CONDITION MONITORING OF A CAPER

Abstract

In a power electronics circuit (1), a spectrum of interference signals generated by an electronic power switch (10) is analyzed. From this, the state of a capacitor (20) in the power electronics circuit (1) is inferred.

Description

[0001] The invention relates to a method for monitoring the state of a capacitor, in particular an intermediate circuit capacitor, in a power electronics circuit.

[0002] Capacitors are used in power electronics circuits, for example, to control an electric motor in automotive applications, for example as intermediate circuit capacitors. These capacitors are often film capacitors. These capacitors are subject to aging, which can impair the functioning of the electronic circuit. Therefore, monitoring the condition of a capacitor is essential.

[0003] Information on the structure, aging processes and known measures for monitoring capacitors can be found in: Gupta, Anunay et al. „A Review of Degradation Behavior and Modeling of Capacitors.“ ASME 2018 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems (2018): n. pag. Film Capacitors: General technical information; EPCOS AG 2018 Gallay, Roland. „Metallized Film Capacitor Lifetime Evaluation and Failure Mode Analysis.“ arXiv: Accelerator Physics 3 (2016): 45-56. Dang, Hoang-Long and Sangshin Kwak. „Review of Health Monitoring Techniques for Capacitors Used in Power Electronics Converters.“ Sensors (Basel, Switzerland) 20 (2020): n. pag.

[0004] Previous measures for monitoring the condition of a capacitor require separate devices for measuring the current through the capacitor and the voltage across the capacitor. This allows the capacitor's impedance to be determined, which changes as the capacitor ages. Therefore, the capacitor's impedance is a suitable parameter for determining the capacitor's condition. However, the separate devices required by the current state of the art represent additional complexity.

[0005] It is therefore an object of the invention to provide a method by which the state of a capacitor can be monitored with reduced additional effort.

[0006] The purpose of the method according to the invention is to monitor the state of a capacitor in a power electronics circuit. The power electronics circuit comprises an electronic power switch, the capacitor to be monitored, and an active DC filter (e.g., an EMC filter) with an analog-to-digital converter. These elements are already present in the power electronics circuit, i.e., independently of the method according to the invention. The method according to the invention utilizes these elements in process steps.

[0007] During normal operation, the power switch generates electromagnetic interference (EMC) signals in the power electronics circuit. A spectrum of these interference signals is captured by the active DC filter. The spectrum depends on the impedance of the capacitor being monitored. Analyzing the spectrum therefore enables monitoring of the capacitor's condition, as changes in the spectrum over time can be used to infer changes in impedance. As mentioned above, changes in impedance are signs of aging processes in the capacitor.

[0008] To capture the spectrum, the interference signals can be extracted inductively or capacitively, for example. The analog / digital converter in the DC filter (EMC filter) is used to generate a digital signal from the extracted interference signals. The spectrum of the interference signals can be determined from the digital signal using discrete Fourier transformation. The steps of the method can be controlled by evaluation logic, which itself also performs data processing operations, such as the calculations for the discrete Fourier transformation. The evaluation logic can be provided in the active DC filter and perform additional tasks related to the filter function of the DC filter that are not related to steps of the method according to the invention.It is particularly conceivable to extend the functionality of a data processing device already present in the DC voltage filter by additional programming so that it can control the implementation of the method according to the invention as evaluation logic and can participate in it, for example, by means of calculation steps.

[0009] In one embodiment, in a reference phase, a spectrum of the interference signals is recorded for a plurality of operating points of the power electronics circuit (for example, by an evaluation logic as described above) and stored as a reference spectrum for the respective operating point (for example, in an evaluation logic as described above). The reference phase can, in particular, correspond to a new state of the capacitor. To monitor the capacitor during subsequent operation of the power electronics circuit, a spectrum is recorded at a time at a respective operating point of the power electronics circuit and compared with a reference spectrum. Deviations between the spectrum and the reference spectrum are used to determine the state of the capacitor. These steps can also be carried out with the assistance of an evaluation logic as mentioned above.The current operating point can correspond to one of the numerous operating points for which a reference spectrum has been stored. In this case, the acquired spectrum is compared with the corresponding reference spectrum. If the current operating point for which a spectrum is being acquired does not correspond to any of the operating points for which a reference spectrum has been stored, interpolation between reference spectra can be performed to subsequently generate a reference spectrum for the respective current operating point. This can also be performed by the evaluation logic.

