DE102021208773B3 - Traction network for an electric vehicle and charging method - Google Patents

Abstract

The invention relates to a traction network (1) for an electric vehicle, comprising a first and second battery unit (2, 3), a 3-level inverter (5), a switching module (4) and a control unit (8), the first and second battery unit (2, 3) have the same nominal voltage, the switching module (4) being arranged between the battery units (2, 3) and the 3-level inverter (5), the switching module (4) having an input for the positive and the negative voltage connection (A+, A-) of the first battery unit (2) and an input for the positive and the negative voltage connection (B+, B-) of the second battery unit (3). The battery units can be connected in series or in parallel using the switching module, with the voltage level being checked during charging before parallel connection, with the battery unit (2, 3) with the higher voltage level being connected to the charging connections (7), whereas the positive voltage connection of the Battery unit (2, 3) with the lower voltage is connected to the inductance (L), the inductance (L) and at least one half-bridge (HB) of the 3-level inverter (5) working as a step-up converter and, after adjustment, the control unit ( 8) the two battery units (2, 3) connected in parallel, and a method for charging battery units (2, 3) of the traction network (1).

Description

The invention relates to a traction network for an electric vehicle and a method for charging such a traction network.

The typical structure of a traction network consists of a traction battery, an inverter and an electric machine. Many different topologies are known as to how the individual components of such a traction network are constructed. In order to reduce the currents, an attempt is made to work with the highest possible voltages on the vehicle side. For example, traction networks are known in which the nominal voltage of the traction battery is 800 V. This leads to problems with the dielectric strength of the semiconductor switches, so that 3-level inverters have been proposed for such traction networks, such as in DE 10 2016 206 945 A1 described. Another problem is the inconsistent charging infrastructure, where both 400 V and 800 V DC charging stations are available. Depending on the topology used, the maximum charging power cannot be fully exploited or the switching effort is very high.

• - mindestens einen ersten Eingang und einen zweiten Eingang zur elektrischen Verbindung mit einer Energiequelle,
• - mindestens einen ersten Ausgang und einen zweiten Ausgang zur elektrischen Verbindung mit einer elektrischen Komponente,
• - mindestens zwei erste Polanschlüsse und zwei zweite Polanschlüsse, wobei jeweils einer der ersten Polanschlüsse mit dem ersten Pol der elektrischen Energiespeichereinheiten und jeweils einer der zweiten Polanschlüsse mit dem zweiten Pol der elektrischen Energiespeichereinheiten elektrisch leitend verbunden sind,
• - mindestens zwei erste Schalter, wobei ein erster Anschluss des ersten Schalters mit dem ersten Polanschluss der ersten Energiespeichereinheit und ein erster Anschluss des ersten Schalters mit dem ersten Eingang elektrisch verbunden ist,
• - mindestens zwei zweite Schalter, wobei ein erster Anschluss des zweiten Schalters mit dem zweiten Polanschluss der ersten Energiespeichereinheit und ein erster Anschluss des zweiten Schalters mit dem zweiten Eingang elektrisch verbunden ist,
• - mindestens einen dritten Schalter, wobei ein erster Anschluss des dritten Schalters mit dem ersten Polanschluss der ersten elektrischen Energiespeichereinheit und ein zweiter Anschluss des dritten Schalters mit dem zweiten Polanschluss der zweiten elektrischen Energiespeichereinheit elektrisch verbunden ist, wobei
• - der erste Ausgang mit jeweils einem zweiten Anschluss der ersten Schalter elektrisch verbunden ist, und
• - der zweite Ausgang mit jeweils einem zweiten Anschluss der zweiten Schalter elektrisch verbunden ist, und
• - die ersten Schalter, die zweiten Schalter, der dritte Schalter sowie weitere Schalter derart geschaltet sind, dass in einem Ladebetrieb der Schaltungsanordnung, zum Laden mindestens einer der elektrischen Energiespeichereinheiten mittels einer an den ersten Eingang und den zweiten Eingang angeschlossenen Energiequelle, und/oder in einem Entladebetrieb der Schaltungsanordnung, zum Betreiben der an den ersten Ausgang und den zweiten Ausgang angeschlossenen elektrischen Komponente, die elektrischen Energiespeichereinheiten in Serie oder parallel geschaltet werden.

