Abstract:
An ungrounded 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. 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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims benefit of U.S. Provisional Application 60/992,752 filed Dec. 6, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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 an 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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    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 is not desirable to shutdown the entire network in the event of a single ground fault, it would be a great advantage to confirm the occurrence of the fault, localize the fault and isolate only the faulty section of the network while keeping the unaffected network functions operative. 
         [0010]    As can be seen, there is a need for a system that may provide for early detection and confirmation of occurrence of single line-ground fault in an ungrounded power distribution system. Furthermore, there is a need for such a system which may provide for detection without interruption of operation of the system. 
       SUMMARY OF THE PRESENT INVENTION 
       [0011]    In one aspect of the present invention an ungrounded electrical power distribution system comprises a switching system for selectively grounding the electrical distribution system in the event of a line to ground fault. 
         [0012]    In a further aspect of the invention a ground fault detection system for an ungrounded power system comprises a plurality of conduction paths between the power distribution system and ground potential. The conduction paths have differing impedances. A switching system progressively connects the power distribution to ground potential through successive ones of conduction paths having successively lower impedance. 
         [0013]    In another aspect of the invention a method of distributing electrical power on an ungrounded distribution system comprises the steps of detecting a variation in line voltage to ground, connecting a high impedance conduction path between the distribution system and ground and declaring presence of line to ground fault in presence of a current change produced by connecting of conduction path. 
         [0014]    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 
         [0015]      FIG. 1  is a block diagram of an ungrounded power distribution system in accordance with the invention; 
           [0016]      FIG. 2  is a switching arrangement that may be employed in the power distribution system of  FIG. 1  in accordance with the invention; and 
           [0017]      FIG. 3  is a flow chart of a method for distributing power in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    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. 
         [0019]    The present invention provides a system for detection of ground faults in an ungrounded 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. The invention may be useful in any ungrounded commercial power system and more particularly in aerospace electrical power system application. 
         [0020]    In contrast to the prior art, which may use line voltage variations to detect general presence of a ground fault in a system, 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. 
         [0021]    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 a 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 . A three-phase AC load  24  may be connected to the inverter  22  through phase conductors  24 - 1 . 
         [0022]    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 isolated. 
         [0023]    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 grounding 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. 
         [0024]    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 charging/discharging 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. 
         [0025]    As shown in  FIG. 1 , three-phase currents and voltages (with respect to ground) may be measured with sensors  43  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 sensors  43  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   
         [0026]    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. 
         [0027]    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 . 
         [0028]    The resistances R 1  and R 2  may be selected such that ground fault detectors, e.g., 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 any 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 . 
         [0029]    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 of a switching system or switch 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. 
         [0030]    If the resultant fault current is found to be very small and fault detection 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 directly ground the generator neutral point  12 - 1 . 
         [0031]    If current at the neutral point  12 - 1  were to remain unchanged or non-significant after high-impedance grounding, 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. Data from the current sensors  50  may be processed in the processor  60  to localize the fault. When the system  10  is provided with the subject progressive grounding arrangement, an operator of the system may have an option to assess the severity of a fault without disrupting operation of the system operation. 
         [0032]    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. 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  64  in one of the switches such as the switch  32  as shown in  FIG. 2 . 
         [0033]    Referring now to  FIG. 3 , which is a flow chart, it may be seen that the present invention also envisions a method  300  of detecting a single line-ground fault in an ungrounded electrical distribution system. In a step  302 , current differential may be detected (e.g., one of the sensors  43  may provide a potential fault signal to the processor  60 ). In a step  304 , 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  306 , determination may be made as to whether the detected current increases significantly (e.g., one of the test points may provide an updated current signal to the processor which may be compared with a previous current signal). 
         [0034]    If current increase is significant, a step  308  may be performed in which localization of a fault may proceed (e.g., the processor  60  may provide location identifying information to an operator of the system  10 ). If a current increase is not significant, a step  310  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 the resistor R 2 ). Steps  306  and  310  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  306  after a solid ground is produced in step  310 , a step  312  is initiated in which a declaration of no-fault is made. 
         [0035]    The method  300  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 
         [0036]    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.