Electrical circuit arrangement and method for insulation measurement on a battery electric vehicle
Patent Information
- Application Number
- EP2023833486
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-12-20
- Filing Date
- 2023-12-20
- Publication Date
- 2025-10-29
AI Technical Summary
Existing electrical circuit arrangements for battery-electric vehicles lack a cost-effective and reliable method for insulation measurement, particularly in transformerless charging systems, which can lead to safety issues due to potential ground faults and high costs associated with insulation faults.
An electrical circuit arrangement with a power converter, DC and AC switches, and a discharge resistor, allowing for insulation measurement by setting DC voltages and recording measurement voltages between EV connections and ground potential, enabling the determination of insulation resistance values and detection of hard ground faults, while utilizing existing components of a transformerless charger.
The solution provides a reliable and cost-effective insulation measurement method for battery-electric vehicles, ensuring proper insulation and safety by detecting insulation faults and limiting ground currents, thus preventing damage and ensuring safe operation of transformerless chargers.
Smart Images
Figure 1.1
Abstract
Description
[0001] ELECTRICAL CIRCUIT ARRANGEMENT AND METHOD FOR INSULATION MEASUREMENT ON A BATTERY ELECTRIC VEHICLE
[0002] TECHNICAL FIELD
[0003] The invention relates to an electrical circuit arrangement and a method for measuring insulation on an electric vehicle, in particular a battery-electric vehicle.
[0004] STATE OF THE ART
[0005] It is well known that an electric vehicle, also known as a battery electric vehicle (BEV), can be charged with electrical power from an AC voltage network using a charger. The AC voltage network can be, for example, an electrical power supply network. A charger can have an electrical circuit arrangement with an all-pole AC disconnect point for the AC connections with pre-charging via resistors, an all-pole DC disconnect point for its DC connections on its direct current side (DC: direct current / direct voltage), a pre-charging circuit for the BEV and a discharging circuit for the BEV. The circuit arrangement can also have voltage measurements on both sides of the DC disconnect point, i.e. at the DC output of a power converter with a converter circuit and at EV connections at the input of the BEV.A DC switch of the all-pole DC isolation point is arranged between each DC connection and a corresponding EV connection. The discharge circuit can have a discharge resistor connecting the two EV connections. Any residual charge in the capacitances of the BEV and its supply lines can be discharged via the discharge resistor. The pre-charging circuit can be arranged in parallel with one of the switches and, in addition to a pre-charging resistor, can have another switch connected in series with the pre-charging resistor.
[0006] EP4057012 describes a method for on-board insulation measurement in a battery-electric vehicle. The insulation measurement is performed by charging and discharging capacitors.
[0007] EP2487496 describes a method for in-vehicle insulation measurement in a battery-electric vehicle. The vehicle has a high-voltage battery, a drive motor, a power converter arranged between the motor and the battery, and a discharge and pre-charge circuit.
[0008] The application is based on the object of providing an electrical circuit arrangement and a method for measuring insulation on an electric vehicle, whereby in particular a reliable measurement and a cost-effective implementation are to be enabled.
[0009] SOLUTION
[0010] The object is achieved by an electrical circuit arrangement having the features of claim 1 and a method having the features of claim 15. Embodiments are specified in the dependent claims.
[0011] DESCRIPTION
[0012] An electrical circuit arrangement for measuring insulation on a battery-electric vehicle can be connected to a high-voltage battery of the vehicle via EV connections. The circuit arrangement has an electrical power converter with AC connections (AC: alternating current) for connection to an electrical AC voltage network and with DC connections. A first DC connection of the DC connections can be connected to a first EV connection of the EV connections via a first DC switch, and a second DC connection of the DC connections can be connected to a second EV connection of the EV connections via a second DC switch or via a parallel connection comprising the second DC switch and a third DC switch. The described DC and AC switches are implemented, for example, by relays.The circuit arrangement is designed and configured to connect one of the DC terminals to the respective EV terminal by closing one of the DC switches when the power converter is connected to the AC voltage network, to set a DC voltage at the DC terminals by clocking the power converter, and to perform the insulation measurement on the connected vehicle.
[0013] The insulation measurement on the vehicle can, in particular, include the measurement of insulation resistance. Insulation resistance can symbolize the electrical resistance of the vehicle to earth potential. The insulation measurement can, for example, detect an insulation fault if the resistance value of one of the insulation resistances between the respective EV connection and earth potential, and thus the insulation resistance of the vehicle to earth potential, is below a minimum value. The circuit arrangement can be part of a charger. For safety reasons, circuit arrangements of chargers can include a transformer to galvanically isolate the electric vehicle from the AC grid. Charging vehicles using a charger is taken into account in standards, e.g., IEC-61851-23, e.g., Section CC.4.
[0014] Compared to chargers with transformers, transformerless charging, i.e. charging using a charger without galvanic isolation by a transformer, would offer advantages in terms of equipment costs, weight, and size of the chargers. Using the described transformerless circuit arrangement for insulation measurement, it can be ensured that the current-carrying cables and the vehicle itself, and in particular its electrical storage device, i.e. the battery, are properly insulated from earth potential. In addition, a so-called residual current monitor can be installed in the connections to the AC voltage network to monitor fault currents during charging and, in the event of a fault, interrupt the charging process by opening the DC switches in the DC isolating point and the AC switches in the AC isolating point. The described circuit arrangement thus enables the safe operation of a transformerless charger.
[0015] The vehicle's electrical storage system typically consists of many cells connected in series, with any insulation faults occurring at any point in this series connection. The described circuit arrangement for insulation measurement enables insulation measurements with such an unknown voltage source. It is advantageous that existing components of a transformerless charger can be used for insulation measurement. Even if minor adjustments are required, costs can be kept low while maintaining a high level of safety.
[0016] In one embodiment of the circuit arrangement, the EV terminals in the circuit arrangement are connected to one another via a discharge resistor, and the circuit arrangement is designed and configured to carry out the insulation measurement using the discharge resistor, in particular by calculating an insulation resistance on the vehicle using the discharge resistor as a component of a voltage divider.
[0017] In one embodiment of the circuit arrangement, the power converter has a split intermediate circuit on its DC side, which is connected to the DC terminals, wherein the potential of the center point of the intermediate circuit has a given, i.e. essentially unchanging, reference to earth potential when the power converter is connected to the AC voltage network on the AC side. A DC-side DC voltage can be set in particular by clocking the power converter, wherein a "half" DC voltage is set at each of the DC terminals. Due to the given earth reference of the intermediate circuit center point, this corresponds to a positive voltage at one of the DC terminals, e.g. the first DC terminal, and an equally large negative voltage at the other of the DC terminals, e.g.the second DC connection, each relative to the earth potential, taking into account the given potential of the intermediate circuit center relative to earth potential.
