Current measuring apparatus

• US 4,847,780 A
• Filed: 08/21/1987
• Issued: 07/11/1989
• Est. Priority Date: 08/21/1987
• Status: Expired due to Fees

• Current Assignee: Tennessee Valley Public Power Association
• Original Assignee: Tennessee Valley Public Power Association
• Inventors: Daharsh, Ross S., Gilker, Clyde, Berezinski, Mark A., Dolnik, Thomas G., Dunk, Michael P.
• Primary Examiner(s): Lall, Parshotam S.
• Assistant Examiner(s): Teska, Kevin J.
• Application Number:
  US07/087,703
• Time in Patent Office: 690 Days
• Field of Search: 364/483, 364/492, 364/571, 324/17, 324/127, 324/133, 340/538, 340/539, 340/657, 340/870.17, 340/870.38, 323/223, 323/301, 323/907
• US Class Current: 702/64
• CPC Class Codes: G01R 15/186 using current transformers with a core consisting of two or more parts, e.g. clamp-on type G01R15/142 - G01R15/16 take precedence; tong testers G01R1/22

Abstract

Apparatus is disclosed for monitoring an A.C. distribution line comprising a novel split transformer core which may be inserted remotely about a distribution feeder line without de-energizing of that line, an encoder for producing a train of voltage signals whose duration are characteristic of the magnitude of the A.C. current flowing through the line and for producing a train of light pulses from the voltage levels which characterize the magnitude of the A.C. current flowing through the line, a decoder which is opticlaly linked to the encoder for measuring the time intervals of the light pulses and for converting the time intervals into a numerical value representing the current flowing through the line.

Claims

1. Apparatus for monitoring an A.C. distribution line having A.C. current which flows therethrough and which is characterized by an RMS value and a magnitude, comprising:

(a) a current transformer having means for removably clamping it about said line while being inductively coupled to and electrically insulated from said line; and

(b) encoding means, carried and powered by said current transformer, for producing a train of voltage signals having a duration which is characteristic of said magnitude of said A.C. current flowing through said line and for producing a train of light pulses from said voltage signals which characterize said magnitude of said A.C. current flowing through said line, said encoding means comprising;

(1) zero-crossing detector means for producing a starting pulse reference to the positive zero-crossing of the A.C. current through the distribution line;

(2) A.C. to D.C. converter means for converting the output of said current transformer to a D.C. voltage signal level which is proportional to said magnitude of said A.C. current;

(3) comparison means, charging a capacitor from a constant current source upon the receipt of said starting pulse, for producing an off-signal when said capacitor is charged to said D.C. voltage signal level; and

(4) means, operatively connected to said zero-crossing detector means and said comparison means, for producing a light pulse which begins upon the receipt of said starting pulse and which ends upon the receipt of said off-signal and for discharging said capacitor upon the receipt of said off-signal.

2. The apparatus of claim 1, wherein said light pulses are separated by time intervals, and further including:

(a) decoding means, spaced apart and electrically isolated from said encoding means, for measuring said time intervals between said light pulses and for converting said train of light pulses into a numerical value representing the current flowing through said line; and

(b) a fiber optic link for transmitting said train of light pulses from said encoding means to said decoding means.

3. The apparatus set forth in claim 2, wherein said decoding means includes a pulse forming network for converting said train of light pulses to a train of voltage pulses of constant width.

4. The apparatus set forth in claim 3, further including:

a microprocessor which has been programmed to accept said train of voltage pulses from said pulse forming network, to count said pulses, to determine the time intervals between said pulses, and to compare said time intervals against a look-up table stored in said microprocessor to calculte said RMS value of said A.C. current in said line.

7. The apparatus of claim 4, wherein said train of voltage signals are initiated at a positive zero-crossing of said A.C. current.

