Method and assessment unit for determining the remaining service life of a capacitor, and system

Abstract

A method for determining the remaining service life of a capacitor is disclosed, wherein the capacitor may be formed by an electrolytic capacitor. The method includes the stages of: measuring a voltage change across the capacitor during a discharging time, determining a discharging current during the discharging time, determining an actual capacitance of the capacitor on the basis of the voltage change, the discharging current and the discharging time, determining a corrected capacitance of the capacitor from the actual capacitance based on an error correction, wherein influences of the temperature on the capacitance of the capacitor are corrected during the error correction, and determining the remaining service life on the basis of a difference between the corrected capacitance and an initial capacitance of the capacitor. A system including an assessment device, configured to perform this method, and a circuit having at least one capacitor to be assessed are also disclosed.

Claims

1. A method of determining a remaining service life of a capacitor, wherein the capacitor is charged and discharged at least once, the method comprising: measuring a voltage change across the capacitor during a discharging time; determining a discharging current during the discharging time; determining an actual capacitance of the capacitor based on the voltage change, the discharging current, and the discharging time; determining a corrected capacitance of the capacitor based on the actual capacitance by determining an error correction, wherein influences of a temperature on the capacitance of the capacitor are corrected by the error correction; and determining the remaining service life based on a difference between the corrected capacitance and an initial capacitance of the capacitor.

2. The method according to claim 1, wherein determining the corrected capacitance further comprises determining an error correction for the capacitance with regard to an influence of one or more of: a frequency of the discharging current during the discharging time; a voltage across the capacitor during the discharging time; and the discharging current during the discharging time.

3. The method according to claim 1, wherein one or more error corrections are carried out based on a characteristic curve of the capacitor, wherein the characteristic curve describes a dependence of the capacitance of the capacitor on a respective corrected influencing variable.

4. The method according to claim 3, wherein the characteristic curve is approximated by several subsections, and wherein the subsections continuously merge into each other and are described by first or second order polynomial functions.

5. The method according to claim 1, further comprising: measuring the temperature of the capacitor, or estimating the temperature based on an ambient temperature of the capacitor, taking into account one or more of: a voltage across the capacitor, the discharging current through the capacitor, and a frequency of the discharging current.

6. The method according to claim 1, further comprising: determining an end of service life to be a time when the corrected capacitance has decreased by more than a predetermined proportion below the initial capacitance, wherein the predetermined proportion is between 15% and 50%.

7. The method according to claim 1, wherein the discharging time is selected such that the capacitor is discharged quasi-linearly during the discharging time.

8. The method according to claim 1, wherein measuring the voltage change further comprises: measuring a voltage across the capacitor at a beginning of the discharging time; measuring a voltage across the capacitor at an end of the discharging time; and calculating the voltage change to be a difference between the voltage measured at the beginning of the discharging time and the voltage measured at the end of the discharging time.

9. The method according to claim 1, further comprising: determining the initial capacitance when the capacitor is first put into operation, the initial capacitance being determined based on an error correction that corrects for influences of a temperature on the capacitance of the capacitor.

10. The method according to claim 1, further comprising: determining the initial capacitance by performing multiple measurements of a capacitance of the capacitor; and generating a mean value of the multiple capacitance measurements; and identifying the mean value of the capacitance as the initial capacitance.

11. The method according to claim 1, further comprising determining the initial capacitance after a burn-in period of time.

12. An assessment device configured to determine a remaining service life of a capacitor, the assessment device comprising: a voltage input device configured to receive a measured value of a voltage change across the capacitor during a discharging time; a current input device configured to receive a measured value of a discharging current, wherein the current input device is configured to measure the discharging current during the discharging time; a capacitance detecting device configured to detect an actual capacitance of the capacitor based on the voltage change, the discharging current, and the discharging time; a correcting device configured to determine a corrected capacitance based on the actual capacitance by performing an error correction, wherein influences of a temperature on a capacitance of the capacitor are corrected by the error correction; and an evaluation device configured to determine the remaining service life of the capacitor based on a difference between the corrected capacitance and an initial capacitance of the capacitor.

