Method and apparatus for detecting AFE filter capacitor degradation

Patent metadata

Field	Value
Publication number	US-9294005-B2
Application number	US-201314042753-A
Country	US
Kind code	B2
Filing date	Oct 1, 2013
Priority date	Oct 1, 2013
Publication date	Mar 22, 2016
Grant date	Mar 22, 2016

Abstract

Methods and systems are disclosed for detecting capacitor degradation in an input filter of an active front end power conversion system in which voltage and current sensing is performed to determine sequence component impedance asymmetry to detect filter capacitor degradation according to the value of an off-axis admittance matrix component for Delta or Y-connected filter capacitor banks without sensitivity to voltage unbalance, and with the capability to identify particular degraded capacitor locations based on individual impedance values.

First claim

The following is claimed:

1. A power conversion system, comprising:
an active front end rectifier, comprising a multiphase AC input, and a plurality of switching devices operative according to a plurality of rectifier switching control signals to convert power received at the multiphase AC input to provide DC output power;
an input filter circuit coupled between a power converter input and the active front end rectifier, the input filter circuit comprising a plurality of filter inductors coupled between a corresponding phase of the power converter input and a corresponding phase of the multiphase AC input of the active front end rectifier, and a capacitor circuit including a plurality of filter capacitors individually connected to at least one of the plurality of filter inductors;
a sensing circuit operative to determine a plurality of measured currents and measured voltages associated with the input filter circuit; and
a degradation detection system operatively coupled with the input filter circuit and comprising at least one processor configured to:
compute fundamental frequency current and voltage phasors based on the measured currents and voltages,
compute positive and negative sequence current and voltage component phasors based on the computed fundamental frequency current and voltage phasors,
compute at least one off-diagonal term of a filter circuit admittance matrix based on at least one of the computed positive and negative sequence current and voltage component phasors,
compare the at least one off-diagonal term to at least one threshold, and
selectively detect degradation of one or more of the filter capacitors if the at least one off-diagonal term exceeds the at least one threshold.

2. The power conversion system of claim 1 , wherein the sensing circuit is operative to determine a plurality of measured capacitor bank branch line currents and measured line-line voltages associated with the input filter circuit, and wherein the at least one processor is configured to compute the fundamental frequency current and voltage phasors based on the plurality of measured capacitor bank branch line currents and measured line-line voltages.

3. The power conversion system of claim 1 , wherein the at least one processor is configured to compute an adjusted off-diagonal term by offsetting the at least one off-diagonal term of the filter circuit admittance matrix based on a predetermined nominal asymmetry value, to compare the adjusted off-diagonal term to the at least one threshold, and to selectively detect degradation of one or more of the filter capacitors if the adjusted off-diagonal term exceeds the at least one threshold.

4. The power conversion system of claim 1 , wherein the at least one processor is configured to compute the at least one off-diagonal term of the filter circuit admittance matrix as an off-diagonal term of the filter circuit admittance matrix representing an effect of positive sequence voltage on negative sequence current in the input filter circuit.

5. The power conversion system of claim 4 , wherein the at least one processor is configured to compute the off-diagonal term Ŷ np of the filter circuit admittance matrix representing the effect of positive sequence voltage on negative sequence current in the input filter circuit based on a positive sequence current phasor Ĩ p , a negative sequence current phasor Ĩ n , a positive sequence voltage phasor {tilde over (V)} p , and a negative sequence voltage phasor {tilde over (V)} n according to the following formula:

Y
^

np

=

I
~

n

⁢

I
~

p

V
~

p

⁢

V
~

n

V
~

p

.

6. The power conversion system of claim 1 , wherein the at least one processor is configured to:
determine individual impedance values for the plurality of filter capacitors, and to
identify at least one particular filter capacitor as being suspected of degradation based on the corresponding determined impedance value.

7. The power conversion system of claim 6 :
wherein the plurality of filter capacitors are connected in a delta configuration;
wherein the at least one processor is configured to determine individual impedance values for the plurality of filter capacitors by:
computing line-neutral admittance matrix elements and line-line positive and negative sequence current and voltage component phasors based on line-neutral positive and negative sequence current and voltage component phasors,
computing line-line admittance matrix components based on the line-neutral admittance matrix components,
computing a zero sequence circulating current phasor based on at least some of the line-line admittance matrix components, and
computing individual Delta line-line impedance values for at least some of the filter capacitors; and

wherein the at least one processor is configured to identify at least one particular filter capacitor as being suspected of degradation by:
comparing individual Delta line-line impedance values with the at least one threshold value, and
identifying at least one of the filter capacitors as being suspected of degradation if a corresponding Delta line-line impedance value exceeds the at least one threshold value.

8. The power conversion system of claim 6 :
wherein the plurality of filter capacitors are connected in a Y configuration;
wherein the at least one processor is configured to determine individual impedance values for the plurality of filter capacitors by:
computing a line-neutral admittance matrix based on line-neutral positive and negative sequence current and voltage component phasors,
computing a line-neutral impedance matrix by inverting the line-neutral admittance matrix,
computing line-neutral positive and negative sequence voltage component phasors based online-line positive and negative sequence voltage component phasors,
computing a neutral zero sequence voltage phasor based on at least some line-neutral impedance matrix components and the computed line-neutral positive and negative sequence voltage component phasors, and
computing individual Y line-neutral impedance values for at least some of the filter capacitors based on the line-neutral positive and negative sequence current and voltage component phasors and the computed neutral zero sequence voltage phasor; and

wherein the at least one processor is configured to identify at least one particular filter capacitor as being suspected of degradation by:
comparing the individual Y line-neutral impedance values with the at least one threshold value,

Assignees

• Tallam Rangarajan
• Kerkman Russel
• Rockwell Automation Tech Inc

Inventors

• Tallam Rangarajan
• Kerkman Russel

Classifications

• G01R31/64 Primary

  Testing of capacitors · CPC title

• G01R31/42

  AC power supplies · CPC title

• H02M5/452

  with automatic control of output waveform · CPC title

• H02M5/4585

  having a rectifier with controlled elements · CPC title

• H02M7/125 Primary

  Avoiding or suppressing excessive transient voltages or currents · CPC title