Method for estimating the lifetime of an intermediate circuit capacitor, computer program product, computing unit, power electronics system and electric drive of a vehicle

Abstract

A method is provided for estimating the service life of an intermediate circuit capacitor that is operatively connected to an inverter of a power electronics system, wherein during each discharge process of the intermediate circuit capacitor, a current capacitance of the intermediate circuit capacitor is determined by a microcontroller using the following equation and compared with a value of the capacitance expected for the service life time, and from this an estimate of the remaining service life of the intermediate circuit capacitor is made: C = T / (R * (In(V) - In(1 * R))), where T is a predetermined time constant, R is a discharge resistance, V is the voltage applied to the intermediate circuit capacitor and I is the discharge current.

Description

[0001] The present invention relates to the field of power electronics.

[0002] Power electronic systems are used worldwide in a wide variety of applications, including electric drive systems for vehicles. One of the key components of such a power electronic system is the DC link capacitor, which is located in the intermediate circuit to the inverter. This capacitor must meet specified lifetime specifications; at the end of its lifetime, its capacitance must be high enough to meet the minimum peak-to-peak voltage requirements at the output of a converter (the inverter). Due to environmental and electrical stresses during operation, the initial capacitance value can decrease significantly. Depending on the operating parameters, the capacitance may decrease by, for example, 30% after a few hours of operation.For this reason, either a thicker, more robust film is used to reduce the influence of environmental conditions, or the capacitor is designed with a significantly higher initial capacitance value. This ensures that the required capacitance is still present at the end of its service life. Both options increase the size, weight, and cost of the capacitor, resulting in oversized systems.

[0003] For example, US 2010 / 0 295 554 A1 discloses a method for detecting the condition of a capacitor pack. The condition of the capacitor pack is determined based on self-discharge.

[0004] The invention is therefore based on the object of providing a lifetime estimate for an intermediate circuit capacitor.

[0005] This problem is solved by the features of the independent claims. Advantageous embodiments are the subject of the dependent claims.

[0006] A method is provided for estimating the service life of an intermediate circuit capacitor that is operatively connected to an inverter of a power electronics system, wherein during each discharge process of the intermediate circuit capacitor, a current capacitance of the intermediate circuit capacitor is determined by a microcontroller using the following equation and compared with a value of the capacitance expected for the service life time, and from this an estimate of the remaining service life of the intermediate circuit capacitor is made: C = T / (R * (In(V) - In(I * R))), where T is a predetermined time constant, R is a discharge resistance, V is the voltage applied to the intermediate circuit capacitor and I is the discharge current.

[0007] In one embodiment, the time constant is either determined during a first discharge process from the discharge resistance and the capacity and stored as a fixed value, or is specified and stored as a fixed value.

[0008] In one embodiment, it is provided that the discharge resistance is specified and stored as a fixed value, or is calculated for each discharge process or after a number of specified discharge processes from the voltage applied to the intermediate circuit capacitor and the discharge current.

[0009] In one embodiment, the discharge current is determined by means of a shunt resistance measurement or by means of an analog circuit or by means of an integrated circuit.

[0010] In one embodiment, it is provided that the current capacitance of the intermediate circuit capacitor determined for each discharge process is stored over the service life of the intermediate circuit capacitor and compared with a value of the capacitance expected for the service life time, wherein in the event that the current capacitance falls below the expected value, an assessment is made as to whether the current value is within a predetermined tolerance and if this is not the case, a measure is initiated.

[0011] In one embodiment, it is provided that a measure includes reducing the power of the inverter.

[0012] Furthermore, a computer program product is provided which carries out the method when implemented on one or more computing units.

[0013] Furthermore, a computing unit is provided on which the computer program is implemented and which is in signal connection with an intermediate circuit of a power electronics system.

[0014] Furthermore, a power electronics system is provided, comprising at least one inverter and an intermediate circuit, as well as the computing unit.

[0015] Furthermore, an electric drive of a vehicle is provided, comprising the power electronics system formed for controlling the electric drive.

[0016] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which illustrate details of the invention, and from the claims. The individual features can be implemented individually or in combination in a variant of the invention.

