Ground fault detecting apparatus including current-responsive threshold detection circuitry

• US 4,114,089 A
• Filed: 12/17/1976
• Issued: 09/12/1978
• Est. Priority Date: 03/21/1975
• Status: Expired due to Term

• Current Assignee: RCA Corporation (General Electric Company)
• Original Assignee: RCA Corporation (General Electric Company)
• Inventors: Ahmed, Adel Abdel Aziz
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/751,588
• Time in Patent Office: 634 Days
• Field of Search: 324/51, 324/133, 340/255, 361/44, 361/45, 361/46, 361/110, 328/151, 328/127, 328/165, 307/350-353
• US Class Current: 324/509
• CPC Class Codes: G01R 19/16571 comparing AC or DC current with one threshold, e.g. load current, over-current, surge current or fault current G01R19/16514, G01R19/16519, G01R19/16528, G01R19/16533, G01R19/1659 take precedence; measuring currents by using elements sensitive to the magnetic field generated G01R15/14; measuring earth resistance G01R27/18; testing for leakage or short circuits in electrical apparatus G01R31/52
• CPC Class Codes: G01R 27/18 Measuring resistance to earth , i.e. line to ground
• CPC Class Codes: H02H 3/337 avoiding disconnection due to reactive fault currents

Abstract

Ground fault detection apparatus improved to include detector circuiry for providing an output current responsive to imbalance in the alternating currents flowing to and from a load only if it be of sustained character and of sufficient amplitude. The detector circuitry discriminates against responses to imbalance current primarily consisting of infrequent short-duration high-amplitude spikes characteristic of transient disturbances in favor of responses primarily consisting of the sustained, relatively long-duration rectified sinusoidal waveforms characteristic of ground fault conditions, even where the average level of the transient disturbance component of the imbalance current is equal to or in excess of the average level of ground fault component.

Claims

1. In ground-fault detection apparatus for a power distribution system with a plurality of a-c power line conductors, said apparatus including differential current sensing means for responding to total current flow through said a-c power line conductors in one direction differing from total current flow through said a-c power line conductors in the opposite direction to provide an a-c signal representative of that difference;

synchronous detection means having a first input circuit to which the a-c signal provided by said differential current sensing means is applied, having a second input circuit to which the a-c potential appearing between a pair of the a-c power line conductors is applied, and having an output circuit for supplying a product signal current substantially proportional up to a maximum limit value of said product signal current to the product of the a-c signal provided by said differential current sensing means times said a-c potential appearing between a pair of the a-c power line conductors; and

means for determining whether or not said product signal current has a direct component of an amplitude and duration indicative of the presence of a resistive ground fault, the improvement wherein said means for determining whether or not said product signal current has a direct component of an amplitude and duration indicative of the presence of said resistive ground fault comprises;

true integrator means having an input circuit connected across the output circuit of said synchronous demodulator means and having an output circuit for supplying an output signal substantially proportional to the time integral of current flow into and out of its input circuit, thus differing from a low-pass filter, which output circuit of said integrator means may be identical to its input circuit as in the case where said integrator means essentially consists of an integrating capacitor;

means connected across the output circuit of said synchronous detection means for continually extracting as much current in a first sense as is available from the output circuit of said synchronous detection means and the input circuit of said integrator means, up to a threshold value of current, to divert product signal current of less than said threshold value away from the input circuit of said integrating means and for reducing the level of output signal from said integrator during the times said product signal current is less than said threshold value at a rate depending upon how much said product signal current is less than said threshold value; and

threshold detection means having an input circuit to which the output circuit of said integrator means is connected, having an output circuit, and including means responsive to as much, if any, of said integrator means output signal which exceeds a predetermined level in a first sense for producing at the output circuit of said threshold detection means a signal indicative of a resistive ground fault condition existing in one of said a-c power line conductors, said predetermined level being chosen such that said product signal current approaching said maximum limit value is required for said signal indicative of a resistive ground fault to be provided within the period defined as acceptable for ground fault currents of the amplitude that generates said maximum limit value of product signal current.

2. The improvement set forth in claim 1 including further means for continually extracting as much current as is available in a second sense opposite to said first sense from the output circuit of said synchronous detection means and the input circuit of said integrator means, up to a threshold value of current, and means included in said threshold detection means responsive to as much, if any, of said integrator output signal which exceeds a predetermined level in a second sense opposite to said first sense, for producing at the output circuit of said threshold detection means a signal indicative of a ground fault condition existing in one of said a-c power line conductors, thereby to lessen the possibility of ground fault conditions failing to be detected because of said secondary winding being erroneously in reverse connection to the input circuit of said synchronous detection means.

