Method for monitoring insulation resistances and corresponding control unit

Abstract

The invention relates to a method for monitoring insulation between two electrical conductors (102, 103), in particular high-voltage electrical conductors, and ground (104) by means of an insulation monitoring circuit, wherein the insulation monitoring circuit has at least two switches (SWTestP, SWTestM), and wherein the method comprises the following steps: (a) switching the at least two switches (SWTestP, SWTestM) according to a first switching sequence (110) consisting of a first sequence of several switching states (112), wherein a respective time duration (113) of each of the switching states (112) is at least as long as a first time interval; (b) determining a parasitic capacitance (Cy1, Cy2) by means of the first switching sequence (110) and checking whether the parasitic capacitance (Cy1, Cy2) exceeds a predetermined threshold value;(c) Switching the at least two switches (SWTestP, SWTestM) according to a second switching sequence (111) consisting of a second sequence of several switching states (112), wherein the respective duration (114) of each of the switching states is at most as long as a second time interval; and (d) Determining an insulation resistance (RisoPP, RisoPM, RisoLP, RisoLM) using the second switching sequence (111); wherein the first time interval is longer than the second time interval. The invention further relates to a corresponding control unit and computer program.

Description

TECHNICAL AREA

[0001] The present disclosure relates to methods and control devices for monitoring insulation resistances, in particular for a high-voltage system of a motor vehicle. BACKGROUND OF THE INVENTION

[0002] In high-voltage (HV) battery and drive systems for motor vehicles, insulation resistances between electrical conductors and ground must be determined to ensure safe and reliable operation. It must be ensured that these insulation resistance values ​​do not fall below the threshold values ​​specified by the vehicle manufacturers. Insulation monitoring methods, such as those described in DE 10 2021 203 919 A1 or DE 10 2018 222 123 B4, can be used for this purpose. However, it has been shown that such methods can produce erroneous insulation resistance values ​​under certain conditions. In such cases, safe operation of the battery and drive systems can no longer be guaranteed. Undetected insulation faults can lead to uncontrolled fault currents, which may endanger human lives, cause fires, or result in other property damage.

[0003] From DE 10 2020 102 658 A1, a method for monitoring y-capacitances in a motor vehicle is known, wherein the motor vehicle has at least one battery and a traction drive powered by electrical energy stored in the battery, as well as at least one electrical system for operating at least the battery and the traction drive. The electrical system has a first high-voltage potential, a second high-voltage potential, and a vehicle ground. Between the first high-voltage potential and the vehicle ground, the electrical system has a first y-capacitance, a first insulation resistance, and a first switch, and between the second high-voltage potential and the vehicle ground, it has a second y-capacitance, a second insulation resistance, and a second switch.

[0004] From DE 10 2021 203 919 A1, an insulation monitor for detecting an insulation fault in the electrical insulation of an electrical system is known, wherein the insulation is arranged between a DC voltage with a positive terminal and a negative terminal of the electrical system. The insulation monitor has test resistors, switches, and a control and evaluation device configured to establish three circuit phases and to determine voltage values ​​within a predetermined time interval in the circuit phases by means of least squares or extrapolation of voltage values, and to determine an insulation fault based on the determined voltage values.

[0005] From DE 10 2023 102 987 A1, a method for detecting insulation faults in a vehicle's high-voltage network is known, wherein the high-voltage network comprises an insulation monitor and a switching component. During a test phase, the switching component is electrically connected to and disconnected from the insulation monitor in a pattern characteristic of the switching component. During this test phase, the insulation monitor measures a charging curve and transmits this measurement to an evaluation unit. The evaluation unit detects an insulation fault in the high-voltage network based on a characteristic deviation in the charging curve measured by the insulation monitor, which corresponds to the pattern characteristic of the switching component.

[0006] From DE 10 2021 106 891 A1, a method for checking the insulation state of a battery or a battery system comprising at least two batteries is known, preferably a high-voltage battery or a high-voltage battery system for use as a traction battery of an electric vehicle or for use in stationary energy storage applications. The method comprises the steps of: measuring an electrical voltage between a terminal element of the battery and a ground over a predetermined time; evaluating the measured voltage and determining whether a change in the measured voltage occurs at a time corresponding to a predetermined time threshold; and outputting a safety signal indicative of the insulation state, depending on the result of the determination.

[0007] From DE 10 2018 222 123 B4 a measuring arrangement is known which is set up to measure a first insulation between a battery arrangement with a first battery connection and a second battery connection and a ground connection of a vehicle and a second insulation between a consumer side with a first consumer connection and a second consumer connection and the ground connection of the vehicle. SUMMARY AND FORMS OF EXECUTION

[0008] Therefore, one objective of the present disclosure is to provide a method and corresponding control device for insulation monitoring which detects situations in which conventional methods for determining insulation resistances may produce erroneous results.