[0018] The power converter can comprise an AC / DC converter, which can be operated in particular as a three-phase AC / DC converter and is configured to transfer electrical power from the AC electrical network connectable to the AC terminals of the power converter to the DC terminals. The power converter can be connected to the AC network via an AC isolating point, wherein the connection is initially established in particular via precharging resistors, which are bridged after precharging by AC switches of the AC isolating point.
[0019] The power transfer can be used to adjust the DC voltage at the DC terminals. If the circuit is part of a charger, the transferred electrical power can be used, in particular, to charge the vehicle's battery.
[0020] The insulation measurement may include recording measured values of a first measurement voltage at the first EV terminal and recording measured values of a second measurement voltage at the second EV terminal. The measurement voltages are preferably recorded between the respective EV terminal and ground potential.
[0021] In addition, the insulation measurement can include determining one or more insulation resistance values on the vehicle using the measurement voltages. The switches of the circuit arrangement are configured so that the discharge resistor acts as a voltage divider and can be used to determine the insulation resistance.
[0022] In embodiments, the circuit arrangement is designed and configured to set the DC voltage successively to a first and a second voltage value when there is an existing connection between one of the DC connections and the respective EV connection via a closed DC switch and to determine a first insulation resistance value for that EV connection which is not connected to the respective DC connection during the setting of the first and second voltage values and the recording of the first and second measured values, based on first and second measured values of the measured voltages recorded at the first and second voltage values, taking into account the discharge resistance.
[0023] In addition, the circuit arrangement can be designed and configured to open the DC switch closed for determining the first insulation resistance value and thus to disconnect the connection between the corresponding DC connection and the respective EV connection, to connect the other DC connection to the respective other EV connection by closing another of the DC switches, to set the DC voltage successively to a third and a fourth voltage value and, based on third and fourth measured values of the measured voltages recorded at the third and fourth voltage value, to determine a second insulation resistance value for that EV connection which is not connected to the respective DC connection during the setting of the third and fourth voltage values and the recording of the third and fourth measured values.
[0024] In some embodiments, the power converter is designed in two stages and has a DC-side DC / DC converter. The DC voltage at the DC terminals can be adjusted, in particular, by clocking the DC / DC converter.
[0025] The circuit arrangement can be further designed and configured to detect a hard ground fault using insulation measurement if no DC voltage can be set while there is an existing connection between one of the DC connections and the respective EV connection. In the event of a hard ground fault at this EV connection, for example, a short circuit of a line between the EV connection and the BEV to ground potential, the connection to ground potential has a very low resistance, so that when the corresponding DC connection is connected to the AC voltage grid via the power converter in a clocked manner, large currents immediately flow to ground and no DC voltage can be set. The insulation measurement can then be terminated upon detection of the hard ground fault.
[0026] The circuit arrangement can in particular be designed and configured to charge the high-voltage battery of the vehicle. In such an embodiment, the circuit arrangement is preferably part of a charger or charging station for a battery-electric vehicle. For the charging process, electrical energy is transferred from the AC voltage network to the vehicle's battery via the power converter when the first and second DC switches are closed. By using the circuit arrangement provided for charging the vehicle, a cost-effective insulation measurement can be realized. An optional pre-charging circuit can have the third DC switch. The circuit arrangement can have a pre-charging resistor in series with the third DC switch, so that the series circuit comprising the pre-charging resistor and the third DC switch is arranged in parallel with the second DC switch.The precharging resistor and the third DC switch can form the precharging circuit. It is understood that, as an alternative to a precharging resistor, other components such as fuses or active circuits can be used to limit the precharging current as needed and / or to ensure that currents flowing through the third switch do not exceed a specified limit or are interrupted if a specified limit is exceeded. Preferably, the third switch is closed for precharging and the second is open. For charging, the second switch can be closed and the third open, or it can remain closed.
[0027] In embodiments, the circuit arrangement is designed and configured to determine the insulation resistance value for the EV terminal not connected to the pre-charging resistor by closing the third switch and based on the measured values of the measurement voltages with the third switch closed, taking into account the discharge resistance and the pre-charging resistor. Using the pre-charging resistor for insulation measurement allows for even more reliable insulation measurement. Furthermore, the pre-charging resistor ensures that any ground currents occurring after the third switch is closed are limited in the event of a hard ground fault, regardless of which EV terminal the ground fault occurs at.If, in a subsequent step, the first switch, which is not connected to the pre-charging resistor, is closed, the previous check already ensures that there is no hard earth fault and, accordingly, no high currents occur.
[0028] A method for measuring insulation on a battery-electric vehicle uses an electrical circuit arrangement that can be connected to a high-voltage battery of the vehicle via EV connections. The circuit arrangement has an electrical power converter with AC connections for connection to an electrical alternating voltage network and with DC connections. A first DC connection can be connected to a first EV connection via a first DC switch, and a second DC connection can be connected to a second EV connection via a second DC switch or via a parallel connection comprising the second and a third DC switch, the EV connections being connected to one another via a discharge resistor. The method is carried out automatically, for example, by a higher-level controller that is connected to the vehicle and communicates with it.The method comprises: • Connecting the power converter to the AC network by closing an AC disconnect point.
[0029] • Connect one of the DC terminals to the respective EV terminal by closing one of the DC switches,
[0030] • Setting a DC voltage at the DC terminals and
[0031] • Carry out the insulation measurement on the connected vehicle.
[0032] The procedure may further include:
[0033] • after setting the DC voltage to a first voltage value:
[0034] • Recording first measured values of a first measuring voltage at the first EV terminal and a second measuring voltage at the second EV terminal.
[0035] The first measured values are recorded when a DC voltage with the first voltage value is applied. The first measured values of the first measured voltage are recorded at the first EV terminal, and the first measured values of the second measured voltage are recorded at the second EV terminal.
[0036] The procedure may further include:
[0037] • after recording the first measured values of the measuring voltages:
[0038] • Setting the DC voltage to a second voltage value and
[0039] • Recording of second measured values of the measuring voltages and
[0040] • Determining a first insulation resistance value taking into account the discharge resistance for the EV terminal that is not connected to the respective DC terminal during the setting of the first and second voltage values and the acquisition of the first and second measured values.