3. The improvement set forth in claim 1 wherein said integrator means includes:

an integrating capacitor connected across the output circuit of said synchronous detection means, across which integrating capacitor a potential is developed proportionally responsive to the time integral of such of said product signal current as exceeds said threshold value of current; and

wherein said threshold detection means includes;

a first voltage-responsive threshold detector having an input circuit to which the potential appearing across said integrating capacitor is applied, having an output circuit coupled to the output circuit of said threshold detection means, and being responsive to that potential exceeding a threshold value of potential in said first sense to supply at least a portion of said signal indicative of a resistive ground fault condition existing in one of said a-c power line conductors to the output circuit of said threshold detection means.

4. The improvement set forth in claim 3 wherein said integrating capacitor is of a value such that the maximum limit value of product signal current will require substantially 10 milliseconds to charge the capacitor from zero potential to said threshold value of voltage.

5. The improvement set forth in claim 3 including:

further means for continually extracting as much current as is available in a second sense opposite to said first sense from the output circuit of said synchronous detection means and the input circuit of said integrator means, up to a threshold value of current;

a second voltage-responsive threshold detector being included in said threshold detection means, having an input circuit to which the potential appearing across said integrating capacitor is applied, having an output circuit coupled to the output circuit of said threshold detection means, and being responsive to that potential exceeding a predetermined threshold potential in a second sense opposite to said first sense to supply a further portion of said signal indicative of a ground fault condition existing in one of said a-c power line conductors, thereby to eliminate the possibility of ground fault conditions failing to be detected because of said secondary winding being erroneously in reverse connection to the input circuit of said synchronous detection means.

6. In ground-fault detection apparatus for a power distribution system with a plurality of a-c power line conductors, a differential current sensing means for responding to total current flow through said a-c power line conductors in one direction differing from the total current flow through said a-c power line conductors in the opposite direction to provide an a-c signal representative of that difference;

synchronous detection means having a first input circuit to which the a-c signal provided by said differential current sensing means is applied, having a second input circuit to which the a-c potential appearing between a pair of the a-c power line conductors is applied, and having an output circuit for supplying a product signal proportional to the product of the a-c signal current provided by said differential current sensing means times said a-c potential appearing between a pair of the a-c power line conductors; and

detection circuitry having an input circuit connected to receive said product signal, having an output circuit, and being responsive to said product signal to provide at its output circuit a signal indicative of a ground fault condition existing in one of said a-c power line conductors, the improvement wherein said detection circuitry comprises;

means for integrating, over periods which extend over more than one cycle of said a-c power, only those portions of said product signal which exceed a predetermined value, thereby integrating both the direct and changing components of those portions of the product signal to provide an integrated signal; and

a threshold detector having an input circuit connected to receive said integrated signal, having an output circuit, including means responsive to said integrated signal exceeding a predetermined threshold value in a first sense to provide at the output circuit of said threshold detector a signal indicative of a ground fault condition existing in one of said a-c power line conductors, including means responsive to said integrated signal as supplied to the input circuit of said threshold detector exceeding a predetermined threshold in a second sense opposite to said first sense to provide at the output circuit of said threshold detector a signal indicative of a ground fault condition existing in one of said a-c power line conductors, and including means for applying whichever of said signals indicative of ground fault conditions obtains to the output circuit of said detection circuitry thereby to forestall the possibility of ground fault conditions failing to be detected because of said secondary winding being erroneously in reverse connection to the input circuit of said controlled gain amplifier.

7. In ground-fault detection apparatus for a power distribution system with a plurality of a-c power line conductors, a differential current sensing means for responding to total current flow through said a-c power line conductors in one direction differing from the total current flow through said a-c power line conductors in the opposite direction to provide an a-c signal representative of that difference;

synchronous detection means having a first input circuit to which the a-c signal provided by said differential current sensing means is applied, having a second input circuit to which the a-c potential appearing between a pair of the a-c power line conductors is applied, and having an output circuit for supplying a product signal current proportional up to a limit to the product of the a-c signal current provided by said differential current sensing means times said a-c potential appearing between a pair of the a-c power line conductors; and

detection circuitry having an input circuit connected to receive said product signal, having an output circuit, and being responsive to said product signal current to provide at its output circuit a signal indicative of a ground fault condition existing in one of said a-c power line conductors, the improvement wherein said detection circuitry comprises;

