Abstract:
A ground fault interruption (GFI) system is incorporated onto a DSP based LRM of an aerospace vehicle. The GFI system operates with digital controls and, unlike the prior art, the system does not employ current transformers. Synchronization pulses are employed to coordinate instantaneous current measurement samplings in each phase of a multi-phase power system. Coordinated sampling may reduce phase angle current differential errors and improve operational precision of the GFI system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/896,213 filed Mar. 21, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is in the field of ground fault interrupters (GFI&#39;s) and, more particularly, GFI&#39;s in power distribution systems which operate in vehicles such as aerospace vehicles. 
     In modern day aerospace vehicles, power distribution systems may incorporate ground fault protection. In a typical prior art vehicle, current transformers are employed as part of the apparatus needed to detect current variations and interrupt current if and when a ground fault event occurs. Prior art ground fault interruption (GFI) is realized by detecting differential current using a current transformer, comparing the differential current with a threshold value, and interrupting current from a power source through a remote power controller when the differential current exceeds the threshold value. Current transformers are expensive and their use adds weight to an aerospace vehicle. Also use of current transformers increases system interconnection complexity and reduces flexibility of SSPC system. As is the case for virtually any type of complexity, interconnection complexity may present opportunities for failures and may contribute to reduced overall reliability of a power system of an aerospace vehicle. 
     An alternate method to current transformer type GFI is provided by performing current sum digitally. But, many aerospace vehicles employ multi-phase power distribution (e.g. 3 phase power). Precision of the digital current sum GFI performance may be affected by errors in detecting actual current differentials between respective phases. Phase angle variations may produce one form of current differential error. Also current transformers may not be capable of perfectly representing actual current in a phase. Collectively, these factors may produce a current differential error. Presence of such potential errors in detecting actual current may adversely affect the precision with which prior art GFI systems may operate. 
     In order to avoid false tripping, a GFI device or system must be allowed to ignore a current differential that is equal to or less than an error differential. For example, if an error differential has a potential for appearing as a current variation of 1% between phases, then a GFI trip level must be set so that the GFI operates only after an actual current variation or current reading differential exceeds 1%. 
     As aerospace vehicles evolve, there is an increased demand for lower weight of components. There is also a developing need for increased reliability of individual systems because there are an increasing number of systems being incorporated into aerospace vehicles. Overall reliability of vehicles with an increasing number of systems may only be sustained if reliability of each system is improved. In that context, interconnection complexity associated with use of current transformers for GFI functions is counterproductive. 
     As can be seen, there is a need to provide for ground fault interruption without use of current transformers. There is also a need to provide trip levels of ground fault interruption devices lower than prior-art differentials in multi-phase power systems. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention a power distribution control system with ground fault interruption (GFI) protection comprises a current measurement sensor for a first conductor, a current measurement sensor for a second conductor, analog to digital converters to convert current measurements to a first digital representation of current in the first conductor and to a second digital representation of current in the second conductor, and a digital processor that receives the first and second digital representations. The processor calculates differentials between the digital representations and produces a current interruption signal in the event that a calculated differential exceeds a pre-defined value. 
     In another aspect of the present invention a control system for multi-phase power distribution with ground fault interruption (GFI) protection comprises a plurality of DSP based trip engines which perform instantaneous sampling of current values of each power feeder of the multi-phase power distribution system. The trip engines produce digital representations of the sampled current values. A processor is interconnected with the trip engines to receive the digital representations and determine if a differential between current values of the power feeders exceeds a predefined limit. Solid-state switches are interconnected with each of the trip engines to interrupt current in the power feeders upon receiving a switch-off signal from an associated one of the trip engines, which switch-off signal is generated responsively to a determination by the processor that the differential exceeds the predefined limit. GFI protection is thus provided without use of current transformers. 
     In still another aspect of the present invention a method for performing ground fault interruption (GFI) functions comprises the steps of measuring a first current in a first conductor, measuring a second current in a second conductor, producing a first digital representation of the measured first current, producing a second digital representation of the measured second current, calculating a differential between the first and the second digital representations, and interrupting current through at least one of the conductors in the event that the differential exceeds a predefined level. 
     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 a GFI-protected power distribution system in accordance with the present invention; 
         FIG. 2  is a block diagram of a portion of the system of  FIG. 1  in accordance with the present invention; and 
         FIG. 3  is a flow chart of a method providing ground fault interruption functionality in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out 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. 
     Broadly, the present invention may be useful in providing ground fault protection in a power distribution system. More particularly, the present invention may provide accurate ground fault protection in multi-phase power distribution systems. The present invention may be particularly useful in aerospace vehicles. 
     In contrast to prior-art ground fault interruption (GFI) systems, among other things, the present invention may provide light weight, non-complex and accurate GFI functionality. The present invention, instead of utilizing heavy current transformers and interconnecting circuitry, may provide GFI functions incorporated as an ancillary feature of a power distribution control line-replaceable-module (LRM) that may otherwise already be incorporated into a design of the vehicle. Consequently, the inventive system for performing GFI functions may be introduced into an aerospace vehicle while adding virtually no weight to the vehicle. Additionally, because the inventive GFI system is based on digital signals processors (DSP&#39;s). GFI functionality may be provided with trip level accuracies that exceed those of the prior art. 
