Traction network and method for operating a traction network of an electric vehicle

Abstract

The invention relates to a traction network (1) for an electric vehicle, comprising a high-voltage source, an inverter (4), an electric machine (5), a separate bidirectional charger (6) and at least one control unit (21), wherein the separate bidirectional charger (6) has a DC voltage side (7) and an AC voltage side (8) with three phase lines, wherein the three phase lines of the charger (6) are connected to the three phase lines of the electric machine (5), wherein at least one switching element (12) is arranged in each of the connecting lines (11), wherein the at least one control unit (21) is designed to switch off the inverter (4) in the event of a fault and to close the switching elements (12) in the connecting lines (11), and to a method for operating such a traction network (1).

Description

The invention relates to a traction network and a method for operating a traction network of an electric vehicle.Traction networks for electric vehicles comprise a high-voltage voltage source, an inverter and an electric machine. The high-voltage voltage source is typically a high-voltage battery. Other fuel cell designs are possible. The inverter is configured bidirectionally, i.e. it can convert DC voltage of the high-voltage voltage source into a typically three-phase AC voltage for the electric machine in order to operate the electric machine by motor. Alternatively, the inverter can convert an AC voltage of the electric machine into a DC voltage in recuperation mode in order to charge the high-voltage voltage source in this way. It is also known to provide charging devices by means of which the high-voltage voltage source can be charged from an AC voltage source. Such chargers are also referred to as on-board chargers (OBC). It is also known here to configure the charging device in a bidirectional manner, so that the electric vehicle can feed from the high-voltage voltage source back into the grid or can supply another AC voltage consumer. It is also known to partially integrate the charging device into the inverter in order to save components. Alternatively, the bidirectional charger is designed as a separate charger.Such separate bidirectional charging devices are known, for example, from U.S. Pat. No. 11,491,883 B2 or U.S. Pat. No. 10,046,656 B2.DE 10 2018 009 840 A1 discloses a 3-level charging device for a traction network.A traction network of an electric vehicle is known from DE 10 2016 209 872 A1, wherein an AC charging connection is provided, wherein rectification in the AC charging mode takes place via an inverter, which energizes an electric machine in the traction mode. Separating elements are arranged between the AC voltage side of the inverter and the connections of the phases of the electric machine in order to switch off the electric machine in the AC charging mode.From CN 1 13 890 407 A an galvanically isolated on-board charger is known and from CN 1 16 208 062 A an galvanically non-isolated on-board charger is known.A traction network of the generic type is known from DE 42 35 531 C2.The invention is based on the technical problem of creating a traction network which is improved with regard to failure safety and of making available a corresponding method for operating a traction network.The solution of the technical problem is achieved by a traction network having the features of claim 1 and a method having the features of claim 6.The traction network for an electric vehicle comprises a high-voltage voltage source, an inverter, an electric machine, a separate bidirectional charging device and at least one control device. The separate bidirectional charger has a DC side and an AC side with three phase lines. The DC voltage side is connected to the high-voltage voltage source. The AC voltage side is connected to connections for an AC voltage grid. Furthermore, the three phase lines of the charger are connected to the three phase lines of the electric machine, wherein at least one switching element is arranged in each case in the connecting lines. Furthermore, the at least one control device is designed in such a way that, in the event of a fault in the inverter, it is switched off and the switching elements in the connecting lines are closed. As a result, an emergency operation for the electric machine can be maintained with limited performance in the event of failure of the inverter. The power of an inverter for electric vehicles is in the range from 100 kW to 200 kW, for example. The power from the chargers can be in the range of 22 kW or 43 kW, so that at least as much power can be provided for the drive in order to drive the electric vehicle into a workshop or to the home. The high-voltage voltage source is preferably a high-voltage battery.In this case, switching elements are arranged on the AC voltage side of the inverter, by means of which switching elements the connection to the electric machine can be disconnected. In addition, switching elements or fuses can also be arranged on the DC voltage side in order to disconnect the inverter from the high-voltage battery.The full driving current must flow via the switching elements on the AC voltage side of the inverter during regular operation. Therefore, the switching elements are embodied as pyro-fuses. These can also be used to switch off the inverter in the case of an active short circuit of the electric machine.In one embodiment, the switching elements are designed as relays. These must be designed only for the power of the charger from the power point of view, and are therefore considerably less expensive than the main contactors for disconnecting the high-voltage battery. In principle, however, the switching elements can also be designed as semiconductor switching elements.In a further embodiment, the separate bidirectional charging device is designed as a galvanically separated charging device, so that in particular in charging operation galvanic decoupling from the AC voltage networks is ensured.In an alternative embodiment, the separate bidirectional charging device is designed as a galvanically non-separated charging device, so that in particular the transformer and further components can be saved.In a further embodiment, the charger is designed as a 3-level charger, so that an improved motor torque can be set at the electric machine.In a further embodiment, an EMC filter is arranged on the AC voltage side of the charger.Preferably, the connecting lines to the electric machine are arranged between