Vorzugsweise wird die Schaltungsanordnung in einem Elektro- oder Hybridfahrzeug eingesetzt, das einen Elektromotor, einen Wechselrichter, einen DC/DC-Wandler für ein 12V-Bordnetz mit Bordnetzbatterie, einen Zwischenkreiskondensator und einen Klimakompressor als elektrische Komponenten aufweisen kann.From the DE 10 2016 225 513 A1 discloses a circuit arrangement for an electrical energy storage system having at least two electrical energy storage units each having a first pole and a second pole, comprising

• - at least one first input and one second input for electrical connection to an energy source,
• - at least one first output and one second output for electrical connection to an electrical component,
• - at least two first pole connections and two second pole connections, one of the first pole connections being electrically conductively connected to the first pole of the electrical energy storage units and one of the second pole connections to the second pole of the electrical energy storage units,
• - at least two first switches, wherein a first connection of the first switch is electrically connected to the first pole connection of the first energy storage unit and a first connection of the first switch is electrically connected to the first input,
• - at least two second switches, wherein a first connection of the second switch is electrically connected to the second pole connection of the first energy storage unit and a first connection of the second switch is electrically connected to the second input,
• - At least one third switch, wherein a first connection of the third switch is electrically connected to the first pole connection of the first electrical energy storage unit and a second connection of the third switch is electrically connected to the second pole connection of the second electrical energy storage unit, wherein
• - the first output is electrically connected to a respective second connection of the first switch, and
• - the second output is electrically connected to a respective second connection of the second switch, and
• - the first switch, the second switch, the third switch and other switches are connected in such a way that, in a charging operation of the circuit arrangement, for charging at least one of the electrical energy storage units by means of an energy source connected to the first input and the second input, and/or in a discharging operation of the circuit arrangement, for operating the electrical components connected to the first output and the second output, the electrical energy storage units are connected in series or in parallel.

The circuit arrangement is preferably used in an electric or hybrid vehicle which can have an electric motor, an inverter, a DC/DC converter for a 12V vehicle electrical system with vehicle electrical system battery, an intermediate circuit capacitor and an air conditioning compressor as electrical components.

• - Anschließen einer Wechselstromquelle über einen Brückengleichrichter an den Wechselrichter,
• - Ansteuern mindestens eines Schaltelements des Wechselrichters und
• - Öffnen der übrigen Schaltelemente des Wechselrichters.

From the DE 10 2015 225 574 A1 a method for charging a battery of a drive train is known, the drive train comprising a battery, a three-level inverter and an electric machine, with the steps:

• - connecting an AC source to the inverter through a bridge rectifier,
• - Driving at least one switching element of the inverter and
• - Opening of the other switching elements of the inverter.