[0010] In such an embodiment, but also more generally, a lifetime model of the capacitor can be used to monitor the condition of the capacitor, which specifies the impedance of the capacitor as a function of the lifetime.

[0011] The capacitor to be monitored can be used in the power electronics circuit, in particular as an intermediate circuit capacitor.

[0012] The invention and its advantages are explained in more detail below with reference to the attached drawing. Fig. 1 shows a power electronics circuit in which the method according to the invention can be used.

[0013] The figure shows only one possible example of the application of the method according to the invention. The method according to the invention is therefore not limited to the circuit shown.

[0014] Fig.1 shows a power electronics circuit 1, with a power switch 10, an intermediate circuit capacitor 20, which is the capacitor to be monitored, and an active DC voltage filter 30 with analog-to-digital converter 31. An equivalent circuit diagram is given for the capacitor 20, which includes capacitance 21, an equivalent resistor 22 and an equivalent inductance 23 in series with the capacitance 21, as well as an equivalent resistor 24 in parallel with the capacitance 21.

[0015] During operation, the circuit breaker 10 generates electromagnetic interference signals that act on the capacitor 20 and are influenced by the capacitor 20. The resulting voltage signals at the capacitor 20 are tapped by the active DC voltage filter 30. The active DC voltage filter 30 is used according to the method according to the invention to generate a spectrum of these voltage signals at the capacitor 20. The spectrum depends on the capacitance 21, equivalent resistances 22 and 24, and equivalent inductance 23, which determine the impedance of the capacitor 20. Aging processes change the capacitance 21 and equivalent resistances 22 and 24, and thus the impedance of the capacitor 20. Changes in the spectrum over time can be used to infer changes in the impedance and thus the aging processes. An evaluation logic 32 is provided to calculate the spectrum and evaluate it.It is conceivable for the evaluation logic 32 to generate a signal to indicate an age-related deterioration of the condition of the capacitor 20 beyond a tolerance limit. Apart from the evaluation logic 32, the method according to the invention only uses components already present in the circuit 1; the evaluation logic 32 can sometimes be implemented by additional programming of a data processing device already present in the DC voltage filter 30 for other purposes. The existing circuit breaker 10 is even used as the source for the signals to be evaluated, and its operation does not need to be modified for the purposes of the method. List of reference symbols 1 power electronics circuit 10 circuit breakers 20 Capacitor 21 capacity 22 equivalent resistor (series) 23 Equivalent inductance (series) 24 equivalent resistor (parallel) 30 active DC filter 31 analog-to-digital converters 32 Evaluation logic QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited non-patent literature

[0000] Gupta, Anunay et al. “A Review of Degradation Behavior and Modeling of Capacitors.” ASME 2018 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems (2018

[0003] Film Capacitors: General technical information; EPCOS AG 2018

[0003] Gallay, Roland. „Metallized Film Capacitor Lifetime Evaluation and Failure Mode Analysis.“ arXiv: Accelerator Physics 3 (2016): 45-56

[0003] Dang, Hoang-Long and Sangshin Kwak. „Review of Health Monitoring Techniques for Capacitors Used in Power Electronics Converters.“ Sensors (Basel, Switzerland) 20 (2020

[0003]

Claims

[1] Method for monitoring a state of a capacitor (20) in a power electronics circuit (1), which an electronic circuit breaker (10), the capacitor to be monitored (20) and an active DC voltage filter (30) with analog-digital converter (31), the method characterized by the following steps: detecting a spectrum of interference signals generated by the circuit breaker (10) with the active DC filter (30); Evaluate the spectrum to monitor the condition of the capacitor (20). [2] The method of claim 1, wherein: in a reference phase, a spectrum is recorded for a plurality of operating points of the power electronics circuit (1) and stored as a reference spectrum; at a later time, at an operating point of the power electronics circuit (1), a spectrum is recorded and compared with a reference spectrum; the state of the capacitor (20) is determined from deviations between the spectrum and the reference spectrum. [3] Method according to one of the preceding claims, wherein, in order to monitor the state of the capacitor (20), use is made of a lifetime model of the capacitor (20) which indicates the impedance of the capacitor (20) as a function of the lifetime.

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