Fault detection

• US 4,837,519 A
• Filed: 12/07/1987
• Issued: 06/06/1989
• Est. Priority Date: 09/06/1985
• Status: Expired due to Term

• Current Assignee: Southern California Edison Company (Edison International)
• Original Assignee: Southern California Edison Company (Edison International)
• Inventors: Clopton, James Jr., Lopetrone, Juan P., Valdes, Oliver
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Solis, Jose M.
• Application Number:
  US07/129,160
• Time in Patent Office: 547 Days
• Field of Search: 324/529, 324/509, 324/530, 324/528, 340/650
• US Class Current: 324/529
• CPC Class Codes: G01R 27/18 Measuring resistance to earth , i.e. line to ground
• CPC Class Codes: G01R 31/52 Testing for short-circuits, leakage current or ground faults
• CPC Class Codes: H02H 3/16 responsive to fault current to earth, frame or mass with balanced or differential arrangement H02H3/26 ; monitoring earth connection H02H5/105
• CPC Class Codes: H02H 3/17 by means of an auxiliary voltage injected into the installation to be protected using summation current transformers H02H3/33

Abstract

Apparatus for the detection of leakage current in a system includes a power supply and conductors from the supply for supplying power to a load connected to the system. There is an interrupter connected in the system and to a ground point such that a ground leakage current in the system has a closed circuit. Activation of the interrupter about every 12 seconds effectively generates a pulse interrupted ground fault signal. Such signal is detected by a magnetic sensor located relative to a multi-feeder system with high capacitive reactance. The sensor is immune to random electromagnetic and electrostatic conditions in the distribution system. The sensor is synchronized to operate with the interrupter so that sensing is effected at a steady level of a square wave interrupted pulse.

Claims

1. Apparatus for the detection of a fault signal in a supply system including conductors from a supply for supplying power to a load connected in the system comprising an impedance element for connection across the power supply, a tapping point to the impedance element, a connector between the tapping point and a point to complete a circuit for a fault signal in the system, an interrupter for periodically pulsing a signal into the system effectively thereby to generate a pulse interrupted fault signal, a detector for location relative to the system for sensing the pulse interrupted fault signal and thereby providing for detecting the location of the fault, and means for operating the detector in synchronism with the interrupter whereby the detector senses a substantially steady state level of the pulse interrupted fault signal.

2. Apparatus as claimed in claim 1 wherein the system is a DC system and the pulse is a DC pulse.

3. Apparatus as claimed in claim 2 wherein the DC pulse level is sensed at a time removed from the time of change of the pulse interrupted signal.

4. Apparatus as claimed in claim 3 wherein the interrupted pulse is substantially a square wave and the detection is effected during the steady state high level of the square wave.

5. Apparatus as claimed in anyone of claims 1, 2, 3 or 4 wherein the interrupter generates a synchronization signal, and including means for feeding the synchronization signal to the detector.

6. Apparatus as claimed in anyone of claims 1, 2, 3 or 4 including a crystal oscillator for generating a synchronization signal, and means for feeding such signal to the detector.

7. Apparatus as claimed in anyone of claims 1, 2, 3 or 4 wherein the detector includes a magnetic sensor for location about a positive and a negative conductor of the conductors between the power supply and load, and including means for determining whether the fault signal is in the positive and/or negative conductor between the power supply and load.

8. Apparatus as claimed in claim 4 wherein the period of the square wave is about 12 seconds.

9. Apparatus as claimed in claim 1 including means for eliminating the effect of electromagnetic changes in the system prior to sampling and measuring the steady state interrupted pulse.

10. Apparatus as claimed in claim 1 including means for detecting fault signals at least as low 2 milliamperes.

11. Apparatus as claimed in claim 1 including means for detecting a fault signal less than 2 milliamperes.

40. Apparatus as claimed in anyone of claims 1, 2, 3, 4, 20, 21, or 22 including means for synchronizing the detector to operate in time advance of the pulse interrupted signal.

41. Apparatus as claimed in claim 5 wherein the synchronization signal leads in time the interrupted pulse signal.

42. Apparatus as claimed in claim 5 including means for delaying in time the interrupted pulse signal relative to the synchronization signal.