[0009] This task is accomplished by a method for insulation monitoring, a computer program, and a control unit according to the independent patent claims. Advantageous embodiments and further developments are described in the respective dependent claims, the following description, and the drawings.

[0010] According to a first aspect, a method is provided for monitoring insulation between two electrical conductors, in particular high-voltage electrical conductors, and ground by means of an insulation monitoring circuit, wherein the insulation monitoring circuit has at least two switches.The method comprises the following steps: (a) switching the at least two switches according to a first switching sequence consisting of a first series of several switching states, wherein the duration of each switching state is greater than a first time interval; (b) determining a parasitic capacitance using the first switching sequence and checking whether the parasitic capacitance exceeds a predetermined limit; (c) switching the at least two switches according to a second switching sequence consisting of a second series of several switching states, wherein the duration of each switching state is less than a second time interval; and (d) determining an insulation resistance using the second switching sequence. The first time interval is greater than the second time interval.

[0011] According to another aspect, a control unit, in particular a battery management system, is provided which is configured to carry out the previously described procedure. According to one embodiment, the control unit includes the insulation monitoring circuit.

[0012] According to another aspect, a battery system is provided which includes the previously described control unit including insulation monitoring circuitry and a battery, in particular a vehicle battery and / or a high-voltage battery.

[0013] According to another aspect, a computer program is provided that includes instructions which, when executed by a computer, cause it to perform the procedure described above. In the context of this disclosure, a computer is defined, for example, as a device that processes data using programmable computational instructions. Computers can be embedded in everyday devices, such as the control units of motor vehicles.

[0014] According to another aspect, a storage medium is provided with a computer program, wherein the computer program includes instructions which, when the computer program is executed by a computer, cause it to carry out the procedure described above.

[0015] In the context of this disclosure, an electrical conductor is defined, for example, as a means for transmitting electrical energy and / or electrical signals. The electrical conductor may be a high-voltage conductor. The electrical conductor may connect a terminal of a battery, in particular a positive terminal or a negative terminal of the battery, to a terminal to which one or more loads may be connected. The terminal may be a link terminal. The two electrical conductors may connect a positive terminal of the battery to a positive terminal and a negative terminal of the battery to a negative terminal.

[0016] In the context of this disclosure, high voltage is defined, for example, as an alternating voltage greater than 30 V, in particular less than 1 kV, and / or a direct voltage greater than 60 V, in particular less than 1.5 kV. An electrical conductor can be a high-voltage conductor if corresponding voltages occur between the electrical conductor and ground.

[0017] In the context of this disclosure, insulation resistance is defined, for example, as the resistance between electrical conductors and / or between an electrical conductor and ground. According to one embodiment, the insulation resistance indicates the degree or state of insulation. Predefined threshold values ​​for the insulation resistance can correlate with various protection classes.

[0018] In the context of this disclosure, an insulation monitoring circuit is defined, for example, as an electrical circuit by means of which one or more insulation resistances can be determined. The insulation monitoring circuit comprises at least two switches. Each switch can be configured to connect one of the two electrical conductors to ground and / or to disconnect it from ground. The switches can be connected in series with a respective test resistor. The switches and / or test resistors can be connected in parallel with corresponding insulation resistances.

[0019] In the context of the present disclosure, a switching state is defined, for example, in relation to the switching position of one or more switches, in particular the at least two switches. The beginning of the switching state can be defined by the switching of at least one of the switches. The end of the switching state can also be defined by the switching of at least one of the switches. The switching state can be defined such that none of the switches are switched during the switching state. In other words, a switching state can be defined over the period between two directly successive switch changes. A switching sequence can be defined as a succession of several switching states, in particular several directly successive switching states.

[0020] In the context of this disclosure, parasitic capacitance is defined, for example, as a capacitance that exists between two electrical conductors and / or between an electrical conductor and ground. Insulation may be provided between the electrical conductors or between the electrical conductor and ground. The parasitic capacitance may be determined by properties of the respective electrical conductor and / or of electrical components in that electrical conductor and / or by properties of ground.

[0021] The previously described method and / or control unit can be advantageous to ensure reliable operation of the insulation monitoring circuit and thus enable safe operation of the battery system.

[0022] Methods for determining insulation resistances, such as those based on the second switching sequence, can produce erroneous results if one or more parasitic capacitances lie outside a predefined range, particularly above the specified limit. In such cases, the duration of the second time interval may be insufficient for a measurement signal, especially a voltage signal, to stabilize, which is necessary for determining the insulation resistances.