5. Apparatus for monitoring an A.C. distribution line having A.C. current which flows therethrough and which is characterized by an RMS value and a magnitude, comprising:

(a) a current transformer having means for removably clamping it about said line while being inductively coupled to and electrically insulated from said line; and

(b) encoding means, carried and powered by said current transformer, for producing a train of voltage signals having a duration which is characteristic of said magnitude of said A.C. current flowing through said line and for producing a train of light pulses from said voltage signals which characterize said magnitude of said A.C. current flowing through said line, said encoding means comprising;

(a) D.C. level means, operated in response to said current transformer, for producing a generally fixed D.C. voltage which is dividable into at least four D.C. voltage levels;

(b) first half-cycle means, operated in response to the output of said current transformer, for producing a first voltage which is proportional to the current flowing through said line over one-half of the period of said A.C. current;

(c) second half-cycle means, operated in response to the output of said current transformer, for producing a second voltage which is proportional to the current flowing through said line over the remaining half of the period of said A.C. current; and

(d) sequential comparison means, comparing said first voltage and at least two of said four D.C. voltage levels and comparing said second voltage and at least the remaining two of said four D.C. voltage levels, for producing said train of voltage signals which are characteristic of said magnitude of said A.C. current flowing through said line.

6. The apparatus of claim 5, wherein said current transformer is electro-magnetically connected to and electrically isolated from said A.C. distribution line.

8. The apparatus set forth in claim 5, wherein said D.C. level means includes:

(a) a first diode in parallel with a resistor;

(b) a second diode in parallel with a second resistor; and

(c) a bridge rectifier having one input connected to the parallel combination of said first diode and first resistor and having a second input connected to the parallel combination of said second diode and said second resistor.

9. The apparatus set forth in claim 8, wherein said current transformer has two output terminals and said bridge rectifier has two outputs, and wherein said first half-cycle means includes:

(a) a third resistor having one end connected to one of said two outputs of said bridge rectifier;

(b) a third diode having one end connected to the other end of said third resistor;

(c) a fourth resistor having one end connected to one of said two output terminals of said current transformer; and

(d) a first NPN transistor having a base connected to the other of said two outputs of said bridge rectifier, a collector connected to other end of said third diode, and having an emitter connected to the other end of said fourth resistor.

10. The apparatus set forth in claim 8, further including a zener diode and a capacitor, wherein the output of said bridge rectifier is connected to the parallel combination of said capacitor and said zener diode, whereby a D.C. voltage level is established across said capacitor and zener diode.

11. The apparatus set forth in claim 10, wherein said first resistor and said second resistor are selected such that said current transformer produces a voltage across them which is proportional to the current through said line.

12. The apparatus set forth in claim 10, further including five resistors connected in series with each other and in parallel with said zener diode and said capacitor.

13. The apparatus set forth in claim 5, wherein said sequential comparison means includes four voltage comparators, each of said voltage comparators having an input connected to one of said at least four D.C. voltage levels, two of said comparators having an input connected to said first half-cycle means, and the remaining two of said four comparators being connected to said second half-cycle means, whereby as line current increases said comparators are turned on sequentially.

14. The apparatus set forth in claim 13, further including:

(a) a first exclusive-OR gate whose inputs are connected to the outputs of said two voltage comparators;

(b) a second exclusive-OR gate whose inputs are connected to the outputs of said two remaining voltage comparators; and

(c) a third exclusive-OR gate whose inputs of which are connected to the outputs of said first exclusive-OR gate and said second exclusive-OR gate.

15. The apparatus set forth in claim 14, further including:

(a) a switch which operates in response to the output of said third exclusive-OR gate; and

(b) a LED in series with said switch, the series combination of said switch and said LED being connected across said D.C. level means.

16. The apparatus set forth in claim 15, wherein said LED is optically connected to a photo-transistor.

17. Apparatus for monitoring an A.C. distribution line having A.C. current which flows therethrough and which is characterized by an RMS value and a magnitude, comprising:

(a) a current transformer having means for removably clamping it about said line while being inductively coupled to and electrically insulated from said line;

(b) encoding means, carried and powered by said current transformer, for producing a train of voltage signals having a duration which is characteristic of said magnitude of said A.C. current flowing through said line and for producing a train of time separated light pulses from said voltage signals which characterize said magnitude of said A.C. current flowing through said line; and

(c) decoding means, spaced apart and electrically isolated from said encoding means, for measuring said time intervals between said light pulses and for converting said train of light pulses into a numerical value representing the current flowing through said line, said decoding means including a pulse forming network for converting said train of light pulses to a train of voltage pulses, said pulse forming network including an exclusive-OR gate having an output to drive a microprocessor; and

(d) a fiber optic link for transmitting said train of light pulses from said encoding means to said decoding means.