[0041] The second measured values are recorded when a DC voltage with a second voltage value is applied. Second measured values of the first measured voltage are recorded at the first EV terminal, and second measured values of the second measured voltage are recorded at the second EV terminal.
[0042] The procedure may further include:
[0043] • Opening the DC switch closed to determine the first insulation resistance value,
[0044] • Connecting the other DC terminal to the respective EV terminal by closing another of the DC switches, • Setting a DC voltage to a third and a fourth voltage value,
[0045] • Recording third and fourth measured values of the measuring voltages at the third and fourth voltage values respectively and
[0046] • Determine a second insulation resistance value taking into account the discharge resistance for the EV terminal that is not connected to the respective DC terminal during the setting of the third and fourth voltage values and the acquisition of the third and fourth measured values.
[0047] The third measured values are recorded when a DC voltage with a third voltage value is applied. Third measured values of the first measured voltage are recorded at the first EV terminal, and third measured values of the second measured voltage are recorded at the second EV terminal.
[0048] The fourth measured values are recorded when a fourth voltage value is applied to a DC voltage. Fourth measured values of the first measured voltage are recorded at the first EV terminal, and fourth measured values of the second measured voltage are recorded at the second EV terminal.
[0049] In one embodiment of the method, a pre-charging resistor is arranged in series with the third DC switch, so that the series circuit comprising the pre-charging resistor and the third DC switch is arranged in parallel with the second DC switch. The insulation resistance value for the EV terminal not connected to the pre-charging resistor is determined by closing the third switch and based on the measured voltages with the third switch closed, taking into account the discharge resistance and the pre-charging resistance. BRIEF DESCRIPTION OF THE FIGURES
[0050] In the following, the application is further explained and described using exemplary embodiments shown in the figures.
[0051] Fig. 1a shows a first embodiment of an electrical circuit arrangement for insulation measurement,
[0052] Fig. 1b shows the first embodiment of the electrical circuit arrangement for insulation measurement with modified self-test circuit,
[0053] Fig. 2 shows a second embodiment of the electrical circuit arrangement for insulation measurement, and Figs. 3 + 4 show exemplary possible voltage curves for possible switch positions.
[0054] In the figures, identical or similar elements are designated by the same reference numerals. Representations in the figures may not be to scale.
[0055] FIGURE DESCRIPTION
[0056] Fig. 1a shows a first embodiment of an electrical circuit arrangement 10 for insulation measurement. Such a circuit arrangement can, for example, be part of a charger for a battery-electric vehicle (EV) and can be used to charge a high-voltage battery 20 of the EV.
[0057] The connection to the vehicle EV is established via a first EV terminal 26 and a second EV terminal 28. In the example shown, the first EV terminal 26 is a positive EV terminal, and the second EV terminal 28 is a negative EV terminal. Additionally, Figure 1a shows two insulation resistances Risol and Riso2 as examples, symbolizing the electrical resistance of the corresponding (here arbitrarily chosen) tapping points to ground potential. An insulation fault would be present if the resistance value of one of the insulation resistances Risol or Riso2 is below a minimum value.
[0058] As an example, for the vehicle EV, a first insulation resistance Risol is shown between a tapping point on the high-voltage battery of the vehicle EV near the negative potential and earth potential PE and a second insulation resistance Riso2 between a tapping point at the positive potential and earth potential PE.
[0059] The battery 20 of the EV vehicle may have many cells connected in series, with a potential insulation fault occurring at any point in this series connection. This is taken into account by the Risol diagram in Figure 1a, as the circuit arrangement and insulation measurement method can also detect insulation faults that occur within the battery.
[0060] A voltmeter V is provided between the first EV terminal 26 and ground potential, and a voltmeter V is provided between the second EV terminal 28 and ground potential. The figure also shows the measuring resistors Rm of the voltmeters V as examples.
[0061] The circuit arrangement 10 is connected to a three-phase AC voltage network G via AC switches ACSW. The AC switches ACSW allow the circuit arrangement to be connected to and disconnected from the AC voltage network G in all phases. The AC voltage network G has three phases L1, L2, L3, and optionally a neutral conductor N. The AC voltage network G has a fixed reference to earth potential, symbolized in Fig. 1a by a protective conductor connection PE.
[0062] The power converter 18 comprises an AC / DC converter 12 (rectifier) with a split intermediate circuit 14. The lower part of Fig. 1a shows two possible topologies for the power converter 18 as examples. The DC output of the AC / DC converter has a first DC terminal 22 and a second DC terminal 24. In the example shown, the first DC terminal 22 is a positive DC terminal, and the second DC terminal 24 is a negative DC terminal.
[0063] A voltmeter V is provided between the first DC terminal 22 and the center point of the split intermediate circuit 14, and a voltmeter V is provided between the second DC terminal 24 and the center point of the split intermediate circuit 14. The measuring resistors of the voltmeters are not shown in the figure.
[0064] The power converter 18 can be connected to and disconnected from the EV terminals 26, 28 via an all-pole disconnection point comprising a first switch SW1 and a second switch SW2. In the example shown, the first DC terminal 22 can be connected to the first EV terminal 26 via the first switch SW1, and the second DC terminal 24 can be connected to the second EV terminal 28 via the second switch SW2.
[0065] A discharge resistor Rdis is arranged between the first EV terminal 26 and the second EV terminal 28, via which any residual charge in the vehicle's EV capacities can be discharged.
[0066] For a method for measuring insulation on the vehicle EV using the circuit arrangement 10, a connection between the power converter 18 and the AC voltage network G is first established by closing the AC switch ACSW, so that the potential of the center point of the intermediate circuit 14 approximately corresponds to the potential of the neutral conductor N of the AC voltage network and thus to ground potential. This is also the case if there is no connection between the intermediate circuit center point and the neutral conductor N of the AC voltage network G.
[0067] The second switch SW2 is then closed and a connection is established between the second DC connection 24 and the second EV connection 28. A DC voltage UDC.22, UDC.24 is then set on the DC side of the power converter 18. The DC voltage UDC.22, UDC.24 is set to a first voltage value by suitable clocking of the AC / DC converter 12. The DC voltage UDC.22, UDC.24 corresponds to a voltage between the first DC connection 22 and the second DC connection 24 with a fixed center point of the intermediate circuit 14. A portion UDC.22 of the DC voltage is set between the first DC connection 22 and the center point of the divided intermediate circuit 14. Another portion UDC.24 of the DC voltage is set between the second DC connection 24 and the center point of the divided intermediate circuit 14.