[0023] In the worst case, such faulty results can lead to insufficient insulation of electrical lines not being detected or not being detected in time, which can cause fires and even endanger human lives, especially when high-voltage lines are involved.

[0024] Parasitic capacitances outside a predefined range can also lead to erroneous results in various other safety diagnostics. In addition to the insulation monitoring described above, for example, a method for detecting stuck contactors, known as contactor stick detection, can also produce erroneous and / or uninterpretable results if the parasitic capacitance is too high or too low. Such contactors are used, for instance, to disconnect a battery from one or more loads.

[0025] The described problem can be remedied by performing the first switching sequence with switching states whose duration is at least as long as a first time interval, where the first time interval is longer than the second time interval. The duration of the first time interval can be sufficient, in particular, for the measurement signal, especially the voltage signal, to stabilize adequately. Based on this, the insulation resistance(s) can be determined with sufficient accuracy, and the respective parasitic capacitances can be calculated based on the insulation resistance(s) and the waveform of the measurement signal.

[0026] It can then be checked whether the parasitic capacitances exceed the specified limit. If this is not the case, it can be assumed that the insulation monitoring using the second switching sequence and / or the contactor sticking detection using a third switching sequence will deliver reliable results that are at least not distorted by excessively high parasitic capacitances.

[0027] According to one embodiment, two insulation resistances are determined, wherein one of the insulation resistances is between one of the two electrical conductors and ground, and the other insulation resistance is between the other of the two electrical conductors and ground.

[0028] According to one embodiment, two parasitic capacitances are determined, wherein one of the parasitic capacitances exists between one of the two electrical conductors and ground, and the other parasitic capacitance exists between the other of the two electrical conductors and ground.

[0029] According to one embodiment, it is also checked whether the parasitic capacitance falls below a further predetermined threshold. Such an embodiment can be advantageous, among other things, with regard to the contactor sticking detection mentioned above. In this method, excessively low parasitic capacitances can also lead to erroneous results. According to one embodiment, a contactor sticking detection method is only carried out if the parasitic capacitance does not exceed the predetermined threshold and does not exceed the further predetermined threshold.

[0030] According to one embodiment, the second switching sequence and / or the determination of the insulation resistance and / or the use of the determined insulation resistance for a safety-critical diagnosis only takes place if the parasitic capacitance does not exceed the specified limit.

[0031] According to one embodiment, the first time interval is at least one and a half times as long as the second time interval, in particular at least twice as long, and in particular at least three times as long. Such values ​​may be necessary to reliably determine insulation resistances using the first switching sequence and, based on the insulation resistances, to calculate the respective parasitic capacitances.

[0032] According to one embodiment, each of the two switching sequences comprises at least three switching states or phases. Each switching state can be defined by the positions of the two switches.

[0033] According to one embodiment, the first switching sequence and the second switching sequence have the same sequence of switching states. For example, the sequence of the first switching sequence can correspond to the sequence of the second switching sequence, except for the duration of the switching states. Such an embodiment can be advantageous if both switching sequences are configured to determine the insulation resistance.

[0034] According to one embodiment, the method further comprises: determining a voltage waveform indicative of a voltage between one of the two electrical conductors and ground, at least partially during the first switching sequence; and determining the insulation resistance(s) taking the voltage waveform into account. The voltage waveform can be discrete or continuous. One of the two electrical conductors can be connected to the negative terminal of a battery. For determining the insulation resistance(s), voltage values ​​at the end of one or more of the switching states of the first switching sequence can be particularly relevant. Such an embodiment can be advantageous because the longer duration of the first switching states allows for the determination of more reliable values ​​of the insulation resistance(s).

[0035] According to one embodiment, the voltage profile is also determined, at least partially, during the second switching sequence. In this case as well, values ​​at the end of one or more of the switching states of the second switching sequence can be particularly relevant for determining the insulation resistance(s).

[0036] According to one embodiment, the method further comprises: determining the parasitic capacitance based on the determined insulation resistance, in particular several determined insulation resistances, and a time parameter of the voltage waveform. The parasitic capacitance can be determined based on the total resistance of a relevant circuit, which includes the determined insulation resistance.

[0037] According to one embodiment, the time parameter is one of the following: a time derivative of the voltage waveform at one or more predetermined points in time during the first switching sequence; a duration until the voltage waveform reaches a predetermined saturation level. The voltage waveform can be determined, and in particular completely determined, by the total resistance of the relevant circuit and the corresponding parasitic capacitance. The voltage waveform can follow an exponential function for one or more, and in particular all, switching states. The slope of the exponential function can depend on the parasitic capacitance and the total resistance. Accordingly, the parasitic capacitance can then be calculated from the time derivative.