From the DE 10 2016 218 304 B3 a device for voltage conversion in an electric vehicle is known. The device for voltage conversion includes a converter and a control unit, the converter having a two-pole DC voltage connection for connecting a battery and a three-pole AC voltage connection for connecting an electric machine. The converter also has three half-br cken, with two transistors with freewheeling diodes being arranged both in the upper and in the lower part of each half-bridge. In this case, the center taps of the half-bridges can be connected to the AC voltage connection or are connected to it. An intermediate circuit capacitor, which consists of at least two capacitors connected in series, is arranged in parallel with the DC voltage connection, with the common connection point forming a neutral point. In this case, a plurality of capacitors can be connected in parallel, with the two parallel circuits then being connected in series. Furthermore, two diodes each are assigned to the half-bridges, the two diodes being connected in series and connecting center taps of the upper and lower parts of the half-bridge. An anode of a first diode is connected to the center tap of the lower part and the cathode of the other diode is connected to the center tap of the upper part. Furthermore, at least one half-bridge has two switching elements. In a first switching position, the first switching element connects the center tap between the two diodes to the neutral point and in a second switching position connects the center tap between the diodes to a positive charging connection for an external DC voltage source. In a first switching position, the second switching element connects the center tap of the half-bridge to a pole of the AC voltage connection and in a second switching position connects the center tap of the half-bridge to the negative pole of the DC voltage connection. The control unit is designed in such a way that it controls the switching elements from the first to the second switching position as a function of an intended external charging process. By using four transistors per half-bridge, the nominal voltage of the battery can be selected to be twice as large. For example, a battery with a nominal voltage of 800 V can be chosen. Another advantage is that a 3-level converter topology is achieved, so that the more favorable characteristics with the voltage edge steepness relax the requirement on the machine-side windings. With a correspondingly modified pulse pattern, harmonic current components can be eliminated in a targeted manner, which improves the overall efficiency of the electric machine, since harmonics usually do not contribute to the generation of torque. The battery is charged with an external DC voltage source, the nominal voltage of which is lower than the nominal voltage of the battery, by operating the transistors in the upper part of the half-bridge as a DC/DC step-up converter in charging mode (second switching position). In this way, for example, the battery with a nominal voltage of 800 V can be charged by an external DC voltage source with a nominal voltage of 400 V. This allows fast charging times with high charging power. The inductance required for the step-up converter can be provided by the external DC voltage source, for example. The size of the inductance required depends on the switching frequency of the transistors, since the impedance is jωL.

From the DE 10 2018 124 787 A1 a charging device for charging a battery of a motor vehicle equipped with an electric drive motor is known, with an inductance, a drive converter, which converts the DC voltage of the battery for the electric drive motor during drive operation of the motor vehicle and has an intermediate circuit center point, the inductance together with the drive converter for a charging operation of the battery is used as a step-up converter, the intermediate circuit center point being connected permanently or temporarily via a compensating conductor to an input voltage of a charge source and/or the inductance. The inductance is preferably formed by at least one winding of the electric drive motor.

A similar circuit with two inverters is known from WO 2021/160 022 A1.

From the DE 10 2019 201 706 A1 discloses an on-board charging circuit with a controllable rectifier, multiple energy stores and a configuration circuit that connects the energy stores in a selectable series or parallel configuration, the multiple energy stores being connected to the controllable rectifier via the configuration circuit, the controllable rectifier having at least one switch cell , wherein at least one of the switch cells is controllable and each has at least one switchable half-bridge branch.

From the DE 10 2015 105 833 A1 is a voltage regulator and from the DE 10 2012 106 866 A1 a switching regulator is known.

The invention is therefore based on the technical problem of creating a traction network that can be flexibly charged at different DC charging stations with little circuit complexity, and of providing a corresponding charging method.

The technical problem is solved by a traction network with the features of claim 1 and a method with the features of claim 10. Further advantageous refinements of the invention result from the dependent claims.

Here, the traction network for an electric vehicle comprises a first battery unit and a second battery unit, a 3-level inverter Switching module and at least one control unit. The first and second battery units have the same nominal voltage of 400 V, for example. The switching module is placed between the battery units and the 3-level inverter. The switching module has an input for the positive and negative voltage connection (battery pole) of the first and second battery unit. The switching module also has a positive voltage connection, a negative voltage connection and a neutral point as an output, with the outputs being connected to the 3-level inverter. The switching module also has a connection for at least one inductance, the inductance being arranged between the neutral point of the 3-level inverter and the connection of the switching module. DC voltage charging connections are also provided, which are connected to the positive voltage connection and the negative voltage connection at the output of the switching module. The control unit is designed in such a way that it activates the switching elements in the switching module in such a way that the two battery units are connected in series when the vehicle is being driven. Likewise, the two battery units are connected in series when charging with a charging voltage that corresponds to twice the nominal voltage of the battery units. The charging voltage is therefore equal to the sum of the nominal voltages of the battery units. In charging mode with a charging voltage equal to the nominal voltage of the battery units, the two battery units are connected in parallel via the switching module. In this case, the control unit is designed in such a way that it checks the current voltage level of the two battery units before the parallel connection. If the voltage level is the same or differs only slightly, the two battery units can be connected in parallel. If, on the other hand, the deviation is greater than a threshold value, the battery unit with the higher voltage level is first connected to the DC voltage charging connections and charged, but preferably not yet with the maximum possible power. On the other hand, the positive voltage terminal of the battery pack with the lower voltage is connected to the inductor. The control unit then controls at least one half-bridge of the inverter in such a way that it works with the inductance as a step-up converter in order to align the voltage levels of the two battery units, with the control unit connecting the two battery units in parallel after the voltage levels have been equalized. The advantage is that this active balancing of the battery units is very fast and no significant losses occur, with the additional circuitry complexity being limited to the inductance and the switching elements in the switching module. This avoids the situation where the battery units, which may differ by a few 10 V to 20 V in the voltage level, have to be connected in parallel immediately, which would cause extreme equalizing currents that could accelerate the aging of the battery units and cause other thermal problems.