13. A system comprising: the assessment device of claim 12; and a circuit comprising at least one capacitor, wherein the system is configured to determine a remaining service life of the capacitor by performing operations including: charging and discharging the capacitor one or more times; and causing the assessment device to determine the remaining service life of the capacitor a respective one or more times.

14. The system according to claim 13, wherein the circuit further comprises a DC intermediate circuit in which at least one capacitor is configured as a buffer capacitor.

15. The system according to claim 13, further comprising an intermediate circuit monitoring device, wherein the intermediate circuit monitoring device is configured to measure a voltage across the at least one capacitor and to feed the measured voltage to the voltage input device of the assessment unit.

16. The system according to claim 13 further comprising a current detection device comprising: a current sensor configured to measure the discharging current; or a current calculating device configured to calculate the discharging current based on other measured values of other physical variables within the circuit, the other measured values including a power which is output by an inverter circuit of the circuit and a voltage of the capacitor.

17. The system according to claim 13, further comprising: a frequency detection device configured to determine a frequency of the discharging current, wherein the frequency detection unit determines the frequency of the discharging current based on a measured frequency of a control signal of a power stage of the circuit.

18. The system according to claim 13, further comprising: an output device configured to indicate the determined remaining service life; and/or configured to issue a warning when an end of the remaining service life is reached.

19. The method of claim 6, wherein the predetermined proportion is between 15% and 30%.

20. The method of claim 6, wherein the predetermined proportion is 20%.

Description

(1) There are various options, then, to design and further develop the teaching of this disclosure in an advantageous manner. For this purpose, we refer, on the one hand, to the claims subordinated to the dependent claims and, on the other hand, to the following explanation exemplary embodiments of the disclosure on the basis of the drawing. In connection with the explanation of the exemplary embodiments of the disclosure on the basis of the drawing, embodiments and further developments of the teaching are explained in general terms. The drawings show:

(2) FIG. 1 A diagram with typical voltage curves at a capacitor to be assessed in a DC intermediate circuit

(3) FIG. 2 A diagram with a current curve with an applied voltage according to FIG. 1.

(4) FIG. 3 A diagram with an approximation of a characteristic curve of a capacitor with respect to the change in capacitance caused by temperature

(5) FIG. 4 A diagram with a service life curve of a capacitor

(6) FIG. 5 A flowchart with an exemplary sequence for generating a characteristic curve of a capacitor approximated by subsections

(7) FIG. 6 A flowchart of an exemplary embodiment of a method according to the disclosure

(8) FIG. 1 shows a diagram with typical voltage curves, as may be present at a capacitor in a DC intermediate circuit. Shown is an application in which a single-phase AC voltage at the input of the circuit is rectified by a bridge rectifier. The progression of the input voltage U.sub.in, is shown as a solid sinusoidal line. By rectifying the input voltage, the voltages represented in the negative area of the voltage are converted to positive voltages, which in FIG. 1 is represented by dotted sinusoidal half waves. This pulsating, rectified voltage U.sub.gI is applied to a capacitor. As soon as the applied voltage exceeds the current value of the voltage across the capacitor, a charging current flows into the capacitor, which charges the capacitor. As soon as the rectified sine voltage assumes a voltage below the voltage across the capacitor, the capacitor is gradually discharged. The resulting voltage curve U.sub.ZK is represented by a dotted line. The discharging of the capacitor is performed according to an exponential function, wherein the discharging curve assumes an approximately linear course at the beginning of the discharging process. The discharging time Δt, during which the voltage difference ΔU is measured, is selected from this range. The range in which this is possible is represented by an ellipse in FIG. 1, In this range, the capacitance is proportional to Δt/ΔU and is approximately constant. An example of a possible discharging time for the determination of the actual capacitance is represented as Δt. The voltage is measured at the beginning of the discharging time Δt and at the end of the discharging time Δt, and the voltage difference ΔU is calculated from these values.

