elapsed between the fault current I j  and a prior fault current I j−1  when I j  does not exceed a predetermined threshold current level and determining if the time elapsed exceeds a predetermined threshold time period and if it does, maintaining the connection of the superconducting electrical cable in the utility power network. Determining the cumulative total energy value includes determining whether the cumulative total energy value is less than a critical threshold value that represents the amount of energy at which the cable would be damaged if a major fault occurs in a time period that is short than a predetermined threshold time period. In another aspect of the invention, a system for providing protection for a superconducting electrical cable located in a utility power network, includes a sensor configured to detect a fault current carried on the superconducting electrical cable; a controller configured to determine the cumulative total energy dissipated in the superconducting electrical cable from the fault current and at least one prior fault current over a predetermined time period; and configured to determine whether to disconnect the superconducting electrical cable from the utility power network on the basis of said cumulative total energy dissipated.  
         [0012]     Embodiments of this aspect of the invention may include one or more of the following features. The controller determines the magnitude of the fault current I j  and a time duration of the fault current t dj  on the superconducting electrical cable. The controller determines if the fault current I j  exceeds a predetermined threshold current level and if it does disconnects the superconducting electrical cable from the utility power network for a time period based on the level of the fault current I j . The time period is based on the geometry of the superconducting electrical cable and associated cooling system. The controller determines the time elapsed between the fault current I j  and a prior fault current I j−1  and determines if the time elapsed exceeds a predetermined threshold time period and if it does, maintaining the connection of the superconducting electrical cable in the utility power network.  
         [0013]     Among other advantages, the method and system described above provides over-current protection of superconducting electrical cables by monitoring the current flowing in a portion of a power network to which the power cable is connected. The method and system accounts for an occurring fault as well as previously occurring faults, even those that do not rise to the level of requiring immediate interruption of current through the cable (i.e., activating a circuit breaker) but nevertheless cause heating of the superconductor in the cable. By doing so, the cumulative heating effects due to previous faults are accounted for when a fault occurs. For example, a fault having a current magnitude and duration that would normally not trip a circuit breaker, might generate sufficient heat to be damaging to the superconductor cable in conjunction with heating from other recently occurring faults. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  is a schematic diagram illustrating a fault management system for providing over-current protection to an HTS cable.  
         [0015]      FIG. 2  is a isometric view of an HTS cable.  
         [0016]      FIG. 3  is a flow diagram showing the operation of the fault management system.  
         [0017]      FIGS. 4A and 4B  are graphs illustrating current as a function of time and HTS temperature as a function of time, respectively, for a first series of fault events.  
         [0018]      FIGS. 5A and 5B  are graphs illustrating current as a function of time and HTS temperature as a function of time, respectively, for a second series of fault events. 
     
