Detecting ground faults in DC grids

Abstract

The invention relates to a method for detecting a ground fault in a load area (3) that can be connected to a DC grid (1), the load area having a first load area line (31) that can be connected to a first main line (7) of the DC grid (1) via a protection switch (45), and having a second load area line (32) that can be connected to a second main line (9) of the DC grid (1) via an interrupter switch (51). In the method, at least one line voltage applied to the load area line (31, 32) is continuously measured. If no ground fault is detected on the main line (7, 9) of the DC grid (1) before the load area (3) is connected, the interrupter switch (51) is closed when the protection switch (45) is open, and if at least one line voltage does not change significantly after the interrupter switch (51) is closed, it is concluded that a ground fault exists in the load area (3).

Description

Technical Field

[0001] The invention relates to a method for detecting a ground fault in a load region which can be connected to a DC grid, and to a connection unit for connecting the load region to the DC grid. Background Art

[0002] A DC grid that is insulated from the ground potential or grounded by a high impedance can continue to operate in the event of a ground fault in the DC voltage potential. During operation, the ground fault itself is easily detected by measuring and evaluating the voltage between the DC voltage potential of the DC grid and the ground potential. However, it is difficult to identify the load area in which the ground fault occurs, which is powered by the DC grid. However, in order to eliminate the ground fault, it is important to identify the load area in which the ground fault occurs. In order to identify the faulty load area, for example, the so-called common mode current, i.e., the fault current, which is equal to the sum of the currents in the two connecting lines of the load area and the main line of the DC grid can be measured at the connection point connected to the DC grid in each load area, such as at the input end of the switch cabinet. If one of the common mode currents is not equal to zero, this indicates that a ground fault has occurred in the associated load area. The measurement of the common mode current can be performed, for example, continuously or manually by maintenance personnel. However, such measurement is costly in any case. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide an improved method for detecting ground faults in the region of a load that can be connected to a DC power system.

[0004] According to the invention, the above-mentioned technical problem is solved by a method having the features according to the invention and an access unit having the features according to the invention.

[0005] The method according to the invention is used to detect a ground fault in a load area that can be connected to a DC power grid, the load area having a first load area line, which is grounded via a parallel circuit formed by a first grounding capacitor and a first grounding resistor and can be connected to a first main line of the DC power grid via a protective switch; and having a second load area line, which is grounded via a parallel circuit formed by a second grounding capacitor and a second grounding resistor and can be connected to a second main line of the DC power grid via an interrupter switch. In the method, at least one line voltage associated with the load area line between the line potential at which the load area line is located and the ground potential is continuously measured. If no ground fault is detected on the main line of the DC power grid before the load area is connected, the interrupter switch is closed when the protective switch is open, and if at least one line voltage does not change significantly after the interrupter switch is closed, it is inferred that there is a ground fault in the load area. It is always assumed here and in the following that the DC power grid is insulated from the ground potential or grounded via a high impedance.

[0006] The method makes use of the fact that before the interrupter switch is switched on, both grounding capacitors are discharged, so that both line voltages are zero, and therefore after the interrupter switch is switched on, if there is a ground fault on one of the load area lines, both line voltages will remain at zero or at a very low value. Thus, a ground fault in the load area can be detected in such a way that at least one line voltage does not change or changes only slightly after the interrupter switch is switched on. Therefore, the method particularly advantageously makes it possible to detect a ground fault in the load area even before the load area is connected to the DC grid via the protection switch.

[0007] One embodiment of the method provides that the associated line voltages are respectively continuously measured and evaluated for both load region lines. This advantageously allows for more reliable detection of ground faults in the load region than if only one line voltage were measured and evaluated.

[0008] Furthermore, it can be provided that a first delay period and a first absolute or relative minimum change are predefined and that a change in the line voltage after switching on the interrupter switch is classified as significant if the change in the line voltage after the first delay period exceeds the first minimum change after switching on the interrupter switch.

[0009] By predetermining the first minimum change in the line voltage, it is possible to quantify when a change in the line voltage after the interrupter switch is switched on is considered significant. As a result, it is advantageously possible to reduce or completely avoid erroneous judgments caused by changes in the line voltage (even if they occur due to a ground fault). Predetermining a first delay duration for evaluating the change in the line voltage advantageously makes it possible to advantageously reduce or completely avoid erroneous detections of ground faults caused by oscillation processes after the protection switch is switched on. The first delay duration is predetermined, for example, in the range of 0.1 ms to 100 ms. In particular, approximately 1 ms can be predetermined as the first delay duration. The first minimum change is predetermined, for example, in the range of 5% to 50% of the rated voltage of the DC grid. In particular, it can be approximately 16% of the rated voltage.

