TRANSFORMER PROTECTION CIRCUIT AND METHOD

• US 20150311697A1
• Filed: 07/06/2015
• Published: 10/29/2015
• Est. Priority Date: 05/21/2012
• Status: Active Grant

• Current Assignee: Techhold, LLC
• Original Assignee: Emprimus, LLC
• Inventors: FAXVOG, FREDERICK R., JENSEN, WALLACE, NOE, TERRANCE R., EID, CRAIG, JACKSON, DAVID BLAKE, FUCHS, GREG, NORDLING, GALE
• Granted Patent: US 9,564,753 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: H02H 3/08 responsive to excess current responsive to abnormal temperature caused by excess current H02H5/04
• CPC Class Codes: H02H 3/105 responsive to excess current and fault current to earth
• CPC Class Codes: H02H 3/16 responsive to fault current to earth, frame or mass with balanced or differential arrangement H02H3/26 ; monitoring earth connection H02H5/105
• CPC Class Codes: H02H 3/33 using summation current transformers H02H3/347 takes precedence
• CPC Class Codes: H02H 3/52 responsive to the appearance of harmonics
• CPC Class Codes: H02H 7/04 for transformers
• CPC Class Codes: H02H 9/08 Limitation or suppression of earth fault currents, e.g. Petersen coil

Abstract

Systems and methods for protecting an electrical component in an alternating current system that includes a transformer are disclosed. In one aspect, a circuit includes a switch assembly connected between a transformer neutral of a transformer and a ground and having an open position and a closed position. In normal operation, the switch remains in a closed position. The circuit further includes a DC blocking component positioned in parallel with the switch assembly and connected between the transformer neutral and the ground, a voltage probe connected between the transformer neutral and ground and configured to monitor an instantaneous voltage level at the transformer neutral, and a control circuit configured to control the switch assembly, the control circuit including a sensor configured to actuate the switch assembly to an open position at least upon detection of a harmonic signal in at least one phase of the transformer or a predetermined threshold of DC or quasi DC current between the transformer neutral and the ground.

Claims

1. An electrical protection circuit for use in an alternating current system including a transformer, the system comprising:

(a) a switch assembly connected between a transformer neutral of a transformer and a ground, the switch assembly having an open position and a closed position, the open position disrupting the path through the switch between the electrical connection and the ground connection, and the closed position establishing a path connecting the electrical connection to the ground connection through the switch assembly, wherein in normal operation the switch remains in a closed position;

(b) a DC blocking component positioned in parallel with the switch assembly and connected between the transformer neutral and the ground;

(c) a voltage probe connected between the transformer neutral and ground and configured to monitor an instantaneous voltage level at the transformer neutral; and

(d) a control circuit configured to control the switch assembly, the control circuit including a sensor configured to actuate the switch assembly to an open position at least upon detection of a harmonic signal in at least one phase of the transformer or a predetermined threshold of DC or quasi DC current between the transformer neutral and the ground.

2. The electrical protection circuit of claim 1, wherein the switch assembly includes an AC switch and a DC switch.

3. The electrical protection circuit of claim 2, wherein the AC switch comprises a circuit breaker.

4. The electrical protection circuit of claim 2, wherein the DC switch comprises a circuit breaker.

5. The electrical protection circuit of claim 1, wherein based on detection of a voltage at the transformer neutral over a threshold level by the voltage probe while the switch assembly is in the open position, the control circuit maintains the switch assembly in the open position for a predetermined amount of time.

6. The electrical protection circuit of claim 1, further comprising a Rogowski coil connected between the ground and the DC blocking component in parallel with the switch assembly.

7. The electrical protection circuit of claim 6, wherein the Rogowski coil is further connected between the ground and the voltage probe.

8. The electrical protection circuit of claim 6, wherein the Rogowski coil monitors transformer neutral conductivity of the transformer neutral.

9. The electrical protection circuit of claim 1, wherein the DC blocking component is a capacitor connected in parallel with the switch assembly, between the transformer neutral and the ground.

10. A method of protecting electrical equipment in an alternating current circuit from damage, the method comprising:

maintaining a switch assembly in a closed position during normal operation of the alternating current circuit, the switch assembly electrically connected between a neutral of the electrical equipment and a ground;

upon detecting either a harmonic signal above a predetermined threshold or a DC or quasi DC current above a predetermined threshold, opening the switch assembly, thereby blocking the DC or quasi DC current to the ground through a DC blocking component electrically connected in parallel with the switch assembly between the neutral of the electrical equipment and the ground;

determining whether a voltage above a predetermined threshold remains at the neutral while the switch assembly is in an opened state; and

based on the voltage remaining above the predetermined threshold, maintaining the switch assembly in the open state.

11. The method of claim 10, wherein determining whether the voltage above the predetermined threshold remains is based on a voltage detected at a voltage probe electrically connected between the neutral and the ground.

12. The method of claim 11, wherein the predetermined threshold is about 8 volts.

13. The method of claim 11, wherein determining whether a voltage above a predetermined threshold occurs within a predetermined time after the switch assembly is opened.

14. The method of claim 13, wherein the predetermined time is less than a reset time maintained by a controller.

15. The method of claim 11, further comprising determining whether current passes from the neutral to the ground via a Rogowski coil electrically connected between a neutral blocking component and the ground, wherein the neutral blocking component is electrically connected to the neutral and in parallel with the switch assembly.

16. The method of claim 15, wherein the Rogowski coil is further electrically connected between the switch assembly and the ground.

17. The method of claim 10, wherein the switch assembly includes an AC switch and a DC switch connected in series.