Abstract:
An ungrounded or floating DC electrical power distribution system may experience a single line to ground fault. Such a fault may not disrupt operation of the system, but its presence may raise a risk of additional problems if left uncorrected. A system for progressively grounding the ungrounded system may be initiated when a line to ground fault is suspected based on the voltage difference measured to a common chassis point. As grounding through successively lower impedance proceeds, fault current may increase and detection of severity of the line to ground fault may be more readily achieved, thus facilitating localization of the fault. Localization may be achieved through an analysis of direction of capacitive currents in isolatable zones of the system.

Description:
RELATED APPLICATIONS 
     The present application claims benefit of U.S. Provisional Application 60/992,752 filed Dec. 6, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to apparatus and methods for providing protection for an electrical system and, more particularly, to apparatus and methods of ground fault detection in an ungrounded electrical system. 
     In a grounded three phase alternating current (AC) electric power system a neutral point may be connected to ground (or chassis ground). In a grounded direct current (DC) power system, either a positive or a return conductor may be grounded. In some cases, a middle point of a DC voltage source may be grounded. 
     A grounded system may be grounded solidly or through some impedance. Grounding through an impedance may help in controlling a level of fault current arising out of a single line-ground fault. An ungrounded system does not have any direct connection to the ground. In other words, the grounding impedance tends to infinity. 
     The purpose of grounding a generator or transformer neutral is to limit a voltage rise among healthy phases when single line to ground fault occurs. It may also provide a path for zero-sequence current which may help in detection of unwanted phase to ground connection in the system. 
     A disadvantage of a grounded system lies in the fact that even a single line-ground fault may lead to heavy fault currents and hence disrupt operation of the entire power system. The fault has to be cleared before the grounded system resumes its normal operation. Ground faults can lead to process disruption and safety hazards such as equipment malfunctions, fire and electric shock. During a ground fault condition, power supply has to be interrupted to limit the damage to equipment. 
     In an ungrounded electrical power system, there may be no intentional connection between the conductors and the ground. However, in any system, stray capacitive coupling may exist between the system conductors and adjacent grounded surfaces. Consequently, an “ungrounded system” is, as a practical matter, a “capacitive grounded system” by virtue of the distributed stray capacitance. 
     The advantage of ungrounded system is that a single line to ground fault may have minimal fault current and hence practically no impact on the system operation. However in the event of a line-ground fault in an ungrounded system, the voltage of the healthy phases may rise to line-to-line voltage. Thus voltage stress on the healthy phase conductors may increase during fault condition. Further, there may be capacitive voltage build up if the fault is of restriking nature. Thus the phase conductors have to be insulated by design for higher voltage stress in an ungrounded system. Also, due to increased voltage stress, the probability of occurrence of second line to ground fault may increase. 
     Though a single line to ground fault may not impact operation of an ungrounded system, a second ground fault may lead to phase-to-phase fault, with very high fault current. In the event of such a phase-to-phase fault, power interruption is required, thus leading to system outage. It is, therefore, desirable to detect and isolate and clear the first line-ground fault as soon as possible, even though it does not affect the system operation. 
     A single line-ground fault in an ungrounded system may produce very small current flowing through any shunt-connected or parasitic and stray capacitances. Therefore the fault detection and in particular localization is a daunting task in a large and complex ungrounded electrical distribution network which may include multiple power sources and utilization systems. Several methods have been proposed in the prior art for single line-to-ground fault detection in ungrounded or floating networks. Most of the ground fault detection methods described in the prior art rely on measurement of positive and negative line voltage with respect to a common chassis or ground potential. During single line-to-ground condition, the faulty terminal voltage may assume ground potential while the ‘healthy’ phase potential may rise to the original line-to-line voltage level. Single line-to-ground fault is declared when the difference between measured voltage at the positive and the negative line with respect to common chassis exceeds a threshold value. However, these methods fail to indicate the localization of the ground fault. Since it is not desirable to shutdown the entire network in the event of a single ground fault it would be a great advantage to localize the fault and isolate only the faulty section of the network while keeping the unaffected network functions operative. 
     Prior-art fault localization methods may include sequential disconnection of network sections until the fault is isolated. However, this procedure leads to unnecessary disconnection of power sources or loads, disrupting the system operation. 
     As can be seen, there is a need for a system that may provide for early detection and isolation of single line-ground fault in an ungrounded power distribution system. Furthermore, there is a need to such a system which may provide both detection and localization of a faulted network section. 
