Electric drive system and method for operating same

Abstract

The invention relates to an electric drive system (1) for a vehicle, comprising two electric machines (3.1, 3.2), each electric machine having three stator windings (L1 to L6); a high-voltage battery (2); and two inverters (4.1, 4.2), each of which is designed to supply one of the electric machines (3.1, 3.2), wherein, in a boost mode, one of the inverters (4.1, 4.2) is designed as a boost stage for a negative high-voltage potential (HV-), and the other inverter (4.1, 4.2) can be operated as a boost stage for a positive high-voltage potential (HV-) for charging the high-voltage battery (2). Each inverter (4.1, 4.2) is designed as a three-level T-type inverter and has three half-bridges, each of which consists of a high-side switch (HS1 to HS6), designed as a semiconductor switch, and a low-side switch (LS1 to LS6), designed as a semiconductor switch, the center taps (M1 to M6) thereof being connected, in each case, to one of the stator windings (L1 to L6) and to a bidirectional switch (BDS1 to BDS6) composed of two semiconductor switches connected anti-serially to one another. A series circuit consisting of a first capacitor (C1, C3) and a second capacitor (C2, C4) is connected between a positive potential (P1+, P2+) and a negative potential (P1-, P2-) of each inverter (4.1, 4.2), the center tap (M) of said series circuit being connected to the bidirectional switches (BDS1 to BDS6) of the respective inverter (4.1, 4.2).

Description

[0001] Electric drive system and method for its operation

[0002] The invention relates to an electric drive system according to the preamble of claim 1 and a method for its operation according to the preamble of claim 6.

[0003] When DC charging an electric vehicle at a DC charging station, it is necessary that the DC charging station covers the vehicle's maximum voltage range. If this is not the case, charging will not be possible at all or will only be possible up to the maximum voltage level of the DC charging station. DC charging can be achieved via a DC boost converter installed in the vehicle or via a switchable HV battery (switching between series and parallel battery strings or alternating charging of battery strings).

[0004] In addition to charging, another goal is to discharge the vehicle battery into the DC charging station via buck mode. This is known as "bidirectional DC charging."

[0005] In the event of an insulation fault in the vehicle, a further insulation fault can occur as a direct consequence in the opposite high-voltage potential on the DC charging station side. To protect the insulation, some DC charging station manufacturers install a varistor between the PA (potential equalization) and the positive high-voltage potential HV+ or between the PA and the negative high-voltage potential HV-. The varistor has a terminal voltage of 500 V to 550 V. If the varistor trips or the insulation is destroyed, a short circuit is created in the high-voltage battery, which, in the Chademo charging standard, will lead to the destruction of the PE (protective conductor) line in the charging cable (the line is very thin). The limit values ​​​​are now defined: 100 mAs to protect the varistor and 7000 As2 to protect the PE cable.

[0006] Known solutions for protecting the cable in the state of the art include: Using a diode in the reverse direction of the expected short-circuit current. The disadvantage of this is that bidirectional DC charging capability is not possible. The diode is conductive as long as the DC booster is driving a charging current. The short-circuit current corresponds exactly to the value of the charging current. If the DC / DC converter is not deactivated in time, the varistor in the DC charging station and / or one of the PE conductors in the charging cable will be destroyed. Using a MOSFET with the body diode or an IGBT with a freewheeling diode in the reverse direction of the expected short-circuit current. The disadvantage of this is that delayed detection of the short circuit leads to very high short-circuit currents that can destroy the PE conductor in the charging cable or parts of the boost converter, the DC charging station, or the vehicle. There is also a risk of fire.

[0007] Use of a PyroFuse (fuse with explosive element). The disadvantage of this is that delayed detection of the short-circuit current leads to excessive demands on the explosive element to interrupt the short circuit. Delayed detection of the short circuit leads to very high short-circuit currents, which can destroy the PE line in the charging cable or parts of the boost converter, the DC charging station, or the vehicle. Furthermore, there is a risk of fire.

[0008] A choke at both HV potentials with a shutdown element (e.g., MOSFET, IGBT) and, if possible, a freewheeling path for the choke current. This is implemented, for example, in quasi-isolated DC / DC converters. The disadvantage is the additional cost of a standalone DC / DC converter.