[0010] Another embodiment of the method provides that the first load area line can be connected to the first main line by a series circuit formed by a pre-charging switch and a pre-charging resistor connected in parallel with the protection switch, in addition to being connected to the first main line via the protection switch, and in the case of a ground fault detected on the second main line of the DC grid before the load area is connected, the pre-charging switch is switched on when the protection switch is opened and the interrupter switch is opened, and if at least one line voltage does not change significantly after the pre-charging switch is switched on, it is inferred that there is a ground fault in the load area. If the two load area lines are connected to each other via a storage capacitor and the charge state of the storage capacitor is acquired, and if the storage capacitor is not significantly charged after the pre-charging switch is switched on, it can be further inferred that there is a ground fault on the first load area line, and / or if the storage capacitor is significantly charged after the pre-charging switch is switched on, it can be inferred that there is a ground fault on the second load area line.

[0011] The above-described embodiment of the method utilizes the fact that, in the case of a ground fault on the second main line, after the precharging switch is switched on, if a ground fault is present on the load area line, the line voltage associated with the load area line remains at zero or a very low value. In this case, the precharging resistor can prevent a short circuit from occurring immediately in the DC grid due to the switching on of the precharging switch. In contrast, if a ground fault is present on the first main line before the load area is switched on, a ground fault detection in the load area cannot be performed in a corresponding manner, because the switching on of the precharging switch does not cause a potential change in the load area, and the switching on of the interrupter switch immediately produces a short circuit in the DC grid.

[0012] Furthermore, it can be provided that a second delay duration and an absolute or relative second minimum change are predefined, and that the change in the line voltage after the pre-charging switch is switched on is classified as significant if the change in the line voltage after the second delay duration exceeds the second minimum change. In this case, the second delay duration can be consistent with the first delay duration, and the second minimum change can be consistent with the first minimum change. For example, the charging of the storage capacitor is classified as significant if the storage capacitor is charged with a predetermined minimum charge amount within a predetermined measuring duration. For example, the measuring duration is predefined in the range of 100 ms to 5 s, in particular, the measuring duration is predefined as approximately 1 s, and the minimum charge amount is predefined in the range of 10% to 50% of the rated voltage of the DC grid, in particular, the minimum charge amount is predefined as approximately 20% of the rated voltage.

[0013] The access unit according to the present invention is used to access a load area to a DC grid, wherein the DC grid has two main lines and a ground fault monitoring unit, wherein the ground fault monitoring unit is designed to detect a ground fault on each main line. The access unit includes:

[0014] a protection switch connected between the first main line and a first load area line of the load area,

[0015] - an interrupter switch connected between the second main line and a second load area line of the load area,

[0016] a measuring unit for at least one load region line, the measuring unit being designed to continuously measure a line voltage associated with the load region line between a line potential at which the load region line is situated and a ground potential, and

[0017] - a control unit, the control unit being designed to control the protection switch and to identify the switching state of the interrupter switch; to receive measurement signals of at least one measuring unit and a ground fault monitoring unit; and to infer the presence of a ground fault in the load area if no ground fault is detected on the main line of the DC grid before the load area is connected, if at least one line voltage does not change significantly after the interrupter switch is turned on when the protection switch is turned off.

[0018] In addition, the access unit can have a series circuit formed by a pre-charging switch and a pre-charging resistor connected in parallel with the protective switch, and the control unit can be designed to identify the switching state of the pre-charging switch and, in the case where a ground fault is detected on the second main line of the DC grid before the load area is connected, infer that there is a ground fault in the load area if, after the pre-charging switch is turned on, the line voltage does not change significantly when the protective switch is disconnected and the interrupter switch is disconnected.

[0019] The access unit according to the invention can implement the method according to the invention with the advantages described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above characteristics, features and advantages of the present invention and the manner in which they are achieved will become clearer and easier to understand in conjunction with the following description of the embodiments explained in more detail in conjunction with the accompanying drawings. In this drawing:

[0021] Figure 1 A block diagram showing a DC grid to which multiple load areas can be connected,

[0022] Figure 2 A block diagram showing the load region,

[0023] Figure 3 The block diagram of a connection unit is shown, with which a load area can be connected to a DC grid.

