Filter capacitor degradation detection apparatus and method

Patent metadata

Field	Value
Publication number	US-9653984-B2
Application number	US-201213570781-A
Country	US
Kind code	B2
Filing date	Aug 9, 2012
Priority date	Apr 30, 2012
Publication date	May 16, 2017
Grant date	May 16, 2017

Abstract

Power conversion systems and methods are presented for detecting input filter capacitor degradation or approach of end of operational life based on filter capacitor current measurements using single and/or dual threshold comparisons for computed instantaneous sum of squares of filter currents or power values.

First claim

The following is claimed:

1. A power conversion system, comprising:
an AC input coupleable to receive AC input power from a power source;
a rectifier operative to convert the AC input power to provide a DC output;
an inverter operatively coupled with the DC output of the rectifier to provide an AC output; and
a filter circuit coupled between the AC input and the rectifier, the filter circuit comprising a plurality of filter capacitors;
a controller operative to identify suspected degradation of at least one of the filter capacitors based at least partially on currents flowing in the plurality of filter capacitors;
wherein the controller is operative to:
compute a computed value as at least one of a sum of the squares value, an instantaneous active power value, and an instantaneous reactive power value according to a plurality of filter capacitor currents associated with the plurality of filter capacitors; and
selectively identify suspected filter capacitor degradation if the computed value is greater than an upper threshold or less than a lower threshold;
measure at least one AC voltage associated with the power conversion system;
selectively adjust at least one of the upper threshold and the lower threshold based at least partially on the at least one AC voltage; and
increase the upper threshold and the lower threshold if the at least one AC voltage is greater than a nominal value, and to decrease the upper threshold and the lower threshold if the at least one AC voltage is less than the nominal value.

2. The power conversion system of claim 1 :
wherein the controller is operative to:
compute the computed value as a sum of the squares of a plurality of filter capacitor currents associated with the plurality of filter capacitors.

3. The power conversion system of claim 2 , wherein the controller is operative to:
measure the plurality of filter capacitor currents;
filter the measured filter capacitor currents using a low pass filter with a cutoff frequency set between second and third harmonics of a fundamental power source frequency; and
compute the sum of the squares of the low pass filtered filter capacitor currents.

4. The power conversion system of claim 3 , wherein the controller is operative to:
determine an AC voltage balance condition;
selectively adjust the upper and lower thresholds based at least partially on the determined AC voltage balance condition.

5. The power conversion system of claim 2 , wherein the controller is operative to:
determine an AC voltage balance condition;
selectively adjust the upper and lower thresholds based at least partially on the determined AC voltage balance condition.

6. The power conversion system of claim 1 , wherein the controller is operative to:
measure a plurality of filter capacitor currents associated with the plurality of filter capacitors;
measure a plurality of AC voltages associated with the power conversion system;
compute at least one of an instantaneous active power value and an instantaneous reactive power value according to the measured filter capacitor currents and the measured AC voltages;
compare the computed power value to a threshold; and
selectively identify suspected filter capacitor degradation if the computed power value exceeds the threshold.

7. The power conversion system of claim 1 , wherein the filter circuit is an LCL circuit with first and second inductors connected in series with one another between each input terminal of the AC input and a corresponding input phase of the rectifier, with at least one of the plurality of filter capacitors connected to a center node between the first and second inductors.

8. The power conversion system of claim 1 , wherein the plurality of filter capacitors are connected in a delta configuration.

9. The power conversion system of claim 1 , wherein the plurality of filter capacitors are connected in a Y configuration.

10. A method for identifying suspected filter capacitor degradation in a power conversion system, the method comprising:
measuring a plurality of filter capacitor currents associated with a plurality of filter capacitors of the power conversion system;
selectively identifying suspected filter capacitor degradation based at least partially on the plurality of filter capacitor currents;
computing a computed value as at least one of a sum of the squares value, an instantaneous active power value, and an instantaneous reactive power value according to the plurality of filter capacitor currents;
selectively identifying suspected filter capacitor degradation if the computed value is greater than an upper threshold or less than a lower threshold;
measuring at least one AC voltage associated with the power conversion system;
selectively adjusting at least one of the upper threshold and the lower threshold based at least partially on the at least one AC voltage;
increasing the upper threshold and the lower threshold if the at least one AC voltage is greater than a nominal value; and
decreasing the upper threshold and the lower threshold if the at least one AC voltage is less than the nominal value.

11. The method of claim 10 , comprising:
computing a sum of the squares of a plurality of filter capacitor currents associated with the plurality of filter capacitors.

12. The method of claim 11 , comprising:
measuring the plurality of filter capacitor currents;
filtering the measured filter capacitor currents using a low pass filter with a cutoff frequency set between second and third harmonics of a fundamental power source frequency; and
computing the sum of the squares of the low pass filtered filter capacitor currents.

13. The method of claim 12 , comprising:
determining an AC voltage balance condition; and
selectively adjusting the upper and lower thresholds based at least partially on the determined AC voltage balance condition to increase the threshold if the AC voltage is unbalanced.

14. The method of claim 11 , comprising:
determining an AC voltage balance condition; and
selectively adjusting the upper and lower thresholds based at least partially on the determined AC voltage balance condition to increase the threshold if the AC voltage is unbalanced.

15. The method of claim 10 , comprising:
measuring a plurality of filter capacitor currents associated with a plurality of filter capacitors of the power conversion system;
measuring a plurality of AC voltages associated with the power conversion system;
computing at least one of an instantaneous active power value and an instantaneous reactive power value according to the measured filter capacitor currents and the measured AC voltages; and
comparing the computed power value to the threshold; and
selectively identifying suspected filter capacitor degradation if the computed power value exceeds the threshold.

Assignees

• Patel Yogesh Popatlal
• Wei Lixiang
• Kerkman Russel J
• Rockwell Automation Tech Inc

Inventors

• Patel Yogesh Popatlal
• Wei Lixiang
• Kerkman Russel J

Classifications

• G01R31/64

  Testing of capacitors · CPC title

• H02M5/4585

  having a rectifier with controlled elements · CPC title

• G01R31/42

  AC power supplies · CPC title

• H02M1/126

  using passive filters · CPC title

• H01G4/40

  Structural combinations of fixed capacitors with other electric elements, the structure mainly consisting of a capacitor, e.g. RC combinations · CPC title