[0017] Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings. Fig. 1 shows a schematic view of important components for carrying out the method according to an embodiment of the present invention.

[0018] In the following descriptions of the figures, the same elements or functions are provided with the same reference symbols.

[0019] As mentioned at the beginning, DC link capacitors must meet certain service life specifications. Therefore, it is useful to estimate the service life of a DC link capacitor. Currently known service life estimates are based on the so-called worst-case scenario, which usually differs significantly from reality, as real-world measurements show that the point of maximum degradation of the DC link is rarely reached in most vehicles.

[0020] The invention is based on the insight that the difference between the worst-case scenario and reality can be used to optimize the value of the DC link capacitance and reduce the initial differences. Information about the remaining resource of the DC link capacitor at any given time provides the opportunity for a software derating function to adapt the operating mode accordingly, e.g., by reducing the temperature or voltage. For example, the current can be limited to a lower level when the capacitor reaches a higher temperature operating point. This information can also be integrated into the vehicle's status parameters, allowing users to adjust their driving behavior accordingly.Ultimately, knowing the exact capacitance value at any point during its lifetime enables significant savings in the cost, weight, and size of a DC link capacitor.

[0021] In order to make the most realistic lifetime estimate (estimate of the remaining lifetime) of the DC link capacitor, it is proposed to monitor the capacitance C of the DC link capacitor by determining it during each discharge process of the DC link capacitor via an active discharge circuit using a microcontroller (µC). An active discharge circuit is present in many applications, including inverters used in the automotive sector. However, in principle, all capacitors present in rectifiers or inverters must be discharged. This discharge function is shown on the right side (HV side) in Fig. 1 and is called the discharge function.

[0022] To determine the current capacitance C of the intermediate circuit capacitor, the discharge equation I=V / R*e^-(T / (R*C)) of the intermediate circuit capacitor is used and rearranged for the capacitance C, resulting in the following equation: C=T / (R*(In(V)−In(I*R))).

[0023] In one version, the discharge current I is measured at the active discharge circuit, e.g. via a shunt resistance measurement, and is fed to the microcontroller µC (LV side, galvanically isolated from HV side, as shown by the dashed line in Fig.1). The applied intermediate circuit voltage V is measured, for example, using a corresponding voltmeter on the inverter. The time constant T is a fixed value that specifies the period of time within which the intermediate circuit capacitor is considered discharged (e.g., when 99% of it is discharged). The discharge resistance R is calculated from the measured voltage and the measured discharge current I. Alternatively, a predefined value stored in the microcontroller µC is used for the discharge resistance R. Alternatively, the discharge current I and the intermediate circuit voltage V can also be determined using analog circuits or integrated circuits such as FPGAs.

[0024] A discharge process is started by a REQUEST, e.g. because the vehicle has been parked. Basically, during each discharge process of the DC link capacitor, the discharge current I is measured, e.g. using a shunt resistance measurement. The applied DC link voltage is determined cyclically during the discharge process using a voltmeter V. From these two values, the exact discharge resistance R can be determined or calculated in a corresponding microcontroller µC. It is advantageous to determine the discharge resistance R for each module and for each discharge process or after a number of specified discharge processes, since the installed discharge resistors R can have very high tolerances. Regularly determining the discharge resistance R makes the calculation of the service life of the DC link capacitor more accurate.

[0025] Furthermore, the time constant T is determined during a first discharge process, e.g., directly after production of the module, by determining the discharge resistance and the current capacitance of the intermediate circuit capacitor. The time constant T is then calculated using the equation T=R*C. Alternatively, the time constant can also be determined in advance or a value specified by the manufacturer, for example, can be used, so that only the discharge current I needs to be measured / determined to determine the service life. The time constant T is stored as a fixed value and used to calculate the capacitance C for each discharge process.