18. Apparatus for monitoring an A.C. distribution line having A.C. current which flows therethrough and which is characterized by an RMS value and a magnitude, comprising:

(a) a current transformer having means for removably clamping it about said line while being inductively coupled to and electrically insulated from said line;

(b) encoding means, carried and powered by said current transformer, for producing a train of voltage signals having a duration which is characteristic of said magnitude of said A.C. current flowing through said line and for producing a train of time separated light pulses from said voltage signals which characterize said magnitude of said A.C. current flowing through said line; and

(c) decoding means, spaced apart and electrically isolated from said encoding means, for measuring said time intervals between said light pulses and for converting said train of light pulses into a numerical value representing the current flowing through said line, said decoding means including;

(1) a pulse forming network for converting said train of light pulses to a train of voltage pulses;

(2) a microprocessor which has been programmed to accept said train of voltage pulses, to count said voltage pulses, to determine the time intervals between said voltage pulses, to compare said time intervals against a look-up table stored in said microprocessor, and to calculate said RMS value of said A.C. current in said line; and

(3) a flip-flop having a RESET-input driven by said train of voltage pulses, a SET-input triggered by an output from said microprocessor, and an OUTPUT which is connected to an input port of said microprocessor, said microprocessor having been programmed to provide an INTERRUPT-output signal which goes to said SET-input which is gated by said OUTPUT of said flip-flop.

19. The apparatus set forth in claim 18, further including a gate having an INPUT connected to the output of said flip-flop and an OUTPUT which is connected to said input port of said microprocessor, said gate being triggered into operation in response to an output signal from said microprocessor.

20. Apparatus, comprising:

(a) a clamp-on transformer which is configured to be removably attached and electrically insulated from an A.C. distribution line having current flowing therethrough, said transformer having a magnetic core and a secondary winding with a primary winding of said transformer being formed by said A.C. distribution line;

(b) encoding means, comparing a voltage which is derived from said secondary of said transformer and which is a function of the magnitude of said A.C. current to one reference voltage which is derived from said secondary of said transformer, for producing a coded sequence of light pulses which are characteristic of said A.C. current flowing through said line, said one reference voltage being produced by a capacitor which is charged by a constant current source beginning at a zero-crossing of said A.C. line current;

(c) microprocessor means, which is physically remote from said transformer and said encoding means, which has been programmed with a look-up table to compute, from the duration of said light pulses, said electrical current flowing through said line; and

(d) means for sending light pulses from said encoding means to said microprocessor means and for converting said light pulses to digital values readable by said microprocessor means.

21. The apparatus of claim 20, wherein said current transformer, comprises:

(a) a split transformer core which is configured to encompass a distribution line conducting A.C electricity and which comprises two parts;

(b) transforming means, carried by one of said split core parts, for transforming current flowing through said distribution line to an electrical conductor having two ends;

(c) connecting means for connecting together said two split core parts such that at least one of said core parts is free to move relative to the other between a closed positon and an open position where said distribution line is free to pass out of said core; and

(d) moving means, carried by said one part and connected to said other core part and operable at a spaced distance from said distribution line, for moving said two core parts between said open position and said closed position and for applying uniform force in holding said core parts together.

22. The apparatus of claim 21, wherein said hinge comprises a circular pin, one hinge plate having a circular aperture for said pin, and a second hinge plate having a generally elongated aperture for receiving said pin, whereby said pin and said one plate are free to rotate and move relative to said second plate along the axis of said elongated aperture.

23. The apparatus of claim 21, wherein each of said core parts is encapsulated with an epoxy electrical insulating material.

24. The apparatus of claim 21, wherein said transforming means comprises a bobbin which is disposed around at least a portion of said core part and comprising an electrical conductor having two ends, at least a portion of said electrical conductor between said two ends being wrapped around said bobbin.

25. The apparatus of claim 21, wherein each core part defines two ends and said one core part has one end which mates with one end of the other core part.

26. The apparatus of claim 21, wherein each core part defines two ends and further including means for aligning one end of said one core part to one end of the other core part.

27. The apparatus of claim 21, wherein said moving means comprises a threaded shaft carried by said frame and engaging said other core part.

28. The apparatus of claim 21, wherein said connecting means is a piano hinge.

29. The transformer of claim 28, wherein said arm has a ring at one end and has said threaded section at the opposite end and wherein said arm is linked to said other core part.