[0009] DE 102022 002 607 B3 describes a vehicle with an electrical circuit arrangement, wherein the electrical circuit arrangement comprises:

[0010] - a traction battery,

[0011] - an electrical connection for electrical coupling with an external electrical unit,

[0012] - a first high-voltage potential, and

[0013] - a second high-voltage potential, wherein the first high-voltage potential is a positive high-voltage potential and the second high-voltage potential is a negative high-voltage potential or vice versa. In this case, the circuit arrangement

[0014] - two electric drive units, each with an inverter and an electrically coupled three-phase electric machine to drive the vehicle, and

[0015] - is designed such that the two electric drive units can be electrically connected in series between the electrical connection and the traction battery in such a way that the first electric drive unit increases the first high-voltage potential and the second electric drive unit increases the second high-voltage potential from the electrical connection to the traction battery. It is provided that the inverters each have a capacitor between a potential line of the first high-voltage potential and a potential line of the second high-voltage potential, wherein the circuit arrangement is designed such that the second electric drive unit can be electrically connected in series with the capacitor connection of the first electric drive unit.

[0016] DE 102019200996 A1 discloses an electrical circuit arrangement and a method for operating such a circuit arrangement, which comprises one or two three-phase electrical machines and a first and second inverter. Each inverter is connected to the electrical machine via three phase connections and is individually connected to a first and second DC voltage source. Via a switch unit between the two inverters, both the phases of the DC voltage sources and one of the three phases of the inverter can be switchably connected between the two inverters.

[0017] The invention is based on the object of providing a novel electric drive system for a vehicle and a novel method for its operation.

[0018] The object is achieved according to the invention by an electric drive system having the features of claim 1 and a method for its operation having the features of claim 6.

[0019] Advantageous embodiments of the invention are the subject of the subclaims.

[0020] An electric drive system for a vehicle is proposed, comprising at least two electric machines, each with three stator windings for driving the vehicle, at least one high-voltage battery and two inverters for converting a direct voltage of the high-voltage battery into an alternating voltage for supplying one of the electric machines each, wherein the inverters can be connected in series with one another in a boost mode and can be controlled and / or regulated such that one of the inverters with the electric machine connected thereto can be operated as a boost stage for a negative high-voltage potential and the other inverter with the electric machine connected thereto can be operated as a boost stage for a positive high-voltage potential for charging the high-voltage battery.According to the invention, the inverters are each designed as a three-level T-type inverter and each have three half-bridges, each formed from a high-side switch designed as a semiconductor switch and a low-side switch designed as a semiconductor switch, to the center taps of which one of the stator windings and a bidirectional switch made up of two semiconductor switches connected anti-serially to one another is connected, wherein between a positive potential and a negative potential of each inverter, between which the half-bridges are arranged, a series circuit comprising a first capacitor and a second capacitor is connected, to the center tap of which the bidirectional switches of the respective inverter are connected.

[0021] In one embodiment, the inverters in boost mode can be controlled and / or regulated in such a way that both high-voltage potentials can be adjusted independently of each other with respect to a potential equalization conductor.

[0022] In one embodiment, the positive high-voltage potential of the high-voltage battery is connected to the positive potential of the first inverter and the negative high-voltage potential of the high-voltage battery is connected to the negative potential of the first inverter, wherein one of the high-voltage potentials of the high-voltage battery is connected to the homopolar potential of the second inverter, wherein the other high-voltage potential of the high-voltage battery is switchably connected to the homopolar high-voltage potential of the second inverter via a first contactor, wherein the first contactor is closed in a charging mode and open in a charging mode, in particular the boost mode.

[0023] In one embodiment, a DC charging connection is provided for connection to a DC charging station, comprising a first positive contact and a first negative contact for charging with a higher voltage (for example the nominal voltage of the high-voltage battery, in particular 800 V) and a second positive contact and a second negative contact for charging with a lower voltage (for example less than the nominal voltage of the high-voltage battery, in particular less than 500 V), wherein the first positive contact is connected to the positive high-voltage potential of the high-voltage battery, wherein the first negative contact is connected to the negative high-voltage potential of the high-voltage battery, wherein the second positive contact is connected to the center tap between the capacitors of the first inverter, wherein the second negative contact is connected to the center tap between the capacitors of the second inverter.

[0024] In one embodiment, the contacts are connected to the inverters and the high-voltage battery via respective contactors. In one embodiment, the semiconductor switches are designed as MOSFETs or IGBTs with a freewheeling diode.

[0025] In one embodiment, the inverter has current measuring devices for measuring the alternating current between the center taps of the half-bridges and the stator windings.