[0068] If the DC voltage UDC.22, UDC.24 cannot be adjusted despite the appropriate timing of the AC / DC converter 12, a hard ground fault with a very low resistance value between the second EV terminal 28 and ground potential is detected, for example, due to a very small Riso2. Alternatively or additionally, such a hard ground fault can be detected if a current flowing through switch SW2 exceeds a specified limit.
[0069] If the DC voltage UDC.22, UDC.24 can be adjusted, the first measured voltage values UEV.26 and UEV.28 are recorded. The measured voltage UEV.26 is recorded between the first EV terminal 26 and ground potential. The measured voltage UEV.28 is recorded between the second EV terminal 28 and ground potential.
[0070] The DC voltage UDC.22, UDC.24 is then set to a second voltage value by suitable timing of the AC / DC converter 12 and second measurement voltage values UEV.26 and UEV.28 are recorded.
[0071] Then, the second switch SW2 is opened, breaking the connection between the second DC terminal 24 and the second EV terminal 28. After that, the first switch SW1 is closed, establishing a connection between the first DC terminal 22 and the first EV terminal 28.
[0072] The DC voltage UDC.22, UDC.24 is then adjusted to a third voltage value by appropriately timing the AC / DC converter 12. The third voltage value can, for example, correspond to the first voltage value.
[0073] If the DC voltage UDC.22, UDC.24 cannot be adjusted despite the clocking being suitable for this purpose or generates a current that increases above a specified limit, a hard earth fault with a very small resistance value between the first EV terminal 26 and earth potential is detected, which occurs, for example, due to a very small Risol.
[0074] If the DC voltage UDC.22, UDC.24 can be adjusted, third measurement voltage values UEV.26 and UEV.28 are recorded. The DC voltage UDC.22, UDC.24 is then adjusted to a fourth DC voltage UDC.22, UDC.24 by appropriately timing the AC / DC converter 12, and fourth measurement voltage values UEV.26 and UEV.28 are recorded. The fourth voltage value can, for example, correspond to the second voltage value.
[0075] Then the first switch SW1 is opened, thereby breaking the connection between the first DC terminal 22 and the first EV terminal 26.
[0076] In the next step, the insulation resistance Riso2 of the positive potential of the vehicle EV and the insulation resistance Risol of the negative potential of the vehicle EV can be calculated from the determined measurement voltage values using known formulas, see below. Alternatively or additionally, the insulation resistance Riso2 of the positive potential can be calculated after the first and second measurement voltage values UEV.26 and UEV.28 have been recorded. The process can be aborted at this point if the insulation resistance Riso2 already has an impermissibly low value.
[0077] Optionally, a self-test of the insulation measuring system can be performed, particularly before the actual insulation measurement, by connecting one of the DC terminals 22, 24 to ground potential via an optional switch SW4 and an optional self-test resistor Rtest with a known resistance value and performing the procedure described above. During such a self-test, switches SW1 and SW2 are open and switch SW4 is closed, so that the procedure described above should determine an insulation resistance that corresponds to the known resistance value of the self-test resistor Rtest. If this is not the case, i.e., if the insulation measurement in the self-test determines an insulation resistance that deviates significantly from the known self-test resistance Rtest, the procedure is aborted with a corresponding error message.
[0078] Fig. 1b again shows the first embodiment of the electrical circuit arrangement 10 for insulation measurement, wherein the discharge resistance Rdis, in contrast to Fig. 1a, consists of two separate partial resistors Rdisl, Rdis2. The optional self-test resistor Rtest is arranged according to Fig. 1b between the connection point of the partial resistors Rdisl, Rdis2 and ground potential, wherein the connection point of the partial resistors Rdisl, Rdis2 can be connected to ground potential via a switch SW4 and the self-test resistor Rtest. During a self-test, one of the switches SW1 or SW2 and the switch SW4 are closed, so that the method described above should determine an insulation resistance which is composed of the known resistance values of the self-test resistor Rtest and the partial resistors Rdisl, Rdis2. If this is not the case, the method can be aborted with an error message.
[0079] Fig. 2 shows a second embodiment of the electrical circuit arrangement 10 for insulation measurement. This embodiment can also be part of a charger for the battery-electric vehicle (EV) and used to charge the high-voltage battery 20 of the EV.
[0080] The connection to the vehicle EV is made via the first EV connection 26 and the second EV connection 28. As an example, a first insulation resistance Risol and a second insulation resistance Riso2 are shown for the vehicle EV.
[0081] A voltmeter V is provided between the first EV terminal 26 and ground potential, and a voltmeter V is provided between the second EV terminal 28 and ground potential. The figure also shows the measuring resistors Rm of the voltmeters as examples. The circuit arrangement 10 is connected to the three-phase AC voltage network G via AC switch ACSW.
[0082] The power converter 18 includes the AC / DC converter 12 (rectifier) with a split intermediate circuit 14 and an optional DC / DC converter 16. The AC / DC converter 12 can have a topology as shown in Fig. 1a. The DC output of the power converter 18 includes the first DC terminal 22 and the second DC terminal 24.
[0083] A voltmeter V is provided between the first DC terminal 22 and the center point of the split intermediate circuit 14, and a voltmeter V is provided between the second DC terminal 24 and the center point of the split intermediate circuit 14. The measuring resistors of the voltmeters are not shown in the figure.
[0084] The power converter 18 can be connected to and disconnected from the EV terminals 26, 28 via an all-pole isolating point comprising a first switch SW1 and a second switch SW2. A discharge resistor Rdis is arranged between the first EV terminal 26 and the second EV terminal 28, through which any residual charge in the vehicle's EV capacitances can be discharged.
[0085] In addition, the embodiment illustrated in Figure 2 includes an optional precharging circuit comprising a precharging resistor Rpchrg and a third switch SW3 arranged in series therewith. Alternatively or in addition to the precharging resistor Rpchrg, other components such as fuses or active circuits may be arranged in series with the switch SW3 to monitor, control, and / or limit a precharging current and / or any ground current.
[0086] In the illustrated embodiment, the first DC connection 22 can be connected to the first EV connection 26 via the first switch SW1. The second DC connection 24 can be connected to the second EV connection 28 via the second switch SW2 and / or via the third switch SW3. If the second DC connection 24 is connected to the second EV connection 28 via the third switch, the second DC connection 24 is connected to the second EV connection 28 via the pre-charging resistor Rpchrg. A current flowing from the second DC connection 24 to the second EV connection 28, which current can be provided in particular for pre-charging capacitors in the vehicle EV, would therefore flow via the pre-charging resistor Rpchrg. A resistor arranged in series with the switch SW3 in addition to or alternatively to the pre-charging resistor Rpchrg can also monitor, control and / or limit this pre-charging current.
