Method and device for operating a motor vehicle

Abstract

The invention relates to a method for operating a motor vehicle (2) which has an electric high-voltage network (3) and an electric low-voltage network (4), wherein the high-voltage network (3) has at least one traction battery (5) and at least one electric drive machine (6), the low-voltage network (4) has a vehicle electrical system battery (9), and the high-voltage network (3) is monitored for an electrical short-circuit and, upon detection of a short-circuit, the traction battery (5) is disconnected from the high-voltage network (3). According to the invention, in order to check the plausibility of the detected short-circuit before the traction battery (5) is reconnected to the high-voltage network (3), a boost current (I.sub.HV) is applied to the high-voltage network (3) through the low-voltage network (4) by means of a DC/DC converter (11), and a boost voltage (U.sub.HV) measured in the high-voltage network (3) is compared with an expected boost voltage (U.sub.HV,soll).

Claims

1. A method for operating a motor vehicle (2) which has an electrical high-voltage network (3) and an electrical low-voltage network (4), wherein the high-voltage network (3) has at least one traction battery (5) and at least one electric drive motor (6), wherein the low-voltage network (4) has a vehicle electrical system battery (9), and wherein the high-voltage network (3) is monitored for an electrical short circuit and when a short circuit is detected the traction battery (5) is isolated from the high-voltage network (3), the method comprising: checking the plausibility of the detected short circuit before reconnecting the traction battery (5) to the high-voltage network (3) by supplying the high-voltage network (3) with a boost current (I.sub.HV) from the low-voltage network (4) via a DC-DC converter (11); and comparing a boost voltage (U.sub.HV) which is measured in the high-voltage network (3) to an expected boost voltage (U.sub.HV,soll).

2. The method as claimed in claim 1, further comprising confirming the short circuit when a difference of the measured boost voltage (U.sub.HV) to the expected boost voltage (U.sub.HV,soll) deviates beyond a threshold value.

3. The method as claimed in claim 1, connecting the traction battery (5) to the high-voltage network (3) when a difference of the measured boost voltage (U.sub.HV) to the expected boost voltage (U.sub.HV,soll) is below a threshold value or corresponds to the threshold value.

4. The method as claimed in claim 1, further comprising determining whether energy (E.sub.v) available in the low-voltage network (4) is greater than stored energy in an intermediate circuit of an intermediate circuit capacitor (C.sub.ZK) of the high-voltage network (3).

5. The method as claimed in claim 4, wherein the energy (E.sub.v) available in the low-voltage network (4) is compared with the stored energy in the intermediate circuit taking based on the efficiency of the DC-DC converter (11) and/or a power loss in the high-voltage network (3).

6. The method as claimed in claim 1, wherein the expected boost voltage (U.sub.HV,soll) is determined based on an input power of the DC-DC converter (11).

7. The method as claimed in claim 1, wherein the expected boost voltage (U.sub.HV,soll) is determined based on the efficiency of the DC-DC converter (11) and/or the power loss in the high-voltage network (3).

8. The method as claimed in claim 1, wherein the expected boost voltage (U.sub.HV,soll) is determined based on a longitudinal resistance (R.sub.1) and a parallel resistance (R.sub.2) relative to the intermediate circuit capacitor (C.sub.ZK).

9. A device for operating a motor vehicle (2) which has an electrical high-voltage network (3) and an electrical low-voltage network (4), wherein the high-voltage network (3) has at least one traction battery (5) and at least one electric drive motor (6), wherein the low-voltage network (4) has a vehicle electrical system battery (9), and wherein the high-voltage network (3) is configured to be connected to the low-voltage network (4) by a DC-DC converter (11), as the device comprising a control unit (12) configured to check the plausibility of the detected short circuit before reconnecting the traction battery (5) to the high-voltage network (3) by supplying the high-voltage network (3) with a boost current (I.sub.HV) from the low-voltage network (4) via the DC-DC converter (11); and compare a boost voltage (U.sub.HV) which is measured in the high-voltage network (3) to an expected boost voltage (U.sub.HV,soll).