[0026] The phases of the three-phase electric machine can, for example, be connected to each other via a common star point

[0027] According to one aspect of the present invention, a method for operating the above-described electric drive system in boost mode is proposed. According to the invention, in one of the inverters, the low-side switch of one of the half-bridges is operated in a cyclic manner, while the bidirectional switches of the two other half-bridges are permanently closed in the forward direction toward the center tap of the respective half-bridge by controlling the corresponding semiconductor switch for the duration of boost mode. In the other inverter, the high-side switch of one of the half-bridges is operated in a cyclic manner, while the bidirectional switches of the two other half-bridges are permanently closed in the forward direction toward the center tap between the capacitors by controlling the corresponding semiconductor switch for the duration of boost mode.

[0028] In one embodiment, the remaining semiconductor switches of the inverter are or remain open or can be closed to optimize efficiency with a conductive body diode or freewheeling diode.

[0029] In one embodiment, to homogenize aging defects after clocking operation of one of the low-side switches and / or high-side switches in boost mode, another of the low-side switches and / or high-side switches is / are used for clocking operation.

[0030] In one embodiment, the clocking of one of the inverters is dependent on a choke current through the stator windings of the connected electrical machine, such that the clocked high-side switch or low-side switch is switched on when the choke current falls below a predetermined lower threshold and switched off when the choke current exceeds a predetermined upper threshold. The clocking of the other inverter can be performed with a fixed clock frequency and a fixed duty cycle.

[0031] In one embodiment, the high-voltage potentials are regulated by regulating a voltage across at least one of the capacitors to a predetermined value.

[0032] It may be advantageous to discontinue boost operation as soon as an insulation fault is detected. This can be done by measuring the high-voltage potential distributions, measuring the insulation resistance, or performing a plausibility check of the currents.

[0033] Using two 3-level T-type inverters and the associated electrical machines, the boost function can be implemented and both high-voltage potentials related to the equipotential bonding conductor and / or the protective conductor can be controlled and / or regulated simultaneously. Each drive (inverter and electrical machine) represents a boost stage, with the first boost stage increasing the first high-voltage potential (e.g., the positive high-voltage potential) and the second boost stage increasing the other high-voltage potential (e.g., the negative high-voltage potential). Both drives are connected in series in such a way that both high-voltage potentials can be adjusted independently of each other relative to the equipotential bonding conductor. This protects the insulation from overload in lower-voltage systems (e.g., a 500 V DC charging station).

[0034] The invention enables the two drives, each consisting of an electric motor and an inverter designed as a T-type 3-level inverter, to be used as a DC booster in addition to their driving function. Therefore, no additional boost converter or switching battery is required. The inventive solution allows for buck and boost operation (bidirectional DC charging) with the continuous power of the drives. The boost function can independently adjust both high-voltage potentials to the equipotential bonding conductor and / or protective conductor, thus preventing insulation overload in the charging station. In the event of an insulation fault in the vehicle, a short-circuit current from the high-voltage battery via the charging station is prevented by blocking semiconductors. In addition, the current rise in the event of an insulation fault is delayed by the two motor main inductances, allowing more time to detect the fault and stop boost operation.The current directions of the individual stator windings represent a normal operating state in the electric machine (the sum of all currents in each electric machine is 0 A). Therefore, the very large main inductance of the electric machine is effective. As a result, current ripple and / or voltage ripple are reduced. There is a possibility of disconnection of a battery short circuit caused by an insulation fault in the vehicle.

[0035] Embodiments of the invention are explained in more detail below with reference to drawings.

[0036] Showing:

[0037] Fig. 1 is a schematic view of an electric drive system for an electrically powered vehicle,

[0038] Fig. 2 is a schematic view of a simulation setup of the electric drive system from Figure 1,

[0039] Fig. 3 is a schematic diagram of signals from the simulation setup of Figure 2, and

[0040] Fig. 4 is a schematic diagram of further signals of the simulation setup from Figure 2.

[0041] Corresponding parts are provided with the same reference numerals in all figures.

[0042] Figure 1 is a schematic view of an electric drive system 1 for an electrically powered vehicle. The electric drive system 1 has at least one electrical energy storage device 2, in particular a high-voltage battery 2, and at least two electric motors 3.1, 3.2, each with three stator windings L1, L2, L3, and L4, L5, L6, which can be supplied with energy from the high-voltage battery 2 to drive the vehicle via an inverter 4.1, 4.2, each embodied as a T-type 3-level inverter.

[0043] The high-voltage battery 2 has a positive high-voltage potential HV+ and a negative high-voltage potential HV-. The vehicle can be an at least partially electrically powered vehicle, such as a hybrid vehicle or electric vehicle, in particular a passenger car, a commercial vehicle, or a bus.

