Method and device for monitoring the insulation resistance of a traction network

Abstract

The invention relates to a method and a device for monitoring the insulation resistance of a traction network (1), wherein the traction network (1) comprises a high-voltage battery (2), a DC / DC converter (3), an intermediate circuit capacitor (C), and an inverter (4), wherein the device comprises a first insulation resistor (R1) with a switching element (S1) and a second insulation resistor (R2) in series with a second switching element (S2), wherein the device has a voltage measuring device (V1, V2) assigned to the first insulation resistor (R1) and the second insulation resistor (R2), respectively, wherein the device further comprises a control and evaluation circuit (6) that controls the switching elements (S1, S2) and reads the voltage values, wherein the control and evaluation circuit (6) is configured such thatat least one measurement with the first switching element (S1) closed and the second switching element (S2) open, and at least one measurement with the second switching element (S2) closed and the first switching element (S1) open, whereby a virtual equivalent circuit diagram for the high-voltage battery (2) is established.

Description

[0001] The invention relates to a method and a device for monitoring an insulation resistance of a traction network of a motor vehicle.

[0002] Monitoring the insulation resistance of a traction network is typically done by connecting an insulation resistor from the HV+ line to ground or an insulation resistor from the HV- line to ground. These connected insulation resistors then form a series circuit with the insulation resistance of the air gap to be monitored. If no fault is present, the voltage across the basic insulation is almost exactly the same as the system voltage in the IT network.

[0003] Another device for monitoring an insulation resistance is known from EP 3 913 388 A1.

[0004] Traction networks are also known in which an additional DC / DC converter is arranged between the high-voltage battery and the inverter. This converter can be used, for example, to adjust the voltage at the inverter to optimize the operating point of an electric motor. However, this requires greater effort for monitoring the insulation resistance. DE 10 2018 217 116 B3 discloses a method and device for monitoring the insulation resistance of such a traction network.

[0005] US Pat. No. 6,998,819 B2 describes a method for detecting leakage currents in a high-voltage battery pack system that reduces detection errors caused by fluctuations in the battery pack voltage during the detection process. During each leakage current measurement, the voltages of the individual modules of the battery pack are measured separately and simultaneously on the high- and low-voltage sides of the leakage point. Measurements of the individual module voltages are combined with the simultaneous leakage current measurements to calculate the leakage resistance.

[0006] US 2016 / 0 091 551 A1 relates to leakage current detection for vehicles with a high-voltage bus. A hybrid-electric or fully electric vehicle consists of a traction battery composed of multiple battery cells connected in series and / or parallel. The traction battery provides power for the vehicle's propulsion and accessory functions. The traction battery typically operates at high voltages. Under normal conditions, a high-voltage system is designed to exhibit high resistance between the traction battery terminals and ground. However, during operation, the leakage resistance may decrease for various reasons. Many high-voltage systems incorporate circuitry to monitor and detect abnormal leakage current in the traction battery.

[0007] The invention is based on the technical problem of providing a method and a device for monitoring an insulation resistance which are of simpler design.

[0008] The solution to the technical problem results from a method having the features of claim 1 and a device having the features of claim 6. Further advantageous embodiments of the invention result from the subclaims.

[0009] The traction network comprises a high-voltage battery, a DC / DC converter, an intermediate circuit capacitor, and an inverter. A first insulation resistor is arranged in series with a first switching element between an HV+ line on the high-voltage battery and ground, and a second insulation resistor is arranged in series with a second switching element between an HV- line on the high-voltage battery and ground. Furthermore, a voltage measuring device or a current measuring device is each assigned to the first insulation resistor and the second insulation resistor, with voltage measuring devices being preferred. Another voltage measuring device measures the battery voltage of the high-voltage battery.To measure the resulting insulation resistance, the switching elements are closed, and the resulting insulation resistances between the HV+ and HV- lines and ground are determined by evaluating the voltages across the first insulation resistance and the second insulation resistance. At least one measurement is performed with the first switching element closed and the second switching element open, and at least one measurement is performed with the second switching element closed and the first switching element open. Furthermore, a virtual equivalent circuit diagram for the high-voltage battery is created, in which the high-voltage battery is connected to the battery voltage U. Bat by two batteries connected in series with voltage n·U Bat and (1-n)·U Batis mapped, assuming a possible insulation resistance between the two batteries and ground, where n is a real number, whereby the insulation fault resistance and n are determined by solving mesh equations of the two measurements, whereby the location of the insulation fault is deduced from n, whereby if n is negative the insulation fault lies between the DC / DC converter and the inverter. This makes it possible in principle to determine from just two measurements whether there is an insulation fault and whether it lies before or after the DC / DC converter. This is possible because the approach with the virtual batteries and the insulation fault resistance per measurement means that two mesh equations in the two measurements are linearly independent. This results in two equations for two unknowns, so that n and the insulation fault resistance can be determined.This represents a very simple and robust method for monitoring insulation resistance.

