Apparatus for sensing short circuit faults in alternating current supply lines

• US 4,251,846 A
• Filed: 05/08/1979
• Issued: 02/17/1981
• Est. Priority Date: 05/12/1978
• Status: Expired due to Term

• Current Assignee: National Research Development Corporation
• Original Assignee: National Research Development Corporation
• Inventors: Lord, Harold, Pearson, Frank K.
• Primary Examiner(s): Moose, Jr., Harry E.
• Application Number:
  US06/037,080
• Time in Patent Office: 651 Days
• Field of Search: 361/30, 361/42, 361/79, 361/80, 361/86, 361/87
• US Class Current: 361/30
• CPC Class Codes: G01R 29/18 Indicating phase sequence; Indicating synchronism
• CPC Class Codes: G01R 31/52 Testing for short-circuits, leakage current or ground faults
• CPC Class Codes: H02H 3/382 involving phase comparison between current and voltage or between values derived from current and voltage

Abstract

Apparatus is provided for sensing and responding to short circuit faults, in particular in three phase alternating current supply lines for direct-on-line starting three phase squirrel cage induction motors. Line current in each line is sensed by a respective current transformer and signals representing both the amplitude and phase of each line current are produced. Line-to-line or line-to-neutral voltage for each phase is also sensed, by photo-electric isolator circuitry, and signals produced representative of the respective phases of these voltages. Logic circuitry is used to produce from the signals representative of current and voltage phases three phase difference signals in the form of currents each varying substantially directly with the difference between the respective current and voltage phases of the respective line. The relationships between these difference signals and the respective current amplitude signals are compared with the relationship which corresponds to conditions from starting to normal running of the motor, and an output signal is produced representing whether there is correspondence or not between the compared relationships. If lack of correspondence is indicated, a circuit breaker is operated to break the supply lines upstream of the sensing equipment.

Claims

1. Apparatus for sensing short circuit faults in alternating current supply lines, the apparatus comprising:

means for sensing line current in an alternating current supply line and producing signals representative of the amplitude and phase of line current flowing in operation;

means for sensing a line voltage of the supply line and producing a signal representative of the phase of the said line voltage;

difference means coupled to said line current sensing means and to said voltage sensing means to receive the signals which are representative of the phases of the said current and voltage and arranged to produce in response thereto a difference output signal having a measure which varies substantially directly with difference between the said phases;

and means for comparing a relationship of the signal representative of current amplitude and the said signal produced by the difference means with a predetermined relationship of amplitude and phase difference and producing in reponse thereto a fault state output signal representative of whether or not the sensed current amplitude is larger than the corresponding amplitude as determined by the said predetermined relationship for the said difference between the said phases, said comparing means being coupled to said line current sensing means to receive said signal representative of current amplitude and to said difference means to receive said difference output signal.

2. Apparatus as claimed in claim 1, wherein the means for sensing line current and producing signals representative of the amplitude and phase of line current flowing in operation include a current transformer core and secondary winding so arranged that the supply line for conducting the line current to be sensed forms the primary of the current transformer thus constituted.

3. Apparatus as claimed in claim 1, wherein the means for sensing voltage includes an electro-optical isolator means.

4. Apparatus as claimed in claim 3, wherein the means for sensing voltage senses the line to neutral voltage of the supply line and the electro-optical isolator means includes an Infra Red Emitting diode connected in series with a resistor to form a series combination which in use is connected between the supply line and a neutral point.

5. Apparatus as claimed in claim 1, wherein the said signals are voltage signals and the said comparing means includes an inverter arranged to receive a signal representative of the amplitude and phase of sensed line current and to produce in response thereto an inverter output signal representative of the amplitude of the sensed line current and of opposite polarity to the said signal proportional to difference, a scaler connected to the inverter to receive the inverter output signal and to divide the magnitude of the inverter output signal by a predetermined factor, and a comparator connected to the scaler and to receive the said signal produced by the difference means and to a source of reference voltage of a predetermined magnitude and polarity in accordance with the said predetermined relationship.

6. Apparatus as claimed in claim 1, wherein the means for sensing line current and producing signals and the means for sensing voltage and producing a signal produce respectively a first square wave in phase with the sensed line current and sensed square wave in phase with the second line to line or line to neutral voltage.