[0087] For the method for measuring insulation on the vehicle EV using the circuit arrangement 10, a connection of the power converter 18—in the example shown, the DC / DC converter 16 of the power converter 18—to the AC voltage network G is first established by closing the AC switch ACSW, so that the potential of the center point of the intermediate circuit 14 approximately corresponds to ground potential. This is also the case if there is no connection between the intermediate circuit center point and the neutral conductor N of the AC network G.
[0088] The third switch SW3 is then closed, and a connection is established between the second DC terminal 24 and the second EV terminal 28 via the pre-charging resistor Rpchrg. Using the pre-charging resistor Rpchrg has the advantage that in the event of a possible hard ground fault, i.e., a very small resistance value between the second EV terminal 28 and ground potential, the flowing current is limited by the pre-charging resistor Rpchrg. It is also advantageous to start the method with the closing of switch SW3, since excessive currents are limited by the pre-charging resistor Rpchrg. Optionally, the measurement for this embodiment can also be performed using the second switch SW2 instead of the third switch SW3.
[0089] The DC / DC converter 16 then sets a DC voltage UDC.22, UDC.24 on the DC side of the power converter 18. The DC voltage UDC.22, UDC.24 is set to the first voltage value by suitable clocking of the DC / DC converter 16. The DC voltage UDC.22, UDC.24 corresponds to a voltage between the first DC connection 22 and the second DC connection 24 with a fixed center point of the intermediate circuit 14. A portion UDC.22 of the DC voltage is set between the first DC connection 22 and the center point of the divided intermediate circuit 14 and thus between the first DC connection 22 and ground potential. Another portion UDC.24 of the DC voltage is set between the second DC connection 24 and the center point of the divided intermediate circuit 14 and thus between the second DC connection 24 and ground potential.
[0090] If the desired DC voltage UDC.22, UDC.24 cannot be achieved despite the clocking being suitable for this purpose, a hard earth fault with a very small Risol and / or very small Riso2 is detected and the process is aborted if necessary.
[0091] With the DC voltage UDC.22, UDC.24 set to the first voltage value, the first measured voltage values UEV.26 and UEV.28 are recorded. The measured voltage UEV.26 is recorded between the first EV terminal 26 and ground potential. The measured voltage UEV.28 is recorded between the second EV terminal 28 and ground potential.
[0092] The DC voltage UDC.22, UDC.24 is then set to the second DC voltage UDC.22, UDC.24 by suitable clocking of the DC / DC converter 16 and second measurement voltage values UEV.26 and UEV.28 are recorded.
[0093] Then, the third switch SW3 is opened, thereby breaking the connection between the second DC terminal 24 and the second EV terminal 28. At this point, the insulation resistance Riso2 can be determined from the measured voltage values, and the process can be aborted if the insulation resistance Riso2 has an impermissibly low value.
[0094] The first switch SW1 is then closed and a connection is established between the first DC terminal 22 and the first EV terminal 28.
[0095] The DC voltage UDC.22, UDC.24 is then set to the third voltage value by appropriately timing the DC / DC converter 16. The third voltage value can, for example, correspond to the first voltage value.
[0096] If the DC voltage UDC.22, UDC.24 cannot be adjusted despite the clocking being suitable for this purpose, a hard earth fault with a very small resistance value between the first EV terminal 26 and earth potential is detected.
[0097] If the third voltage value can be set, third measurement voltage values UEV.26 and UEV.28 are recorded. The DC voltage UDC.22, UDC.24 is then set to the fourth DC voltage UDC.22, UDC.24 by appropriately timing the DC / DC converter 16, and fourth measurement voltage values UEV.26 and UEV.28 are recorded. The fourth voltage value can, for example, correspond to the second voltage value.
[0098] Then, optionally, the first switch SW1 is opened, thereby breaking the connection between the first DC terminal 22 and the first EV terminal 26.
[0099] In a next step, the equivalent source voltage Viso2 and the insulation resistance Riso2 of the positive potential of the vehicle EV as well as the equivalent source voltage Visol and the insulation resistance Risol of the negative potential of the vehicle EV can be calculated from the measured voltage values.
[0100] The calculation is based on the fact that the discharge resistor and, if applicable, the pre-charge resistor act as a voltage divider with the insulation resistance. The calculation can be performed in a similar and, given knowledge of the components involved, conventional manner, as described in relation to Figure 4.
[0101] Optionally, a self-test can be carried out, particularly before the actual insulation measurement, by connecting one of the DC terminals 22, 24 to earth potential via an optional switch SW4 and an optional self-test resistor Rtest with a known resistance value and carrying out the procedure described above. During such a self-test, the switches SW1, SW2 and SW3 are open and the switch SW4 is closed, so that the procedure described above should determine an insulation resistance that corresponds to the self-test resistance Rtest. If this is not the case, i.e. if the insulation measurement in the self-test determines an insulation resistance that deviates significantly from the known resistance value of the self-test resistor Rtest, the procedure is aborted with a corresponding error message.
[0102] Figures 3 and 4 show exemplary voltage curves for the DC voltage UDC.22, UDC.24 and the measurement voltages UEV.26 and UEV.28. Figures 3 and 4 also show the switch positions for switches SW1, SW2, and SW3, as they can be used for the insulation measurement method described above.
[0103] Figure 3 shows example voltage curves for a system comprising circuit arrangement 10, vehicle EV and AC network G, in which the insulation on the vehicle EV is faulty, i.e. has a very high resistance throughout. Figure 4 shows example voltage curves for a system comprising circuit arrangement 10, vehicle EV and AC network G, in which the insulation on the vehicle EV is faulty, i.e. has a comparatively low resistance. In Figures 3 and 4, the upper part shows voltage curves at the first, positive DC connection 22 and the measurement voltage UEV.26 at the first, positive EV connection. The middle part shows voltage curves at the second, negative DC connection 24 and the measurement voltage UEV.28 at the second, negative EV connection. The lower part shows switch positions for the first switch SW1 and the second switch SW2 for the first exemplary embodiment of Figures 1a and 1b, respectively.The lower section also shows the switch position for the third switch SW3 for the second embodiment of Figure 2. A value of "1" corresponds to a closed switch. A value of "0" corresponds to an open switch.