In one embodiment, the switching module has three switching elements, with the first switching element being designed in such a way that in a first switching position the positive voltage connection of the second battery unit is connected to the inductor, in a second switching position it is connected to the positive voltage connection at the output of the switching module and is connected in a third switch position mi to the negative voltage connection of the first battery unit. The second switching element is designed such that in a first switching position the positive voltage connection of the first battery unit is connected to the inductor and in a second switching position it is connected to the positive voltage connection at the output of the switching module. The third switching module is designed in such a way that the negative DC voltage charging connection is connected to or disconnected from the neutral point at the output of the switching module.

The switching elements are preferably in the form of relays in order to implement reliable galvanic isolation.

The switching module and the two battery units are preferably integrated in a battery module. The inductance is preferably also integrated into the battery module.

In a further embodiment, the traction network also has AC voltage charging connections, with a unidirectional or bidirectional rectifier with at least one downstream DC/DC converter being arranged at the AC voltage charging connections. The advantage of a bidirectional rectifier is that the traction network can then be used as a power source for external devices, although this increases the circuit complexity compared to unidirectional embodiments.

The rectifier is preferably designed as a Vienna rectifier with a power factor correction. The advantage of Vienna rectifiers are the low switching losses, low interference radiation and simple control as well as high reliability.

In a further embodiment, two DC/DC converters are connected in series. The advantage is that the rectified voltage at the output of the rectifier can then be divided, so that the requirements for the voltage strength are met capability of the transistors in the DC/DC converter are reduced.

In a further embodiment, the DC/DC converter has a plurality of half-bridges, each of which has an inductance connected to its center tap, with the inductances being connected at a common point. It should be noted that if there are two DC/DC converters, the number of half-bridges in both DC/DC converters should be the same. Due to the number of half-bridges, the traction network can easily be scaled to the possible or desired charging capacity. For example, if a half-bridge can transmit 3.6 kW of power, then three half-bridges are required for 11 kW, correspondingly six half-bridges for 22 kW and twelve half-bridges for 43 kW.

In a further embodiment, the transistors of the traction network are designed as GaN transistors (gallium nitride), which have particularly low losses. In this case, embodiments are also possible where individual transistors are used in a different technology (e.g. SiC) in order to reduce the number of components.

With regard to the procedural design of the invention, reference is made in full to the preceding statements.

• 1 ein schematisches Blockschaltbild eines Traktionsnetzes,
• 2 eine Darstellung einer Schalterkonfiguration zum Angleichen in einer ersten Konfiguration,
• 3 eine Darstellung einer Schalterkonfiguration zum Ausgleichen in einer zweiten Konfiguration,
• 4 ein erweitertes schematisches Blockschaltbild eines Traktionsnetzes,
• 5 eine mögliche schaltungstechnische Darstellung eines Traktionsnetzes,
• 6 eine alternative schaltungstechnische Darstellung eines Traktionsnetzes gemäß 4 mit einem unidirektionalen Gleichrichter,
• 7 eine alternative schaltungstechnische Darstellung eines Traktionsnetzes und
• 8 eine weitere alternative schaltungstechnische Darstellung eines Traktionsnetzes.

