GROUND FAULT DETECTOR CIRCUIT WITH FEEDBACK TO SENSOR

• US 3,848,159 A
• Filed: 06/18/1973
• Issued: 11/12/1974
• Est. Priority Date: 06/18/1973
• Status: Expired due to Term

• Current Assignee: Daniel J. Eisenstadt, Dudley D. Nye Jr
• Original Assignee: Daniel J. Eisenstadt, Dudley D. Nye Jr
• Inventors: Nye, Dudley D. Jr., Eisenstadt, Daniel J.
• Primary Examiner(s): Trammell, James D.
• Application Number:
  US05/370,754
• Time in Patent Office: 512 Days
• Field of Search: 317/18D,18R,27R
• US Class Current: 361/45
• CPC Class Codes: H02H 1/06 Arrangements for supplying operative power power supply arrangements in general G05F, H02M
• CPC Class Codes: H02H 3/33 using summation current transformers H02H3/347 takes precedence
• CPC Class Codes: H02H 3/331 responsive to earthing of the neutral conductor H02H3/338 takes precedence

Abstract

A ground fault circuit breaker is disclosed in which a differential transformer used to sense ground faults is provided with feedback current for generating a magnetic flux which opposes the ground fault induced flux. The resulting flux nulling cancels out the effects of variable core permeability on the breaker trip signal. This permits the use of less expensive and smaller sensor cores together with more consistant and stable calibration. The feedback current is derived from the output signal of a high gain differential amplifier connected to a secondary winding on the core. The output signal from the amplifier is also utilized for actuating a trip circuit which interrupts power and prevents electrocution. Because of the flux nulling, the output signal from the amplifier is an extremely linear and stable function of the ground fault current which facilitates calibration of the circuit breaker. This circuit also permits the use of clamp on ground fault sensor cores in conjunction with a portable ground fault indicator instrument. The instrument reading is then unaffected by dirt and variation of preasure of the split sensor core.

Claims

1. Apparatus for detecting ground faults between one of a plurality of transmission lines and earth-ground comprising:

a magnetically permeable core surrounding the transmission lines for generating a magnetic flux proportional to a current differential between the transmission lines;

means coupled to said core for producing a signal having an amplitude related to said magnetic flux; and

means responsive to said signal for generating a flux opposing said differential current induced flux, the amplitude of said signal being proportional to said current differential with core permeability characteristics being substantially removed.

2. The apparatus according to claim 1 and further including means for interrupting power in said transmission lines responsive to the amplitude of said signal exceeding a threshold indicative of a ground fault induced current imbalance in the transmission lines.

3. The apparatus according to claim 1 wherein said means for producing said signal includes a high gain amplifier and wherein said means for generating the opposing flux includes a separate winding on said core.

4. The apparatus according to claim 3 and further including means for superimposing a high frequency signal on the transmission lines whereby neutral to earth-ground faults are detected.

5. The apparatus according to claim 1 wherein said means for producing said signal includes a high gain amplifier having at least one input terminal and an output terminal and a feedback circuit from said output terminal to said input terminal;

and wherein said means for generating the opposing flux includes a secondary winding connected to said amplifier for coupling flux-generated current to said amplifier and for coupling feedback signals in said feedback circuit through said secondary winding for generating said opposing flux.

6. The apparatus according to claim 5 and further including means for superimposing on said transmission lines a high frequency signal whereby neutral to earth-ground faults are detected.

7. The apparatus according to claim 1 wherein said means for producing said signal is a differential amplifier having two input terminals, wherein said core has a secondary winding and further including transient filters comprising a resistor and capacitor connected in series between each end of said secondary winding and system common, the interconnection points between said resistor and capacitor being connected to different input terminal.

8. The apparatus according to claim 7 and further including a resistor across said secondary winding to prevent circuit ringing and overshoot.