[0044] The electrical machines 3.1, 3.2 for driving the vehicle can each be designed as three-phase electrical machines. In particular, this three-phase electrical machine is an electric motor. In particular, the three-phase electrical machine can be operated in motor mode and thus as an electric motor. In order to operate the three-phase electrical machine in motor mode, the three-phase electrical machine can be supplied with an electrical alternating voltage, in particular with a high-voltage electrical alternating voltage, via its phases. The phases of the three-phase electrical machine can be connected to one another, for example, via a common star point.

[0045] To ensure that the electric motors 3.1, 3.2 can be supplied with an alternating voltage, the electric drive system 1 and thus the vehicle have at least one high-voltage battery 2. Using the high-voltage battery 2, the electric motors 3.1, 3.2 and, if applicable, other vehicle components and / or vehicle systems and / or on-board electrical systems can be supplied with electrical energy.

[0046] A battery voltage can be provided using the high-voltage battery 2. In particular, the vehicle can be a battery-powered vehicle with a voltage level of 800 volts. A voltage of essentially 800 volts can be provided using the battery voltage.

[0047] The electric machines 3.1, 3.2 require an alternating voltage for their operating state. This alternating voltage can be provided by the respective inverter 4.1, 4.2. This involves converting the battery voltage into an alternating voltage. In particular, the alternating voltage for the electric machines 3.1, 3.2 is provided by a respective primary function or main function of the inverter 4.1, 4.2.

[0048] For example, the inverter 4.1, 4.2 can be connected or arranged between the high-voltage battery 2 and the respective electric machine 3.1, 3.2. A series circuit comprising a first capacitor C1, C3 and a second capacitor C2, C4 is connected or arranged between a positive potential P1+, P2+ and a negative potential P1-, P2- of each inverter 4.1, 4.2. With regard to the electrical energy storage device 2, this series circuit is located at the input of the inverter 4.1, 4.2. In particular, the positive potential of the first capacitor C1, C3 is connected to the positive potential P1+, P2+ of the respective

[0049] Inverters 4.1, 4.2 are connected. The negative potential of the first capacitor C1, C3 is connected to the positive potential of the second capacitor C2, C4. Consequently, the negative potential of the second capacitor C2, C4 is connected to the negative potential P1-, P2- of the respective inverter 4.1, 4.2. A center tap M is located between each of the first capacitors C1, C3 and the second capacitor C2, C4.

[0050] For example, the inverters 4.1, 4.2 can each be designed as a three-level T-type inverter. The inverter 4.1, 4.2 can be three half-bridges, each consisting of a high-side switch HS1 to HS6 designed as a semiconductor switch and a low-side switch LS1 to LS6 designed as a semiconductor switch with a

[0051] Center tap M1 to M6. One of the stator windings L1 to L6 is connected to the center tap M1 to M6 of each half-bridge. Furthermore, between the center tap M1 to M6 of each half-bridge and the center tap M, the

[0052] Capacitors C1, C2 or C3, C4 are each connected to a bidirectional switch BDS 1 to BDS 6 consisting of two semiconductor switches connected anti-serially to each other.

[0053] In order to be able to convert an input voltage of a DC charging station 5 for charging the electrical energy storage device 2 by means of the inverters 4.1, 4.2, each inverter 4.1, 4.2 has three switching arrangements for each of the three phases of the respective electrical machine 3.1, 3.2. Each of these switching arrangements can have a plurality of different semiconductors, such as IGBTs or MOSFETs. For example, the capacitors C1, C2 and C3, C4 form an intermediate circuit of the respective inverter 4.1, 4.2 with the respective center tap M. In particular, the inverter 4.1, 4.2 can be configured to charge the first capacitor C1, C3 and / or the second capacitor C2, C4 selectively, in particular cyclically. Thus, for example, an output voltage of the inverter 4.1, 4.2, a sum of a first voltage of the first capacitor C1, C3 and a second voltage of the second capacitor C2, C4 can be generated or provided for charging the electrical energy storage device 2. In particular, the inverter 4.1, 4.2 can charge the first capacitor C1, C3 or the second capacitor C2, C4 with the input voltage, depending on which semiconductor switches of the inverter 4.1, 4.2 are clocked. Thus, with the help of the series connection of C1 and C2 or C3 and C4, an output voltage can be provided which corresponds to the battery voltage. Consequently, the electrical energy storage device 2 can be charged via the capacitors C1, C2 or C3, C4 of the inverters 4.1, 4.2. The inverters 4.1, 4.2 therefore have the secondary function of charging the electrical energy storage device 2 if a charging voltage of 500 V or less can be provided by means of the DC charging station 5.