[0010] In one embodiment, the insulation resistance measurements are also performed when both switching elements are open and closed, with the measurement results of these two measurements being used to verify the plausibility of the determined values ​​for n and the insulation fault resistance. This allows, for example, differences in the resistances of the individual measuring devices to be recorded and taken into account. In this case, four measurements are performed, with the possible switching positions of the two switching elements being permuted.

[0011] In a further embodiment, the different series of measurements (2 or 4) are carried out repeatedly with the different switching positions of the switching elements, which improves the significance of the results.

[0012] In another embodiment, if n changes in the various measurement series, an insulation fault at the inverter output is inferred, since only AC components can lead to changing voltage levels. Therefore, the voltage measuring points with a low cutoff frequency are also suitable for evaluating insulation resistance in the AC range of the PWR or for evaluating interference voltage input from this range.

[0013] In a further embodiment, a third insulation resistor R3 is arranged in series with a third switching element between the HV++ line at the output of the DC / DC converter and ground, wherein the measurements are used for further plausibility check of the insulation fault and n.

[0014] The invention is explained in more detail below using preferred embodiments. The figures show: Fig. 1 a schematic representation of a traction network, Fig. 2 a virtual equivalent circuit diagram of a high-voltage battery and Fig. 3 another representation of the equivalent circuit according to Fig. 2 to show the mesh equations.

[0015] In the Fig. Figure 1 schematically shows a traction network 1 for an electric or hybrid vehicle. The traction network 1 has a high-voltage battery 2 with a voltage U Bat, a DC / DC converter 3 and an inverter 4, which converts a direct voltage into a three-phase alternating voltage, which can then be fed to an electric machine (not shown) or converts a three-phase alternating voltage into a direct voltage in recuperation mode. An intermediate circuit capacitor C is arranged between the DC / DC converter 3 and the inverter 4. Two contactors 5 are also shown, by means of which the high-voltage battery 2 can be separated from the rest of the traction network 1. Further fuses and / or switching elements can be present, but these are not shown for reasons of clarity. A voltage measuring device V is arranged between the HV+ line and the HV- line, which measures the voltage U Batof the high-voltage battery 2. Furthermore, a first insulation resistor R1 is connected in series with a first switching element S1 between the HV+ line and ground, wherein a first voltage measuring device V1 is assigned to the series connection. A second insulation resistor R2 is arranged in series with a second switching element S2 between the HV- line and ground, wherein a second voltage measuring device V2 is assigned to this series connection. A third insulation resistor R3 with a third switching element S3 is arranged between the HV++ line at the output of the DC / DC converter 3 and ground, wherein a third voltage measuring device V3 is assigned to the series connection. Also shown is a control and evaluation circuit 6, which controls the switching elements S1, S2, S3 and evaluates the measured voltages of the voltage measuring devices V, V1, V2, V3.The control and evaluation unit 6 can be a separate unit or can be integrated into a battery management control unit.

[0016] The basic idea of ​​the invention is that an insulation fault R f and its location can be determined. For this purpose, a virtual equivalent circuit diagram is set up for the high-voltage battery 2, wherein the high-voltage battery 2 is divided into two sub-batteries 11, 12 in series, wherein one sub-battery 11 has a voltage n·U Bat and the partial battery 12 has a voltage (1-n)·U Bat where a possible insulation fault resistance R f between the two sub-batteries 11, 12 and ground, which is Fig. 2. In addition, internal resistances R m1 , R m2of the voltage measuring devices V1, V2. Here, n is a real number and can therefore also be negative.