43. Apparatus as claimed in claim 40 wherein the time advance is about 200 milliseconds.

12. Apparatus for the detection of a fault signal in a supply system including conductors from a power supply for supplying power to a load and having stray capacitance in the system comprising an impedance element for connection across the power supply, a tapping point to the impedance element, a connector between the tapping point and a point to complete a circuit for a fault signal in the system, an interrupter for periodically pulsing a signal into the system effectively thereby to generate a pulse interrupted fault signal, a detector for location relative to the system for sensing the pulse interrupted fault signal and thereby providing for detecting the location of the fault, and including means for substantially discharging the stray capacitance in the system.

13. Apparatus as claimed in claim 12 wherein the power supply is a DC power supply and the pulse is a DC pulse, and wherein the detector is a magnetic detector and detection is effected after the stray capacitance in the system has been substantially fully charged or discharged.

14. Apparatus as claimed in claim 13 wherein the discharging means includes a resistor bank for selective connection across the power supply.

15. Apparatus as claimed in claim 14 wherein the resistor bank is part of the impedance element and includes four resistors, a pair of resistors being connected to each conductor, and including a switch between at least some of the resistors whereby in one position of the switch a first resistor is connected with one conductor and another resistor is connected with second conductor, and in a second position of the switch a third resistor is connected with the first conductor, and a fourth resistor is connected with the second conductor.

16. Apparatus as claimed in claim 15 wherein in the second position of the switch at least one of the resistors acts to discharge stray capacitance while in the first position at least one of the resistors acts as a current limiting resistor.

17. Apparatus as claimed in claim 16 wherein the value of the discharge resistor is determined to be substantially equal to the current limiting resistor such that the time constant to charge the circuit is substantially equal to the time constant to discharge the circuit.

18. Apparatus as claimed in claim 17 wherein the cycle for interrupting the circuit is about 12 seconds, and wherein about 6 seconds is for the fault signal to flow thorugh the limiting resistor and about 6 seconds is for the stray capacitance to discharge through the discharge resistor.

19. Apparatus as claimed in claim 18 including means for detecting a fault signal as low as about 2 milliamperes.

44. Apparatus as claimed in anyone of claims 12, 13, 14, 15, 16, 17, 30, 31, 32, 33 or 34 including means for synchronizing the detector to operate in synchronization with the interrupter whereby the detector senses a substantially steady state level of the pulse interrupted fault signal.

45. Apparatus as claimed in claim 44 including means for synchronizing the detector to operate in time advance of the pulse interrupted signal.

46. Apparatus as claimed in claim 45 including means for generating a synchronization signal leading in time of the interrupted pulse signal.

71. Apparatus as claimed in claim 14 wherein the resistor bank is part of the impedance element and includes four resistors, a pair of resistors being connected to each conductor, and including a switch between at least some of the resistors whereby the switch selectively connects each resistor in turn to complete the circuit for the fault system.

72. Apparatus as claimed in claim 71 wherein one resistor discharges stray capacitance and another resistor limits current.

73. Apparatus as claimed in claim 72 wherein the value of the discharge resistor is determined such that the time constant to charge the circuit is substantially equal to the time constant to discharge the circuit, interrupting the circuit is about 12 seconds.

74. A method as claimed in claim 73 including having the resistor bank contain four resistors and connecting a pair of resistors to each conductor, and including switching between at least some of the resistors whereby, in turn, a first resistor is connected with one conductor and another resistor is connected with a second conductor, and then a third resistor is connected with the first conductor, and a fourth resistor is connected with a second conductor.

75. A method as claimed in claim 74 including selectively switching resistors to discharge stray capacitance, and to limit current.

76. A method as claimed in claim 75 including selecting the value of the discharge resistor such that the time constant to charge the circuit is substantially at least equal or greater than the time constant to discharge the circuit.

20. Apparatus for the detection of a ground fault signal in a normally ungrounded DC system including conductors from a supply for supplying power to a load in the DC system comprising an impedance element for connection across the DC power supply, a tapping point to the impedance element, a connector between the tapping point and a ground point to complete a ground circuit for a ground fault in the system, a relay between the tapping point and ground, an interrupter for periodically opening and closing the relay while the ungrounded DC system remains substantially closed and operational, the opening and closing of the relay interrupting the ground circuit effectively thereby to generate a DC pulse interrupted ground fault signal, a magnetic detector for location relative to the DC system for sensing the DC pulse interrupted ground fault signal and thereby providing for detecting the location of the ground fault, the DC system remaining substantially closed and operational during the ground fault detection, and means for operating the detector in synchronism with the interrupter whereby the detector senses a substantially steady state level of the DC pulse interrupted fault signal.

21. Apparatus as claimed in claim 20 wherein the DC pulse level is sensed at a time removed from the time of change of the interrupted signal.