[0024] In the drawings, parts corresponding to each other have the same reference numerals. DETAILED DESCRIPTION

[0025] Figure 1 A DC grid 1 is schematically shown, to which a plurality of load areas 3, 4, 5 can be connected. The DC grid 1 is insulated from ground potential or is grounded via a high impedance.

[0026] The DC grid 1 has two main lines 7 , 9 , a connection unit 10 , 11 , 12 for each load area 3 , 4 , 5 , as well as a grid control unit 13 and a ground fault monitoring unit 15 .

[0027] In the figure, three load areas 3, 4, 5 are shown by way of example. The first load area 3 and the second load area 4 respectively have converters 19, 20 and motors 21, 22. The third load area 5 has a converter 23, a switch and protection unit 25 for AC power, and an AC grid 27, wherein the switch and protection unit 25 is connected between the AC grid 27 and the converter 23. Each converter 19, 20, 23 is connected to a connection unit 10, 11, 12.

[0028] The first main line 7 is at a first DC voltage potential of the DC grid 1 . The second main line 9 is at a second DC voltage potential of the DC grid 1 .

[0029] Each access unit 10 , 11 , 12 is associated with a load area 3 , 4 , 5 which can be connected to the DC grid 1 via the access unit 10 , 11 , 12 .

[0030] The grid control unit 13 is connected to the access units 10, 11, 12 and the ground fault monitoring unit 15. The access of the load areas 3, 4, 5 is controlled by the grid control unit 13.

[0031] The ground fault monitoring unit 15 is designed to detect ground faults on the main lines 7, 9. To this end, the ground fault monitoring unit 15 is connected to each main line 7, 9 and is designed to measure the voltage between the ground potential and the DC voltage potential on the main line 7, 9. In addition, the ground fault monitoring unit 15 is designed to report the detected ground fault to the access units 10, 11, 12 and the grid control unit 13.

[0032] Figure 2 The block diagram of the load region 3 is shown by way of example. The load region 3 has a first load region line 31, which is ohmically capacitively grounded with high impedance via a parallel circuit formed by a first grounding capacitor 33 and a first grounding resistor 35. In addition, the load region 3 has a second load region line 32, which is ohmically capacitively grounded with high impedance via a parallel circuit formed by a second grounding capacitor 34 and a second grounding resistor 36. The grounding capacitors 33, 34 have the same capacitance so that during fault-free operation, the line potential at which the load region lines 31, 32 are located remains symmetrical with respect to the ground potential. In addition, the load region lines 31, 32 are connected to one another via a storage capacitor 37.

[0033] Figure 3 The block diagram shows a connection unit 10 associated with the load area 3. The connection unit 10 has two grid-side connections 41, 42, two load area-side connections 43, 44, a protection switch 45, a pre-charging switch 47, a pre-charging resistor 49, an interrupter switch 51, a control unit 52 and two measuring units 53, 54.

[0034] The first grid-side connection 41 is connected to the first main line 7. The second connection 42 is connected to the second main line 9. The first load-area-side connection 43 is connected to the first load-area line 31. The second load-area-side connection 44 is connected to the second load-area line 32.

[0035] The protection switch 45 is connected between the first grid-side connection 41 and the first load-area-side connection 43. The protection switch 45 has two semiconductor switches 55, 56 that can be turned off and two diodes 57, 58. The semiconductor switches 55, 56 are connected in anti-series, that is, the semiconductor switches 55, 56 are connected in series but have opposite conduction directions. Each diode 57, 58 is connected in anti-parallel with one of the semiconductor switches 55, 56. The protection switch 45 is designed as a fast switch that can turn off a short-circuit current.

[0036] The pre-charging switch 47 and the pre-charging resistor 49 are connected in series, wherein the series circuit formed by the pre-charging switch 47 and the pre-charging resistor 49 is connected in parallel to the protection switch 45 between the first grid-side connection 41 and the first load area-side connection 43. The pre-charging switch 47 is, for example, a mechanical switch.

[0037] The interrupter switch 51 is connected between the second grid-side connection 42 and the second load-range-side connection 44. The interrupter switch 51 is, for example, a mechanical load disconnect switch.

[0038] The first measuring unit 53 is designed to continuously measure a first line voltage between a line potential of the first load region line 31 and a ground potential, which is associated with the first load region line 31. The second measuring unit 54 is designed to continuously measure a second line voltage between a line potential of the second load region line 32 and a ground potential, which is associated with the second load region line 32. The measuring units 53 and 54 transmit the measurement signals they acquire to the control unit 52.