     Referring now to  FIG. 1 , there is shown a system for providing ground fault interruption (GFI) protection for a power distribution control system  10 . The power distribution control system  10  may be a solid state power control (hereinafter SSPC  10 ). The SSPC  10  may be incorporated into a Line Replaceable Module (hereinafter LRM)  11  that may be employed on an aerospace vehicle for power distribution control. The SSPC  10  may comprise power switches  12 ,  14  and  16 . The switches  12 ,  14  and  16  may comprise conventional solid state switching devices such as metal oxide field effect transistors (MOSFET&#39;s). The power switches  12 ,  14  and  16  may be positioned to interrupt current in conductors or power feeders  18 ,  20  and  22 , respectively. The power feeders  18 ,  20  and  22  may be interconnected, through current feedback sensors  24 , to trip engines  26 ,  28  and  30  respectively. A processor such as a supervisory control unit  32  may interconnected to the trip engines  26 ,  28 , and  30  through galvanic interfaces  34  and synchronization pulse interfaces  36 . Each of the power feeders  18 ,  20  and  22  may comprise one phase of a multi-phase power distribution system. 
     In the exemplary configuration of  FIG. 1 , the power feeders  18 ,  20  and  22  may be interconnected to power loads  40 ,  42  and  44 . One of the power loads, the power load  42  as an example, is illustrated, symbolically, with a leakage path  46  going to ground. For purposes of illustration the present invention is described in the context of the leakage path  46  developing between ground and the power load  42 . 
     In operation, the trip engines  26 ,  28 , and  30  may periodically perform sampling operations and acquire instantaneous current readings from the current feedback sensors  24 . The sampling operations may be performed at intervals of 1 milliseconds (msec) to 5 msec between each sampling operation. The trip engines  26 - 30  may then transmit a digital representation of the current reading to the supervisory control unit  32 . The supervisory control unit  32  may then perform a current sum calculation. A fault may be declared when current differential between phases (a so-called sum error) exceeds a predefined limit for a predefined period of time. Typically, such a sum error may be found when ground leakage current develops between one of the loads  40 ,  42  or  44  and ground and when such leakage current continues for two or more of the periodic sampling operations of the trip engines  26 ,  28  and  30 . 
     In the event of a fault declaration, the supervisory control unit  32  may produce a current interruption signal and transmit the signal to the trip engines  26 ,  28  and  30 . The signaled trip engines  26 ,  28  and  30  may then produce switch-off signals for transmission to their respective power switches  12 ,  14  and  16 . The power switches  12 ,  14  and  16  may then interrupt current flowing through the power feeders  18 ,  20  and  22 . 
     Referring now to  FIG. 2  there is shown a drawing of one of the trip engines, in an exemplary case, the trip engine  28 . The trip engine  28  is described herein in its role as providing GFI protection. It should be noted that the trip engine  28  need not be dedicated exclusively to GFI functionality. The trip engine  28  may also perform other power control tasks of the SPPC  10 . For example, the trip engine  28  may perform a circuit breaker function (not described herein) or a contactor control function (not described herein) in addition to the GFI function which is presently being considered herein. 
     The trip engine  28  may comprise a DSP  50  and one or more current processing blocks  52  and  54  which are tuned to process differing ranges of currents. The current processing blocks  52  and  54  may be interconnected with the DSP  50  through analog to digital (A/D) converters  50   a . The current processing blocks  52  and  54  may be interconnected with a current sensing resistor  56  of one of the current sensors  24 . 
     The current processing block  52  may be tuned to process current feedback in a range of zero to nominal current. For example, if the SSPC  10  is set with a 15 ampere (A.) range, the current processing block  52  might be tuned to process currents up to 10% greater than 15 A. The current processing block  54  may be configured to process current that may be higher than nominal. For example, if the SSPC  10  is set with a maximum trip rating of 1000%, then the current processing block  54  may be tuned to process currents up to 1000% of 15 A or 150 A. Tuning as described above may be accomplished by constructing the current processing blocks  52  and  54  with components which are selected for particular current ranges in a manner familiar to those skilled in the art of power distribution control. 
     Referring back now to  FIG. 1 , the utility of the synchronization pulse interfaces  36  may be better understood. The supervisory control unit  32  may be interconnected with the trip engines  26 ,  28  and  30  through the synchronization pulse interfaces  36 . Because of this interconnection the supervisory control unit  32  may provide a synchronization pulse through one of the synchronization pulse interfaces  36  to each of the trip engines  26 ,  28 , and  30 . Upon receiving the synchronization pulse, each of the trip engines  26 ,  28 , and  30  may acquire instantaneous readings of current feedback from their respective power feeders  18 ,  20  and  22 . 