[0017] In the Fig. 3 is now the equivalent circuit diagram from Fig. 2 more generally, where Z1, Z2 vary depending on which switching element S1, S2 is open or closed, for example, if both switching elements S1, S2 are open, Z1=R m1 and Z2=R m2 is.

[0018] The following generally applies: I1=(n⋅UBat)⋅(Z2+Rf)+(1−n)⋅UBat⋅Rf(Z1+Rf)⋅(Z2+Rf)−(Rf)2 and I2=(Z1+Rf)⋅(1−n)⋅UBat+(Rf⋅n⋅UBat)(Z1+Rf)⋅(Z2+Rf)−(Rf)2

[0019] Now, two measurements are performed, one with the first switching element S1 being closed and the second switching element S2 remaining open. During the second measurement, the first switching element S1 is open and the second switching element S2 is closed. For each measurement, there are two mesh equations, each with an equation for R. f can be established, which still depends on n. As already noted, Z1 and Z2 are also different in the two measurements. Rf(S1=1,S2=0)=Z1⋅Z2⋅(n⋅UBat−U1)U1⋅(Z1+Z2)−Z1⋅UBat Rf(S1=0,S2=1)=Z2⋅Z1⋅((1−n)⋅UBat−U2)U2⋅(Z1+Z2)−Z2⋅UBat

[0020] Since the two values ​​for R determined in this way f but result from different measurements, these are linearly independent and can be used to determine the uniform insulation fault or insulation fault resistance R fand solved for n. This n can then be converted into one of the two equations for R f be used and thus determined. The n now indicates the location of the insulation fault (if one has occurred). At n = ½, the insulation fault is in the middle of the high-voltage battery 2. With smaller n, the insulation fault moves upwards and correspondingly with larger n, downwards. At n = 0, the insulation fault is at HV+. If n becomes negative, the short-circuit voltage is greater than U Bat and must therefore come from behind the DC / DC converter 3 as a boost converter from HV++, which can be verified by a measurement with the third insulation resistor R3, but this is not mandatory.

[0021] It can also be provided that the measurements are carried out repeatedly, whereby the two other measurements S1 = S2 = 0 and S2 = S2 = 1 are also carried out. These measurements can then be used to determine resistance ratios and to check the plausibility of the determined values ​​for n and R f In addition, the measurement S1 = S2 = 1 can be used to actively discharge Y capacitors not shown.

[0022] If n varies in the series of measurements, this is an indication that the insulation fault is located at the output of inverter 4, i.e. is caused by an alternating voltage. List of reference symbols 1 traction network 2 high-voltage batteries 3 DC / DC converters 4 inverters 5 contactor 6 Control and evaluation circuit 11 Partial battery 12 partial batteries S1, S2, S3 switching elements V, V1, V2, V3 voltage measuring devices R f Insulation fault resistance R1, R2, R3 insulation resistances U Bat Tension C DC link capacitor