a threshold detector having an input circuit, having an output circuit coupled to the output circuit of said detection circuitry, and including means responsive to signal as supplied to the input circuit of said threshold detector exceeding a predetermined threshold value in a first sense to provide at the output circuit of said threshold detector a signal indicative of a ground fault condition existing in one of said a-c power line conductors;

a capacitor connected across the output circuit of said synchronous detection means, across which capacitor an integrated signal is developed that is applied to the input circuit of said threshold detector, said capacitor having a capacitance sufficiently large that the maximum rate at which said capacitor can be charged from said synchronous detection means is limited sufficiently that said predetermined threshold value of said integrated signal cannot be attained proceeding from zero in a period appreciably less than 10 milliseconds; and

means connected to said capacitor for discharging it sufficiently over a period, said period being of the duration required to charge the capacitor to said predetermined threshold level responsive to the minimum level of the a-c signal provided by said differential current sensing means to be detected, thereby to forestall accumulation of charge on said capacitor by noise generated internally to said synchronous detection means or to `hash` on said a-c power line conductors sufficient to cause said integrated signal to approach its said predetermined threshold value.

8. In ground-fault detection apparatus for a power distribution system with a plurality of a-c power line conductors, a differential current sensing means for responding to total current flow through said a-c power line conductors in one direction differing from the total current flow through said a-c power line conductors in the opposite direction to provide an a-c signal representative of that difference;

synchronous detection means having a first input circuit to which the a-c signal provided by said differential current sensing means is applied, having a second input circuit to which the a-c potential appearing between a pair of the a-c power line conductors is applied, and having an output circuit for supplying a product signal current proportional up to a limit to the product of the a-c signal current provided by said differential current sensing means times said a-c potential appearing between a pair of the a-c power line conductors; and

detection circuitry having an input circuit connected to receive said product signal, having an output circuit, and being responsive to said product signal to provide at its output circuit a signal indicative of a ground fault condition existing in one of said a-c power line conductors, the improvement wherein said detection circuitry comprises;

a threshold detector having an input circuit, having an output circuit coupled to the output circuit of said detection circuitry, and including means responsive to the signal as supplied to the input circuit of said threshold detector exceeding a predetermined threshold value in a first sense to provide at the output circuit of said threshold detector signal indicative of a ground fault condition existing in one of said a-c power line conductors;

a capacitor connected across the output circuit of said synchronous detection means, across which capacitor an integrated signal is developed which is applied to the input circuit of said detector means, said capacitor having a capacitance sufficiently large that the maximum rate at which said capacitor can be charged from said synchronous detection means is limited sufficiently that said predetermined threshold value of said integrated signal cannot be attained proceeding from zero in a period appreciably less than 10 milliseconds; and

means connected to said capacitor for continually discharging it at a predetermined rate independent of the voltage across said capacitor so long as charge is available from said capacitor, whereby when the rate at which said capacitor is charged by said integrated signal exceeds said predetermined rate for a sufficiently long period of time, the integrated signal exceeds said predetermined threshold value.

9. In ground-fault detection apparatus for a power distribution system with a plurality of a-c power line conductors, said apparatus including differential current sensing means for responding to total current flow through said a-c power line conductors in one direction differing from total current flow through said a-c power line conductors in the opposite direction to provide an a-c signal representative of that difference;

synchronous detection means having a first input circuit to which the a-c signal provided by said differential current sensing means is applied, having a second input circuit to which the a-c potential appearing between a pair of the a-c power line conductors is applied, and having an output circuit for supplying a product signal proportional to the product of the a-c signal provided by said differential current sensing means times said a-c potential appearing between a pair of the a-c power line conductors; and

means for determining whether or not said product signal has a direct component of an amplitude and duration indicative of the presence of a resistive ground fault, the improvement wherein said means for determining whether or not said product signal has a direct component of an amplitude and duration indicative of the presence of said resistive ground fault comprises;

integrating means having an input circuit to which said product signal is applied, having an output circuit, and being of a type responsive to the product signal applied to its input circuit to provide at its output circuit integrated response to rectified sinusoidal components of the product signal which components are of amplitude greater than a threshold value and of duration of at least the better part of a half cycle of a-c power line potentials, which response is of a relatively high sensitivity compared to the sensitivity of its integrated response to short-lived transient components of the product signal which have similar average power level; and

threshold detector means having an input circuit to which the output circuit of said integrating means is directly coupled, and having an output circuit exhibiting response to the signal applied to its input circuit exceeding a threshold value, for providing an indication of whether or not a resistive ground fault is present on any of the a-c power line conductors.