30. The apparatus of claim 21, wherein said moving means comprises:

(a) a frame carried by said one core part;

(b) a rod carried by said frame defining a threaded section; and

(c) an arm linked to the other of said core parts, carried by said frame and engaging said threaded section of said rod, whereby rotation of said rod in one direction brings said two core parts to their open position and rotation in the opposite direction brings said two core parts to their closed position.

31. The apparatus of claim 30, wherein said encoding means is carried by said frame.

32. The apparatus of claim 21, wherein:

(a) said connecting means comprises a frame carried by said one core part;

(b) said one core part is generally U-shaped and is fixed to said frame; and

(c) the other core part is carried by said frame so as to be free to move toward and away from the open end of said U-shaped core part.

33. The apparatus of claim 32, wherein said one end of said one core part defines a groove and said one end of the other core part defines a land which is configured to fit within said groove when said core is in its closed position, whereby said two core parts are self-aligning.

34. Apparatus for monitoring an A.C. distribution line having A.C. current which flows therethrough and which is characterized by an RMS value and a magnitude, comprising:

(a) a current transformer having means for removably clamping it about said line while being inductively coupled to and electrically insulated from said line; and

(b) encoding means, carried and powered by said current transformer, for producing a train of voltage signals having a duration which is characteristic of said magnitude of said A.C. current flowing through said line and for producing a train of light pulses from said voltage signals which characterize said magnitude of said A.C. current flowing through said line, each of said voltage signals being initiated at a positive zero-crossing of said A.C. current and only one voltage signal being produced for each cycle of said A.C. current.

35. Apparatus for monitoring an A.C. distribution line having A.C. current which flows therethrough and which is characterized by an RMS value and a magnitude, comprising:

(a) a current transformer having means for removably clamping it about said line while being inductively coupled to and electrically insulated from said line; and

(b) encoding means, carried and powered by said current transformer, for producing a train of voltage signals having a duration which is characteristic of said magnitude of said A.C. current flowing through said line and for producing a train of light pulses from said voltage signals which characterize said magnitude of said A.C. current flowing through said line, said encoding means comprising;

(a) a zero-crossing detector powered by said current transformer;

(b) an A.C. to D.C. converter for producing a voltage level which is proportional to the current flowing through said A.C. distribution line;

(c) a capacitor and means for producing a constant current to charge said capacitor and establish a threshold voltage, whereby a time to reach said threshold voltage is established; and

(d) signaling means, operating in response to said zero-crossing detector, for turning ON a light source at the beginning of said A.C. current wave form and for shutting OFF said light source when said threshold voltage is generally equal to said voltage level, whereby the duration that said light source was ON is a characteristic of said magnitude of said A.C. current flowing through said line.

36. The apparatus of claim 35, wherein said signaling means comprises:

(a) a mono-stable multi-vibrator which has an output connected to an LED and an input which is connected to said zero-crossing detector and which is used for triggering the multi-vibrator to one state and turning ON said LED; and

(b) means, connected to said threshold voltage and said voltage level, for controlling the duration that said multi-vibrator is in said one state.

37. The apparatus of claim 35, wherein said means for producing a constant current is a temperature compensated constant current source.

38. Apparatus for metering the A.C. current flowing through a line conductor, said current being characterized by a magnitude, a period, and an RMS value, comprising:

(a) a current transformer removably carried by the line;

(b) D.C. level means, operated in response to said current transformer, for producing a D.C. voltage level;

(c) voltage means, operated in response to said current transformer for producing a voltage level characteristic of the current through said line;

(d) first half-cycle means, operated in response to said voltage means, for producing a current signal through a first resistor and thereby a first voltage, said current signal being proportional to the current flowing through said line over the one half of said period of said A.C. current;

(e) second half-cycle means, operated in response to said voltage means, for producing a current signal through a second resistor and thereby a second voltage, said current signal being proportional to the current flowing through said line over the remaining half of said period of said A.C. current;

(f) voltage dropping means, across said D.C. level means, for producing at least four levels of voltage;

(g) sequential comparison means, comparing said first voltage with two of said four level voltages and comparing said second voltage with the remaining two of said four level voltages, for producing a train of light pulses whose duration is characteristic of said magnitude of said current flowing through said line;

(h) a pulse forming network for taking said train of light pulses and converting them to a train of voltage pulses;

(i) a flip-flop characterized by transitions and having an OUTPUT, a RESET-input driven by said pulse forming network and having a SET-input; and

(j) a microprocessor which has been programmed to provide for an interrupt which is gated by the OUTPUT of said flip-flop, said microprocesor having an output which is connected to said SET-input of said flip-flop and being programmed to gate said flip-flop to an ON condition for at least one electrical cycle, to count the number of transitions of said flip-flop which are gated into said microprocessor by said interrupt, to determine time intervals between transitions of said train of pulses, and to calculate said RMS value of said current flowing through said line using said time intervals.