    
       [0019]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0020]     Referring to  FIG. 1 , a high temperature superconductor (HTS) cable  100  is shown connected within a portion of a utility power grid  10 . HTS cable  100  has a length here of several hundred meters and provides a relatively high current and low resistance electrical path while occupying a fraction of the space required by a conventional copper cable carrying the same amount of current. As will be discussed in greater detail below, HTS wire used in HTS cable  100  has a design and structure capable of carrying as much as 150 time the electrical current of similar sized copper wire. With a relatively small amount of HTS wire rather than large quantities of copper stranded into the core of an AC cable, three to five times more power can be carried through an equivalent sized line.  
         [0021]     Referring to  FIG. 2 , HTS cable  100  includes a strand copper core  102  surrounded in radial succession by a first high temperature superconductor layer  104 , a second high temperature superconductor layer  105 , a high voltage insulation layer  106 , a high temperature superconductor shield layer  108 , an outer copper shield layer  109 , a protection layer  110 , a coolant envelope  111 , an inner cryostat wall  112 , a vacuum space  113 , an outer cryostat wall  114  and an outer cable sheath  115 . In operation, a refrigerant (e.g., liquid nitrogen) is supplied from an external coolant source (not shown) to circulate inside and along the length of coolant envelop  111 . This type of HTS power cable is available from Nexans, Paris France. Other companies such as Sumitomo Electric Industries, Ltd., Osaka, Japan and Ultera™ a joint venture of Southwire Company of Carrollton, Ga. and nkt cables, Asnaes Denmark also produce HTS power cables.  
         [0022]     Referring again to  FIG. 1 , HTS cable  100  is connected within a transmission grid segment  120  that carries voltages at a level of about 138 kV and extends from grid segment  122  to grid segment  124  which carries voltages at a lower level of about 69 kV. A step-down transformer  126  drops the voltage from the 138 kV transmission line voltage to the lower 69 kV transmission line voltage. In this particular application, transmission grid segment  124  feeds a number of leads  128  as well as a substation  129 , A pair of circuit breakers  130 ,  132  are connected at each end of HTS cable  100  to allow the HTS cable to be quickly disconnected from the utility power grid.  
         [0023]     A fault management system  200  provides over-current protection to HTS cable  100  by ensuring that HTS cable  100  is maintained below a temperature where the cable could be subjected to damage. Fault management system  200  provides such over-current protection by monitoring the current flowing in the segment of the utility grid to which HTS cable  100  is connected. In particular, fault management system  200  senses the current from a pair of current transformers  134  at one end of HTS cable  100 . Fault management system includes a memory  210  including at least a portion for storing a computer program  212  for controlling circuit breakers  130 ,  132 , a processor  214  to execute the computer-readable instructions, and a bus  216  connecting the memory to the processor. Fault management system also includes a standard GPS clock  133  to determine both the fault duration and the time between the faults. Specifically, the GPS clock provides a time stamp for processor  214  in establishing the fault duration and the time between the faults. The stored program includes computer-readable instructions which, on the basis of the signals detected at transformers  134  and the standard GPS clock, provide control signals to circuit breakers  130 ,  132  to disconnect HTS cable from transmission line  120 .  
         [0024]     With reference to  FIG. 3 , the operation of fault management system  200  will be described in conjunction with HTS cable  100  having a structure as shown in  FIG. 2 . In this example, HTS cable  100  is designed to withstand a fault current as high as 69 kA fault current with a duration of 200 ms (12 cycle) assuming that the HTS cable was not subjected to more than the predetermined allowable through-fault limits by which the temperature rise of HTS layer  105  is higher than the allowable temperature margin. It should be understood that this invention is generally applicable to HTS power cables and the specific parameters used in this example are for this particular cable design and a particular transmission grid configuration. The parameters needed to practice this invention for a given cable design and grid will be readily determined by those skilled in the art.  
         [0025]     In operation, fault management system  200  includes an energy register  220  for storing the value of the total energy per length expected to be dissipated by HTS cable (step  302 ) at its most vulnerable location (typically at the end of the cable where the coolant has the highest temperature). Fault management system continuously monitors the level of current at current transformers  134  within a time window provided by the GPS clock signal. If the current in this window exceeds the normal operating current a time stamp is issued when the current exceeds the normal level and when it returns to the normal level. This allows the fault management system to determine the total time of the particular fault, (step  304 ). In this embodiment, a fault current greater than 69 kA would cause the temperature of superconducting layer  105  to exceed the saturated temperature of the coolant, thereby possibly causing permanent damage in the cable due to formation of bubbles within high voltage insulation layer  106 . However, even if the current level is less than 69 kA but above a predetermined threshold current level for a particular cable/grid (in this example 20 KA), HTS cable  100  could be susceptible to damage if the level of current carried by the cable and time duration of the fault were significant enough to raise its temperature without sufficient time for the cable to cool to a level at which it can withstand a fault current at the 69 kA level. Thus, fault management system  200  determines whether the monitored current is less than 20 kA (step  306 ). If it is greater than 20 kA, fault management system  200  refers to a cable off-line timetable  224  to establish a time period necessary for HTS cable to cool without regard to a subsequent fault. The cable is then taken off line for the determined time period by sending a signal to the breakers commanding them to open.  
         [0026]     For this particular example, HTS cable uses a cable off-line timetable lookup table as follows:  
                                                                     Approximate Time           Current (kA)   (hours)                                        69   8           60   5.8           50   3.8           40   2.3           30   1.2           20   0.5                      
 