10. An electrical system (1) of a motor vehicle (2) with the high-voltage network (3) for the traction battery (5) and the low-voltage network (4) for the vehicle electrical system battery (9), wherein the high-voltage network (3) is connected to the low-voltage network (4) by the DC-DC converter (11), comprising the device as claimed in claim 9.

11. The electrical system (1) according to claim 10, wherein the control unit (12) is configured to confirm the short circuit when a difference of the measured boost voltage (U.sub.HV) to the expected boost voltage (U.sub.HV,soll) deviates beyond a threshold value.

12. The electrical system (1) according to claim 10, wherein the control unit (12) is configured to connect the traction battery (5) to the high-voltage network (3) when a difference of the measured boost voltage (U.sub.HV) to the expected boost voltage (U.sub.HV,soll) is below a threshold value or corresponds to the threshold value.

13. The electrical system (1) according to claim 10, wherein the control unit (12) is configured to determine whether energy (E.sub.v) available in the low-voltage network (4) is greater than stored energy in an intermediate circuit of an intermediate circuit capacitor (C.sub.ZK) of the high-voltage network (3) to check the plausibility of the detected short circuit.

14. The electrical system (1) according to claim 13, wherein the control unit (12) is configured to compare the energy (E.sub.v) available in the low-voltage network (4) with the stored energy in the intermediate circuit taking based on the efficiency of the DC-DC converter (11) and/or a power loss in the high-voltage network (3).

15. The electrical system (1) according to claim 10, wherein the control unit (12) is configured to determine the expected boost voltage (U.sub.HV,soll) based on an input power of the DC-DC converter (11).

16. The electrical system (1) according to claim 10, wherein the control unit (12) is configured to determine the expected boost voltage (U.sub.HV,soll) based on a longitudinal resistance (R.sub.1) and a parallel resistance (R.sub.2) relative to the intermediate circuit capacitor (C.sub.ZK).

17. The method as claimed in claim 2, further including reconnecting the traction battery with the high-voltage network (3) when the difference of the measured boost voltage (UHV) to the expected boost voltage (UHV,soll) is below the threshold value or corresponds to the threshold value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is to be explained in greater detail hereinafter using the drawings. In the drawings:

(2) FIG. 1 shows an electrical system of a motor vehicle in a simplified representation,

(3) FIG. 2 shows a simplified representation of an exemplary embodiment of the DC-DC converter of the electrical system,

(4) FIG. 3 shows a further exemplary embodiment of the DC-DC converter in a simplified representation,

(5) FIG. 4 shows voltage curves of the DC-DC converter and

(6) FIG. 5 shows an advantageous method for operating the motor vehicle.

DETAILED DESCRIPTION

(7) In a simplified representation, FIG. 1 shows the electrical system 1 of a motor vehicle 2 which is not represented in greater detail here. The system 1 has a high-voltage network 3 and a low-voltage network 4. A traction battery 5, an electric machine 6, an inverter 7 which is connected upstream of the electric machine 6, as well as at least one high-voltage consumer 8 are associated with the high-voltage network 3. The low-voltage network 4 has a vehicle electrical system battery 9 as well as at least one low-voltage consumer 10.

(8) The two networks 3, 4 are or can be electrically connected to one another by a controllable DC-DC converter 11.

(9) Furthermore, a control unit 12 is present which actuates the DC-DC converter 11, the inverter 7 as well as the consumers 8, 10 and a switching device 13 associated with the high-voltage battery. The switching device 13 is designed to isolate the traction battery from the high-voltage network 3 if required. For this purpose, the switching device 13 in particular has actuatable battery contactors. Moreover, the control unit 12 is designed to carry out short circuit monitoring for the high-voltage network 3. Monitoring will check whether a short circuit is present in the high-voltage network 3. If this is the case, the switching device 13 is actuated and the traction battery 5 is isolated from the high-voltage network 3. This results in an optionally present short circuit current being interrupted and the high-voltage network 3 being disconnected and transferred into a safe state. However, the short circuit identification function can also have false triggers which are not caused by an actual short circuit in the high-voltage network 3. It is therefore advantageous to restart the high-voltage network 3 after opening the battery contactors and after a predeterminable period of time has passed, for example, by actuating the switching device 13 to restore the connection between the traction battery 5 and the high-voltage network 3. As a result, the availability of the entire system 1 is increased.

