Abstract:
A protected electrical power system may comprise a feeder between a power source and an electrical load. A first current transformer may be positioned on the feeder in a first location. The first current transformer may have a shunt resistor electrically connected across its winding. A second current transformer may be positioned at a second location on the feeder. A control unit may be interposed between the first and second current transformers and may be interconnected with the first and second current transformers on current-monitoring loops independent from the feeder. The control unit may be responsive to a predetermined differential in feeder (DF) current between the first and second current transformers to disconnect the power source from the electrical load. The control unit may have a compensation network for reducing DF error resulting from presence of the shunt resistor in the first current transformer.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to electrical systems which employ current transformers. More particularly the present invention relates to current transformers employed as circuit protection devices. 
     In some electrical distribution systems a protection system may be used to assure that electrical failures of wiring may be isolated. For example, in an aircraft, protection may be established between a generator and one or more electrical loads that may receive current from the generator. In a typical three-phase system, first current transformers may be placed around output conductors at the generator. Second current transformers may be placed on the conductors at a position remote from the generator. A monitoring system may detect any current imbalance between the first and second current transformers if and when a fault may develop in a zone between the first and second current transformers. 
     Windings of the current transformers may be interconnected to a control unit which may operate disconnection contactors in the event of a fault. In this context, the windings may be considered to be connected to a load with low impedance. The current transformer design and the impedance of the control unit may be selected so that, in normal operation, the voltage developed across the windings does not exceed insulation breakdown limits of the windings. 
     It is possible that a so-called “open circuit” failure may occur in the control unit or in a current-monitoring loop between one of the current transformers and the control unit. In such an event, the impedance across a winding of the current transformer may become infinite. Continued passage of current from the generator to the electrical loads may then produce extremely high voltages across the winding. As a result of such high voltages, insulation in the winding may break down and the current transformer may become inoperative. 
     In a typical aircraft generator, current transformers may be incorporated directly in the generator. Consequently, failure of one of the current transformers in the generator may result in a requirement to remove the generator from the aircraft to replace the defective current transformer. 
     As can be seen, there is a need to provide protection of current transformers from damage resulting from open circuit failures in a control unit to which the winding of the current transformer may be connected. Additionally, there is a need to assure that the current transformers may accurately detect fault conditions in protection zone. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, an electrical power system may comprise: a feeder between a power source and an electrical load; a first current transformer positioned at a first location on the feeder, the first current transformer comprising a first shunt resistor electrically connected across the first current transformer; a second current transformer positioned at a second location on the feeder; and a control unit interposed between the first and second current transformers; the control unit being interconnected with the first and second current transformers on current-monitoring loops independent from the feeder; the control unit being responsive to a predetermined differential in feeder (DF) current between the first and second current transformers to disconnect the power source from the electrical load; and the control unit comprising a compensation network for reducing DF error resulting from presence of the shunt resistor in the first current transformer. 
     In another aspect of the present invention, a protection system for a feeder may comprise: a first current transformer at a first location on the feeder; and a control unit that acts responsively to a predetermined DF to interrupt feeder current; wherein the first current transformer is connected to a first shunt resistor; wherein the control unit interrupts feeder current within a time period T after development of an open fault in a first current-transformer loop; and wherein the time T is less than a time period in which a rate of power dissipation within the first shunt resistor reaches a power dissipation rate limit for the first shunt resistor. 
     In still another aspect of the present invention, a method for operating an electrical power system may comprise the steps of: passing electrical power on a feeder from a power source to electrical loads through a first current transformer to produce a first monitoring current in a first current-transformer loop; passing a portion of the monitoring current through a first shunt resistor connected across the first current transformer; interrupting power passage on the feeder in the event of an open circuit fault in the first current-transformer loop; wherein the step of interrupting power passage is performed within a time T after development of an open fault in the first current-transformer loop; and wherein the time T is less than a time period in which a rate of within the first shunt resistor reaches a rated limit for power dissipation rate for the first shunt resistor. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an electrical power system in accordance with an embodiment of the invention; 