(9) FIG. 2 shows a progression of an exemplary current I (C_ZK). In addition, the input voltage U.sub.in and the voltage across the capacitor U.sub.ZK are marked in FIG. 2. The curve shows that the current rises briefly during the charging phases and otherwise runs approximately constant at about −0.3 A. This means that the capacitor is charged during the short current peaks, while the capacitor is discharged during the approximately horizontal areas (in this example, via a resistive load).

(10) FIG. 3 shows an exemplary progression of a change in capacitance over temperature. The temperature of 20° C. is selected as a reference temperature. As the temperature rises, so does the capacitance, and as the temperature drops, so does the capacitance. For example, at a temperature of about 70° C., the capacitance value would increase by almost 4% over the capacitance value at 20° C., The temperature characteristic curve of the capacitor displayed in FIG. 3 is approximated in three subsections. A first subsection runs from −25° C. to −5° C., a second subsection between −5° C. and +85° C. and a third subsection above +85° C. The first subsection is represented by a dashed line, the second subsection by a solid line, and the third subsection by a dotted line. The first and third parts can be described by a second-degree polynomial function, while the second part can be described by a first-degree polynomial function. Using the exemplary characteristic curve approximation shown in FIG. 3, the capacitance corrected for the influence of temperature, for example, can be calculated in the first subsection with the formula
C.sub.x,T,−25° C. . . . −5° C.=(1−7.Math.10.sup.−5.Math.T.sup.2−2.Math.10.sup.−4.Math.T−0.0186).Math.C.sub.measured

(11) and in the second subsection with the formula
C.sub.x,T,−5° C. . . . −85° C.−(1+8.Math.10.sup.−4.Math.T−0.0151).Math.C.sub.measured

(12) As the third portion above +85° C. is usually irrelevant due to a very rapid aging of the capacitor, a formulaic description of it is omitted here. It is clear that this approximation can be used to calculate a capacitance corrected for temperature influence quickly and with little effort. In this manner, a capacitance measured at a given temperature will be corrected to a reference temperature value, here 20° C. The adjusted capacitance is then corrected for the influence of the temperature.

(13) If an error correction additionally is to be performed according to other influencing factors, this can be done accordingly. This additional error correction can also be performed based on characteristic curves formed section by section, Such an additional error correction would then include the capacitance value calculated in the previous correction stage in the calculation. After all error correction stages are performed, the corrected capacitance C.sub.X results.

(14) FIG. 4A shows a diagram with a service life characteristic curve of a capacitor. This, too, may include an approximated characteristic curve (in sections). The initial capacitance is C.sub.0, which decreases over the course of the capacitor's operating life. It is assumed that the capacitor is always operated under similar ambient conditions. If the capacitor is operated in such a way that the operating conditions lead to increased aging, for example in the range with high temperatures of the capacitor, the characteristic curve would be shortened in the time direction t. Under operating conditions causing less aging, the characteristic curve would be extended in the time direction t. It is clear that the capacitance is hardly reduced over a relatively long period of the service life. The capacitance only decreases significantly in the last quarter of the service life. However, it also is clear that the characteristic curve moves continuously and steadily down toward lower capacitance values. If the end of the service life L.sub.rE is defined by a 20% drop in actual capacitance below the initial capacitance C.sub.0, the result is an end of service life L.sub.rE as marked in FIG. 4. Thus, the inventive method can be used to form an assessment about the remaining service life L.sub.r at any time. The only value that must be known in addition to the current measured values is the initial capacitance C.sub.0. This shows that no repetitive procedures and no complex calculations are required.

(15) FIG. 5A shows a flowchart with an exemplary sequence for generating a characteristic curve approximated sector-by-sector of a capacitor. In a first stage, the capacitance of the capacitor is measured depending on the temperature T, the voltage U, the current I or the frequency f. For this purpose, the value whose dependency is to be determined is changed continuously or in discrete stages, while the other quantities are kept approximately constant. In this context, it is recommended to keep the other values at predetermined reference values. For example, if a characteristic curve for the dependence of the capacitance on the temperature is to be measured, the temperature is changed and the voltage U, the current I and the frequency f are kept largely constant. For example, the temperature can be changed by single degrees or in jumps of 5° C. The selection of the measurement intervals depends on the desired accuracy and the maximum measurement duration for capturing the characteristic curve.