(10) In order to ensure that the switching device 13 has not been actuated or opened as a result of a short circuit which is actually present and that reconnecting the traction battery 5 to the high-voltage network does not lead to a further short circuit, the method described hereinafter is provided, by means of which a short circuit test or rather a plausibility check is carried out before reconnecting the traction battery 5 to the high-voltage network 3.

(11) For this purpose, all consumers 8 and 10 are firstly deactivated by the control unit 12 and a so-called boost operation is set up by means of the DC-DC converter 11, by means of which boost operation the high-voltage network 3 is fed or supplied with energy by the low-voltage network 4.

(12) FIG. 2 shows, in a simplified manner, the DC-DC converter 11 which is connected between the high-voltage network 3 and the low-voltage network 4. The DC-DC converter 11 is designed as a bidirectionally operating DC-DC converter. In order to carry out the boost process, it has a rectifier on the low-voltage side and an inverter 14 on the high-voltage side. A transformer 15 is connected between the rectifier 13 and inverter 14, which transformer raises the voltage level of the low-voltage network 4 to that of the high-voltage network 3. An intermediate circuit capacitor C.sub.zk is connected on the high-voltage side, parallel to the inverter 14.

(13) The control unit 12 firstly checks whether the electrical energy in the low-voltage network 4 is sufficient in order to charge the high-voltage network 3. The available electrical energy in the low-voltage network depends heavily on the current charge state of the vehicle electrical system battery 9. Depending on the operating state, the voltage in the low-voltage network should at least maintain one voltage in order to supply the consumer 10, in particular the safety-relevant consumer 10. The electrical energy which is actually available in the vehicle electrical system battery 9 should be determined for the boost operation. Taking into account the current operating temperature T, the charge state SOC of the vehicle electrical system battery 9 (SOC=state of charge), the operative condition SOH of the vehicle electrical system battery 9 (SOH—state of health) and the lower battery voltage threshold U.sub.S, which depends on the current operating state, the available electrical energy emerges from:
E.sub.v=f(T,SOC,SOH,U.sub.S)

(14) A short circuit point does not always have good conductivity if the voltage acting on the short circuit point is small. The voltage in the high-voltage network 3 is therefore increased to such an extent that at least one normal operating voltage U.sub.HV,min is reached. Therefore, the following applies to the energy E.sub.zk,min stored in the intermediate circuit or intermediate circuit capacitor C.sub.zk:
E.sub.zx, min=½C.sub.zkU.sub.HV, min.sup.2

(15) Taking into account the efficiency n of the DC-DC converter 11 and the power loss E.sub.vl in the high-voltage network 3, which results from a self-discharge and/or the discharge resistor, for example, the following condition should be checked:

(16) $E_v>\frac{E_{\mathrm{zk},\min }}{n}+E_{\mathrm{vl}}$

(17) If this condition is met, the boost operation can be carried out. Correspondingly, the control unit 12 triggers the DC-DC converter 11 to drive a boost current I.sub.HV into the high-voltage network 3. With the traction battery 5 isolated from the high-voltage network 3 and the disconnected consumers 8, the intermediate circuit voltage would have to increase during the boost operation.

(18) Hereinafter, an expected boost voltage U.sub.HV,soll is compared with a measured boost voltage U.sub.HV,mess If the difference between these two exceeds a predeterminable threshold value U.sub.HV,S, a malfunction is identified or the short circuit which is present in the high-voltage network 3 is confirmed/checked for plausibility. As a result, the DC-DC converter 11 is deactivated again by the control unit 12 and the boost operation is stopped. Moreover, the traction battery is prevented from being reconnected to the high-voltage network 3.