[0026] By determining the current capacitance C of the DC link capacitor during each discharge process, a better estimate of the service life of the DC link capacitor can be made. The parameters of the expected service life of the DC link capacitor are known, and so is the decrease in capacitance C over the service life. Thus, by comparing the current capacitance C with the expected capacitance C at this point in time (depending on the service life elapsed since commissioning), a deviation can be determined. If the deviation is negative, i.e. if the capacitance C is below an expected value, measures can be taken. First, the deviation can be assessed in the microcontroller µC, for example, whether the value of the deviation is within a specified tolerance.If this is the case, it can be assumed that the capacity C and thus the voltage ripple for the battery are still within a range that does not require replacement, for example. If the value is no longer within a specified tolerance, measures can be taken. These could, for example, simply save the value and wait for measured values ​​from further discharge processes, or warn the driver that they should visit a workshop, or notify a maintenance service / workshop / the manufacturer via a wireless connection (or a combination of several measures). The temperature of the battery can also be reduced. The measures to be taken are initiated by a control unit, which receives the information and, if applicable, the assessment from the microcontroller µC and processes it accordingly.

[0027] In one embodiment, if the current capacitance C is determined to be below a specified value (possibly with a tolerance), the inverter's power is reduced for a specific operating point. This can extend the service life of the intermediate circuit capacitor.

[0028] The ability to estimate service life more accurately also allows for a reduction in the weight, size, and cost of the DC link capacitor, as it no longer needs to be oversized. Another advantage is that existing hardware circuits can be used to determine the required parameters I, V, R, and T to calculate the current capacitance C.

[0029] The described method is used in power electronics systems of an at least partially electrically powered vehicle. In particular, the intermediate circuit capacitor is operatively connected to an inverter (DC / AC converter), which serves to convert a direct current supplied by a battery into an alternating current suitable for an electric drive.

[0030] In addition to the inverter and the intermediate circuit capacitor, the power electronics system has other components such as EMC filters, heat sinks, AC / DC rectifiers, DC / DC converters, direct AC-AC converters (cycloconverters, matrix converters) and / or other electrical converters. List of reference symbols C Capacitance of the intermediate circuit capacitor I discharge current R discharge resistance V DC link capacitor applied voltage T time constant µC microcontroller Discharge Function Discharge circuit

Claims

[1] Method for estimating the service life of an intermediate circuit capacitor that is operatively connected to an inverter of a power electronics system, whereby during each discharge process of the intermediate circuit capacitor, a current capacitance (C) of the intermediate circuit capacitor is determined by a microcontroller (µC) using the following equation and compared with a value of the capacitance (C) expected for the service life, and from this an estimate of the remaining service life of the intermediate circuit capacitor is made: C=T / (R*(In(V)−In(I*R))), where T is a given time constant, R is a discharge resistor, V is the voltage applied to the intermediate circuit capacitor and I is the discharge current. [2] Method according to claim 1, wherein the time constant (T) - either determined during a first discharge process from the discharge resistance (R) and the capacity (C) and stored as a fixed value, or - is specified and stored as a fixed value. [3] Method according to claim 1 or 2, wherein the discharge resistance (R) - is specified and stored as a fixed value, or - is calculated from the voltage (V) and the discharge current (I) applied to the intermediate circuit capacitor during each discharge process or after a number of specified discharge processes. [4] Method according to one of the preceding claims, wherein the discharge current (I) is determined by means of a shunt resistance measurement or by means of an analog circuit or by means of an integrated circuit. [5] Method according to one of the preceding claims, wherein the current capacitance (C) of the intermediate circuit capacitor determined for each discharge process is stored over the service life of the intermediate circuit capacitor and compared with a value of the capacitance (C) expected for the service life time, wherein in the event that the current capacitance (C) falls below the expected value, an assessment is made as to whether the current value is within a predetermined tolerance and if this is not the case, a measure is initiated. [6] The method of claim 5, wherein one measure comprises reducing the power of the inverter. [7] Computer program product which carries out the method according to one of the preceding claims when implemented on one or more computing units. [8] Computing unit (µC) on which the computer program according to claim 7 is implemented and which is in signal connection with an intermediate circuit of a power electronics system. [9] Power electronics system, comprising at least one inverter and one intermediate circuit, and the computing unit (µC) according to claim 8. [10] Electric drive of a vehicle, comprising the power electronics system according to claim 9 formed for controlling the electric drive.

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