[0104] Figure 3 shows that first the second switch SW2 or the third switch SW3 is closed and then the DC voltage is set to the first voltage value for a time period T1. The DC voltage is applied between the DC terminals 22 and 24, with half the DC voltage being applied to each of the two DC terminals 22, 24 - in the positive direction at the positive DC terminal 22 and in the negative direction at the negative DC terminal 24. Afterwards - still with the second switch SW2 or third switch SW3 closed - the second voltage value is set as the DC voltage UDC.22, LIDC24 for a time period T2. It can be seen that the measured voltages LIEV26 and LIEV28 follow the voltage on the negative DC terminal 24 quite closely. This is because when the second or third switch SW2 or SW3 is closedSW3 and intact insulation, the two EV terminals 26, 28 are connected to the potential of the second, negative DC terminal 24 via the discharge resistance Rdis. Since the discharge resistance Rdis is small compared to the insulation resistance of the vehicle EV with intact insulation, the positive measurement voltage UEV.26 also follows the voltage UDC.24 applied to the negative DC terminal 24 quite closely.
[0105] After that, the DC voltage adjustment is stopped and the second or third switch SW2 or SW3 is opened.
[0106] In the following measuring cycle, the first switch SW1 is closed and the DC voltage is then set to the third voltage value for a time period T3. The DC voltage is applied between the DC terminals 22 and 24, with half the DC voltage being applied to each of the two DC terminals 22, 24 - in the positive direction at the positive DC terminal 22 and in the negative direction at the negative DC terminal 24. Afterwards - still with the first switch SW1 closed - the fourth voltage value is set as the DC voltage for a time period T4. It can be seen that the measured voltages UEV26 and UEV28 follow the voltage on the positive DC terminal 22 quite closely. This is because when the first switch SW1 is closed and the insulation is intact, the two EV terminals 26, 28 are connected to the potential of the first, positive DC terminal 22 via the discharge resistor Rdis.Since the discharge resistance Rdis is small compared to the insulation resistance of the vehicle EV with intact insulation, the negative measuring voltage UEV.28 also follows quite closely the voltage UDC.22 applied to the positive DC terminal 22.
[0107] The DC voltage adjustment is then stopped and the first switch SW1 is opened. The insulation resistances Risol and Riso2 can then be calculated using the recorded measured values.
[0108] Figure 4 shows voltage waveforms for an exemplary fault scenario. In the example shown, a finite resistance insulation with a resistance value of 166 kiloohms is present between the negative conductor at EV terminal 28 and ground potential (see Figures 1a, 1b, or 2), so the insulation resistance at the negative potential of the vehicle's EV is faulty, i.e., too low. The resistance value of the discharge resistor Rdis is also 166 kiloohms in this example.
[0109] First, the second switch SW2 (embodiment according to Fig. 1a, 1b) or the third switch SW3 (embodiment according to Fig. 2) is closed and then the DC voltage is set to the first voltage value for a time period T1. After that - still with the second switch SW2 or third switch SW3 closed - the second voltage value is set as the DC voltage for a time period T2. It can be seen that the measured voltages UEV.26 and UEV.28 follow the voltage on the negative DC terminal 24 quite closely. This is because when the second or third switch SW2 or SW3 is closed and the insulation is intact, at least with regard to the positive EV terminal 26, the EV terminals 26, 28 are connected to the potential of the second, negative DC terminal 24 via the discharge resistor Rdis.Since the discharge resistance Rdis is small compared to the insulation resistance of the vehicle EV relative to the positive EV terminal 26, the positive measurement voltage UEV.26 also follows quite closely the voltage UDC.24 applied to the negative DC terminal 24.
[0110] After that, the DC voltage adjustment is stopped and the second or third switch SW2 or SW3 is opened.
[0111] In the following measuring cycle, the first switch SW1 is then closed and the DC voltage is set to the third voltage value for a time period T3. The DC voltage is applied between the DC terminals 22 and 24, with half the DC voltage being applied to each of the two DC terminals 22, 24 - in the positive direction at the positive DC terminal 22 and in the negative direction at the negative DC terminal 24. It can be seen that the recorded measuring voltage UEV.26 follows the applied DC voltage UDC.22 exactly because the first switch SW1 is closed, while the recorded measuring voltage UEV.28 follows the applied DC voltage UDC.22 much more weakly than in Figure 3. This is because the insulation resistance Risol is in series with the discharge resistor Rdis when the first switch SW1 is closed and an earth current flows through this series connection. The applied DC voltage UDC.22 also drops across this series circuit, which thus forms a voltage divider and thus lowers the potential of the positive conductor and the connected first, positive EV terminal 26. In the example shown in Fig. 4, the insulation resistance Risol is approximately equal to the discharge resistance Rdis. Therefore, the negative measurement voltage UEV.28, which is tapped in the middle of the series circuit of discharge resistance Rdis and insulation resistance Risol, has approximately half the value of the DC voltage UDC.22 applied to the positive DC terminal 22.
[0112] Afterwards, still with the first switch SW1 closed, the fourth voltage value is set as the DC voltage for a period of time T4. It can be seen again that, due to the insulation fault, the measured voltage UEV28 follows the voltage on the positive DC terminal 22 significantly less than in Figure 3, and is only approximately half the applied DC voltage UDC22.
[0113] Once the error is detected, the measurement can be stopped and the first switch SW1 opened. In the example sequence shown in Fig. 4, the DC / DC converter is deactivated simultaneously with the opening of switch SW1, and the voltages UEV.26 and UEV.28 are rapidly reduced via the discharge resistor Rdis. The voltages UDC.22 and UDC.24, in contrast, decrease only slowly, especially if the power converter 18 itself does not have an internal discharge resistor.
[0114] This can be followed by the calculation of the insulation resistances Risol and Riso2 using the recorded measured values. Without considering the measuring resistance Rm, an equivalent source voltage Viso2 and the insulation resistance Riso2 of the positive potential of the vehicle EV can be calculated as follows:
[0115] Viso2 = (UEV.28(T1) * UEV.26(T2) - UEV.28(T2) * UEV.26(T1)) / (UEV.28(T1) - UEV.28(T2) - UEV.26(T1) + UEV.16(T2))
[0116] Riso2 = Rdis * (UEV.26(T1) - UEV.26(T2)) / (UEV.28(T1) - UEV.28(T2) - UEV.26(T1) + UEV.16(T2)) with T1 : period with first voltage value for UDC.22 + UDC.24 and with T2: period with second voltage value for UDC.22 + UDC.24.