9. A ground fault interruption system for use in the interruption of power to power lines in the event of the provision of a leakage path between one of said lines and earth-ground comprising:

a differential transformer having a toroidal core surrounding said power lines, said power lines forming a primary winding for said transformer, said core having secondary and tertiary windings;

a differential amplifier having input terminals connected to different ends of said secondary winding;

a circuit breaker and a trip circuit for actuating said circuit bReaker;

a D.C. power supply, said power supply and said trip circuit energized from said power lines, said amplifier being supplied from said power supply and having an output terminal connected to said trip circuit for interrupting power in said power lines when the signal at said output terminal exceeds a predetermined threshold, the signal at said output terminal being connected to said tertiary winding for causing a magnetic flux which opposes that caused by a current imbalance in said power lines.

10. The ground fault interruption system according to claim 9 wherein said secondary winding includes a center tap and wherein said differential amplifier includes an emitter-follower differential amplifier connected across the ends of said secondary winding with said center tap connected between the positive side of said power supply and system common;

a high gain differential amplifier having two input terminals;

two capacitors, different capacitors connected between different input terminals of said high gain differential amplifier and different output terminals of said emitter-follower amplifier;

a feedback resistor connected between the output terminal of said high gain differential amplifier and one of its input terminals, a resistor between the other of its input terminals and system common; and

a control resistor between said output terminal and said tertiary winding.

11. The ground fault interruption system according to claim 10 wherein the reactance of the capacitor connected to the input terminal to which said feedback resistor is connected and the resistance to said feedback resistor is such that 60 Hertz signals are not attenuated.

12. The ground fault interruption system according to claim 10 and further including a Zener diode and two resistors connected in series between said output terminal and system common, the signal applied to said trip circuit being tapped from the interconnection point of said last mentioned resistors.

13. The ground fault interruption system according to claim 12 wherein said trip circuit and power supply includes:

a trip coil, a diode full-wave rectifying bridge having one input terminal connected to one power line and its other input terminal connected through said trip coil to another power line;

a capacitor connected between said input terminals, said bridge circuit having a pair of output terminals;

a silicon-controlled rectifier connected across said output terminals and having a trigger terminal connected to said interconnection point, whereby whenever a voltage exceeding a predetermined potential is delivered to the trigger terminal of said silicon-controlled rectifier said output terminals are shorted thereby to cause the entire line voltage to be applied across said trip coil.

14. Apparatus for producing an electrical output in response to a ground fault between one of a plurality of transmission lines and earth-ground comprising:

a magnetically permeable split core having at least one movable section whereby the core may be opened and placed around a plurality of transmission lines for generating a magnetic flux proportional to a current differential between the transmission lines;

means coupled to said core for producing signal having an amplitude related to said magnetic flux; and

means responsive to said signal for generating a flux opposing said differential current induced flux, the amplitude of said signal being proportional to said current differential with core permeability characteristics being substantially removed.

15. Apparatus according to claim 14 wherein said means for producing said signal includes a high gain amplifier and wherein said means for generating the opposing flux includes a separate winding on said core. 16.Apparatus according to claim 15 including a trip coil coupled to the output of said amplifier.

17. Apparatus according to claim 15 including an electrical indicating meter coupled to the output of said amplifier.

18. Apparatus according to claim 14, with a non-splitting core substituted for said split core.

19. A ground fault interruption system for use in the interruption of power to power lines in the event of an impedance path between one of said lines and earth-ground comprising:

a differential transformer having a core surrounding said power lines and having a secondary winding;

a differential amplifier having two input terminals, a capacitor connecting one of said input terminals to one end of said secondary winding, the other end of said secondary winding being connected to the other of said input terminals, a feedback resistor connected between the output terminal of said amplifier and said one input terminal;

a resistor connected between said other input terminal and system common;

means for interrupting power line current responsively to the signal from said amplifier exceeding a predetermined amplitude, said core having a tertiary winding and a resistor connected between one end of said tertiary winding and the output terminal of said amplifier, the other end of said tertiary winding being connected to system common for generating a flux opposing that generated by a current imbalance in said power lines.