Current ratio device

• US 4,841,236 A
• Filed: 03/22/1988
• Issued: 06/20/1989
• Est. Priority Date: 03/22/1988
• Status: Expired due to Term

• Current Assignee: National Research Council Canada
• Original Assignee: Canadian Patents and Development Limited
• Inventors: Miljanic, Petar N., Moore, William J. M.
• Primary Examiner(s): Tokar, Michael J.
• Application Number:
  US07/171,765
• Time in Patent Office: 455 Days
• Field of Search: 324/55, 324/117 R, 324/117 H, 324/115, 324/127, 324/60 C, 307/512, 307/350, 328/162, 328/165, 328/166, 330/293
• US Class Current: 324/127
• CPC Class Codes: G01R 1/20 Modifications of basic electric elements for use in electric measuring instruments; Structural combinations of such elements with such instruments

Abstract

A current ratio device for use in constructing a current transformer comprises a first magnetic core, a second magnetic core, a measuring winding electromagnetically coupled with the first core but not with the second core, and a compensating winding electromagnetically coupled with both cores. An externally powered amplifier has its input connected to receive the voltage waveform across the measuring winding. Its output provides a compensating current through the compensating winding equal to a magnetizing current required to magnetize the second core. The amplifier output also provides a measuring current through the measuring winding in the direction opposite that of the compensating current and equal to the compensating current multiplied by the ratio of turns in the compensating and measuring windings. This device is a self-contained article of commerce that, when electromagnetically coupled (without electrical connection) to conventional primary and secondary windings and a burden, provides a current transformer with improved qualities. In particular, the magnetizing current needed by the transformer is provided by the separate current ratio device, thus avoiding the error inherent in current transformers that arises from the need to supply the magnetizing current from the primary circuit. Another disclosed feature is the ability to examine the waveform of the magnetizing current and in particular to detect the presence of even harmonics and hence a probability of direct current in the primary circuit.

Claims

1. A device for use in constructing an alternating current, current transformer, said device comprising

(a) a first magnetic core,

(b) a second magnetic core,

(c) a measuring winding having Nm turns electromagnetically coupled with the first core but not with the second core,

(d) a compensating winding having Nc turns electromagnetically coupled with both cores,

(e) an amplifier having an input and an output,

(f) said input being connected to receive the waveform across the measuring winding,

(g) said output being connected to the compensating winding to provide a compensating currect Ic through the conpensating winding equal to a magnetizing current Iμ

required to magnetize the second core, and

(h) said output being connected to the measuring winding to provide a measuring current Im through the measuring winding in a direction opposite to that of the compensating current and equal to ##EQU5## whereby, when primary and secondary windings are electromagnetically coupled with the cores to form an alternating current, current transformer and the secondary winding is connected to a burden, the magnetizing current Iμ

for such transformer is provided by the device.

2. A device as in claim 1, wherein the measuring and compensating windings are connected in series with each other across said output and Nm=Nc.

3. A device as in claim 1, including a first resistance of value R1 connected to the amplifier output so that the measuring current flows through such resistance, and a second resistance of value R2 connected to the amplifier output so that the compensating current flows through such second resistance, wherein R1/R2=Nm/Nc.

4. A device as in claim 3, including means connected across a series connection of the amplifier output and the second resistance for causing the compensating current to be independent of the impedance of the compensating winding.

5. A device as in claim 4, wherein said means comprise

(i) a series connection of a pair of third resistances of values having a predetermined ratio to each other,

(ii) a second amplifier having an input and an output,

(iii) the input of the second amplifier being connected between a center point of said series connection of resistances and a junction point between the compensating winding and the second resistance,

(iv) the output of the second amplifier being connected across said series connection of resistances, and

(v) a fourth resistance having a value relative to that of the second resistance equal to said predetermined ratio, said fourth resistance being connected between the output of the second amplifier and said junction point.

6. A device as in claim 1, including means for observing the waveform of at least one of the measuring and compensating currents.

7. A device as in claim 1, including means for observing the presence of even harmonics in at least one of the measuring and compensating currents.

8. A device as in claim 7, wherein said observing means comprise a peak detector connected in series with one of the measuring and compensating windings.

9. A device as in claim 7, including a D.C. feedback winding electromagnetically coupled with both the cores, and a D.C. amplifier having an input and an output, said input of the D.C. amplifier being connected to said observing means for detecting even harmonics, and said output of the D.C. amplifier being connected across a D.C. feedback circuit comprising a current measuring device in series with said D.C. feedback winding.

10. A current transformer comprising a device as in claim 1, and primary and secondary windings electromagnetically coupled with the cores of said device.

11. A current transformer comprising a device as in claim 2, and primary and secondary windings electromagnetically coupled with the cores of said device.

12. A current transformer comprising a device as in claim 3, and primary and secondary windings electromagnetically coupled with the cores of said device.

13. A current transformer comprising a device as in claim 4, and primary and secondary windings electromagnetically coupled with the cores of said device.

14. A current transformer comprising a device as in claim 5, and primary and secondary windings electromagnetically coupled with the cores of said device.

15. A current transformer comprising a device as in claim 6, and primary and secondary windings electromagnetically coupled with the cores of said device.

16. A current transformer comprising a device as in claim 7, and primary and secondary windings electromagnetically coupled with the cores of said device.

17. A current transformer comprising a device as in claim 8, and primary and secondary windings electromagnetically coupled with the cores of said device.

18. A current transformer comprising a device as in claim 9, and primary and secondary windings electromagnetically coupled with the cores of said device.

19. A current transformer comprising:

(a) a first magnetic core,

(b) a second magnetic core,

(c) a measuring winding electromagnetically coupled with the first core but not the second core,

(d) a primary winding electromagnetically coupled with both cores,

(e) a secondary winding electromagnetically coupled with both cores to provide a secondary current Is when connected to a burden,

(f) an amplifier having an input and an output,

(g) said input being connected to receive the waveform across the measuring winding,

(h) said output being connected to provide a current in the measuring winding equal to a magnetizing current Iμ

required to magnetize the second core,

(i) the measuring and secondary windings being connected in parallel across the burden, and

(j) means for observing the waveform of the current in the measuring winding.

20. A current transformer as in claim 19, wherein said observing means includes means for observing the presence of even harmonics in the current in the measuring winding.

21. A current transformer as in claim 20, including a D.C. feedback winding electromagnetically coupled with both the cores, said D.C. feedback winding being connected in parallel with the secondary winding across the burden, and a D.C. amplifier having an input and an output, said input of the D.C. amplifier being connected to said observing means for detecting even harmonics, and said output of the D.C. amplifier being connected to provide a D.C. current in said D.C. feedback winding whereby the current in the burden is a replica of both the A.C. and D.C. components of the current in the primary winding.