Capacitance-coupled voltage transformer monitoring

1. A system to monitor the health of a capacitance-coupled voltage transformer (CCVT) of an electric power delivery system, comprising:

a CCVT in electrical communication with the electric power delivery system, the CCVT comprising a stack of capacitors and an electrical contact to a first ground connection;

a first current measurement device disposed between the stack of capacitors and the first ground connection, the first current measurement device to provide a first electrical signal corresponding to a first current flowing through the CCVT;

a step-down transformer comprising a primary winding electrically connected to the stack of capacitors;

a second current measurement device electrically coupled to the step-down transformer, the second current measurement device to provide a second electrical signal corresponding to a second current flowing through the step-down transformer;

an intelligent electronic device (IED) in electrical communication with the first and second current measurement devices, to:

receive the first electrical signal from the first current measurement device;

receive the second electrical signal from the second current measurement device;

calculate a health factor for a common frequency from the first and second electrical signals;

compare the health factor against a health range; and

when the health factor exceeds the health range, signal an alarm.

2. The system of claim 1 , wherein the health factor comprises a ratio of currents from the first and second current measurements.

3. The system of claim 1 , wherein the IED is further to:

calculate first and second currents at a fundamental frequency using cosine filters.

4. The system of claim 3 , wherein the health factor comprises a ratio of current magnitudes from the first and second currents at the fundamental frequency.

5. The system of claim 1 , wherein the IED is further configured to:

calculate a rate of change of the health factor;

compare the rate of change to a limit; and,

signal an alarm when the rate of change exceeds the limit.

6. The system of claim 1 , wherein the common frequency comprises a common frequency range.

7. The system of claim 1 , wherein the health range is calculated from measurements obtained during healthy operation of the CCVT.

8. The system of claim 1 , wherein the health factor at the common frequency comprises a ratio of voltages calculated by integrating the first and second current measurements.

9. The system of claim 1 , wherein the IED is further configured to monitor the electric power delivery system using the first and second current measurements.

10. The system of claim 1 , wherein the second current measurement device is electrically coupled between a primary winding of the step-down transformer and ground.

11. The system of claim 10 , wherein the second current measurement device is electrically coupled between the primary winding and the first ground connection.

12. The system of claim 10 , wherein the second current measurement device is electrically coupled between the primary winding and a second ground connection.

13. The system of claim 1 , wherein:

the step-down transformer comprises:

a primary winding electrically connected to the stack of capacitors; and

a secondary winding electrically connected to a third ground connection; and,

a third current measurement device is disposed between the secondary winding and the third ground connection to provide a third electrical signal corresponding to a third current flowing through the primary winding.

14. The system of claim 1 , wherein:

the first ground connection is electrically connected to the CCVT chassis;

the CCVT chassis is electrically connected to a third ground; and,

the first current measurement device is disposed between the CCVT chassis and the third ground.

15. The system of claim 1 , wherein:

the step-down transformer comprises a voltage output; and,

the IED is further configured to:

receive a voltage signal from the voltage output;

calculate a health supervisory signal using the voltage signal and the second electrical signal from the second current transformer; and,

compare the health supervisory signal with an acceptable range; and

signal the alarm when the health supervisory signal exceeds the acceptable range.

16. The system of claim 1 , further comprising a communication system to communicate the alarm to other devices associated with the electric power delivery system.

17. A method for monitoring the health of a capacitance-coupled voltage transformer (CCVT) of an electric power delivery system, comprising:

obtaining a first current measurement from a first current measurement device in electrical communication with the CCVT between a stack of capacitors of the CCVT and an electrical ground;

obtaining a second current measurement from a second current measurement device in electrical communication with a step-down transformer of the CCVT;

calculating a health factor from the first current measurement to the second current measurement for a common frequency;

comparing the health factor against a range; and,

when the ratio exceeds the range, signaling an alarm.

18. The method of claim 17 , further comprising:

calculating a rate of change from the health factor;

comparing the rate of change to a limit; and,

signaling an alarm when the rate of change exceeds the limit.

19. The method of claim 17 , wherein the second current measurement device is electrically coupled between a primary winding of the step-down transformer and ground.

20. The method of claim 17 , wherein the range is calculated from measurements obtained during healthy operation of the CCVT.