[0054] Using two inverters 4.1, 4.2 designed as 3-level T-type inverters and the associated electrical machines 3.1, 3.2, the boost function can be implemented and, at the same time, both high-voltage potentials HV+, HV- can be controlled and / or regulated with respect to the equipotential bonding conductor PA and / or the protective conductor PE. Each drive, i.e., each pair of inverters 4.1, 4.2 and the associated electrical machine 3.1, 3.2, represents a boost stage, with the first boost stage increasing the magnitude of one of the high-voltage potentials (e.g., HV+) and the second boost stage increasing the magnitude of the other high-voltage potential (e.g., HV-).Both drives are connected in series so that both high-voltage potentials HV+, HV- can be adjusted independently of each other with respect to the equipotential bonding conductor PA, thus protecting the insulation from overload in a lower-voltage system, for example, a DC charging station 5 with a voltage of 500 V. The advantage here is that no intervention is required on the electrical machine 3.1, 3.2, and the current flow represents a normal operating point, meaning that the entire inductances of the electrical machine 3.1, 3.2 are used, not just the very low stray inductance.

[0055] The order of the boost levels can also be reversed even if the function is the same, i.e. starting from the DC charging station 5, the positive high-voltage potential HV+ can be boosted first and then the negative high-voltage potential HV-.

[0056] For boosting, in one of the inverters 4.1, 4.2, for example in the first inverter 4.1, the low-side switch LS1, LS2, LS3 of one of the half-bridges, in this example the low-side switch LS1, is operated in a clocked manner, while the bidirectional switches BDS2, BDS3 of the two other half-bridges are permanently closed in the forward direction towards the center tap M2, M3 of the respective half-bridge by controlling the corresponding semiconductor switch for the duration of the boost. The other semiconductor switches of the first inverter 4.1 can remain open or, to optimize efficiency, closed with a conductive body diode or freewheeling diode. In the other inverter 4.1, 4.2, for example in the second inverter 4.2, the high-side switch HS4, HS5, HS6 of one of the half-bridges, in this example the high-side switch HS4, is operated in a pulsed manner, while the bidirectional switches BDS5, BDS6 of the other two half-bridges are permanently closed in the forward direction toward the center tap M between the capacitors C3, C4 by controlling the corresponding semiconductor switch for the duration of the boost. The remaining semiconductor switches of the second inverter 4.2 can remain open or be closed to optimize efficiency with a conductive body diode or freewheeling diode.

[0057] Instead of the low-side switch LS1, one of the low-side switches LS2, LS3 of the first inverter 4.1 can also be operated in a clocked mode, while the bidirectional switches BDS1 to BDS3 of the remaining half-bridges are permanently closed for the duration of the boost. Likewise, instead of the high-side switch HS4, one of the high-side switches HS5, HS6 of the second inverter 4.2 can be operated in a clocked mode, while the bidirectional switches BDS4 to BDS6 of the remaining half-bridges are permanently closed for the duration of the boost. Alternating between the clocked low-side switch LS1 to LS3 and / or the high-side switches HS4 to HS6 can be advantageous for homogenizing aging defects.

[0058] The positive high-voltage potential HV+ of the high-voltage battery 2 is connected to the positive high-voltage potential P1+ of the first inverter 4.1. The negative high-voltage potential HV- of the high-voltage battery 2 is connected to the negative high-voltage potential P1- of the first inverter 4.1. One of the high-voltage potentials HV+, HV- of the high-voltage battery 2, for example the negative high-voltage potential HV-, is connected to the same-pole high-voltage potential P2- of the second inverter 4.2. The other high-voltage potential HV+, HV- of the high-voltage battery 2, for example the positive high-voltage potential HV+, is switchably connected to the same-pole high-voltage potential P2+ of the second inverter 4.2 via a contactor S1. Alternatively, the contactor S1 can also be arranged between the negative high-voltage potentials HV-, P2-.

[0059] A DC charging connection 6 for connection to a DC charging station 5 has a first positive contact K+1 for charging with a higher voltage (for example 800 V) and a first negative contact K-1 for charging with the higher voltage as well as a second positive contact K+2 for charging with a lower voltage (for example 400 V) and a second negative contact K-2 for charging with the lower voltage. The contacts K+1, K-1, K+2, K-2 are connected to the electric drive system 1 via respective contactors S2 to S5. The first positive contact K+1 is connected to the positive high-voltage potential HV+ of the high-voltage battery 2. The first negative contact K-1 is connected to the negative high-voltage potential HV- of the high-voltage battery 2. The second positive contact K+2 is connected to the center tap M of the first inverter 4.1. The second negative contact K-2 is connected to the center tap M of the second inverter 4.2.