[0117] Without taking into account the measuring resistance Rm, an equivalent source voltage Visol and the insulation resistance Risol of the negative potential of the vehicle EV can be calculated as follows:
[0118] Viso1 = (UEV.28(T3) * UEV.26(T4) - UEV.28(T4) * UEV.26(T3)) / (UEV.28(T3) - UEV.28(T4)
[0119] - UEV.26(T3) + UEV.16(T4))
[0120] Risol = Rdis * (UEV.28(T3) - UEV.28(T4)) / (UEV.28(T3) - UEV.28(T4) - UEV.26(T3) + UEV.16(T4)) with T3: period with third voltage value for UDC.22 + UDC.24 (where the third voltage value can correspond to the first voltage value) and with T4: period with fourth voltage value for UDC.22 + UDC.24 (where the fourth voltage value can correspond to the second voltage value).
[0121] Specifically, the following values result from the measurement according to Fig. 4:
[0122] The first measurement in the second room T1 is performed with UDC.24 = 22.5V and results in the measured values UEV.28 = 21.5V and UEV.26 = -20.2V. The second measurement in the period T2 is performed with UDC.24 = 202.5V and results in the measured values UEV.28 = 201.5V and UEV.26 = -189.2V. From this, an insulation resistance Riso2 = 34.9 Gohm is calculated with an equivalent source voltage Uiso2 = 0V.
[0123] The third measurement in period T3 is taken with UDC.22 = 22.5V and results in the measured values UEV.26 = 22.5V and UEV.28 = -10.9V. The fourth measurement in period T4 is taken with UDC.22 = 202.5V and results in the measured values UEV.26 = 202.5V and UEV.28 = -98.2V. This results in an insulation resistance of Risol = 166 kOhm with an equivalent source voltage of Uisol = 0V.
[0124] The value of the equivalent source voltages Visol, Viso2 corresponds to the voltage of a voltage source connected in series with the respective insulation resistance Risol, Riso2. In the example of Fig. 1a, 1b, and Fig. 2, the equivalent source voltage Viso2 = 0V, since the insulation resistance Riso2 is directly connected to the EV terminal 26, and the equivalent source voltage Visol corresponds to the voltage of a battery cell of the vehicle's battery EV. Therefore, during periods T1 and T2, the first, positive EV terminal 26 is tested, and during periods T3 and T4, the second, negative EV terminal 28 is tested. It is understood that the order of switching operations of switches SW1, SW2 and the associated measurements can also be reversed, so that the insulation resistance of the negative EV terminal 28 can be tested first and then the insulation resistance of the positive EV terminal 26.
[0125] The described method can, in particular, be part of a higher-level method for charging a battery 20 of a battery-electric vehicle (EV). The higher-level method can, in particular, comprise the following steps:
[0126] Connecting the vehicle EV to the circuit arrangement 10, in particular by plugging a charging cable into the EV connections 26, 28 of the circuit arrangement and / or into a charging connection of the vehicle EV,
[0127] Communication between the vehicle EV and the circuit arrangement 10, in particular with exchange of suitable charging parameters,
[0128] Locking the connection between the vehicle EV and the circuit arrangement 10, in particular by locking a plug connection between the charging cable and the EV terminals 26, 28 of the circuit arrangement 10 and / or between the charging cable and the charging connection of the vehicle EV,
[0129] Insulation measurement on the vehicle EV in one of the variants described above, if necessary with prior self-test of the insulation measurement on the optional self-test resistor Rtest by closing the optional switch SW4,
[0130] Precharging the entire system, in particular via the closed switch SW3 and the precharging resistor Rpchrg, and
[0131] Transfer of electrical power from the AC supply grid G via the power converter 18, the closed switches SW1 and SW2, the EV terminals 26,28 and the charging cable to the battery 20 of the vehicle EV.
[0132] LIST OF REFERENCE SYMBOLS
[0133] 10 Electrical circuit arrangement
[0134] 12 AC / DC converters
[0135] 14 intermediate circuit
[0136] 16 DC / DC converters
[0137] 18 power converters
[0138] 20 high-voltage battery
[0139] 22, 24 DC connectors
[0140] 26, 28 EV connectors
[0141] ACSW AC switch
[0142] EV battery electric vehicle
[0143] G AC supply network
[0144] Rdis discharge resistance
[0145] Risol, Riso2 insulation resistance
[0146] Rm measuring resistance
[0147] Rpchrg Precharge resistor Rtest Self-test resistor
[0148] SW1, SW2, SW3, SW4 switches
[0149] T1, T2, T3, T4 periods
[0150] UDC.22, UDC.24 DC voltage
[0151] UEV.26, UEV.28 measuring voltage
Claims
PATENT CLAIMS 1. Electrical circuit arrangement (10) for insulation measurement on a battery-electric vehicle (EV), which can be connected to a high-voltage battery (20) of the vehicle (EV) via EV connections (26, 28), wherein the circuit arrangement (10) has an electrical power converter (18) with AC connections for connection to an electrical alternating voltage network (G) and with DC connections (22, 24), wherein a first DC connection (22) can be connected to a first EV connection (26) via a first DC switch (SW1) and a second DC connection (24) can be connected to a second EV connection (28) via a second DC switch (SW2) or via a parallel connection of the second and a third DC switch (SW2, SW3), wherein the circuit arrangement (10) is designed and configured, when the power converter (18) is connected to the alternating voltage network (G), to one of the DC terminals (22,24) with the respective EV terminal (26,28) by closing one of the DC switches (SW1,SW2, SW3), to set a DC voltage (UDC.22, UDC.24) at the DC terminals (22, 24) by clocking the power converter (18) and to carry out the insulation measurement on the connected vehicle (EV).
2. Circuit arrangement (10) according to claim 1, wherein the EV terminals (26, 28) are connected to one another via a discharge resistor (Rdis), and wherein the circuit arrangement (10) is further designed and configured to carry out the insulation measurement using the discharge resistor (Rdis).
3. Circuit arrangement (10) according to claim 1 or 2, wherein the power converter (18) has a split intermediate circuit (14) which is connected to the DC terminals (22, 24), wherein the potential of the center point of the intermediate circuit (14) has a given reference to the earth potential when the power converter (18) is connected to the AC voltage network (G).
4. Circuit arrangement according to one of the preceding claims, wherein the power converter (18) has an AC / DC converter (12) which can be operated in particular as a three-phase AC / DC converter and is designed to transfer electrical power from the electrical AC voltage network (G) connectable to the AC terminals of the power converter (18) to the DC terminals (22, 24).
5. Circuit arrangement according to one of the preceding claims, wherein the insulation measurement comprises detecting measured values of a first measuring voltage (UEV.26) at the first EV terminal (26) and a detection of measured values of a second measuring voltage (UEV.28) at the second EV terminal (28).