22. Apparatus as claimed in claim 21 wherein the interrupted pulse is substantially a square wave and the detection is effected during the steady state high level of the square wave.

23. Apparatus as claimed in anyone of claims 20, 21 or 22 wherein the interrupter generates a synchronization signal, and including means for feeding the synchronization signal to the detector.

24. Apparatus as claimed in anyone of claims 20, 21 or 22 including a crystal oscillator for generating a synchronization signal, and means for feeding the synchronization signal to the detector.

25. Apparatus as claimed in anyone of claims 20, 21 or 22 wherein the detector means includes a sensor for location about a positive and a negative conductor of the conductors between the power supply and load, and including means for determining whether the fault signal is in the positive and/or negative conductor between the power supply and load.

26. Apparatus as claimed in claim 22 wherein the period of the square wave is about 12 seconds.

27. Apparatus as claimed in claim 20 including means for eliminating the effect of electromagnetic changes in the system prior to sampling and measuring the steady state interrupted pulse.

28. Apparatus as claimed in claim 20 including means for detecting ground fault signal at least as low as 2 milliamperes.

29. Apparatus as claimed in claim 20 including means for detecting a ground fault signal less than 2 milliamperes.

30. Apparatus for the detection of a ground fault signal in a normally ungrounded DC system including conductors from a supply for supplying power to loads in the DC system having stray capacitance comprising an impedance element for connection across the D.C. power supply, a tapping point to the impedance element, a connector between the tapping point and a ground point such that a ground fault in the system completes a ground circuit, a relay between the tapping point and ground, an interrupter for periodically opening and closing the relay while the ungrounded DC system remains substantially closed and operational, the opening and closing of the relay interrupting the ground circuit effectively thereby to generate a DC pulse interrupted ground fault signal, a magnetic detector for location relative to the DC system for sensing the DC pulse interrupted ground fault signal and thereby providing for detecting the location of the ground fault, the DC system remaining substantially closed and operational during the ground fault detection, and including means for substantially discharging the stray capacitance in the system.

31. Apparatus as claimed in claim 30 wherein the discharging means includes a resistor bank for selective connection across the power supply, and detection is effected after the stray capacitance in the system has been substantially fully charged or discharged.

32. Apparatus as claimed in claim 31 wherein the resistor bank is part of the impedance element and includes four resistors, a pair of resistors being connected to each conductor, and including a switch between at least some of the resistors whereby in one position of the switch a first resistor is connected with one conductor and another resistor is connected with a second conductor, and in a second position of the switch a third resistor is connected with the first conductor, and a fourth resistor is connected with the second conductor.

33. Apparatus as claimed in claim 32 wherein the second position of the switch connects at least one of the resistors to discharge stray capacitance, while in the first position at least one of the resistors acts as a current limiting resistor.

34. Apparatus as claimed in claim 33 wherein the value of the discharge resistor is determined to be substantially equal to the current limiting resistor such that the time constant to charge the circuit is substantially equal to the time constant to discharge the circuit.

35. Apparatus as claimed in claim 34 wherein the cycle for interrupting the circuit is about 12 seconds, and wherein about 6 seconds is for fault signal to flow through the limiting resistor and about 6 seconds is for the stray capacitance to discharge through the discharge resistor.

36. Apparatus as claimed in claim 35 including means for detecting a fault signal as low as about 2 milliamperes.

77. Apparatus as claimed in claim 30 wherein the resistor bank is part of the impedance element and includes four resistors, a pair of resistors being connected to each conductor, and including a switch between at least some of the resistors whereby the switch selectively connects each resistor in turn to complete the circuit for the fault signal.

78. Apparatus as claimed in claim 77 wherein different resistors act to discharge stray capacitance and limit current.

37. Apparatus for the detection of low level ground leakage current in a supply system including conductors from a supply for supplying power to a load connected in the system comprising:

resistor elements for connection at one end to conductors from the supply;

a selector relay connected to the resistors at ends opposite to the connection with the conductors;

an interrupter replay connected with the selector relay for periodically connecting selected resistor elements to ground thereby creating a ground circuit for the ground leakage current through selected resistors; and

a detector for location relative to the system for sensing the ground current when the current is in a substantially steady state level.

38. Apparatus as claimed in claim 37 including delay means for operating the detector before the interrupter interrupts the ground circuit.

39. Apparatus as claimed in claim 38 including means for generating a synchronous signal, the synchronous signal activating the interrupter and activating the detector through the delay means.