     SUMMARY OF THE PRESENT INVENTION 
     In one aspect of the present invention an electrical power distribution system comprises at least one power distribution zone. The at least one zone has at least two test points positioned at separate locations in the zone. Each of the at least two test points is capable of measuring a direction of capacitive current. The at least one power distribution zone is electrically isolatable from the power distribution system responsively to a presence of capacitive current at one of the test points being opposite in direction from capacitive current at the other test point. 
     In a further aspect of the invention a ground fault localization system for an ungrounded power distribution system comprises at least two sensors for measuring direction of capacitive current in a conductor of the power distribution system. At least one isolating device for isolating the conductor from the power distribution system responsively to a determination by that direction of capacitive current at a first one of the at least two sensors is opposite to direction of capacitive current at a second one the sensors. 
     In another aspect of the invention a method of localizing a ground fault in an ungrounded electrical power distribution system comprises the steps of sensing direction of capacitive currents at two separate testing points of a zone of the distribution system and declaring the zone to have a ground fault upon determination that the capacitive currents are in opposite directions. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, descriptions and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an electrical power distribution system in accordance with the invention; 
         FIG. 2  is a block diagram of a DC portion of the power distribution system of  FIG. 1  in accordance with the invention; 
         FIG. 3  is a graph portraying a relationship of capacitive current directions in a first portion of the system of  FIG. 1  in accordance with the invention; 
         FIG. 4  is a graph portraying a relationship of capacitive current directions a second portion of the system of  FIG. 1  in accordance with the invention; 
         FIG. 5  is a graph portraying a relationship of capacitive current directions a third portion of the system of  FIG. 1  in accordance with the invention; 
         FIG. 6  is a graph portraying a relationship of capacitive current directions a fourth portion of the system of  FIG. 1  in accordance with the invention; 
         FIG. 7  is graph portraying a relationship of line voltage to ground in accordance with the invention; and 
         FIG. 8  is a flow chart of a method for localizing ground faults in accordance. 
     
    
    
     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. 
     The present invention provides a system for detection and isolation of ground faults in an ungrounded or floating DC electrical power distribution system or network. 
     The inventive system may gradually ground an ungrounded system in a controlled manner for early detection and isolation of a single line-ground fault. Additionally the inventive system may provide localization of a single ground fault in a complex floating network. The invention may be useful in any ungrounded commercial power system and more particularly in aerospace electrical power system application. 
     In contrast to the prior art, which may use sequential disconnection of network sections to isolate a fault, the present invention may preclude unnecessary disconnection of power sources or loads and the resultant disruption of system operation. In the event of detection of a line-ground fault, a circuit path to ground may be produced through a collection of sequentially switched resistors. Ground fault current may then increase and localization of the ground fault may be more readily achieved. Capacitive currents and their respective directions may be measured. Data obtained in such measurements may be used to determine a zone of a network in which a fault may exist. 
     Referring now to  FIG. 1  there is shown a simplified form of a typical commercial or aerospace ungrounded power system  10 . The power system  10  may comprise a three-phase synchronous generator  12  connected to a rectifier  14 . The generator  12  and the rectifier  14  may be collectively referred to as a DC generator  16 . Power distribution may take place through DC feeders  18  and  20 . Power transmission in the system  10  may be considered to be two-wire DC floating power transmission. At suitable points, inverters may used to convert the DC electric power from the feeders  18  and  20  into AC power of desired voltage and frequency. In an illustrative embodiment of  FIG. 1 , one inverter  22  may be connected to the feeders  18  and  20 . It should be understood that the system  10  may comprise more than one of the inverters  22  and more than one of the DC generators  16 . One or more three-phase AC loads  24  may be connected to the inverter  22  through phase conductors  24 - 1 . 
     A neutral point  12 - 1  of the generator  12  may be connectable to ground through controlled switches  32 ,  34  and  36  and ground conduction paths  38 ,  40  and  42 . While the system  10  may operate normally as an ungrounded system, controlled switches  32 ,  34  and  40  may be used to progressively ground the system  10  when and if a single line-ground fault may develop. As grounding impedance decreases, fault current may increase correspondingly and the fault may thus be more readily detected and isolated. 