The invention is explained in more detail below with reference to preferred embodiments. The individual figures show:

• 1 a schematic block diagram of a traction network,
• 2 a representation of a switch configuration for matching in a first configuration,
• 3 a representation of a switch configuration for balancing in a second configuration,
• 4 an extended schematic block diagram of a traction network,
• 5 a possible circuit representation of a traction network,
• 6 an alternative circuit representation of a traction network according to 4 with a unidirectional rectifier,
• 7 an alternative circuit representation of a traction network and
• 8th Another alternative circuit representation of a traction network.

In the 1 a traction network 1 is shown in a highly simplified manner. The traction network 1 has a first battery unit 2 and a second battery unit 3, both of which are structurally identical. The voltage at the first battery unit is 2 V_A and the voltage at the second battery unit is 3 V_B (see Fig 2 and 3 ). In the following it should be assumed that the nominal voltage of the two battery units is 2, 3 400 V. The traction network 1 also has a switching module 4 , an inductance L, a 3-level inverter 5 and an electric machine 6 . On the input side, the switching module 4 has an input for the positive voltage connection A+ and the negative voltage connection A− of the first battery unit 2 and a positive voltage connection B+ and negative voltage connection B− for the second battery unit 3 . Furthermore, the switching module 4 has a positive voltage connection DC+, a negative voltage connection DC- and a neutral point N as an output. Finally, the switching module 4 also has a connection L+ for the inductance L, the other end of the inductance L being connected to the neutral point N. DC voltage charging connections 7 are arranged at the outputs for the positive voltage connection DC+ and the negative voltage connection DC-.

The operation of the switching module 4 is now based on the 2 and 3 be explained in more detail. The switching module 4 has three switching elements S1-S3. The first switching element S1 has three switching positions. In the first switch position (as in 2 shown), the positive voltage terminal of the second battery unit 2 is connected to the inductance L. In the second switching position, the positive voltage connection is connected to the positive voltage connection DC+ and in the third switching position to the negative voltage connection of the first battery unit 2 . The second switching element S2 has two switching positions, with the positive voltage connection of the first battery unit 2 being connected to the inductor L in a first switching position and to the positive voltage connection DC+ in the second switching position. The neutral point N can be interrupted by means of the third switching element S3.

When driving, the two battery units 2, 3 are to be connected in series so that 800 V are available. For this purpose, the first switching element S1 is switched to the third switching position, the second switching element S2 is switched to the second switching position and the third switching element S3 is closed. In this switch position, 800 V can also be charged via the DC voltage charging connections 7 . The switching elements S1-S3 are controlled by a control unit 8, which is only in 2 is shown schematically and is otherwise no longer shown for reasons of clarity, the control unit 8 can be integrated into a battery management control unit.

If, on the other hand, charging is to be carried out at a 400 V charging station, three cases must be distinguished, namely V_A = V_B, V_A > V_B and V_A < V_B. In the first case, the two battery units 2, 3 can simply be connected in parallel, for which purpose the first switching element S1 is switched to the second switching position. The third switching element S3 can be opened. If, on the other hand, the voltage levels are different, direct parallel switching would lead to compensating currents, which should be avoided.

where is in 2 the circuit configuration for the case U_A > U_B and in 3 shown for U_A < U_B. Based on driving operations, in 2 the first switching element S1 is switched from the third switching position to the first switching position and the third switching element S3 is opened. In this switch position, the first battery unit 2 is in contact with the DC voltage charging connections 7 and can be charged with a moderate 400 V. The second battery unit 3, on the other hand, is connected to the inductor L, which can be operated together with the half-bridges HB of the 3-level inverter 5 as a step-up converter. where is in 2 and 3 one half-bridge HB of the 3-level inverter 5 is shown in each case, with only one correspondingly controlled half-bridge HB being sufficient in principle. The second battery unit 3 is virtually boosted via the step-up converter and adjusted to the voltage level of the first battery unit 2 . If the voltages are equal and the first switching element S1 is switched to the second switching position, then the charging power at the DC voltage charging connections 7 can be regulated to maximum charging power.