[0038] The time constant of the exponential function can be given by the product of the total resistance and the parasitic capacitance. Based on the simple time constant calculated from the beginning of the switching state, approximately 63 percent of the step response may be reached; after three times the time constant, 95 percent of the step response may be reached; and after five times the time constant, over 99 percent of the step response may be reached. The time constant can therefore be derived from the degree of saturation, which can be defined, for example, by a fraction of the step response reached, and the parasitic capacitance can then be derived from this, based on the total resistance.

[0039] Such an embodiment can be advantageous because both time parameters can be easily determined from the voltage curve, especially with the limited storage and computing capacities of an in-vehicle control unit.

[0040] According to one embodiment, the method further includes: determining whether the voltage profile is approximately constant in at least one of the switching states of the first switching sequence in an end section. For example, this can be used to verify whether a predetermined saturation level has been reached. This ensures that a voltage value measured in the end section, particularly at the end of the corresponding switching state, deviates from the final voltage only by a predetermined voltage threshold. Based on the measured voltage value or a correspondingly corrected voltage value, one or more insulation resistances can be determined, for example.

[0041] According to one embodiment, the voltage profile in the end section is approximately constant if a change in the voltage profile in the end section represents at most ten percent of the total change during the corresponding switching state, in particular at most five percent, and more specifically at most two percent. The duration of the end section can, for example, be at least two-thirds of the duration of the corresponding switching state, and in particular at least four-fifths. The end section can encompass a portion of the switching state that extends to the end of the switching state. The change and / or total change can be defined based on a difference between the maximum and minimum voltage values ​​during the corresponding section. Such an embodiment can be advantageous because it allows for simple verification of the achieved degree of saturation.

[0042] According to one embodiment, if the voltage profile in the end section is not approximately constant, an error is indicated, for example by means of an optical, acoustic, and / or haptic signal and / or by setting an error variable. Such an embodiment can be advantageous because a very large parasitic capacitance and / or interference with the voltage measurement signal can be detected in a simple manner.

[0043] According to one embodiment, the method further comprises: determining a voltage between the two electrical conductors, in particular a battery voltage, at least partially during the first switching sequence; and determining whether a change in the voltage during the first switching sequence is below a predetermined voltage threshold. The voltage can be determined continuously or at discrete intervals during the first switching sequence. The change in voltage can be defined by the difference between a maximum measured voltage value and a minimum measured voltage value during the first switching sequence. The predetermined voltage threshold can be, for example, 20 V, in particular 10 V, and in particular 5 V. Such an embodiment can be advantageous because voltage fluctuations above the voltage threshold can result in incorrect values ​​for the insulation resistances and / or the parasitic capacitances.

[0044] According to one embodiment, one of the two switches is configured to disconnect an electrical connection between one of the electrical conductors and ground, and / or the other of the two switches is configured to disconnect an electrical connection between the other of the electrical conductors and ground. The two electrical conductors can be connected to a positive terminal or a negative terminal of a battery, in particular a vehicle battery, or can be connected by means of a switch, in particular a contactor. The switches can be connected in series with a respective test resistor. The switches and / or test resistors can be connected in parallel with the corresponding insulation resistances.

[0045] According to one embodiment, ground is assigned to the chassis of a motor vehicle. In particular, one terminal of each switch can be electrically connected to the chassis. An opposite terminal can be connected to the corresponding electrical line. Such a configuration can be particularly advantageous in high-voltage systems because, for safety reasons, none of the battery terminals may be connected to ground.

[0046] According to one embodiment, the durations of at least two of the switching states in the first switching sequence differ. Alternatively, these durations are the same. According to one embodiment, the durations of at least two of the switching states in the second switching sequence differ. Alternatively, these durations are the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Further advantages and beneficial designs and further developments of the method and the control unit result from the following exemplary embodiments shown in connection with the figures.

[0048] They show: Fig. 1 a battery system with an insulation monitoring circuit for use in an insulation monitoring method according to an embodiment of the present disclosure; Fig. 2 a first switching sequence and a subsequent second switching sequence for a method for insulation monitoring according to an embodiment of the present disclosure; Fig. 3 a switching sequence for determining insulation resistances and corresponding voltage waveforms in the case of proper or excessive parasitic capacitances for use in a method for insulation monitoring according to an embodiment of the present disclosure; Fig. 4 a switching sequence for contactor adhesive determination as well as corresponding voltage waveforms in case of proper or too small or too large parasitic capacitances for use in a method for insulation monitoring according to an embodiment of the present disclosure.