         [0027]     The approximate time values for the cable off-line timetable are generated from cable energy balance equations.  
                   A   j     ⁢     k   j     ⁢         ⅆ   2     ⁢     T   j         ⅆ     x   2           +       (       T     j   -   1       -     T   j       )       R     i   ,     j   -   1           -       (       T   j     -     T     j   +   1         )       R     i   ,     j   +   1             =       ρ   j     ⁢     C   j     ⁢     A   j     ⁢       ⅆ     T   j         ⅆ   t                 (   1   )             
 
 where A j  is cross section area of layer j, k j  is the conductivity of layer j, T j  is local temperature of layer j which is a function of position and time, R jj−1  is the thermal resistance between layer j−1 and layer j, R jj+1  is the thermal resistance between layer j and j+1, ρ j  is the density of the layer j, C j  is specific heat of layer j 
 
         [0028]     The stored energy within HTS cable  100  due to a fault has to be removed before the cable can be re-energized. The time duration to cool HTS cable  100  back down to a normal operating mode depends on several factors. These factors include available cooling power, coolant flow rate as well as the total energy dissipated within the cable during a fault. The more energy stored inside the cable, the longer the time required to cool the cable will be. To calculate the time duration for cool down, a transient thermal model is necessary. The calculated results of the time duration of each fault shown above can be used by a cable operator to determine how long the cable has to be off-line for a given fault current.  
         [0029]     If the monitored current is less than 20 kA, the cable may not have to be taken off line immediately. Thus, fault management system  200  then determines whether the time between a previous fault is less than a predetermined time threshold, here 20 minutes (step  308 ). If the time between faults is greater than 20 minutes the energy per length value stored in the energy register is reset to a zero value as if there had not been a previous fault due to cool back down effect. If the time between fault has been less than twenty minutes, fault management system assumes that HTS cable  100  continues to have some level of stored energy per length. Fault management system  200  uses an algorithm to determine the level of that stored energy (step  310 ): 
 
 E   j   =ΣE   j−1 +( I   j   /F   s )* t   d,j  
        where; E j  is the stored energy per unit length (J/m)     E j−1  is the stored energy per unit length from the previous fault (J/m)     I j  is the magnitude of the fault current     F s  is a current sharing factor (A*sec*m/J)     t dj  is the duration of the fault (seconds)        
 
         [0035]     Note that the above equation includes a current sharing factor F s  which is dependent on a number of factors. The Current Sharing Factor Table is generated by modeling various fault conditions and finding the correlation between the energy dissipated at certain HTS layer and a given fault current and fault duration. As described above, HTS cable  100  consists of several layers. During a fault condition, the current will be distributed among superconductor layers  104 ,  105 ,  108 , former  112  and copper shield  114  based on the cable configuration. Due to the geometry and design of the cable, the temperature of superconductor layer  105  will be higher than that of superconductor layer  104  and, therefore, damage caused by any formation of a bubble will be most likely near this layer. Thus, superconductor layer  105  is used as the critical layer in ensuring that HTS cable  100  is protected from damage. For the particular HTS cable  100  shown in  FIG. 2 , a current sharing factor table associating fault current levels with current sharing factors is as follows:  
                                                                 Current (kA)   F s  (Asm/J)                                        1   554700           2   5371           3   667           4   329           5   95           6   57           7   43           8   36           9   32           10   30                      
 