[0060] Figure 2 is a schematic view of a simulation setup of the electric drive system 1 from Figure 1 and largely corresponds to the setup shown in Figure 1. Contactors S1 to S5 are replaced by open states or connections for the 400V boost state (DC charging with lower voltage). The simulation setup was expanded to include Y-capacitances CY_P1, CY_N1, CY_P2, CY_N2, where CY_P1 and CY_N1 correspond to the Y-capacitances of the positive and negative high-voltage potentials HV+ and HV- in the vehicle, respectively, and CY_P2 and CY_N2 correspond to the Y-capacitances in the DC charging station 5. By determining the voltage U_P1, U_N1, U_P2, U_N2 across the Y-capacitances CY_P1, CY_N1, CY_P2, CY_N2, conclusions can be drawn about the high-voltage potential distribution.

[0061] The boost stage (inverter 4.2) of the negative high-voltage potential HV- is clocked depending on a choke current l_L4 through the stator windings L4 to L6 (between 430A and 450A). The boost stage (inverter 4.1) of the positive high-voltage potential HV+ is clocked with a fixed clock frequency (e.g., 10kHz) and a fixed duty cycle of, for example, 0.3. In applications for insulation protection, the high-voltage potentials HV+ and HV- can be regulated here, for example, by adjusting the voltage U_C2 across capacitor C2 to a specified value.

[0062] Figure 3 is a schematic diagram of signals from the simulation setup shown in Figure 2.

[0063] The top line shows the current l_DC of the DC charging station 5. This current l_DC is primarily determined by the pulsed operation of the semiconductor switch HS4 of the second inverter 4.2, which is controlled by the control signal Gate_HS4, and the resulting inductor current l_L4. For example, the control signal Gate_HS4 is switched on at an inductor current l_L4 of at least 430 A and switched off at an inductor current l_L4 of at most 450 A. Oscillations occur due to the combination of the inductances of the stator windings L4 to L6 with the capacitances of the capacitors C3, C4 present in the circuit, which is why the current l_DC of the DC charging station 5 does not exactly correspond to the inductor current l_L4. The pulsed semiconductor switch LS1 of the first inverter 4.1 is controlled by the control signal Gate_LS1. A duty cycle of 0.3 was selected here, meaning it is switched on for 30% of the period.

[0064] Also shown are the voltages U_C2, U_C3 across capacitors C2 and C3. The potential difference between high-voltage battery 2 and DC charging station 5 is set via capacitor C2 for the positive high-voltage potential HV+. The potential difference between high-voltage battery 2 and DC charging station 5 is set via capacitor C3 for the negative high-voltage potential HV-. Furthermore, a choke current l_L1 through the stator windings L1 to L3 of electric machine 3.1 and a battery current l_Batt through high-voltage battery 2 are shown. The negative sign of battery current l_Batt indicates that high-voltage battery 2 is being charged.

[0065] Figure 4 is a schematic diagram of additional signals from the simulation setup shown in Figure 2.

[0066] The voltages U_P1, U_N1, U_P2, U_N2 shown here across the Y-

[0067] Capacitances CY_P1, CY_N1, CY_P2, CY_N2 correspond to the voltage of the respective high-voltage potential HV+, HV- related to the equipotential bonding conductor PA and / or the protective conductor PE.

[0068] From the comparison of the voltages U_P1, U_N1 it becomes clear that the potential distribution in the vehicle in this example is (almost) symmetrical, i.e. the positive high-voltage potential HV+ is approximately 415 V higher than the potential of the equipotential bonding conductor PA and / or the protective conductor PE and the negative high-voltage potential HV- is approximately 404 V below the potential of the equipotential bonding conductor PA and / or the protective conductor PE.

[0069] A comparison of the voltages U_P2 and U_N2 illustrates the potential distribution in DC charging station 5. Here, too, a nearly symmetrical high-voltage potential distribution has been achieved. The voltage of the positive high-voltage potential HV+ is approximately 171 V higher than the potential of the equipotential bonding conductor PA and / or the protective conductor PE. The voltage of the negative high-voltage potential HV- is approximately 184 V lower than the potential of the equipotential bonding conductor PA and / or the protective conductor PE. Due to the at least approximately balanced potential distribution in the vehicle, no overload of the insulation occurs at the possibly weakly insulated DC charging station 5. If, on the other hand, one of the boost stages boosts with a lower transformation ratio than the other, or is even passive, the resulting uneven potential distribution in the vehicle can lead to an overload of the insulation in DC charging station 5.