6. Circuit arrangement according to one of the preceding claims, wherein the insulation measurement comprises the determination of insulation resistance values (Risol, Riso2) on the vehicle (EV) using the measurement voltages (UEV.26, UEV.28).
7. Circuit arrangement according to one of the preceding claims, further designed and configured, when there is an existing connection between one of the DC connections (22, 24) and the respective EV connection (26, 28) via a closed DC switch (SW1, SW2, SW3), to set the DC voltage (UDC.22, UDC.24) successively to a first and a second voltage value and, based on first and second measured values of the measured voltages (UEV.26, UEV.28) recorded at the first and second voltage value, taking into account the discharge resistance (Rdis), to determine a first insulation resistance value (Risol, Riso2) for that EV connection (26, 28) which is not connected to the respective DC connection (22, 24) during the setting of the first and second voltage values and the recording of the first and second measured values.
8. Circuit arrangement according to claim 7, further designed and configured to open the DC switch (SW1, SW2, SW3) closed to determine the first insulation resistance value (Risol, Riso2) and thus to separate the connection between the corresponding DC connection (22, 24) and the respective EV connection (26, 28), to connect the other DC connection (22, 24) to the respective other EV connection (26, 28) by closing another one of the DC switches (SW1, SW2, SW3), to set the DC voltage (UDC.22, UDC.24) successively to a third and to a fourth voltage value and to determine the third and fourth measured values of the measured voltages (UEV) recorded at the third and fourth voltage value.26, UEV28) taking into account the discharge resistance (Rdis) to determine a second insulation resistance value (Risol , Riso2) for that EV terminal (26, 28) which is not connected to the respective DC terminal (22, 24) during the setting of the third and fourth voltage values and the recording of the third and fourth measured values.
9. Circuit arrangement according to one of the preceding claims, wherein the power converter (18) is designed in two stages and has a DC-side DC / DC converter (16), wherein the DC voltage (UDC.22, UDC.24) at the DC terminals (22, 24) is adjustable in particular by clocking the DC / DC converter (16).
10. Circuit arrangement according to one of the preceding claims, further designed and configured to detect a hard earth fault by means of the insulation measurement if no DC voltage (UDC.22, UDC.24) can be set and / or if an earth current above a specified limit value is detected when setting the DC voltage (UDC.22, UDC.24).
11. Circuit arrangement according to one of the preceding claims, further designed and configured to charge the high-voltage battery (20) of the vehicle (EV).
12. Circuit arrangement (10) according to one of the preceding claims, characterized in that a pre-charging resistor (Rprchrg) is arranged in series with the third DC switch (SW3), so that the series circuit comprising the pre-charging resistor (Rprchrg) and the third DC switch (SW3) is arranged in parallel with the second DC switch (SW2).
13. Circuit arrangement (10) according to claim 12, characterized in that the precharging resistor (Rprchrg) and the third DC switch (SW3) form a precharging circuit.
14. Circuit arrangement (10) according to claim 12 or 13, further designed and configured to determine the insulation resistance value (Risol, Riso2) for that EV terminal (26, 28) which is not connected to the pre-charging resistor (Rprchrg) by closing the third switch (SW3) and based on the measured values of the measuring voltages (UEV.26, UEV.28) with the third switch (SW3) closed, taking into account the discharge resistance (Rdis) and the pre-charging resistor (Rprchrg).
15. A method for measuring insulation on a battery-electric vehicle (EV) by means of an electrical circuit arrangement (10) which can be connected to a high-voltage battery (20) of the vehicle (EV) via EV connections (26, 28), wherein the circuit arrangement (10) has an electrical power converter (18) with AC connections for connection to an electrical alternating voltage network (G) and with DC connections (22, 24), wherein a first DC connection (22) can be connected to a first EV connection (26) via a first DC switch (SW1) and a second DC connection (24) can be connected to a second EV connection (28) via a second DC switch (SW2) or via a parallel connection of the second and a third DC switch (SW2, SW3), wherein the EV connections (26, 28) are connected to one another via a discharge resistor (Rdis), comprising: Connecting the power converter (18) to the AC voltage network (G), connecting one of the DC connections (22, 24) to the respective EV connection (26, 28) by closing one of the DC switches (SW1, SW2, SW3), Setting a DC voltage (UDC.22, UDC.24) at the DC terminals (22, 24) and performing the insulation measurement on the connected vehicle (EV).
16. The method according to claim 15, further comprising: after setting the DC voltage (UDC.22, UDC.24) to a first voltage value: Detecting first measured values of a first measuring voltage (UEV.26) at the first EV terminal (26) and a second measuring voltage (UEV.28) at the second EV terminal (28).
17. The method according to claim 16, further comprising: after detecting the first measured values of the measurement voltages (UEV.26, UEV.28), setting the DC voltage (UDC.22, UDC.24) to a second voltage value and detecting second measured values of the measurement voltages (UEV.26, UEV.28) and determining a first insulation resistance value (Risol, Riso2) taking into account the discharge resistance (Rdis) for that EV terminal (26, 28) which is not connected to the respective DC terminal (22, 24) during the setting of the first and second voltage values and the detection of the first and second measured values.
18. The method of claim 17, further comprising: Opening the DC switch (SW1, SW2, SW3) closed to determine the first insulation resistance value (Risol, Riso2), Connecting the other DC terminal (22, 24) to the respective EV terminal (26, 28) by closing another of the DC switches (SW1, SW2, SW3), setting a DC voltage (UDC.22, UDC.24) to a third and a fourth voltage value, Recording third and fourth measured values of the measuring voltages (UEV.26, UEV.28) at the third and fourth voltage values respectively and Determining a second insulation resistance value (Risol , Riso2) taking into account the discharge resistance (Rdis) for that EV terminal (26, 28) which is not connected to the respective DC terminal (22, 24) during the setting of the third and fourth voltage values and the acquisition of the third and fourth measured values.
19. The method according to any one of claims 15 to 18, wherein a pre-charging resistor (Rprchrg) is arranged in series with the third DC switch (SW3) so that the series circuit comprising the pre-charging resistor (Rprchrg) and the third DC switch (SW3) is connected in parallel with the second DC switch (SW2) is arranged, wherein the insulation resistance value (Risol, Riso2) for that EV terminal (26, 28) which is not connected to the pre-charging resistor (Rprchrg) is determined by closing the third switch (SW3) and based on the measured values of the measuring voltages (UEV.26, UEV.28) with the third switch (SW3) closed, taking into account the discharge resistance (Rdis) and the pre-charging resistor (Rprchrg).