7. Apparatus as claimed in claim 6, wherein the said difference means includes logic gating means connected to receive the said square waves and in response thereto producing logic signals which in combination are representative of the difference in phase between the sensed current and voltage.

8. Apparatus as claimed in claim 7, wherein said logic signals are voltage signals of which the average algebraic sum over one cycle varies directly with the said difference in phase.

9. Apparatus as claimed in claim 8, wherein the difference means includes a plurality of summing resistors connected at a common junction thereof to one input of an operational amplifier included in the comparing means whereby the signal produced by the difference means is a current signal the average magnitude of which over one cycle varies substantially directly with the said difference in phase.

10. Apparatus as claimed in claim 9, wherein the operational amplifier is a differential amplifier and the said one input thereof is an inverting input, and a source of reference voltage is connected to a non-inverting input of the said amplifier so as to supply thereto a reference signal in accordance with the said predetermined relationship.

11. Apparatus as claimed in claim 7, wherein the logic gating means comprises four two input NAND gates so arranged that two of the four gates act as logic inverters connected to respective ones of the inputs of the other two gates.

12. Apparatus as claimed in claim 10, wherein the said predetermined relationship is of the form ##EQU4## where I_L is line current amplitude in amperes,

n is a current transformer ratio,

R_B is burden resistance in ohms,

m is a dimensionless constant,

K is a further constant in volts per degree,

Φ

is the phase difference between the phase, voltage and the line current in degrees, and

Φ

_c is a phase constant in degrees.

13. Apparatus as claimed in claim 1, further including means for interrupting at least the said supply line in response to the said fault state output signal being representative of the sensed current amplitude being larger than the said corresponding amplitude, the interrupting means being coupled to the comparing means to receive the said fault state output signal.

14. Apparatus for interrupting three phase supply lines having short circuit faults, the apparatus comprising:

means for sensing each line current and producing signals representative of the sensed amplitudes and phases of the line currents flowing in operation;

means for sensing the line to line or line to neutral voltages of the supply lines and producing signals representative of the phases of line voltages;

difference means coupled to said line current sensing means and to said voltage sensing means to receive the signals which are representative of the said phases and arranged to produce in response thereto three difference output signals each of which has a measure which varies substantially directly with difference in phase between the respective line current and line voltage of a respective one of the three phase lines;

means for comparing for each phase line a relationship of the respective one of the signals representative of sensed current amplitude and the respective one of the signals produced by the difference means with a predetermined relationship of amplitude and phase difference and producing in response thereto a fault state output signal representative of whether or not any one or more of the sensed current amplitudes is larger than the corresponding amplitude as determined by the respective predetermined relationship for the respective one of the said differences in phase between the respective line current and line voltage, said comprising means being coupled to said line current sensing means to receive said signal representative of current amplitude and to said difference means to receive said difference output signals;

and means for interrupting the three supply lines in response to the said fault state output signal being representative of any one or more of the sensed current amplitudes being larger than the said corresponding amplitude, the interrupting means being coupled to the comparing means to receive the said fault state output signal.

15. Apparatus as claimed in claim 14, wherein override means are provided so coupling the means for sensing each line current to the means for interrupting each phase line that if one or more of the sensed current amplitudes exceeds a predetermined value, the override means actuates the interrupting means before the said fault state output signal is representative of the one or more of the sensed current amplitudes being larger than the said corresponding amplitude or amplitudes.

16. Apparatus as claimed in claim 14, wherein the means for sensing each line current and producing signals representative of the sensed current amplitude and phases includes three current transformer cores with respective secondary windings, each of the cores being, in use, so arranged with a respective one of three phase lines that the phase line and the secondary winding constitute a current transformer, there being connected across the secondary winding of each of the said cores a respective series combination of a resistor and a diode bridge such that for the smallest current able to flow in the secondary winding a square wave voltage is provided across the respective series combination by the forward voltage drops of a plurality of the said diodes and the means for sensing voltages and producing signals representative of the phases of the voltages includes three electro-optical isolators each of which includes an Infra Red Emitting diode which is connected in series with a capacitor and a resistor and has one electrode connected to a neutral point, the impedance of the three series combinations formed by the Infra Red Diodes and the respective capacitors and resistors being such that in use with the said three series combinations star connected to three phase lines the Infra Red Emitting diodes emit radiation during half cycles of the like polarity of respective line to line voltages.