         FIG. 2  is a block diagram of a current-transformer protection system in accordance with an embodiment of the invention; 
         FIG. 3  is a model of a current-transformer loop in accordance with an embodiment of the invention; 
         FIG. 4  is a block diagram of a feeder protection system in accordance with an embodiment of the invention; 
         FIG. 5  is model of the current-transformer protection system of  FIG. 2  in accordance with an embodiment of the invention; 
         FIG. 6  is a graph illustrating a relationship between a rate of power dissipation of a shunt resistor and a power dissipation rate limit for the resistor in accordance with an embodiment of the invention; and 
         FIG. 7  is a flow chart of a method for operating an electrical system in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. 
     Broadly, embodiments of the present invention generally provide power systems in which protective shunt resistors may be incorporated into a current-transformer monitoring circuit or loop. More particularly, embodiments of the present invention may provide a methodology for utilizing such shunt resistors while minimizing adverse effects on accuracy of the monitoring system that may otherwise be produced by presence of the shunt resistors. 
     Referring now to  FIG. 1 , a block diagram may illustrate an exemplary embodiment of an electrical power system  10 . The power system  10  may be, for example, a portion of a vehicular power system such as an aircraft power system. The power system  10  may comprise a power source  12  connected to electrical loads  14  through a feeder  20 . The power system  10  may be provided with a feeder-protection system  11 . In the case of a vehicular or aircraft power system the power source  12  may be an electric machine such as a generator or starter generator. The power system  10  may be a single phase system or a multi-phase system. For purposes of simplicity, the feeder  20  is shown in  FIG. 1  as a single conductor. It is to be understood however that in the case of, for example, a three phase system the feeder  20  may comprise four conductors. 
     The feeder-protection system  11  may comprise a current transformer  16  positioned at or near the power source  12  and a current transformer  18  at or near the electrical loads  14 . The current transformer at or near the power source  12  may be referred to as the power-source current transformer  16 . The current transformer at or near the electrical loads  14  may be referred to as the remote current transformer  18 . Portions of the feeder  20  between the power-source current transformers  16  and the remote current transformers  18  may be referred to as a protected zone  24 . 
     It may be seen that the current transformer  16  may be interconnected to a control unit  22  on a current-monitoring loop  30 . Similarly, the current transformer  18  may be interconnected to the control unit  22  on a current-monitoring loop  31 . As current passes through the feeder  20 , the current transformer  16  may develop a monitoring current that may be proportional to the current in the feeder  20 . The current transformer  18  may develop a similar monitoring current. The control unit  22  may compare the monitoring currents of both of the current transformers  16  and  18 . A differential between the monitoring currents of the current transformers  16  and  18  may be indicative of a differential in current along the feeder  20  and may be hereinafter referred to as differential feeder current or DF. If current in one of the current transformers, e.g., the current transformer  18 , becomes lower than that of the current transformer  16 , the control unit  22  may treat this condition as indicative of a short to ground in the feeder  20 . In such an event, the control unit  22  may activate contactors (not shown) so that the protected zone  24  may be isolated. Similarly, phase-to-phase faults may also be isolated. Additionally, the power source  12  may be shut down so that damage to the power source  12  may be avoided. 
     Under some circumstances, the current-monitoring loop  30  may develop an open circuit fault. It may also be the case that current continues to flow in the feeder  20 . Under these conditions, extremely high voltages may develop in a winding of the current transformer  16 . Such high resultant voltage, in the order of Kilovolts, may damage insulation within the current transformer  16  and cause failure of the current transformer  16 . 
     In some aircraft power systems, the current transformer  16  may be integral with the power source  12  (e.g., a generator). Failure of one or more of the current transformers  16  may result in a requirement to remove the generator from an aircraft and repair it. 
     Referring now to  FIG. 2  an exemplary embodiment of a current-transformer protection system  40  may be seen. In the current-transformer loop  30 , a burden resistor  32  may represent a resistive load that may be presented within the control unit  22 . A parallel or shunt resistor  34  may be connected across the current transformer  16 . In normal operation, the current transformer  16  may be presented with finite impedance, (e.g., the burden resistor  32 ). In the event of an open fault in the loop  30 , the burden resistor  32  may no longer present finite impedance to the current transformer  16 . However, the shunt resistor  34  may continue to present finite impedance to the current transformer  16  even if an open fault develops between the current transformer  16  and the burden resistor  32 . Thus, even though current may continue flowing through the feeder  20 , voltage across the current transformer  16  may not rise to a level that may damage insulation in the current transformer  16 . 
     Referring now to  FIG. 3 , it may be noted that a reduction in sensitivity in the loop  30  may arise from introduction of the shunt resistor  34 . This may be understood by considering the following analysis of a model of the loop  30 . A resistor  36  may represent resistance of the wiring between the burden resistor  32  and the current transformer  16 . Current in the parallel or shunt resistor  34  may be determined in accordance with the expression:
 