connections for an AC voltage grid and the EMC filter, wherein the connecting lines are designed as unshielded lines. This makes use of the fact that there must be grid conformance at the connections for the AC voltage grid.With regard to the embodiment according to the method, reference is made in full to the preceding explanations.The invention is explained in more detail below with reference to preferred exemplary embodiments. The figures show: FIG. 1 shows a schematic illustration of a traction network in a first embodiment according to the invention, FIG. 2 shows a schematic illustration of a traction network in a second embodiment according to the invention, FIG. 3 shows a circuit of a separate galvanically isolated bidirectional 2-level charger, FIG. 4 shows a circuit of a separate galvanically isolated bi-directional 3-level charger, FIG. 5 shows a circuit of a separate galvanically non-separated bi-directional 2-level charger, and FIG. 6 shows a circuit of a separate galvanically non-separated bi-directional 3-level charger.FIG. 1 shows a traction network 1 of an electric vehicle in a greatly simplified manner. The traction network 1 comprises a high-voltage battery 2 which is formed from a plurality of battery cells 3. The traction network 1 also has an inverter 4 which consists of three half bridges, the center taps of which are connected to terminals of an electric machine 5. The traction network 1 further comprises a separate, bidirectional charging device 6. The charging device 6 is connected to the high-voltage battery 2 on a DC voltage side 7. On an AC voltage side 8, the charger 6 is connected to terminals 9 for an external AC voltage network 10. The charger 6 is connected on the AC voltage side 8 via connecting lines 11 to the terminals of the electric machine 5, wherein at least one switching element 12 is arranged in each case in the connecting lines 11. Finally, the traction network 1 has at least one control unit 21 which actuates the inverter 4, the charger 6 and the switching elements 12. In this case, the functions explained below can also be distributed to a plurality of control units which then communicate with one another.If the inverter 4 operates without errors, the switching elements 12 are open during the driving operation and the drive power for the electric machine 5 is made available by the high-voltage battery 2 via the inverter 4, wherein the drive power is, for example, between 100 kW and 200 kW. The inverter is configured bidirectionally, so that in recuperation mode electrical power can be fed back from the electric machine 5 via the inverter 4 to the high-voltage battery 2. In the AC charging mode, the traction network 1 is connected to the external AC voltage network 10, wherein the switching elements 12 are open again. The high-voltage battery 2 can then be charged via the charging device 6 or else energy can be fed back from the high-voltage battery 2 into the AC voltage grid 10.If inverter 4 fails partially or completely during the driving operation, switching elements 12 are closed and inverter 4 is switched off, the electric machine 5 then being supplied from high-voltage battery 2 via charging device 6. As a result, a driving operation which is limited in terms of performance can be maintained. It is assumed here that the failure of individual transistors in the half bridges of the inverter 4 does not lead to short circuits, but rather these are blocked in the event of a fault, which is usually the case with GaN transistors.FIG. 2 shows an embodiment according to the invention, wherein further switching elements 13 are additionally arranged between the AC voltage side of the inverter 4 and the electric machine 5, so that in the event of faults which lead to a short circuit in the inverter 4, the latter can be disconnected. The switching elements 13 must be matched to the high current during regular driving operation. Therefore, the switching elements 13 are embodied as pyro-fuses. In this case, it can additionally be provided that the inverter 4 can also be disconnected from the high-voltage battery 2 on the DC voltage side via further switching elements.FIG. 3 shows a bidirectional charging device 6, which is designed as a galvanically isolated 2-level charging device 14. In this case, an EMC filter 15 is arranged on the input side of the AC voltage side 8 of the charger 6, wherein the connecting lines 11 to the electric machine 5 are arranged between the connections 9 for the AC voltage network 10 and the EMC filter 15, with the result that the connecting lines 11 can be designed as unshielded lines. The shift elements 12 are not shown here for reasons of clarity. Behind the EMC filter, an active bidirectional inverter 16 having two smoothing capacitors C is arranged. This is followed by a galvanically separated bidirectional DC / DC converter 17 which has a transformer with two active half bridges each side. According to the current direction, two half-bridges serve to generate an alternating voltage which is transformed to the other side and rectified there by the two other half-bridges. The DC / DC converter 17 can also be designed as a resonant converter.FIG. 4 shows an alternative embodiment of a bidirectional charging device 6, which is designed as a galvanically separated 3-level charging device 18, wherein the inverter 16 in each case additionally has two MOSFETs connected in series in opposite directions, which are in each case connected to the center taps of the half bridges of the inverter 16 and are connected to a common new point N.FIG. 5 shows a further alternative embodiment of a bidirectional charging device 6, which is designed as a galvanically non-separated 2-level charging device 19. In comparison with FIG. 3, the galvanically separated DC / DC converter 17 is thus completely omitted.Finally, FIG. 6 shows a further alternative embodiment of a bidirectional charging device 6, which is designed as a galvanically non-separated 3-level charging device 20. In comparison with FIG. 4, the galvanically separated DC / DC converter 17 is thus completely omitted.List of reference characters1 Traction network 2 High-voltage battery 3 Battery cell 4 Inverter 5 Electric machine 6 Charger 7 Direct-voltage side 8 Alternating-voltage side 9 Terminal 10 Alternating-voltage network 11 Connecting line 12 Switching element 13 Switching element 14 2-level charger 15 EMC filter 16 Inverter 17 DC / DC converter 18 3-level charger 19 2-level charger 20 3-level charger 21 Control device C Smoothing capacitor N Neutral point