[0049] Identical, similar, or similarly effective elements are marked with the same reference symbols in the figures. In some figures, individual reference symbols have been omitted for clarity. The figures and the relative sizes of the elements depicted within them are not to be considered to scale. Rather, individual elements may be exaggerated for better representation and / or comprehensibility. DETAILED DESCRIPTION OF EXAMPLES OF EXECUTION

[0050] Fig. Figure 1 shows a battery system 100 of a motor vehicle with a high-voltage battery 101 and an insulation monitoring circuit. The battery system 100 has a first electrical conductor 102, which connects a positive terminal B1+ of the battery 101 to a left-side or load-side positive terminal A1+ / P1+ of the battery system 101, and a second electrical conductor 103, which connects a negative terminal B1- of the battery 101 to a left-side or load-side negative terminal A1- / P1-. Both electrical conductors 102 and 103 are insulated from ground 104 by a battery-side insulation 105 with corresponding insulation resistances RisoPP and RisoPM, and a left-side insulation 106 with corresponding insulation resistances RisoLP and RisoLM. Ground 104 is connected to the vehicle chassis.The positive-side electrical conductor 102 is insulated from ground 104 by the insulation resistance RisoPP on the battery side and by the insulation resistance RisoLP on the left side. The negative-side electrical conductor 103 is insulated from ground 104 by the insulation resistance RisoPM on the battery side and by the insulation resistance RisoLM on the left side. A positive-side parasitic capacitance Cy1 exists between the positive-side electrical conductor 102 and ground 104. A negative-side parasitic capacitance Cy2 exists between the negative-side electrical conductor 103 and ground 104. Here, too, a distinction can be made between left-side and battery-side parasitic capacitances, if necessary.

[0051] The insulation monitoring circuit, or insulation monitor, has a positive-side electrical connection between the positive-side electrical line 102 and ground 104. This positive-side electrical connection includes a positive-side test switch SWTestP and a positive-side test resistor RTestP connected in series with it. Both the positive-side test switch SWTestP and the positive-side test resistor RTestP are connected in parallel with the positive-side insulation resistances RisoPP and RisoLP.

[0052] Similarly, the insulation monitoring circuit or insulation monitor has a negative-side electrical connection between the negative-side electrical line 103 and ground 104. This negative-side electrical connection comprises a negative-side test switch SWTestM and a negative-side test resistor RTestM connected in series with it. Both the negative-side test switch SWTestM and the negative-side test resistor RTestM are connected in parallel with the negative-side insulation resistors RisoPM and RisoLM.

[0053] The insulation monitoring circuit further comprises a voltage measurement circuit by means of which a voltage UMEAS_KL31 can be measured between the negative-side electrical conductor 103 and ground 104, in particular arranged between the electrical connection with the negative-side test switch SWTestM and the negative link terminal A1- / P1-. The voltage measurement device includes measuring resistors RMeasU, RMeasL and, for example, an analog-to-digital converter. By means of this and / or another measurement circuit, the ground voltage or chassis voltage UChsGnd between ground 104 and negative-side electrical conductor 103 can be determined.

[0054] The battery system 100 also features one or more measuring circuits for the battery voltage UBat or the link voltage ULink, which may include corresponding measuring resistors RRlink, RLink and / or analog-to-digital converters. Furthermore, contactors SWK1 and SWK3, respectively, are arranged in the positive-side electrical line 102 and the negative-side electrical line 103, respectively, which can be used to disconnect the connection between the battery and the respective link terminal or corresponding loads connected to the link terminals. A pre-charge switch SWK2, connected in parallel to the negative-side contactor SWK3, with a pre-charge resistor RK2 connected in series with it, can be used, among other things, for contactor weld detection, i.e., for detecting a malfunctioning contactor, for example, one that has been welded shut.

[0055] A high-voltage (HV) line 102, 103 to be monitored has an insulation resistance (RisoPP, RisoPM, RisoLP, RisoLM) of several hundred megaohms to several gigahertz as a parasitic component in a fault-free state. Additionally, the lines 102, 103 exhibit parasitic capacitances (Cy1, Cy2) as a physical property. This analysis considers the parasitic capacitances (Cy1, Cy2) located between the HV plus (HV+) line 102 and chassis ground (ChsGnd) 104, as well as between chassis ground 104 and the HV minus (HV-) line 103. These capacitances can range from a few nanofarads to several microfarads.

[0056] In the referenced insulation monitoring system, a measuring circuit is connected in parallel to the insulation resistance RisoPM, RisoLM. This circuit is characterized by the fact that the measuring resistors RTestP, RTestM, connected in parallel to the insulation resistances RisoPP, RisoPM, RisoLP, RisoLM, generate different voltage levels on a measurement signal through alternating switching. The insulation resistances between HV+ and ChsGnd, and ChsGnd and HV-, are determined from the ratio of the resulting measuring voltages UChsGnd and the battery voltage UBat. The typical measuring resistors RTestP, RTestM for such a system range from several hundred kΩ to several MΩ.

