Method and system for monitoring the condition of capacitors in a DC-voltage intermediate circuit
Method and system for monitoring the condition of the capacitor arrangement ( 14 - 16 ) of the DC-voltage intermediate circuit of a power electronics appliance, such as of a frequency converter, at the place of usage, in which method the discharge voltage over the capacitor arrangement as a function of time is measured, and in which method the intermediate circuit is pre-charged with a pre-determined DC voltage, the pre-charging is removed from the intermediate circuit, the voltage of the intermediate circuit is measured by sampling at regular intervals, the voltage drop as a function of time is determined on the basis of the measured voltage of the intermediate circuit, the capacitance or the relative change in it is determined on the basis of the voltage drop, the value of the determined capacitance or of the relative change in it is compared to a pre-determined limit value on the basis of the voltage drop, and the necessary condition monitoring procedure is performed when the value determined with the measurement reaches the pre-determined limit value or is close to it.
1. A method for monitoring a condition of a capacitor in a capacitor arrangement of a DC-voltage intermediate circuit of a power electronics appliance at a place of usage by measuring a DC voltage over the capacitor arrangement as the capacitor discharges the DC voltage as a function of time,
the method comprising the following steps:
pre-charging the capacitor in the intermediate circuit to a pre-determined DC voltage by using a pre-charging circuit,
removing the pre-charging circuit from the intermediate circuit,
measuring the DC voltage of the intermediate circuit by sampling at regular time intervals,
determining a drop of the DC voltage as a function of time based on the measured DC voltage of the intermediate circuit,
determining an actual capacitance value of the capacitor in the intermediate circuit based on the drop of the DC voltage,
comparing the actual capacitance value of the capacitor, to a first pre-determined limit of the actual capacitance value of the capacitor, based on the drop of the DC voltage,
performing a first necessary condition monitoring procedure when the actual capacitance value of the capacitor, as determined by a measurement of the DC voltage, indicates that the actual capacitance value of the capacitor is substantially close to the first pre-determined limit,
determining the actual capacitance value of the capacitor in the intermediate circuit, and the pre-determined change of the actual capacitance value of the capacitor in the intermediate circuit with an equation as follows:
C
tot
=
-
t
R
tot
·
ln
v
where
“C tot ” is the actual capacitance value of the intermediate circuit,
“v” is a pre-determined change in the DC voltage of the intermediate circuit,
“t” is a related discharge time, and
“R tot ” is an actual resistance of trimmer resistors of the capacitor arrangement.
2. The method according to claim 1 , measuring the DC voltage with an internal voltage measuring arrangement of the appliance.
3. The method according to claim 1 , determining the first pre-determined limit of the actual capacitance value of the capacitor by testing the appliance by pre-charging the capacitor in the intermediate circuit with a specific DC voltage in a standard ambient temperature,
removing the pre-charging circuit from the intermediate circuit, and
monitoring how quickly the DC voltage of the intermediate circuit decreases with the measurements of the DC voltage, and then
determining the actual capacitance value of the of the capacitor in the intermediate circuit in a control unit.
4. The method according to claim 3 , recording the first pre-determined limit of the actual capacitance value of the capacitor in a memory of the control unit.
5. The method according to claim 1 , calculating the pre-determined change of the actual capacitance value of the capacitor by omitting the actual resistance R tot of the trimmer resistors of the intermediate circuit from the equation
C
tot
=
-
t
R
tot
·
ln
v
.
6. The method according to claim 1 , wherein the appliance is a frequency converter.
7. The method according to claim 1 , wherein the capacitor comprises:
multiple capacitors in series and parallel-connected capacitor packs, a part of which is a brake chopper unit.