97. Apparatus as claimed in claim 39 wherein the delay means causes the detector to sense the ground current after a substantially steady state is reached.

98. Apparatus as claimed in claim 97 wherein the time delay permits the stray capacitance in the system to be substantially charged prior to the detector detecting the ground current.

47. A method for the detection of a fault signal in a system including conductors from a supply for supplying power to a load connected in the system comprising connecting an impedance element across the power supply, providing a tapping point to the impedance element to complete a circuit between the tapping point and a point for a fault signal in the system, periodically interrupting the system with a pulsing signal thereby to generate a pulse interrupted signal in the system, and detecting the location relative to the system of the pulse interrupted signal, the detection being in synchronism with the interrupting signal whereby the detection is effected at substantially steady state level of the pulse interrupted signal.

48. A method as claimed in claim 47 wherein the system is a DC system and including detecting a DC pulse.

49. A method as claimed in claim 48 including sensing the DC pulse level at a time removed from the time of change of the interrupted signal.

50. A method as claimed in claim 49 including generating an interrupted pulse which is substantially a square wave, and effecting detection during the steady state high level of the square wave.

51. A method as claimed in anyone of claims 47 to 50 including generating a synchronization signal, and feeding the synchronization signal to a detector.

52. A method as claimed in anyone of claims 47 to 50 including generating a synchronization signal from a resistor/capacitor oscillator or a crystal oscillator and feeding such signal to a detector.

53. A method as claimed in anyone of claims 47 to 50 including sensing the fault current in a positive and/or negative conductor between the power supply and load.

54. A method as claimed in claim 50 wherein the period of the square wave is about 12 seconds.

55. A method as claimed in claim 47 including eliminating the effect of electromagnetic changes in the system prior to sampling and measuring the steady state of the pulse interrupted signal.

56. A method as claimed in claim 47 including detecting fault signals at least as low as 2 milliamperes.

57. A method for the detection of a fault signal system including conductors from a supply for supplying power to a load in the system having stray capacitance comprising connecting an impedance element across the power supply, providing a tapping point to the impedance element to complete a circuit between the tapping point and a point for a fault signal in the system periodically interrupting the system with a pulsing signal effectively thereby to generate a pluse interrupted fault signal, detecting the location relative to the system of the pulse interrupted signal and thereby providing for detecting the location of the fault, and substantially charging or discharging substantially the stray capacitance prior to detection of a fault in the system.

58. A method as claimed in claimed in claim 57 including selectively connecting a resistor bank across the power supply for discharging the stray capacitance.

59. A method as claimed in claim 58 wherein the resistor bank includes four resistors and connecting a pair of resistors to each conductor, and including switching between at least some for the resistors whereby in one position of the switch a first resistor whereby in one position of the switch a first resistor is connected with one conductor and another resistor is connected with a second conductor, and in a second position connecting a third resistor with the first conductor, and a fourth resistor with a second conductor.

60. A method as claimed in claim 59 including switching in a second position at least one of the resistors to discharge stray capacitance, and while in the first position at least one of the resistors acts to limit current.

61. A method as claimed in claim 60 including selecting the value of the discharge resistor to be substantially equal to the current limiting resistor such that the time constant to charge the circuit is substantially equal to the time constant to discharge the circuit.

62. A method as claimed in claim 61 including interrupting the circuit with a cycle time of about 12 seconds, and wherein about 6 seconds is for the fault signal to flow through the limiting resistor and about 6 seconds is for the stray capacitance to discharge through the discharge resistor.

63. A method as claimed in claim 62 including detecting a fault signal about as low as 2 milliamperes.

64. A method for the detection of a ground fault signal in a normally ungrounded DC system including conductors from a supply for supplying power to a load in the DC system comprising connecting an impedance element across the DC power supply, providing a tapping point to the impedance element to complete a circuit between the tapping point and a ground point for a ground fault signal in the system having a relay between the tapping point and ground, periodically opening and closing the relay while the ungrounded DC system remains substantially closed and operational, the opening and closing of the relay interrupting the ground circuit effectively thereby to generate a DC pulse interrupted ground fault signal, and magnetically detecting the location relative to the DC system of the DC pulse interrupted ground fault signal and thereby providing for detecting the location of the ground fault, the detection being in synchronism with the interrupting signal whereby the detection is at a substantially steady state level of the DC pulse interrupted ground fault.

65. A method as claimed in claim 64 including generating an synchronization signal, and feeding the synchronization signal to the detector.