     In an illustrative embodiment of  FIG. 1 , three conduction paths  38 ,  40  and  42  are shown. Conduction paths  38  and  40  may comprise resistors R 1  and R 2  respectively. Conduction path  42  may comprise a solid grounding connection, i.e., a conduction path with zero impedance. Thus the embodiment of  FIG. 1  may be considered to have three stages of progressive grounding. However, depending upon system requirements, “n” stages of resistors may be used to give a smooth transition from ungrounded to grounded system. Also, pulse width modulation (PWM) switching of resistors may be employed to provide progressive resistance variation that is smooth. 
     It may be noted that a single line-ground fault in an ungrounded AC system may produce very small current flowing through shunt-connected or any parasitic and stray capacitances. In an ungrounded DC system, the fault current may be characterized by transient capacitive current of shunt capacitance. However, in either system the faulted phase may come to ground potential, while voltages in non-faulted phases may rise to line-voltage levels. 
     As shown in  FIG. 1 , three-phase currents and voltages (with respect to ground) may be measured at testing points  43 - 1 ,  43 - 6  and  43 - 7  in the AC phase conductors  16 - 1  of the DC generator  16  and the phase conductors  24 - 1  which may be connected to the three phase load  24 . In the DC feeders  18  and  20 , line voltages (with respect to the ground) and currents may measured at testing points  43 - 2 ,  43 - 3 ,  43 - 4  and  43 - 5 . Data from the test points may be transmitted continually to a processor  60 . It may be noted that, for purposes of simplicity,  FIG. 1  shows the processor  60  being connected to only one of the sensors  43 . In an actual embodiment of the invention, all of the sensors  43  may be connected with the processor  60 . 
     The resistances R 1  and R 2  may be selected so as to limit fault current through the neutral point  12 - 1  to specified values. A sufficiently high value may be chosen for R 1 , such that the fault current through the neutral point  12 - 1  may be limited to a maximum specified limit. If a fault were to develop at a location far from the DC generator  16  or through some high impedance, a high value of R 1  may restrict the fault current through the neutral point  12 - 1  to an insignificant level. In such cases, this fault current may be increased by closing the switch  34 , which may connect the neutral point  12 - 1  to ground through a lower resistance R 2 . 
     The resistances R 1  and R 2  may be selected such that differential current sensors  50  around the fault may provide current differential signal to the processor  60 . The values of R 1  and R 2  may be selected to suit a particular configuration of the system  10 . In general, R 2  may be about one-tenth of R 1 . It may be noted that, for purposes of simplicity,  FIG. 1  shows the processor  60  being connected to only one of the differential current sensors  50 . In an actual embodiment of the invention, all of the differential current sensors  50  may be connected with the processor  60 . 
     Once a line-ground fault is suspected by observing some line potential coming close to ground potential or abnormal voltages at any one or more of the test points, switch  32  may be closed e.g., through activation by a switching system or switching controller  62  that may be responsive to the processor  60 . The neutral point  12 - 1  of the generator  12  may be thus grounded through a high resistance, R 1 . The resistance, R 1  may be selected such that in a worst case fault situation, the resultant fault current through the resistor R 1  is no higher than about one (1) ampere (A) to about five (5) A. 
     If the resultant fault current is found to be very small and fault localization appears to be difficult, then the second switch  34  may be closed. This may connect the neutral point  12 - 1  with ground through the resistance R 2 . The switch  32  may be left closed or it may be opened before closing the switch  34 . If there is no rise in neutral current, even after closing the switches  32  and  34 , the switch  36  may be closed. This may connect the generator neutral point  12 - 1  to ground with a zero impedance connection. 
     If current at the neutral point  12 - 1  were to remain non-significant, then the system  10  could be declared free from faults. When the system  10  becomes grounded and an appreciable amount of fault current flows in the system  10 , the current sensors  50  may pick up the fault current. 
     The illustrative embodiment of the system  10  is described as one that employs switched resistance grounding. In this illustrative embodiment the grounding resistors R 1 , R 2  . . . Rn may be changed in fixed steps as switches are closed and/or opened by the switching controller  62 . As described above, PWM controlled switching of the resistors R 1 , R 2  . . . Rn may be employed so that resistance variation is continuous and smooth. In other words, the switching controller  62  may be a PWM-based controller. PWM control of switching may be readily implemented with switches such as metal oxide field effect transistors (MOSFET&#39;s) or insulated gate bipolar transistors (IGBT&#39;s) or as a pair of back-back connected thyristors. 