 I   rparallel   =I   generator   /N ×( R   feeder   +R   burden )/( R   feeder   +R   burden   +R   parallel );  1).
 
     where I rparallel  is the current through the resistor  34 ; 
     where I generator  is the current through the main power feeder  20 ; 
     where R feeder  is the resistance of the feeder wiring for the current transformer  16  (i.e. resistor  36 ); 
     where R burden  is the resistance of the burden resistor  32 ; and 
     where R parallel  is the resistance of the parallel resistor  34  which protects the current transformer from over-voltage during open circuit failure. 
     A current reading through the burden resistor  32  may be altered by presence of the shunt resistor  34 . The magnitude of such alteration may be referred to as an error in reading and may be determined in accordance with the expression:
 
Error in reading= I   parallel   /I   generator   /N =( R   feeder   +R   burden )/( R   feeder   +R   burden   +R   parallel )≈( R   feeder   +R   burden )/ R   paralel ;  2).
 
     where N is an effective number of turns in the current transformer  16  in the current transformer  16 . 
     Referring now to  FIG. 4 , it may be seen how the error in reading described above in equation 2 may affect sensitivity of the feeder protection system  11 . An error in current differential between the current transformer  16  and the current transformer  18  may be expressed as:
 
 I   differential  error= I   gen ×[( R   gf   +R   gb )/( R   gf   +R   gb   +R   gp )];  3).
 
     where I gen  is current through the main power feeder  20 ; 
     where R gf  is resistance of the feeder wiring for the current transformer  16  (i.e. resistor  36  of  FIG. 3 ); 
     where R gb  is resistance of the burden resistor  32 ; and 
     where R gp  is resistance of the parallel resistor  34 , which protects the current transformer from over-voltage during open circuit failure. 
     In an example of application of the above analysis, the following table shows numerical relationships of various values of the resistor  34  and their effects on accuracy of the feeder protection system  11  (accuracy being equivalent to a variation in differential feeder current or DF). 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Current in the feeder 20 equals 210 Amps (A) 
               
             
          
           
               
                   
                   
                 DF Inaccuracy 
                   
                   
               
               
                   
                 Rparallel 
                 1st sigma 
                   
                 Power 
               
               
                   
                 (Resistor 
                 84% 
                   
                 Dissipation 
               
               
                   
                 34) 
                 probability 
                 DF Error 
                 Watts 
               
               
                   
                   
               
             
          
           
               
                   
                 50 
                 15%  
                 31.5 
                 3 
               
               
                   
                 100 
                 8% 
                 16.8 
                 5 
               
               
                   
                 200 
                 4% 
                 8.82 
                 10 
               
               
                   
                 300 
                 3% 
                 6.3 
                 14 
               
               
                   
                 500 
                 2% 
                 4.2 
                 25 
               
               
                   
                 1000 
                 1% 
                 2.1 
                 50 
               
               
                   
                   
               
             
          
         
       
     
     It may be seen that when the resistor  34  has a high value, then power dissipation may be high. Conversely when the resistor  34  may have a low value, power dissipation may be low, but inaccuracy may be high. In order to achieve a reasonable accuracy to meet aerospace requirements of +/−5 A of DF protection at  210 A, the resistor  34  may need to be greater than 500 ohms. Such a resistor may produce high power dissipation and thus may be too large to fit in the generator  12 . 
     Referring back now to  FIG. 2 , it may be seen that the current-transformer protection system  40  may incorporate a compensation network  50  which may diminish DF error when a low value one of resistors  34  may be employed as a protective shunt resistor in the current transformer  16 . The compensation network  50  may comprise a series resistor  52  which may have a value that, when added to resistance of wiring between the current transformer  18  and the burden resistor  56 , produces a net resistance equal to that of the resistor  36  of  FIG. 3 . The resistor  52  may be placed in the control unit  22  and connected in series with the current transformer  18 . The resistor  52  may be selected so its resistance may be equal to the resistance of the resistor  36  of  FIG. 3 . The compensation network  50  may also comprise a shunt resistor  54  which may be placed in the control unit  22  and which may be connected across the current transformer  18 . The resistance of the shunt resistor  54  may be equal to the resistance of the shunt resistor  34 . 
     Referring now to  FIG. 5 , which shows a model of the current-transformer protection system  40 , it may be noted that through use of the compensation network  50 , differential current error may be determined in accordance with:
 
 I   differential  error= I   gen ×[( R   gf   +R   gb )/( R   gf   +R   gb   +R   gp )−( R   ff   +R   fb )/( R   ff   +R   fb   +R   fp )];  4).
 