Claims

Traction network (1) for an electric vehicle, comprising a high-voltage voltage source, an inverter (4), an electric machine (5), a separate bidirectional charger (6) and at least one control device 21, wherein the separate bidirectional charger (6) has a DC voltage side (7) and an AC voltage side (8) with three phase lines, wherein the three phase lines of the charger (6) are connected to the three phase lines of the electric machine (5), wherein at least one switching element (12) is arranged in each case in the connecting lines (11), wherein the at least one control device (21) is designed in such a way that, in the event of a fault in the inverter (4), it is disconnected and the switching elements (12) are closed in the connecting lines (11), wherein switching elements (13) are arranged on the AC voltage side of the inverter (4), by means of which switching elements the connection to the electric machine (5) can be disconnected, characterized in that, the switching elements (13) are designed as pyro-fuses.Traction network (1) according to Claim 1, characterized in that the switching elements (12) in the connecting lines (11) are designed as relays.Traction network (1) according to one of the preceding claims, characterized in that the separate bidirectional charging device (6) is designed as a galvanically separated charging device (14, 18).Traction network (1) according to either of Claims 1 and 2, characterized in that the separate bidirectional charging device (6) is designed as a galvanically non-separated charging device (19, 20).Traction network (1) according to one of the preceding claims, characterized in that the charging device (6) is designed as a 3-level charging device (18, 20).Traction network (1) according to one of the preceding claims, characterized in that an EMC filter (15) is arranged on the AC voltage side (8) of the charging device (6).Traction network (1) according to Claim 6, characterized in that the connecting lines (11) to the electric machine (5) are arranged between connections (9) for an AC voltage network (10) and the EMC filter (15) and are designed as unshielded lines.Method for operating a traction grid (1) of an electric vehicle, wherein the traction grid (1) has a high-voltage voltage source, an inverter (4), an electric machine (5), a separate bidirectional charger (6) and at least one control device (21), wherein the separate bidirectional charger (6) has a DC voltage side (7) and an AC voltage side (8) having three phase lines, wherein the three phase lines of the charger (6) are connected to the three phase lines of the electric machine (5), wherein in each case at least one switching element (12) is arranged in the connection lines (11), wherein the control device (21) switches off the inverter and closes the switching elements (12) in the connection lines (11), wherein switching elements (13) designed as pyro-fuses are arranged on the AC voltage side of the inverter (4), which are triggered in the event of a fault of the inverter (4).

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