         [0036]     Once the stored energy per unit length E j  is calculated using the algorithm shown above it is compared with a predetermined energy per unit length threshold, here 74 J/meter. If the energy per unit length threshold is less than 74 J/meter in this example, HTS cable  100  remains connected within transmission segment  120 . If it greater than 74 J/meter HTS cable  100  is disconnected from transmission segment  120  for 20 minutes before being reconnected. The time duration of 20 min is the minimum requirement of removing the stored energy from the cable  100  and is based on the refrigeration system design. To better understand the operation of fault management system  200 , reference is made to  FIGS. 4A-4B  and  FIGS. 5A-5B . With the first fault protection example,  FIG. 4A  shows the fault current levels of three successive fault current events as a function of time while  FIG. 4B  shows the corresponding temperature of superconductor layer  105  within HTS cable  100  as a function of time. At time=0, HTS cable  100  is carrying a current level of approximately 300 A ( FIG. 4A ) and the highest temperature of the cable is approximately 74 K ( FIG. 4B ). One second later t=1 sec), a 3 kA fault current lasting five seconds occurs. As shown in  FIG. 4B , during the five seconds, the temperature of the HTS cable steadily rises to about 74.6 K. When the fault clears at t=6 seconds, the current level drops immediately to the normal operating current of 300 A. However, the temperature of the cable only drops slightly when the second of the three faults occurs at t=6.5 seconds. As shown in  FIG. 4A , the second fault is a 5 kA fault and lasts 0.4 seconds and as shown in  FIG. 4B , the temperature of HTS cable  100  increases relatively rapidly to about 75.2 K. At t= 7  seconds, the final fault current event occurs, a 4 kA fault lasts for 1 second before returning to normal current conditions. Once again, as shown in  FIG. 4B , the temperature of the cable rises once again to about 75.5 K, which is still below the predetermined 76 K off-line threshold. The temperature of the HTS cable is related to the energy by following equation E=∫          ρAC p dT, where E is the energy per length dissipated to HTS layer  105 , T init  is the initial temperature, T f  is the final temperature of HTS layer  105 , ρ is HTS wire density, A is HTS wire sectional area) Therefore, in this first example, the particular combination of fault current events was not sufficient to cause the fault management system  200  to remove HTS cable  100  from transmission line  120 . This means even if there is a major 69 KA fault just after 4 kA fault, the cable will not be damaged.  
         [0037]     Referring now to  FIGS. 5A and 5B , in a second fault protection example, at time=0, HTS cable  100  is carrying a current level of approximately 300 A ( FIG. 5A ) and the temperature of the cable is approximately 74 K ( FIG. 5B ). One second later (t=1), a 6 kA fault lasting 0.4 seconds occurs. As shown in  FIG. 5B , during the five seconds, the temperature of the HTS cable rapidly rises to about 75.2 K. When the fault clears at t=1.5 seconds, the current level drops immediately to the normal operating current of 300 A. However, the temperature of the cable drops very little when the second of the three faults occurs at t=2 seconds. As shown in  FIG. 5A , the second fault is a 4 kA fault and lasts 2 seconds and us shown in  FIG. 4B , the temperature of HTS cable  100  increases relatively steadily to about 75.8 K. At t=4.5 seconds, the final fault current event in which a 3 kA fault lasts for 5 second before returning to normal current conditions. Not that the temperature of HTS cable  100  remains relatively high at the time of the third fault current event. As shown in  FIG. 5B , at about t=5.5 seconds, the temperature of the cable is seen to exceed the off-line temperature of 76 K. Thus, unlike the example shown in  FIGS. 4A and 4B , fault management system  200  sends control signals to circuit breakers  130 ,  132  to disconnect HTS cable  100  from transmission line  120 . If the cable remains on line, a major fault of 69 kA just after 3 kA fault could possibly cause damage to the cable  100 .  
         [0038]     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, it is important to appreciate that values described above for the level of over-current protection depend on the particular design and structure of the HTS cable and how and where it is used within the utility power grid. Therefore, the values for the current and approximate cooling periods shown in the cable off-line table as the values for current sharing factors table are determined on the basis of the design of the HTS cable as well as its application in a utility power network.