[0057] Switching a measuring resistor RTestP, RTestM initiates a charging process whose charging time is determined by the RC circuit consisting of the insulation resistance RisoPP, RisoPM, RisoLP, RisoLM and the measuring resistor RTestP, RTestM, and the parasitic capacitance Cy1, Cy2. Similarly, with any other fluctuation in the battery voltage UBat, such as those caused by dynamic processes during acceleration or recuperation of the HV battery 101, the measuring voltage is subject to a delay by the RC circuit compared to the battery voltage UBat.

[0058] The charging process via the RC circuit can typically take several seconds with fault-free insulation. In the referenced circuit, the insulation resistances RisoPP, RisoPM, RisoLP, and RisoLM are always determined after switching operations by waiting until a saturated measurement voltage (5-τ value) can be assumed, even with very high-resistance insulation.

[0059] It is important that the parasitic capacitances Cy1 and Cy2 remain within a certain, predefined range throughout the entire product lifecycle. If this is not the case, the system design for charging the measurement voltage for insulation measurement is no longer correct. The measurement voltage can no longer reach its saturation value during the charging process, and the insulation measurement becomes inaccurate.

[0060] Furthermore, parasitic capacitances Cy1, Cy2, which lie outside their specified value, can have a negative impact on possible measurement voltage corrections, which mainly occur in dynamic operation with fluctuations in the battery voltage UBat.

[0061] Another problem can arise when determining whether contactors SWK3 are stuck. In the system described, a circuit is closed from HV+ via the insulation measuring circuit and a pre-charge switch SWK2, which is connected in parallel to the contactor SWK3 under test, to HV-. Due to the high resistances of the overall circuit, the parasitic capacitances Cy1 and Cy2 have a significant influence on the signal waveform during this test sequence. This will be discussed in more detail below in connection with Fig. 4 explained.

[0062] If, over the product lifecycle, the parasitic capacities Cy1, Cy2 change so significantly due to aging, damage or deliberate intervention that they are no longer within the range intended for this product, the aforementioned tests may become inaccurate or faulty and may produce false or misleading results.

[0063] Fig. Figure 2 illustrates a method which uses the circuit made of Fig. 1 enables the monitoring of one or more insulation resistances RisoPP, RisoPM, RisoLP, RisoLM, depending, among other things, on the switch positions of the contactors SWK1, SWK3 and the pre-charge switch SWK2. The method comprises the following steps: (i) switching at least two switches SWTestP, SWTestM of the insulation monitoring circuit according to a first switching sequence 110 consisting of a first sequence of several switching states 112, wherein a respective time duration 113 of each of the switching states (112) is at least as long as a first time interval; (ii) determining at least one parasitic capacitance Cy1, Cy2 by means of the first switching sequence 110 and checking whether the at least one parasitic capacitance Cy1, Cy2 exceeds a predetermined threshold value;(iii) Switching the at least two switches SWTestP, SWTestM according to a second switching sequence 111 consisting of a second sequence of several switching states 112, wherein a respective time duration 114 of each of the switching states is at most as long as a second time interval; and (iv) Determining one or more insulation resistances RisoPP, RisoPM, RisoLP, RisoLM by means of the second switching sequence 111; wherein the first predetermined time interval is greater than the second predetermined time interval.

[0064] The various switching states 112 are determined by the state 115 of the positive-side test switch RTestP and the state 116 of the negative-side test switch RTestM. An upper line indicates a closed switch and a lower line an open switch. For example, in the first switching state 112, the positive-side switch RTestP is closed and the negative-side switch RTestM is open; in the second switching state 112, the positive-side switch RTestP is open and the negative-side switch RTestM is closed; in the third switching state 112, both the positive-side switch RTestP and the negative-side switch RTestM are closed; and in the fourth switching state 112, the positive-side switch RTestP is closed and the negative-side switch RTestM is open.

[0065] In the lower part of Fig. Figures 2 and 119 show corresponding voltage waveforms of the ground or chassis voltage UChsGnd, determined, for example, based on the KL31 voltage UMEAS_KL31. The voltage waveform 118 with a solid line represents the ground voltage UChsGnd with proper parasitic capacitances Cy1 and Cy2 within specified limits. In contrast, the voltage waveform 119 represents the ground voltage UChsGnd when at least one of the parasitic capacitances Cy1 or Cy2 is too large. In the latter case, sufficient saturation of the voltage signal is no longer achieved during the shorter duration 114 of the switching states 112 of the second switching sequence 111. Because the saturation values ​​are crucial for determining the insulation resistances RisoPP, RisoPM, RisoLP, and RisoLM, the determined values ​​of these resistances may be incorrect.During the longer duration 113 of the switching states 112 of the first switching sequence 110, sufficient saturation of the voltage signal is achieved, so that the insulation resistances RisoPP, RisoPM, RisoLP, RisoLM can be correctly determined and the parasitic capacitances Cy1, Cy2 can be derived from them. By comparing the parasitic capacitances Cy1, Cy2 with one or more threshold values, it can be predicted whether sufficient saturation of the voltage signal can be expected during the second switching sequence 111 with the shorter duration 114.