Claims

[1] Method for monitoring an insulation resistance of a traction network (1), wherein the traction network (1) has a high-voltage battery (2), a DC / DC converter (3), an intermediate circuit capacitor (C) and an inverter (4), wherein a first insulation resistance (R1) is arranged in series with a first switching element (S1) between an HV+ line on the high-voltage battery (2) and a ground, and a second insulation resistance (R2) is arranged in series with a second switching element (S2) between an HV- line on the high-voltage battery (2) and the ground, wherein a voltage measuring device (V1, V2) or a current measuring device is assigned to the first insulation resistance (R1) and the second insulation resistance (R2), wherein the switching elements (S1, S2) are closed to measure the resulting insulation resistance,whereby the resulting insulation resistances between the HV+ and HV- lines and ground are determined by evaluating the voltages across the first insulation resistance (R1) and the second insulation resistance (R2), characterized by that at least one measurement is carried out with the first switching element (S1) closed and the second switching element (S2) open and at least one measurement is carried out with the second switching element (S2) closed and the first switching element (S1) open, wherein a virtual equivalent circuit diagram is set up for the high-voltage battery (2), in which the high-voltage battery (2) is connected to the battery voltage U Bat by two series-connected partial batteries (11, 12) with voltages n·U Bat and (1-n)·U Bat is shown, whereby a possible insulation fault resistance (R f) between the two partial batteries (11, 12) and ground, where n is a real number, whereby by solving mesh equations of the two measurements the insulation fault resistance (R f ) and n, whereby the location of the insulation fault is determined on the basis of n, whereby with a negative n the insulation fault lies between the DC / DC converter (3) and the inverter (4). [2] Method according to claim 1, characterized by that the insulation resistance measurements are also carried out when both switching elements (S1, S2) are open and closed, whereby the measurement results of these two measurements are used to check the plausibility of the determined values ​​for n and the insulation fault resistance (R f ) be used. [3] Method according to claim 1 or 2, characterized by that the series of measurements are carried out repeatedly with the different switching positions of the switching elements (S1, S2). [4] Method according to claim 3, characterized by that if n changes in the different measurement series, an insulation fault at the output of the inverter (4) is concluded. [5] Method according to one of the preceding claims, characterized by that a third insulation resistor (R3) is arranged in series with a third switching element (S3) between the HV++ line at the output of the DC / DC converter (3) and ground, wherein the measurements are used for further plausibility checks of the insulation fault and n. [6] Device for monitoring an insulation resistance of a traction network (1), wherein the traction network (1) has a high-voltage battery (2), a DC / DC converter (3), an intermediate circuit capacitor (C) and an inverter (4), wherein the device comprises a first insulation resistor (R1) with a switching element (S1), which are arranged between an HV+ line on the high-voltage battery (2) and a ground, wherein the device further comprises a second insulation resistor (R2) in series with a second switching element (S2), which are arranged between an HV- line on the high-voltage battery (2) and the ground, wherein the device further comprises a voltage measuring device (V) for detecting the voltage (U Bat) of the high-voltage battery (2), and each having a voltage measuring device (V1, V2) or a current measuring device which are assigned to the first insulation resistance (R1) and the second insulation resistance (R2), wherein the device further comprises a control and evaluation circuit (6), wherein the control and evaluation circuit (6) is designed to control the switching elements (S1, S2) and to read out the voltage values, wherein by evaluating the voltages the resulting insulation resistances between the HV+ and HV- lines and ground are deduced, characterized bythat the control and evaluation circuit (6) is designed to carry out at least one measurement with a closed first switching element (S1) and an open second switching element (S2) and at least one measurement with a closed second switching element (S2) and an open first switching element (S1), wherein a virtual equivalent circuit diagram for the high-voltage battery (2) is set up in which the high-voltage battery (2) is connected to the battery voltage U Bat by two series-connected partial batteries (11, 12) and voltages n·U Bat and (1-n)·U Bat is shown, whereby a possible insulation fault resistance (R f ) between the two partial batteries (11, 12) and ground, where n is a real number, wherein the control and evaluation circuit (6) is further designed to determine the insulation fault resistance (R f) and n, whereby the location of the insulation fault is determined on the basis of n, whereby with a negative n the insulation fault lies between the DC / DC converter (3) and the inverter (4). [7] Device according to claim 6, characterized by that the control and evaluation circuit (6) is designed in such a way that the insulation resistance measurements are also carried out when both switching elements (S1, S2) are open and closed, the measurement results of these two measurements being used to check the plausibility of the determined values ​​for n and the insulation fault resistance (R f ) be used. [8] Device according to claim 6 or 7, characterized by that the control and evaluation circuit (6) is designed such that the series of measurements are carried out repeatedly with the different switching positions of the switching elements (S1, S2). [9] Device according to claim 8, characterized bythat the control and evaluation circuit (6) is designed in such a way that, in the event of a changing n in the various measurement series, it concludes that there is an insulation fault at the output of the inverter (4). [10] Device according to one of claims 6 to 9, characterized by that a third insulation resistor (R3) is arranged in series with a third switching element (S3) between the HV++ line at the output of the DC / DC converter (3) and ground, wherein the control and evaluation circuit (6) is designed to use the measurements for further plausibility checks of the insulation fault and n.

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