[0070] An insulation fault in the vehicle from one of the high-voltage potentials HV+, HV-, for example, from the positive high-voltage potential HV+, to the equipotential bonding conductor PA inevitably leads to a second insulation fault in the DC charging station 5 from the opposite pole (in this case the negative) high-voltage potential HV- to the equipotential bonding conductor PA and / or protective conductor PE. However, no battery short-circuit current occurs because the low-side switches LS4 to LS6 of the half-bridges of the second inverter 4.2, the capacitor C4, and the semiconductor switches of the bidirectional switches BDS4 to BDS6, whose body diodes or freewheeling diodes point in the forward direction to the center tap M between the capacitors C3, C4, prevent current flow. However, as long as boost operation exists, a current flows through the body diodes or freewheeling diodes of the semiconductor switches at the same level as the boost current.Therefore, it is advantageous to discontinue boost operation as soon as an insulation fault is detected. This can be done by measuring the high-voltage potential distributions, measuring the insulation resistance, or performing a plausibility check of the currents.

[0071] An insulation fault in the vehicle from the negative high-voltage potential HV- to the equipotential bonding conductor PA inevitably leads to a second insulation fault in the DC charging station 5 from the opposite pole (in this case the positive) high-voltage potential HV+ to the equipotential bonding conductor PA and / or protective conductor PE. However, no battery short-circuit current occurs because the high-side switches HS1 to HS3 of the half-bridges of the first inverter 4.1, the capacitor C1 and the semiconductor switches of the bidirectional switches BDS1 to BDS3, whose body diodes or freewheeling diodes point in the forward direction to the center tap M between the capacitors C1, C2, prevent any current flow. However, as long as boost mode is active, a current flows through the body diodes or freewheeling diodes of the semiconductor switches at the same level as the boost current. It is therefore advantageous to stop boost mode as soon as an insulation fault is detected.This can be done by measuring the high-voltage potential distributions, by measuring the insulation resistance or by checking the plausibility of the currents.

[0072] Each of the semiconductor switches can be designed, for example, as a MOSFET or as an IGBT with a freewheeling diode.

[0073] List of reference symbols

[0074] 1 Drive system 2 Energy storage, high-voltage battery

[0075] 3.1 , 3.2 electrical machine

[0076] 4.1, 4.2 Inverter 5 DC charging station

[0077] 6 DC charging port

[0078] BDS 1 to BDS 6 bidirectional switch C1 to C4 capacitor

[0079] CY_P1, CY_N1, CY_P2, CY N2 Y-capacitance Gate_HS4 Control signal Gate_LS1 Control signal HS1 to HS6 High-side switch HV+ High-voltage potential, positive high-voltage potential HV- High-voltage potential, negative high-voltage potential l_Batt Battery current l_DC Current l_L1 , l_L4 Inductor current K+1 Contact, first positive contact K-1 Contact, first negative contact K+2 Contact, second positive contact K-2 Contact, second negative contact

[0080] LS1 to LS6 low-side switch

[0081] L1, L2, L3, L4, L5, L6 Stator winding M Center tap

[0082] M1 to M6 center tap P1+, P2+ high voltage potential, positive potential, positive

[0083] High-voltage potential

[0084] P1, P2 high-voltage potential, negative potential, negative high-voltage potential

[0085] S1 to S5 contactor U_C2, U_C3, U_P1, U_N1, l _P2, U_N2 voltage

Claims

Patent claims 1. Electric drive system (1) for a vehicle, with at least two electric machines (3.1, 3.2) each having three stator windings (L1 to L6) for driving the vehicle, at least one high-voltage battery (2) and two inverters (4.1, 4.2) for converting a direct voltage of the high-voltage battery (2) into an alternating voltage for supplying one of the electric machines (3.1, 3.2), wherein the inverters (4.1, 4.2) can be connected in series to one another in a boost mode and can be controlled and / or regulated in such a way that one of the inverters (4.1, 4.2) with the electric machine (3.1, 3.2) connected thereto can be used as a boost stage for a negative high-voltage potential (HV-) and the other inverter (4.1, 4.2) with the electric machine (3.1, 3.2) connected thereto can be used as a boost stage for a positive high-voltage potential (HV+) for charging the high-voltage battery (2), characterized in that the inverters (4.1, 4.2) are each designed as a three-level T-type inverter and each have three half-bridges, each of which is formed from a high-side switch (HS1 to HS6) designed as a semiconductor switch and a low-side switch (LS1 to LS6) designed as a semiconductor switch, to the center taps (M1 to M6) of which one of the stator windings (L1 to L6) and a bidirectional switch (BDS 1 to BDS 6) made up of two semiconductor switches connected anti-serially to one another are connected, wherein between a positive potential (P1+, P2+) and a negative potential (P1-, P2-) of each inverter (4.1, 4.2) a series circuit of a first capacitor (C1, C3) and a second capacitor (C2, C4) is connected, to the center tap (M) of which the bidirectional switches (BDS1 to BDS6) of the respective inverter (4.1, 4.2) are connected.