[0066] In other words, the procedure is carried out as follows: 1. Determining the total resistances and capacitances Cy1, Cy2 of the entire circuit via a reliable measurement sequence by: a) Determining a stable input state by ensuring that the battery voltage UBat is within a stable range (battery voltage change < 10V) during the subsequent signal acquisition sequence; b) Extending the typical settling times of the measured voltages 118, 119 by a multiple of the normal value (factor 1.5 - 3) to achieve a reliably steady state; c) Acquiring the measured voltages 118, 119 and their respective time profiles; d) Calculating the insulation resistance RisoPP, RisoPM, RisoLP, RisoLM; e) Calculation of the resistances RTestP, RTestM from the measuring circuit and insulation resistance RisoPP, RisoPM, RisoLP, RisoLM, which is relevant for the RC element of the respective capacitance Cy1, Cy2;f) Calculation of the capacitance Cy1, Cy2 from the time-dependent change of the measurement signal 118, 119 and the resistance calculated under e); g) Monitoring of the signal acquisition process by ensuring that the time-dependent change of the measurement signal is approximately stable (< 2% of the measurement signal change) after 4 / 5 of the extended waiting time until its end. If this is not the case, a very large capacitance or a disturbance of the measurement signal can be assumed. An implausible state is detected; 2. Monitoring of parasitic capacitances. It is checked whether the determined capacitances Cy1, Cy2 are within the range specified for the circuit for insulation resistance measurement, contactor sticking detection, and other functions such as dynamic correction; 3. If a capacitance Cy1, Cy2 is outside its specified range or an implausible state is detected according to 1g), an error is reported;4. The monitoring sequence can occur once or several times in a system activation or driving cycle.

[0067] Fig. 3 shows analogous to Fig. 2. A repeated, here twice, execution of a second measurement sequence 111 with switching states 112 of a shorter second state duration 114. The second measurement sequence 111 consists of the following three directly consecutive switching states: i) positive-side test switch SWTestP closed, negative-side test switch SWTestM open; ii) positive-side test switch SWTestP open, negative-side test switch SWTestM closed; iii) positive-side test switch SWTestP and negative-side test switch SWTestM closed. Again, a voltage waveform 118 of the voltage UChsGrnd with correct parasitic capacitances Cy1, Cy2 and a voltage waveform 119 of the voltage UChsGrnd with excessively large parasitic capacitances Cy1, Cy2 are shown. Fig. Figure 3 illustrates that if the parasitic capacitances Cy1, Cy2 are too large, the insulation resistances RisoPP, RisoPM, RisoLP, RisoLM and thus insulation monitoring can no longer be reliably determined because the voltage signal does not reach a sufficient degree of saturation within the switching states 112.

[0068] Fig.Figure 4 illustrates problems with contactor stick detection, particularly of the negative-side contactor SWK3, when parasitic capacitances Cy1, Cy2 are too high or too low. The upper part of the figure shows the sequence 120 for contactor stick detection, which also takes into account a state 117 of the pre-charge switch SWK2. The lower part shows the corresponding voltage signal of the voltage UChsGrnd; the voltage waveform 118 represents a diagnosis when parasitic capacitance Cy1, Cy2 is within the normal range, and the voltage waveforms 119 represent diagnoses when parasitic capacitance Cy1, Cy2 is too low (dotted-dashed) and too high (dashed)

[0069] It is a typical feature of high-voltage drive systems that two switches or contactors (main contactors) SWK1 and SWK3 are used to open and close the circuit. These connect the HV-positive and HV-negative sides of the battery to the link or load side of the load. A typical characteristic is that these contactors SWK1 and SWK3 must be monitored for faults caused by sticking or welding. This monitoring of a contactor SWK1 or SWK3 can be achieved using a parallel pre-charge switch SWK2 with a series resistor. In this case, the circuit for contactor sticking detection is not closed via the load, but rather via the insulation monitoring circuit connected in parallel. REFERENCE MARK 100 battery system 101 Battery 102 positive-side electrical line 103 negative-side electrical conduction 104 Mass / Chassis 105 battery-side insulation 106 left-side insulation 110 first switching sequence 111 second switching sequence 112 Switching state 113 first state duration 114 second state duration 115 Positive-side test switch status 116 Negative-side test switch status 117 Pre-charge switch status 118 chassis voltage with proper parasitic capacitance 119 Chassis voltage with faulty parasitic capacitance 120 sequence contactor adhesive detection B1+ positive battery terminal B1 negative battery terminal A1+ / P1+ positive link A1 / P1 negative link connection R isoPP Battery-side positive insulation resistance R isoPM Battery-side negative insulation resistance R isoLP left-side positive insulation resistance R isoLMleft-side negative insulation resistance C y1 positive-side parasitic capacity C y2 negative-side parasitic capacity SW TestP positive-side test switch R TestP positive-side test resistor SW TestM negative-side test switch R TestM negative-side test resistor SW K1 positive-side protector SW K3 negative-side contactor SW K2 Pre-charge switch R PreChg Pre-charge resistor UMEAS_KL31 KL31 voltage U ChsGnd Chassis voltage R MeasU , R MeasL Chassis measuring resistors U Link Link voltage U Bat Battery voltage R RLink , R Link Link measuring resistors