In the 3 the switching sequence from the driving mode is corresponding, with the first switching element S1 being switched from the third switching position to the second switching position. The second switching element S2 is switched from the second switching position to the first switching position and the third switching element S3 is opened again. The second battery unit 3 can then be charged moderately with 400V from the charging station, with the first battery unit 2 being boosted via the step-up converter until the voltage level of the second battery unit 3 is reached. Then the second switching element S2 is switched to the second switching position. As a result, the traction network 1 can be adapted very easily and the maximum charging power can be called up both at 800 V and at 400 V charging stations.

In the 4 an extended traction network 1 is shown, which also enables charging from an AC voltage source 9 . For this purpose, the traction network 1 has a rectifier 10, at the input of which three inductances L1-L3 are connected upstream. The rectifier 10 can be bidirectional or unidirectional and is preferably embodied as a Vienna rectifier with a power factor correction circuit (PFC). A neutral point N is looped through from the rectifier 10 to the switching module 4 . Two DC/DC converters 11 are arranged at the output of the rectifier 10, so that the output voltage of the rectifier 10 can be divided, which reduces the requirements for the dielectric strength of the power semiconductors used in the DC/DC converter 11.

In the 5 is a possible specific circuit configuration of the traction network 1 according to 4 shown. The 3-level inverter 5 is designed as a 3L-ANPC inverter, in which the neutral point N is actively balanced by the transistors. It is also shown how the switching module 4 and the battery units 2, 3 can be integrated in a battery module 12. In this battery module 12, the main contactors for galvanic isolation of the battery units 2, 3 and a battery management control device or the control unit 8 (see 2 ) to be integrated. For reasons of clarity, the inductance L is no longer shown below. The rectifier 10 is designed as a bidirectional Vienna rectifier with PFC. The two DC/DC converters 11 each comprise half-bridge circuits, at the center tap of which an inductance L _DC/DC is arranged, with the other ends being connected to one another at a common point P. For reasons of clarity, in each case only the upper inductance L _DC/DC is identified with a reference symbol. The power consumption can be scaled via the number of half-bridge circuits.

In the 6 an alternative embodiment of the traction network 1 is shown, in contrast to 5 the rectifier 10 is designed as a unidirectional Vienna rectifier 10, which requires considerably fewer transistors, namely only three. The 3-level inverter 5 is in the form of a 3L-NPC inverter 5 in which the diodes D carry out the passive balancing.

In the 7 a further alternative embodiment is shown, with the rectifier 10 again being in the form of a bidirectional Vienna rectifier 10 and the 3-level inverter 5 being in the form of a 3L-NPC inverter 5 .

In the 8th a further alternative embodiment is shown, in which, in contrast to the previous embodiments, only one DC/DC converter 11 is arranged between the rectifier 10 and the battery module 12 . Included its transistors are preferably designed as SiC transistors, which can have a dielectric strength of 1,200 V. The 3-level inverter 5 is designed as a 3L-T inverter, with the transistors of the half bridges also being designed as SiC transistors. The advantage is that this can reduce the total number of transistors, but the cost is higher than that of GaN transistors.

Reference List

1

traction network

2

first battery unit

3

second battery unit

4

switching module

5

3 level inverter

6

electric machine

7

DC charging port

8th

control unit

9

AC power source

10

rectifier

11

DC/DC converter

12

battery module

L

inductance

L1-L3

inductance

LDC/DC

inductance

D

diode

P

Point

N

neutral point

Claims (10)