     Localization of a single line-ground fault in a floating DC network may be achieved in a manner depicted by reference to  FIG. 2 . In  FIG. 2 , an illustrative ungrounded or floating DC electrical network  70  is depicted. The DC network  70  may comprise a portion of the power distribution system  10  of  FIG. 1 . Ungrounded DC source  72  and source  74  may be connected to a positive DC bus-bar  76  via the generator feeder  18 . The ungrounded DC sources  72  and  74  may be connected to a negative DC bus-bar  78  via the generator feeder  20 . The DC source  72  and  74  may be a floating battery system, a DC generator or an AC generator followed by a rectifier system. In the illustrative embodiment of  FIG. 2 , two loads  80  and  82  are shown connected to the DC bus bar  76  and  78  through the feeders  18  and  20 . It may be understood that there could be any number of DC sources and any number of loads connected to the DC bus-bars  76  and  78 . 
     For illustrative purposes, four zones may be defined in network  70  A zone  70 - 1  may be defined between floating DC power source  72  and the bus bars  76  and  78 . A zone  70 - 2  may be defined between floating DC power source  74  and the bus bars  76  and  78 . A zone  70 - 3  may be defined between bus bars  76  and  78  and the load  80 . A zone  70 - 4  may be defined between bus bar  76  and  78  and the load  82 . 
     Each of the zones  70 - 1 ,  70 - 2 ,  70 - 3  and  70 - 4  may have two of the sensors  43  installed for fault detection. In zone  70 - 1 , line voltages (with respect to the ground) and currents may be measured at test points indicated by the numerals  90 - 1  and  90 - 2 . In zone  70 - 2 , line voltages (with respect to the ground) and currents may be measured at test points indicated by the numerals  90 - 3  and  90 - 4  In zone  70 - 3 , line voltages (with respect to the ground) and currents may be measured at test points indicated by the numerals  90 - 5  and  90 - 6 . In zone  70 - 4 , line voltages (with respect to the ground) and currents may be measured at test points indicated by the numerals  90 - 7  and  90 - 8 . The sensors  43  may be interconnected with a fault detection system  92 , which may be, in an illustrative embodiment, a portion of the processor  60  of  FIG. 1 . 
     A detected ground fault in zone  70 - 1  may be isolated by opening isolating devices such as contactor sets K 1 A and K 1 B. Similarly, ground fault in zone  70 - 2  may be isolated by opening both contactor set K 2 A and contactor set K 2 B. Ground fault in zone  70 - 3  may be isolated by opening both contactor set K 3 A and contactor set K 3 B. Ground fault in zone  70 - 4  may be isolated by opening both contactor set K 4 A and contactor set K 4 B. 
     During a ground fault condition, it may be desirable to localize and isolate only the faulted zone while keeping other zones in normal operation. For example, if a ground fault were to be detected in zone  70 - 1  it may be desired to isolate only zone  70 - 1 , by opening both contactor set K 1 A and contactor set K 1 B. In this scenario the loads  80  and  82  may be fed from DC power source  74 . Similarly if a fault develops in zone  70 - 4  then only zone  70 - 4  may be isolated from the network  70  In that case, the DC power sources  72  and  74  and the load  80  may remain connected to the network  70 . 
     As described above, a single line-ground fault condition may be declared if a difference between measured positive voltage (Vp) and negative voltage (Vn) with respect to ground or a common chassis exceeds a threshold value. However, in the case of a line to ground failure, all of the test points may measure a voltage difference with respect to ground or a common chassis. This may be indicative of presence of a ground fault somewhere in the power system network  70 . But, localization of such a fault may not achieved merely by measurement of Vp and Vn. 
     Localization of a fault may be achieved by considering that shunt or stray capacitance may exist between ground and positive and negative conductors throughout the network  70 . Fault current during single line-ground fault may be characterized by the transient capacitive current through the shunt or stray capacitance. This current may be referred to as capacitive current. 
     The capacitive current during a single line-ground fault may be characterized by equation (1) below
 
 Ic=C*dV/dt   (1)
 
     where Ic=capacitive current;
         C=shunt or stray capacitance; and   V=line voltage       

     Current at each of the test points may be measured and analyzed continuously on fault detection system (FDS)  92 . The FDS  92  may comprise a software module which may run on a common micro-controller or DSP (Digital Signal Processing) platform such as the processor  60  of  FIG. 1 . Current signals may be processed and analyzed continuously to detect presence and direction of any capacitive current direction at each test point during fault condition. To avoid nuisance capacitive current detection during intermittent fault condition, voltage signals from at least one test point may also be analyzed for ground fault confirmation. 