2. Electric drive system (1) according to claim 1, characterized in that the inverters (4.1, 4.2) in boost mode can be controlled so and / or are adjustable so that both high-voltage potentials (HV+, HV-) can be adjusted independently of each other relative to a potential equalization conductor.

3. Electric drive system (1) according to claim 1 or 2, characterized in that the positive high-voltage potential (HV+) of the high-voltage battery (2) is connected to the positive potential (P1+) of the first inverter (4.1), and in that the negative high-voltage potential (HV-) of the high-voltage battery (2) is connected to the negative potential (P1-) of the first inverter (4.1), wherein one of the high-voltage potentials (HV+, HV-) of the high-voltage battery (2) is connected to the homopolar potential (P2+, P2-) of the second inverter (4.2), wherein the other high-voltage potential (HV+, HV-) of the high-voltage battery (2) is switchably connected to the homopolar high-voltage potential (P2+) of the second inverter (4.2) via a first contactor (S1), wherein the first contactor (S1) is closed in a driving mode and open in a charging mode, in particular the boost mode.

4. Electric drive system (1) according to one of the preceding claims, characterized by a DC charging connection (6) for connection to a DC charging station (5), with a first positive contact (K+1) and a first negative contact (K-1) for charging with a higher voltage and a second positive contact (K+2) and a second negative contact (K-2) for charging with a lower voltage, wherein the first positive contact (K+1) is connected to the positive high-voltage potential (HV+) of the high-voltage battery (2), wherein the first negative contact (K-1) is connected to the negative high-voltage potential (HV-) of the high-voltage battery (2), wherein the second positive contact (K+2) is connected to the center tap (M) between the capacitors (C1, C2) of the first inverter (4.1), wherein the second negative contact (K-2) is connected to the center tap (M) between the capacitors (C3, C4) of the second inverter (4.2).

5. Electric drive system (1) according to claim 4, characterized in that the contacts (K+1, K-1, K+2, K-2) are connected to the inverters (4.1, 4.2) via respective contactors (S2 to S5).

6. Method for operating the electric drive system (1) according to one of the preceding claims in a boost mode, characterized in that in one of the inverters (4.1, 4.2) the low-side switch (LS1 to L6) of one of the half-bridges is operated in a clocked manner, while the bidirectional switches (BDS 1 to BDS6) of the two other half-bridges are permanently closed in the forward direction towards the center tap (M2, M3) of the respective half-bridge by controlling the corresponding semiconductor switch for the duration of the boost operation, wherein in the other inverter (4.1, 4.2) the high-side switch (HS1 to HS6) of one of the half-bridges is operated in a clocked manner, while the bidirectional switches (BDS1 to BDS 6) of the two other half-bridges are permanently closed in the forward direction towards the center tap (M) between the capacitors (C1 to C4) by controlling the corresponding semiconductor switch for the duration of the boost.

7. Method according to claim 6, characterized in that the remaining semiconductor switches of the inverters (4.1, 4.2) are or remain opened or are closed to optimize efficiency with a conductive body diode or freewheeling diode.

8. The method according to claim 6 or 7, characterized in that for homogenizing aging defects after clocking operation of one of the low-side switches (LS1 to LS6) and / or high-side switches (HS1 to HS6) in boost mode, another of the low-side switches (LS1 to LS6) and / or high-side switches (HS1 to HS6) is used for clocking operation.

9. Method according to one of claims 6 to 8, characterized in that the clocking of one of the inverters (4.1, 4.2) takes place as a function of a choke current (l_L1, l_L4) through the stator windings (L1 to L6) of the electrical machine (3.1, 3.2) connected thereto, in such a way that the clock-operated high-side switch (HS1 to HS6) or low-side switch (LS1 to LS6) is switched on when the choke current (l_L1, l_L4) falls below a predetermined lower threshold value and is switched off when the choke current (l_L1, l_L4) exceeds a predetermined upper threshold value and / or that the clocking of the other inverter (4.1, 4.2) takes place with a fixed clock frequency and a fixed pulse duty factor.

10. Method according to one of claims 6 to 9, characterized in that a regulation of the high-voltage potentials (HV+, HV-) by regulation of a voltage (U_C1, U_C2, U_C3, U_C4) over at least one of the capacitors (C1, C2, C3, C4) to a predetermined value.

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