Traction network (1) for an electric vehicle, comprising a first battery unit (2) and a second battery unit (3), a 3-level inverter (5), a switching module (4) and at least one control unit (8), the first battery unit (2) and the second battery unit (3) have the same nominal voltage, the switching module (4) being arranged between the battery units (2, 3) and the 3-level inverter (5), the switching module (4) having an input for the positive voltage connection (A+) and the negative voltage connection (A-) of the first battery unit (2) and an input for the positive voltage connection (B+) and the negative voltage connection (B-) of the second battery unit (3), the switching module (4) further has a positive voltage terminal (DC+), a negative voltage terminal (DC-) and a neutral point (N) as an output, the outputs being connected to the 3-level inverter (5), the switching module (4) far it has a connection for at least one inductance (L), with the inductance (L) being arranged between the neutral point (N) of the 3-level inverter (5) and the connection of the switching module (4), with two DC voltage charging connections (7) are provided, which are connected to the positive voltage connection (DC+) and the negative voltage connection (DC-) at the output of the switching module (4), the control unit (8) being designed in such a way that these switching elements (S1-S3) in the switching module ( 4) controlled in such a way that when driving, the two battery units (2, 3) are connected in series, that when charging with a charging voltage of twice the nominal voltage, the two battery units (2, 3) are connected in series, with charging with a charging voltage equal to the nominal voltage of the battery units (2, 3), these are connected in parallel, the control unit (8) being further designed in such a way that the current voltage level of the at the battery units (2, 3), whereby the battery unit (2, 3) with the higher voltage level is first connected to the DC voltage charging connections (7), whereas the positive voltage connection of the battery unit (2, 3) with the lower voltage is connected to the inductance (L) is connected, with the inductance (L) and at least one half-bridge (HB) of the 3-level inverter (5) working as a step-up converter in order to align the voltage levels of the two battery units (2, 3), after the leveling of the voltage levels the control unit (8) connects the two battery units (2, 3) in parallel. traction network claim 1 , characterized in that the switching module (4) has three switching elements (S1-S3), the first switching element (S1) being designed such that in a first switching position the positive voltage connection (B+) of the second battery unit (3) with the inductance (L), is connected to the positive voltage connection (DC+) at the output of the switching module (4) in the second switching position and is connected to the negative voltage connection (A-) of the first battery unit (2) in the third switching position, with the second switching element (S2) is designed in such a way that in a first switching position the positive voltage connection (A+) of the first battery unit (2) is connected to the inductance (L) and in a second switching position to the positive voltage connection (DC+) at the output of the switching module (4) is connected, the third switching element (S3) being designed in such a way that the negative DC voltage connection (7) is connected to the neutral point (N) at the output ng of the switching module (4) to connect or disconnect. traction network claim 1 or 2 , characterized in that the switching elements (S1-S3) of the switching module (4) are designed as relays. Traction network according to one of the preceding claims, characterized in that the switching module (4) and the two battery units (2, 3) are integrated in one battery module (12). Traction network according to one of the preceding claims, characterized in that the traction network (1) additionally has AC voltage charging connections, a unidirectional or bidirectional rectifier (10) with at least one downstream DC/DC converter (11) being arranged at the AC voltage charging connections. traction network claim 5 , characterized in that the rectifier (10) is designed as a Vienna rectifier. Traction network according to one of Claims 5 or 6 , characterized in that two DC/DC converters (11) are connected in series. Traction network according to one of Claims 5 until 7 , characterized in that the DC / DC converter (11) has a plurality of half-bridges, at whose center tap in each case an inductance (L _{DC / DC} ) is connected, the inductances at a point (P) are connected. Traction network according to one of the preceding claims, characterized in that the transistors of the traction network (1) are designed as GaN transistors. Method for loading a traction network (1) with the features of claim 1 , whereby the current voltage level of the battery units (2, 3) is checked during DC voltage charging with a charging voltage equal to the nominal voltage of the battery units, the battery unit (2, 3) with the higher voltage level being first connected to the DC voltage charging connections (7), whereas the positive one is connected Voltage connection of the battery unit (3, 2) with the lower voltage is connected to the inductance (L), the control unit (8) driving at least one half-bridge (HB) of the 3-level inverter (5) as a step-up converter, which the battery unit ( 3, 2) with the lower voltage until the voltage level of the two battery units (2, 3) is the same, in which case the two battery units (2, 3) are connected in parallel and the charging process is continued.

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