Current transformer with enhanced temperature measurement functions

Patent metadata

Field	Value
Publication number	US-10132697-B2
Application number	US-201514757395-A
Country	US
Kind code	B2
Filing date	Dec 23, 2015
Priority date	Dec 23, 2015
Publication date	Nov 20, 2018
Grant date	Nov 20, 2018

Abstract

The present invention provides a method of using a current transformer for a temperature sensing device. The method determines the temperature of the current transformer's secondary winding by injecting a DC current into the secondary winding, measuring a voltage across the secondary winding, calculating the resistance of the secondary winding from the voltage induced into the secondary winding by the injected DC current and determining the secondary winding temperature by calculations or a comparison with verified resistance/temperature combinations.

First claim

We claim:

1. A current transformer (CT) providing both current and temperature measurements comprising:
a current transformer having a primary winding and a secondary winding;
a circuit for injecting a DC current into the secondary winding;
a circuit for measuring a voltage across the secondary winding;
a processor for calculating a resistance of the secondary winding from the measured voltage;
a memory for storing the calculated resistance of the secondary winding;
determining, by the processor, a temperature of the secondary winding; and
storing the determined temperature of the secondary winding in the memory, the processor using the stored resistance and temperature values of the secondary winding with other stored characteristics of the CT to compensate for temperature related current sensing errors of the CT and to diagnose potential electrical problems in the primary winding of the CT.

2. The current transformer of claim 1 , wherein the injected DC current is a known current provided by the circuit for injecting a DC current.

3. The current transformer of claim 1 , wherein a voltage is induced into the secondary winding by the injected DC current and measured by the voltage measuring circuit.

4. The current transformer of claim 1 , wherein the circuit for measuring the voltage includes a buffer and an analog to digital converter.

5. The current transformer of claim 1 , wherein the determining includes comparing the calculated resistance of the secondary winding with a previously recorded resistance of the secondary winding measured at a verified secondary winding temperature.

6. The current transformer of claim 1 , wherein the determining includes calculating the temperature of the secondary winding.

7. The current transformer of claim 1 , wherein a most recently stored temperature and sensed current of the secondary winding is compared with an expected temperature for the sensed current stored in the memory, if the most recent stored temperature is greater than the expected temperature a possible loose electrical connection in the primary winding can be detected by the processor.

8. The current transformer of claim 1 , wherein if a comparison, by the processor, of several recently stored temperatures of the secondary winding are cyclic between a temperature expected for the sensed current and a temperature greater than expected for the sensed current, an indication of arcing in the primary winding can be detected by the processor.

9. The current transformer of claim 1 , wherein a most recent stored temperature of the secondary winding is compared with a threshold temperature, stored in the memory, known to affect current sensing accuracy of the CT.

10. The current transformer of claim 9 , wherein if the most recent stored temperature of the secondary winding is greater than the threshold temperature the processor accesses data stored in the memory associating a known secondary winding temperature with a known temperature related current sensing error and adjusts accordingly the most recently stored sensed current to compensate for the temperature related current sensing error.

11. A method for determining a temperature of a secondary winding of a current transformer (CT) comprising the steps of:
injecting a direct current (DC) signal into the secondary winding of the CT;
measuring a voltage across the secondary winding induced by the injected DC current;
calculating, by a processor, a resistance of the secondary winding from the measured voltage;
determining, by the processor, a temperature of the secondary winding;
storing the calculated resistance and determined temperature of the secondary winding in a memory; and
using, by the processor, the stored resistance and temperature values of the secondary winding with other stored characteristics of the CT to compensate for temperature related current sensing errors of the CT and to diagnose potential electrical problems in a primary winding of the CT.

12. The method of claim 11 , wherein the determining includes comparing the calculated resistance of the secondary winding with a previously recorded resistance of the secondary winding measured at a verified secondary winding temperature.

13. The method of claim 11 , wherein the temperature of the secondary winding is calculated.

14. The method of claim 11 , wherein diagnosing potential electrical problems in the primary winding of the CT by the processor comprises
detecting at least one of:
a temperature related current sensing error and compensating for the temperature related current sensing error,
a potential loose electrical connection in a primary winding circuit;
a potential arcing condition in the primary winding circuit;
a deteriorating performance of the CT;
a subtle change to the CT, or
a sudden deviation from the established characterization of the CT.

15. The method of claim 11 , wherein the injected DC current is provided by a DC current circuit.

16. The method of claim 15 , wherein the DC current circuit is controlled by a measuring/monitoring device such that the injected DC current can be turned off, increased or decreased as required to increase the measurement accuracy.

17. The method of claim 11 , wherein the voltage is measured by a voltage measuring circuit.

18. The method of claim 17 , wherein the voltage measuring circuit includes a buffer and an analog to digital converter.

Assignees

• Schneider Electric Usa Inc

Inventors

• Jefferies Kevin M
• Edwards Benjamin W
• White Matthew L
• Filippenko Konstantin Alexander
• Weiler Richard Karl

Classifications

• G01R31/62

  Testing of transformers · CPC title

• G01R15/14

  Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks · CPC title

• G01R19/0092

  Measuring current only · CPC title

• G01R21/133

  by using digital technique · CPC title

• G01R21/14

  Compensating for temperature change · CPC title