     In the present invention, ground fault in the floating DC network  70  may be localized based on the detection of direction of capacitive current. If the same direction of the capacitive current is measured at both of the test points in a particular one of the zones, then ground fault is declared outside that zone. If, on the other hand, opposite direction of capacitive current is measured at the two test points, then ground fault is declared within that particular zone. For example, consider if a single line to ground fault were to occur in zone  70 - 3  between positive feeder  18  and ground. The direction of the capacitive current measured between a positive terminal of test point  90 - 3  and a positive terminal of test point  90 - 6  would indicate opposite direction of capacitive current, i.e. an opposite signature. The analyzed current signal at test point  90 - 3  may show a positive direction while a current signal at test point  90 - 6  may indicate negative charging direction. Sampled current signals at other test points in zone  70 - 1 , zone  70 - 2  and zone  70 - 4  may indicate capacitive current directions that are the same direction. So based the above fault signature the single line-to-ground fault could be localized in zone  70 - 3 . 
     Referring now to  FIGS. 3 through 7  and back to  FIG. 2 , simulation results produced in a Matlab/Simulink™ environment are shown. A simulated single line-to-ground fault may be injected in zone  70 - 3 . Line to ground stray capacitance may be simulated as a lumped parameter. 
     The simulation result indicates that the capacitive current direction measured at test points  90 - 1  and  90 - 2  are the same (see  FIG. 3 ). Capacitive current direction measured at test points  90 - 3  and  90 - 4  are the same (see  FIG. 4 ). Capacitive current direction measured at test points  90 - 7  and  90 - 8  are the same (see  FIG. 5 ) This confirms that the ground fault lies outside zone  70 - 1 , zone  70 - 2  and zone  70 - 4 . 
     But the capacitive current direction measured at test points  90 - 5  and  90 - 6  shows opposite direction (see  FIG. 6 ). So the current signal analysis confirms fault location within zone  70 - 3 . As described above, a single line-ground fault condition may be declared if a difference between measured positive voltage (Vp) and negative voltage (Vn) with respect to ground or a common chassis exceeds a threshold value. As shown in  FIG. 7 , there is a change that may exceed a predetermined threshold value in voltage measurement between line-to-ground or chassis. This may confirm that a ground fault does exist in the floating DC electrical network  70 . 
     Referring now to  FIG. 8 , which is a flow chart, it may be seen that the present invention also envisions a method  800  of detecting a single line-ground fault in an ungrounded electrical distribution system. In a step  802 , DC bus voltage with respect to common chassis may be detected (e.g., one of the test points may provide a potential fault signal to the processor  60 ). In a step  804 , a high resistance path to ground may be produced (e.g., the neutral point  12 - 1  may be connected to ground through the resistor R 1 ). In a step  806 , determination may be made as to whether the detected voltage increases significantly (e.g., one of the test points may provide an updated voltage signal to the processor which may be compared with a previous voltage signal). 
     If voltage increase is significant, a step  808  may be performed in which localization of a fault may proceed in a step  814  (e.g., the processor  60  may provide location identifying information as described hereinbelow). If a voltage increase is not significant, a step  810  may be performed in which a lower resistance path to ground may be produced (e.g., the neutral point  12 - 1  may be connected to ground through successively lower impedances, resistors R 2  . . . Rn). Steps  806  and  810  may be cyclically repeated until either a solid path to ground is established or a significant current increase is found. In the event of finding no significant current increase in step  806  after a solid ground is produced in step  810 , a step  812  is initiated in which a declaration of no-fault is made. 
     In a step  814 , which may be initiated after step  808 , capacitor charge or discharge current (capacitive current) direction may be measured within a network zone (e.g. sensor  43  at test point  90 - 1  and  90 - 2  may determine direction of capacitive current at both of the test points). In a step  816 , the current directions may be compared. In the event that capacitive current for both of the test points are in the same direction then a step  818  may be initiated to declare that the zone is fault-free. In the event that capacitive currents are determined to be in opposite directions in step  816 , a step  820  may be initiated in which a zone may be declared to contain a ground fault. 
     The method  800  may be performed continuously and repetitively so that the ungrounded electrical system may be continuously monitored to determine if a single line-ground fault may exist 
     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.