66. A method for detecting a ground fault signal in a normally ungronded DC system having conductors from a supply for supplying power to loads in the DC system with stray capacitance comprising connecting an impedance element across the DC power supply, providing a tapping point to the impedance element to complete a circuit between the tapping point and a ground point for the ground fault, providing a relay between the tapping point and ground, periodically opening and closing the relay while the ungrounded DC system remains substantially closed and operational, the opening and closing of the relay interrupting the ground circuit effectively thereby to generate a DC pulse interrupted ground fault signal, magnetically detecting the DC pulse interrupted ground fault signal and thereby providing for detecting the location of the ground fault, and substantially charging or discharging the stray capacitance prior to detection of DC pulse interrupted ground fault in the system.

67. A method as claimed in claim 66 including synchronizing detection to operate in time lag of the pulse interrupted signal.

68. A method as claimed in claim 67 including generating a synchronization signal to lead in time the interrupted pulse signal.

69. A method as claimed in claim 68 wherein the time lag is about 200 milliseconds.

70. A method as claimed in claim 69 including synchronizing the detector to operate in synchronism with the interrupter pulse whereby the detector senses a substantially steady state pulse level.

79. Apparatus for the detection of low level ground leakage in a supply system including conductors from the supply for supplying power to loads connecting to the system comprising:

resistor elements connected at one end to conductors from the supply;

a selector relay connected to the resistors at ends opposite to the resistor connection with the conductors;

an interrupter relay connected with the selector relay for periodically connecting selected resistor elements through a function switch to ground thereby creating a ground circuit for ground leakage current through the selected resistors when the function switch is closed; and

a detector for location relative to the conductors for sensing the ground leakage current when the ground current is at a substantially steady state level.

80. Apparatus as claimed in claim 79 wherein the interrupter relay acts to cause the ground circuit to be periodically interrupted thereby to generate a pulse interrupted ground fault current.

81. Apparatus as claimed in claim 80 wherein the pulse interrupted ground fault current has a period of about 12 seconds.

82. Apparatus as claimed in claim 80 including delay means for operating the detector before the interrupter interrupts the ground circuit.

83. Apparatus as claimed in claim 82 including means for generating a synchronized signal, the synchronized signal activating the interrupter relay and activating the delay means.

84. Apparatus as claimed in claim 83 wherein the delay means causes the detector to sense the ground leakage current after a substantially steady state level is reached.

85. Apparatus as claimed in claim 84 wherein the delay is sufficient in time so that stray capacitive components in the system are substantially charged whereby the detector senses a current level substantially unaffected by stray capacitance.

86. Apparatus as claimed in claim 85 wherein the pulse interrupted ground fault current has a period of about 12 seconds.

87. Apparatus as claimed in claim 86 wherein the resistor elements include four resistors, the one end of a pair of resistors being connected to a first conductor and the one end of another pair of resistors being connected to a second conductor, and wherein the selector relay in turn connects the interrupter relay and function switch through different resistors of the pairs to complete the ground circuit.

88. Apparatus as claimed in claim 87 wherein the different resistors act to discharge stray capacitance to limit current in the system.

89. Apparatus as claimed in claim 88 wherein the resistor elements include discharge resistors and the value of the discharge resistors is determined such that the time constant to charge the system is substantially equal to the time constant to discharge the system.

90. A method for the detection of low level ground leakage current in a supply system including conductors between a supply loads connected in the system comprising:

connecting one end of resistor elements to the conductors from the supply;

connecting a selector relay to the resistors at ends opposite to the resistor connection with the conductors;

connecting an interrupter relay with the selector relay for periodically connecting selector relay and through a function switch to ground thereby creating a ground circuit for the ground leakage current through selected resistors when the function switch is closed; and

locating a detector relative to the system for sensing the ground leakage current when the current is at a substantially steady state level.

91. A method as claimed in claim 90 including periodically interrupting the group circuit with the interrupter relay thereby to generate a pulse interrupted ground fault current.

92. A method as claimed in claim 91 wherein the pulse current has a period of about 12 seconds.

93. A method as claimed in claim 92 including a delay period for operating the detector before the interrupter interrupts the ground circuit.

94. A method as claimed in claim 93 including generating a synchronous signal for activating the interrupter and for activating the detector before the delay period.

95. A method as claimed in claim 94 wherein the delay period causes the detector to sense the ground current after substantially steady state is reached.

96. A method as claimed in claim 95 wherein the delay period is sufficient so that the stray capacitance of the system is substantially charged.