     where R ff  is the combined resistance of wiring between the destination current transformer  18  (typically located in the distribution panel) and the compensation resistor  52 ; 
     where R fp  is resistance of the parallel resistor  54  which is part of the compensation network  50 ; 
     where R gp  is resistance of the resistor  34  which protects the current transformer  16  from over-voltage during open circuit failure; and 
     where R fb  is resistance of a burden resistor  56  used for sensing destination current. 
     In an example of application of the above described compensation network  50 , the following table shows numerical relationships of various values of the resistor  34  and their effects on accuracy of the protection system (i.e., DF error). Table 2 may be illustrative of current in current-monitoring loop  20  being equal to 210 A. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Inaccuracy with Compensation Network. 
               
             
          
           
               
                   
                 DF 
                   
               
               
                   
                 Inaccuracy 
               
               
                 Rparallel 
                 1st sigma 
                 Power 
               
               
                 Resistor 
                 84% 
                 Dissipation 
               
               
                 34 
                 probability 
                 Watts 
               
               
                   
               
             
          
           
               
                 50 
                 1.60% 
                 3 
               
               
                 100 
                 0.90% 
                 5 
               
               
                 200 
                 0.50% 
                 10 
               
               
                 300 
                 0.30% 
                 14 
               
               
                 500 
                 0.18% 
                 25 
               
               
                 1000 
                 0.10% 
                 50 
               
               
                   
               
               
                 (R fp  = R gp , R ff  = R gf ) 
               
             
          
         
       
     
     It may be seen that adding of the compensation network  50  (with settings R ff =R gf  and R fp =R gp  at nominal values) inaccuracy may be reduced from 2% to 0.18% for a 500 ohm Rparallel resistor. (Compare Table 2 with Table 1 for 500 ohm Rparallel.) 
     Referring now to  FIG. 6 , it may be seen how operation of the current-protection system  40  may be further improved by limiting response time of the feeder protection system  11 . A power rating graph  60  may illustrate how power rating of a nominally rated 4 watt (W) power resistor may vary as a function of time. A horizontal line segment  62  may illustrate that the 4 W resistor may be rated to dissipate power at a rate of about 5000 W for a brief period of time, about 0.001 seconds. After the brief period of about 0.001 seconds, the power rating of the resistor may diminish, along a sloped line segment  64 , to a nominal power rating of 4 W; illustrated by a horizontal line segment  66 . 
     A horizontal line  68  line may represent a power dissipation rate of about 25 W. The horizontal line  68  may intersect the line segment  64  at a time T represented by a vertical line  70 . The time T may be about 0.08 seconds. Thus, the graph  60  may illustrate the concept that the 4 W resistor may be allowed to behave like a 25 W resistor for a period of time that may be less than about 0.08 seconds. 
     Referring back to Table 2 it may be seen that an exemplary embodiment of the system  40  may be provided with accuracy of about 0.18% while utilizing a 500 ohm resistor that may be rated for only 4 W. Of course the system must have a response time no greater than the time T, i.e., about 0.08 seconds. In other words, if feeder current is stopped within the time T of occurrence of an open failure, then the resistor  34  may be selected to have a resistance as great as 500 ohms while having a power rating of only 4 W. Such a resistor may be small enough to be readily placed in the generator  12 . 
     Referring now to  FIG. 7 , a flow chart  700  may illustrate an exemplary method which may be employed to operate the electrical system  10  in accordance with an embodiment the invention. In a step  702 , electrical power may be passed on a feeder from a power source to electrical loads through a current transformer to produce a monitoring current in a current-transformer loop (e.g., The generator  12  may produce power for transmission through the current transformer  16  and the feeder  20 . The current transformer  16  may produce monitoring current in the current-transformer loop  30 ). In a step  704 , a portion of the monitoring current may be passed through a shunt resistor connected across the current transformer (e.g., the portion of monitoring current which may pass through the shunt resistor  34  be determined in accordance with equation 1). In a step  706 , an open fault may be detected in the current-transformer loop. In a step  708 , power passage on the feeder may be interrupted in the event of an open circuit fault in the current-transformer loop. The step of interrupting power passage may be performed within a time T after development of an open fault in the current-transformer loop. The time T may less than a time period in which a power dissipation rate within the shunt resistor reaches a rated limit for power dissipation rate for the first shunt resistor, (e.g. the power dissipation of the shunt resistor  34  may be as great as the horizontal line segment  68  as long as the rated limit  64  for the resistor  34  is not exceeded). 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.