(19) The expected boost voltage U.sub.HV,Soll is preferably determined by one of the two methods described hereinafter.

(20) For the first method, the input power P.sub.DCDC,in of the DC-DC converter 11 is determined in the boost operation by measuring the input voltage of the low-voltage network U.sub.lv and measuring the input current I.sub.lv:
P.sub.DCDC,in=U.sub.LVI.sub.LV

(21) Taking into account the efficiency n and the power loss in the high-voltage network 3 without the traction battery 5 P.sub.vl, the power for charging the high-voltage network is to be calculated as follows:
P.sub.HV=nU.sub.LV−P.sub.vl

(22) From the energy balance, the following then applies:

(23) ${\int }_{t=t1}^x{P}_{\mathrm{HV}}\mathrm{dt}=\frac{1}{2}{C}_{\mathrm{ZK}}{U}_{\mathrm{HV},\mathrm{soll}}^2$

(24) The expected voltage U.sub.HV,soll is calculated at any time by adjusting the above formula.

(25) In the second method, a simplified circuit model is set up as shown in FIG. 3. A high-voltage current I.sub.HVDCDC as well as the boost voltage U.sub.HV(t=t.sub.1.sub.) at the starting point of the boost operation are measured. A longitudinal resistance R1 and parallel resistance R2 are preferably estimated or determined by way of experiments. The intermediate circuit capacitor C.sub.ZK is already taken into account in the circuit design. The following therefore applies:
U.sub.HV,soll=U.sub.HVDCDC; Mod=f(I.sub.RVDCDC(t),R.sub.1R.sub.2,C,U.sub.HV(t=t.sub.1))

(26) The expected boost voltage U.sub.HV,soll is compared with the measured boost voltage U.sub.HV. If their difference exceeds U.sub.HV,diff then a short circuit is ascertained or checked for plausibility in the high-voltage network 3. In this case, in order to avoid a possible thermal overload of components in the high-voltage network 3, the boost operation is interrupted. As already mentioned previously, the switching device 13 is also prevented from being actuated again in order to connect the traction battery 5 to the high-voltage network 3.

(27) If the difference between the expected boost voltage and the measured boost voltage remains below the threshold value U.sub.HV,diff during the boost operation, the high-voltage network 3 can be reactivated and in addition the switching device 13 can be actuated, so that the traction battery 5 is reconnected to the high-voltage network 3.

(28) FIG. 4 shows exemplary voltage curves U over time t by means of two error situations. In the first case, the short circuit occurs only after reaching a specific voltage in the high-voltage network 3 (U.sub.HV,mess1). and, in the second case, the short circuit permanently precharges (U.sub.HV,mess2).

(29) FIG. 5 shows the method described above once again summarized in a simplified manner. In step S1, the system 1 is put into operation and normal operation is assumed. In step S2, the high-voltage network 3 is monitored for short circuits. Step S1 continues if no short circuit is present. However, if a short circuit is identified (j), the switching device 13 is actuated in the subsequent step S3 in order to isolate the traction battery 5 from the high-voltage network 3.

(30) In a step S4, it is then checked whether the energy which is available in the low-voltage network is sufficient in order to charge the high-voltage network 3. If this is not the case (n), the high-voltage network 3 is permanently deactivated in a step S5 by permanently preventing the traction battery 5 from being reconnected to the high-voltage network 3.

(31) However, if the energy is sufficient (j), the boost operation is carried out in a step S6, as explained previously, and the above-described query to check the plausibility of the short circuit is carried out in a step S7 by comparing the expected boost voltage with a measured boost voltage. In this case, if the short circuit is confirmed/checked for plausibility, the system 1 is disconnected according to step S5. However, if the short circuit is not confirmed (n), step S1 continues by actuating the switching device 13 in order to connect the traction battery 5 to the high-voltage network 3.