39. The apparatus set forth in claim 38, wherein said D.C. level means produces a generally fixed D.C. voltage level.

40. The apparatus set forth in claim 38, wherein said sequential comparison means includes three exclusive-OR gates with the outputs of two exclusive-OR gates being the inputs of the remaining exclusive-OR gate.

41. The apparatus set forth in claim 38, wherein said pulse forming network is optically isolated and physically remote from said sequential comparison means.

42. The apparatus set forth in claim 38, further including gating means, operated in response to an output of said microprocessor, for inputting the output of said flip-flop to said interrupt of said microprocessor.

43. The apparatus set forth in claim 38, wherein said microprocessor has been programmed to include a look-up table for correlating said time intervals to RMS values of A.C. current.

44. Apparatus, comprising:

(a) a current transducer having means for removably connecting it around an A.C. current conducting line while remaining electrically insulated from said line, said transducer having two output terminals and an iron core comprising two parts which are movable between an open position to accept said line and a closed position around said line;

(b) a D.C. power supply, connected to the output terminals of said transducer, for producing at least four D.C. voltage levels, said D.C. power supply including a bridge rectifier and a parallel combination of a zener diode and a capacitor connected to output terminals of said bridge rectifier;

(c) first half-cylce means, operated in response to the output of said current transducer, for producing a first voltage which is proportional to said A.C. electrical current flowing through said line over one-half of the period of said A.C. electrical current;

(d) second half-cycle means, operated in response to the output of said current transducer, for producing a second voltage which is proportional to said A.C. current flowing through said line over the remaining half of the period of said A.C. electrical current;

(e) comparison means for comparing said first voltage and at least two of said four D.C. voltage levels, for comparing said second voltage and at least the remaining two of said four D.C. voltage levels, and for producing a train of pulses whose separation in time is a characteristic of said A.C. current flowing through said line; and

(f) a microprocessor which has been programed to receive said train of time separated pulses from said comparison means, to count said pulses, to determine time intervals between said pulses, and to calculate an RMS value of the current flowing through said line using said time intervals.

45. Apparatus, comprising:

(a) a clamp-on transformer which is configured to be removably attached and electrically insulated from an A.C. distribution line having current flowing therethrough, said transformer having a magnetic core and a secondary winding with a primary winding of said transformer being formed by said A.C. distribution line;

(b) encoding means, carried by said clamp-on transformer, for comparing a voltage which is derived from said secondary of said transformer and which is a function of the magnitude of said A.C. current to one reference voltage which is derived from said secondary of said transformer, and for producing a coded sequence of light pulses which are characteristic of said A.C. current flowing through said line and which are initiated at the zero-crossing of said A.C. current flowing through said line;

(c) a microprocessor, which is physically remote from said transformer and said encoding means, which has been programmed with a look-up table to compute, from the duration of said light pulses, said electrical current flowing through said line; and

(d) means for sending light pulses from said encoding means to said microprocessor and for converting said light pulses to digital values readable by said microprocessor.

46. Apparatus, comprising:

(a) a clamp-on transformer which is configured to be removably attached and electrically insulated from an A.C. distribution line having current flowing therethrough, said transformer having a magnetic core and a secondary winding with a primary winding of said transformer being formed by said A.C. distribution line;

(b) encoding means, comparing a voltage which is derived from said secondary of said transformer and which is a function of the magnitude of said A.C. current to at least four generally constant reference voltages which are derived from said secondary of said transformer, for producing a coded sequence of light pulses which are characteristic of said A.C. current flowing through said line;

(c) microprocessor means, which is physically remote from said transformer and said encoding means, which has been programmed with a look-up table to compute, from the duration of said light pulses, said electrical current flowing through said line; and

(d) means for sending light pulses from said encoding means to said microprocessor means and for converting said light pulses to digital values readable by said microprocessor means.