8. A system for monitoring a condition of a capacitor in a capacitor arrangement of a DC-voltage intermediate circuit of a power electronics appliance at a place of usage, comprising:
a control unit,
a pre-charging circuit for pre-charging the capacitor in the intermediate circuit, and
a measuring arrangement by which the DC voltage over the capacitor arrangement is measured in relation to time,
wherein the control unit of the system is adapted:
to pre-charge the capacitor in the intermediate circuit with a pre-determined DC voltage,
to remove the pre-charge circuit from the intermediate circuit,
to measure the DC voltage of the intermediate circuit at regular intervals,
to determine a drop of the DC voltage based on the measured voltage of the intermediate circuit,
to determine an actual capacitance value of the capacitor in the intermediate circuit based on the drop of the DC voltage,
to compare the actual capacitance value of the capacitor to a first pre-determined limit based on the drop of the DC voltage,
to perform a first necessary condition monitoring procedure when the actual capacitance value of the capacitor, as determined by a measurement of the DC voltage, indicates that the actual capacitance value of the capacitor is substantially close to the first pre-determined limit value, and
to determine the actual capacitance value of the capacitor in the intermediate circuit, and to determine the pre-determined change of the actual capacitance value of the capacitor in the intermediate circuit with an equation as follows:
C
tot
=
-
t
R
tot
·
ln
v
where
“C tot ” is the actual value of the capacitance of the intermediate circuit,
“v” is a pre-determined change in the DC voltage of the intermediate circuit,
“t” is a related discharge time, and
“R tot ” is an actual resistance of trimmer resistors T r of the capacitor arrangement.
9. The system according to claim 8 , wherein the system is adapted to measure the voltage with the measuring arrangement which is internal of the appliance.
10. The system according to claim 8 , wherein the system is adapted:
to determine the first pre-determined limit of the actual capacitance value
by testing the appliance by pre-charging the capacitor in the intermediate circuit with a specific DC voltage in a standard ambient temperature, and to measure how quickly the DC voltage of the intermediate circuit decreases by the measuring arrangement of the system,
after which the actual capacitance of the capacitor in the intermediate circuit is determined in the control unit.
11. The system according to claim 10 , wherein the first pre-determined limit of the actual capacitance value of the capacitor is recorded as a memory of the control unit.
12. The system according to claim 8 , wherein the system is adapted to calculate the pre-determined change of the actual capacitance value of the capacitor by omitting the resistance R tot of the trimmer resistors T r of the intermediate circuit from the equation
C
tot
=
-
t
R
tot
·
ln
v
.
13. The system according to claim 8 , wherein the appliance is a frequency converter.
14. The system according to claim 8 , wherein the capacitor comprises:
multiple capacitors in series and parallel-connected capacitor packs, a part of which is a brake chopper unit.
15. A method for monitoring a condition of a capacitor in a capacitor arrangement of a DC-voltage intermediate circuit of a power electronics appliance at a place of usage by measuring a DC voltage over the capacitor arrangement as the capacitor discharges the DC voltage as a function of time,
the method comprising the following steps:
pre-charging the capacitor in the intermediate circuit to a pre-determined DC voltage by using a pre-charging circuit,
removing the pre-charging circuit from the intermediate circuit,
measuring the DC voltage of the intermediate circuit by sampling at regular time intervals,
determining a drop of the DC voltage as a function of time based on the measured DC voltage of the intermediate circuit,
determining a pre-determined change of the actual capacitance value of the capacitor in the intermediate circuit based on a pre-determined change of the DC voltage,
comparing the pre-determined change of the actual capacitance value of the capacitor to another pre-determined limit of the pre-determined change of the DC voltage,
performing a second necessary condition monitoring procedure when the pre-determined change of the actual capacitance value of the capacitor, as determined by a measurement of the pre-determined change of the DC voltage, indicates that the pre-determined change of the actual capacitance value of the capacitor is substantially close to the another pre-determined limit,
determining the actual capacitance value of the capacitor in the intermediate circuit, and the pre-determined change of the actual capacitance value of the capacitor in the intermediate circuit with an equation as follows:
C
tot
=
-
t
R
tot
·
ln
v
where
“Ctot” is the actual capacitance value of the intermediate circuit,
“v” is a pre-determined change in the DC voltage of the intermediate circuit,
“t” is a related discharge time, and
“Rtot” is an actual resistance of trimmer resistors of the capacitor arrangement.
16. The method according to claim 15 , wherein the capacitor comprises:
multiple capacitors in series and parallel-connected capacitor packs, a part of which is a brake chopper unit.