     The trip engines  26 ,  28  and  30  may then transmit the digital representations of their respective current readings to the supervisory control unit  32 . The supervisory control unit  32  may then perform conventional current sum calculations based on these digital representations to determine if a ground fault should be declared. 
     Because instantaneous readings of current may be made periodically on a sampling basis, current sum calculations may be prone to certain inaccuracy. This inaccuracy may result if phase differential is allowed to develop between samplings of current. Synchronization pulses reduce such inaccuracy by providing timing coordination between all of the trip engines  26 ,  28  and  30 . 
     If each of the trip engines  26 ,  28  and  30  were to sample current based on its own independent timing, the current samplings might be performed at slightly different times. If the trip engine  26 , for example, sampled current at a time (T 0 ) different from a sampling time (T 1 ) of trip engine  28 , there may a change in phase angle of the power transmitted within the power feeders  18  and  20  during the time interval T 0  minus T 1 . Consider for example, a 50 microsecond (μsec) time differential that may be experienced between current samplings. At 400 Hertz (Hz) this time differential may correspond to a phase differential of 7 degrees. This may translate to an error of 1% in the current sum calculation. At 10 A of phase current, the 1% error may correspond to 100 milliamp (mA.). A 100 mA error may be unsuitable for many aerospace vehicle applications. 
     If a phase differential error of 100 mA were to develop as described above, GFI functionality would need to be withheld for any current differential lower than 100 mA. In other words, any ground-fault induced current differential lower than 100 mA would need to be treated as not representative of a ground fault event. Thus an actual ground fault event that produced a current differential of 75 mA would not trigger a current interruption action in this example. 
     A modest reduction of magnitude of such an error may be provided by increasing sampling rate but this may produce an intolerable processing load. A more desirable way of reducing this error is through a synchronization scheme of the present invention. 
     The supervisory control unit  32  may emit simultaneous synchronization pulses to each of the trip engines  26 ,  28  and  30 . The synchronization pulses may provide commands to the trip engines  26 ,  28  and  30  to sample current in their respective power feeders  18 ,  20  and  22 . This may assure that sampling from all phases is performed virtually simultaneously. The time differences between current samplings by the trip engines  26 ,  28  and  30  may be reduced to an interval of 500 ns to 1000 ns. This corresponds to an error of 0.07 degrees or a 0.0008% error. At 10 A, this is only 0.1 mA error. In this inventive synchronization pulse mode of operation, GFI functionality may be allowed to proceed for any current differential greater than 0.1 mA. 
     A further improvement in operational accuracy of the trip engines  26 ,  28  and  30  may be achieved by individually calibrating each of the DSP&#39;s  50  against a known resistance at the time that the LRM&#39;s  10  are manufactured. A calculated gain may be determined for each individual DSP  50  and stored in a conventional non-volatile memory (not shown) within the individual DSP  50 . In this way, compensation may be made for any physical variations of one of the DSP&#39;s  50  as compared to any of the other DSP&#39;s  50 . 
     In one embodiment of the present invention, a method is provided for GFI functions, for example, on an aerospace vehicle. In that regard, the method may be understood by referring to  FIG. 3 . In  FIG. 3 , a flow chart portrays various aspects of an inventive method  300 . In a step  302 , current in a first power feeder (e.g. the power feeder  18 ) may be instantaneously measured (e.g., by use of one of the feedback sensors  24  and the trip engine  26 ). In a step  304 , current in a second power feeder (e.g. the power feeder  20 ) may be simultaneously measured (e.g., by use of one of the feedback sensors  24  and the trip engine  28 ). In a step  306 , a digital representation of the current in the first power feeder may be produced (e.g. in the A/D converter  50   a ). In step  308 , a digital representation of the current in the second power feeder may be produced in the same manner as step  306 . In a step  310 , the digital current representations may be transmitted (e.g. from the A/D converter  50   a ) to a processor (e.g. the supervisory control unit  32 ). In a step  312 , a calculation may be performed (e.g. in the supervisory processor  32 ) to determine a differential between the digital representations of currents in the first and second power feeders. 
     In the event that the differential calculated in step  312  exceeds a predefined level for a predetermined time interval, a step  316  may be initiated by which power transmission to a load through the first and second power feeders may be interrupted (e.g. by operation of the trip engines  26  and  28  and the power switches  12  and  14 ). In the event that the calculated differential is below the predefined level or does not continue beyond the predetermined time, the interruption step  316  may not be performed. In that case, a step  318  may be performed in which synchronization pulses may be generated and transmitted to trigger operation of steps  302  and  304  in which the trip engines may perform current sampling. 
     It should be noted that the step  318  may be performed by generating a separate synchronization pulse for each of the trip engines. In this way current differential error associated with phase differential may be substantially reduced as described hereinabove. 
     It should also be noted that the foregoing description of the method  300  discusses an exemplary collection of only two power feeders. It should be clear to those skilled in the art that the method  300  may be practiced with any number of power feeders and that current differentials among any combinations of power feeders may be used to trigger GFI functions 
     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.