Claims

[1] Method for monitoring insulation between two electrical conductors (102, 103), in particular high-voltage electrical conductors, and ground (104) by means of an insulation monitoring circuit, wherein the insulation monitoring circuit has at least two switches (SWTestP, SWTestM), the method comprising the following steps: - Switching the at least two switches (SWTestP, SWTestM) according to a first switching sequence (110) consisting of a first sequence of several switching states (112), wherein a respective time duration (113) of each of the switching states (112) is at least as large as a first time interval; - Determining a parasitic capacitance (Cy1, Cy2) using the first switching sequence (110) and checking whether the parasitic capacitance (Cy1, Cy2) exceeds a predetermined threshold; - Switching the at least two switches (SWTestP, SWTestM) according to a second switching sequence (111) consisting of a second sequence of several switching states (112), wherein a respective time duration (114) of each of the switching states is at most as long as a second time interval; and - Determining an insulation resistance (RisoPP, RisoPM, RisoLP, RisoLM) using the second switching sequence (111); where the first time interval is greater than the second time interval. [2] Method according to the preceding claim, wherein it is also checked whether the parasitic capacity (Cy1, Cy2) falls below a further predetermined threshold. [3] Method according to any of the preceding claims, wherein the first time interval is at least one and a half times as long as the second time interval. [4] Method according to one of the preceding claims, wherein the first switching sequence (110) and the second switching sequence (111) have the same sequence of switching states (112). [5] Method according to one of the preceding claims, the method further comprising: determining a voltage profile (118, 119) of a voltage (UChsGnd, UMEAS_KL31) indicative of a voltage between one of the two electrical conductors (102, 103) and ground (104), at least partially during the first switching sequence (110); determining the insulation resistance (RisoPP, RisoPM, RisoLP, RisoLM) taking into account the voltage profile (118, 119). [6] Method according to the preceding claim, further comprising: determining the parasitic capacitance (Cy1, Cy2) based on the determined insulation resistance (RisoPP, RisoPM, RisoLP, RisoLM) and a time parameter of the voltage waveform (118, 199). [7] Method according to the preceding claim, wherein the time parameter is one of the following parameters: a time derivative of the voltage profile (118, 119) at one or more predetermined time points during the first switching sequence (110); a duration until the voltage profile has reached a predetermined saturation level. [8] Method according to any one of claims 5 to 7, further comprising: Determine whether in a final section at least one of the switching states (112) of the first switching sequence (110) the voltage profile (118, 119) is approximately constant. [9] Method according to the preceding claim, wherein the voltage profile (118, 119) in the end section is approximately constant if a change in the voltage profile (118, 119) in the end section is at most ten percent of the total change during the corresponding switching state (112), in particular at most two percent. [10] Method according to the preceding claim, wherein if the stress profile (118, 119) in the end section is not approximately constant, an error is indicated. [11] Method according to one of the preceding claims, further comprising: determining a voltage (UBat, ULink) between the two electrical lines (102, 103) at least partially during the first switching sequence (110); and determining whether a change in the voltage (UBat, ULink) during the first switching sequence (110) is below a predetermined voltage threshold. [12] Method according to one of the preceding claims, wherein one of the two switches (SWTestP, SWTestM) is configured to disconnect an electrical connection between one of the electrical lines (102, 103) and ground (104), and / or the other of the two switches (SWTestP, SWTestM) is configured to disconnect an electrical connection between the other of the electrical lines (102, 103) and ground (104). [13] Method according to one of the preceding claims, wherein mass (104) is assigned to the chassis of a motor vehicle. [14] Computer program comprising instructions which, when executed by a computer, cause the computer to carry out the method according to any one of claims 1 to 13. [15] Control unit, in particular battery management system, which is configured to carry out a method according to any one of claims 1 to 13.

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