Prognostic health monitoring for heater

1. A system for monitoring a health of a heater having a resistive heating element, electrical insulation surrounding the resistive heating element, and a metallic sheath surrounding the electrical insulation, the heater connected to a power supply by a first power lead conducting an inlet current and a second power lead conducting an outlet current, the system comprising:

an injection transformer, comprising a plurality of primary turns that are inductively coupled to the first power lead;

a signal generator, configured to:

generate a time-varying injection signal; and

supply the time-varying injection signal to the plurality of primary turns, thereby impressing the time-varying injection signal on the inlet current; and

a signal reader configured to receive a diagnostic signal from the heater, the signal reader comprising:

a band pass filter, configured to filter the diagnostic signal to pass a frequency associated with the time-varying injection signal while attenuating frequencies less than and greater than the frequency associated with the time-varying injection signal; and

a capacitance measurement circuit configured to produce a heater capacitance signal in response to a capacitive reactance of the heater, the capacitance signal that is indicative of a capacitance value of the heater;

wherein:

the capacitive reactance depends at least in part on a condition of the electrical insulation, and

the heater capacitance signal is indicative of the health of the heater.

2. The system of claim 1 , wherein the injection transformer comprises a ferromagnetic core configured to produce a mutual inductive coupling between the plurality of primary turns and the first power lead.

3. The system of claim 1 , wherein:

the injection transformer comprises a toroid core defining a center region;

the plurality of primary windings are wound around the toroid core; and

the first power lead is configured to pass through the center region one or more times.

4. The system of claim 1 , wherein the signal reader further comprises an amplifier, configured to:

amplify the frequency associated with the time-varying injection signal; and

provide the amplified frequency associated with the time-varying injection signal to the capacitance measurement circuit.

5. The system of claim 4 , wherein the signal reader further comprises an analog-to-digital converter (ADC), configured to convert the heater capacitance signal to a digitized heater capacitance signal.

6. The system of claim 5 , further comprising a processor, configured to:

receive the digitized heater capacitance signal; and

produce a prognostic data output representative of the heater health.

7. The system of claim 1 , wherein the primary winding comprises a number of primary turns that range from 500-6,000.

8. The system of claim 7 , wherein the primary winding comprises a number of primary turns that range from 2,000-3,000.

9. The system of claim 1 , wherein the time-varying injection signal comprises a frequency that ranges from 5-25 KHz.

10. The system of claim 1 , wherein the injection transformer comprises a square core or an EI-core.

11. The system of claim 1 , wherein the injection transformer comprises an air core.

12. The method of claim 1 , wherein:

the injection transformer comprises a toroid core defining a center region;

the plurality of primary windings are wound around the toroid core; and

the first power lead is configured to pass through the center region one or more times.

13. A method of monitoring a health of a heater having a resistive heating element, electrical insulation surrounding the resistive heating element, and a metallic sheath surrounding the electrical insulation, the heater connected to a power supply by a first power lead conducting an inlet current and a second power lead conducting an outlet current, the method comprising:

generating, with a signal generator, a time-varying injection signal;

supplying, to an injection transformer comprising a plurality of primary turns that are inductively coupled to the first power lead, the time-varying injection signal;

receiving, by a signal reader, a diagnostic signal from the heater;

passing, by the signal reader, a frequency associated with the time-varying injection signal;

passing, by a band pass filter, the frequency associated with the time-varying injection signal while attenuating frequencies less than and greater than the frequency associated with the time-varying injection signal; and

producing, by a capacitance measurement circuit, the heater capacitance signal in response to a capacitive reactance of the heater that depends at least in part on a condition for the electrical insulation; and

producing, by a capacitance measurement circuit of the signal reader, a heater capacitance signal in response to a capacitive reactance of the heater, the capacitance signal indicative of a capacitance value of the heater;

wherein:

the capacitive reactance depends at least in part on a condition of the electrical insulation, and

the heater capacitance signal is indicative of the health of the heater.

14. The method of claim 13 , wherein the injection transformer comprises a ferromagnetic core configured to produce a mutual inductive coupling between the plurality of primary turns and the first power lead.

15. The method of claim 13 , further comprising converting, by an analog-to-digital converter (ADC), the heater capacitance signal into a digitized heater capacitance signal.

16. The method of claim 15 , further comprising:

receiving, by a processor, the digitized heater capacitance signal; and

producing, by the processor, a prognostic data output representative of the heater health.

17. The method of claim 13 , wherein the primary winding comprises a number of primary turns that range from 500-6,000.

18. The method of claim 13 , wherein the time-varying injection signal comprises a frequency that ranges from 5-25 KHz.