(16) In a next stage, the measurement results are visualized. Values that fall between two measured values can be interpolated, for example by linear interpolation. This stage is especially necessary if the subsections and possibly the approximation are to be done manually. Otherwise, this stage can also be skipped.

(17) In a further stage, the curve generated from the measured values is divided into subsections and an approximation of the subsections is determined based on a linear or polynomial function. In this process, a polynomial function of no higher than second order may be used. The determination of the subsections and the approximations can be done manually. However, automated methods are also known that can determine such approximations.

(18) FIG. 6 shows a flowchart, which represents an exemplary embodiment of a method according to the disclosure. At the beginning of the process, reference measurements of the capacitance are performed as a first stage. The capacitor is already installed in the circuit. In a next stage, the initial capacitance C.sub.0 is calculated from these measurements. These stages can be repeated several times, wherein an average of all previously determined initial capacitance values would then be formed at the end of the repetitions.

(19) The next stage is to calculate the remaining service life L.sub.r. For this purpose, a voltage change ΔU of the voltage across the capacitor is measured over a discharging time Δt in the quasi-linear range of the discharging phase of the capacitor. For this purpose, the intermediate circuit voltage is measured at the beginning and end of the discharging time Δt, and the voltage change ΔU is determined as the difference between the two voltage values. Furthermore, the temperature T, the discharging current I and the frequency of the discharging current are determined or measured. In this case, corrections are to be performed for temperature dependence, frequency dependence, voltage dependence, and current dependence. For this purpose, a characteristic curve is available for each influencing variable, which is approximated in sections. Depending on the measured value for temperature, frequency, voltage or current, the appropriate subsection of the respective characteristic curve must be selected. For example, if a temperature of 43° C. was measured, the second subsection should be selected—assuming the use of the characteristic curve in FIG. 3.

(20) In a next stage, the corrected capacitance C.sub.X would then be calculated. For this purpose, the four error corrections are performed one by one, wherein the corrected capacitance value of the previous correction is included in the current error correction. For example, if a correction for the temperature influence is performed, followed by a correction for the frequency influence, the capacitance value C.sub.x,T adjusted for the temperature influence would be entered as a value to be corrected in the correction for the frequency influence.

(21) In a further stage, the corrected capacitance C.sub.X is compared with the initial capacitance C.sub.0, and the deviation of the two values from each other is determined. Usually, this deviation will be given as a percentage. This deviation is then used to calculate the remaining service life L.sub.r and/or L.sub.rx, based on a service life characteristic curve. The remaining service life L.sub.r represents the expected remaining service life if the capacitor were to continue to operate under current operating conditions. The remaining service life L.sub.rx is converted to reference operating conditions. After successfully calculating the remaining service life, the process rests until the initialization of the next repetition and then restarts the stages for calculating the remaining service life. In this context, the repetition can be retriggered by the expiration of a certain amount of time, for example.

(22) With regard to further advantageous embodiments of the teaching according to the disclosure, reference is made to the general part of the description as well as to the attached claims, in order to avoid repetition.

(23) Finally, it must be stated expressly that the exemplary embodiments described above merely serve to discuss the claimed teaching, but do not limit the same to these exemplary embodiments.

LIST OF REFERENCE SYMBOLS

(24) T Temperature of the capacitor Δt Discharging time (during which the measurement takes place) ΔU Voltage change across the capacitor during a discharging time Δt I Discharging current during the discharging time Δt f Frequency of the discharging current I C.sub.measured Measured capacitance C.sub.x Corrected capacitance C.sub.0 Initial capacitance L.sub.r Remaining service life (under the current operating conditions) L.sub.rx Remaining service life (under reference boundary conditions) L.sub.rE End of service life U.sub.in Input voltage to the circuit U.sub.gI Rectified input voltage U.sub.ZK Voltage across the capacitor/intermediate circuit voltage