[0039] The control unit 52 is designed to control the protection switch 45 or its semiconductor switches 55, 56 and to recognize the switching states of the pre-charging switch 47 and the interrupter switch 51. In addition, the control unit 52 has an interface 57, through which the control unit is connected to the grid control unit 13 and the ground fault monitoring unit 15, and through which the control signal of the grid control unit 13 is transmitted to the control unit and the ground fault detected by the ground fault monitoring unit 15 is reported.

[0040] If the load area 3 is disconnected from the DC grid 1, the protective switch 45 (or the two semiconductor switches 55, 56), the pre-charging switch 47 and the interrupter switch 51 are opened. If there is no ground fault, in order to connect the load area 3, the interrupter switch 51 is first turned on, then the pre-charging switch 47 is turned on, and finally the protective switch 45 is turned on after the pre-charging of the capacitors 33, 34, 37 is completed.

[0041] According to the invention, if no ground fault is detected on the main lines 7, 9 of the DC grid before the load area 3 is connected, the line voltage is evaluated by the control unit 52 after the interrupter 51 is switched on, but before the pre-charging switch 47 and the protective switch 45 are switched on, wherein a check is made as to whether a significant change in the line voltage occurs after the interrupter 51 is switched on. If at least one line voltage does not change significantly, it is inferred that a ground fault exists in the load area 3. For this purpose, for example, a first delay duration and an absolute or relative first minimum change are predefined for each line voltage, and if a change in the line voltage after the first delay duration after the interrupter 51 is switched on exceeds the first minimum change, the change in the line voltage after the interrupter 51 is switched on is classified as significant.

[0042] In the event that a ground fault is detected on the second main line 9 of the DC grid 1 before the load area 3 is connected, the pre-charging switch 47 is first switched on instead of the interrupting switch 51, and the line voltage is evaluated before the interrupting switch 51 and the protection switch 45 are switched on. If the line voltage does not change significantly after the pre-charging switch 47 is switched on, it is inferred that there is a ground fault in the load area 3. For this purpose, for example, a second delay duration and an absolute or relative second minimum change are predefined for each line voltage, and if the change in the line voltage after the second delay duration after the pre-charging switch 47 is switched on exceeds the second minimum change, the change in the line voltage after the pre-charging switch 47 is switched on is classified as significant. In this case, the pre-charging resistor 49 prevents the switching on of the pre-charging switch 47 from immediately causing a short circuit in the DC grid 1. In addition, the charging state of the storage capacitor 37 can be acquired, and if the storage capacitor 37 is not significantly charged after the pre-charging switch 47 is turned on, it can be inferred that there is a ground fault on the first load area line 31, and if the storage capacitor 37 is significantly charged after the pre-charging switch 47 is turned on, it can be inferred that there is a ground fault on the second load area line 32.

[0043] If there is a ground fault on the first main line 7 before the load area 3 is connected, it is not possible to perform ground fault detection in the load area 3 in a corresponding manner, because the connection of the pre-charging switch 47 will not cause a potential change in the load area 3, and if there is already a ground fault on the load area line 32, the connection of the interrupter switch 51 will immediately produce a short circuit in the DC grid 1.

[0044] A ground fault in the load range 3 detected by the control unit 52 is reported by the control unit 52 to the grid control unit 13 via the interface 57 .

[0045] The embodiments of the method according to the invention and the access unit 10 according to the invention which have been described with reference to the drawings can be modified in different ways to form further embodiments. For example, it can be provided that the line voltage acquired by the measuring units 53, 54 is evaluated by the grid control unit 13 instead of being evaluated locally by the control unit 52 of the access unit 10.

[0046] Alternatively or additionally, provision can be made for only the first line voltage or only the second line voltage to be measured and evaluated.

[0047] For example, it can be provided that only the first line voltage is measured and evaluated. In the case that no ground fault is detected on the main lines 7, 9 of the DC grid before the load area 3 is connected, the interrupter switch 51 is first switched on again, and the first line voltage is evaluated before the pre-charging switch 47 and the protective switch 45 are switched on. If the first line voltage does not change significantly after the interrupter switch 51 is switched on, it is inferred that there is a ground fault in the load area 3. In the case that a ground fault is detected on the second main line 9 of the DC grid 1 before the load area 3 is connected, the pre-charging switch 47 is first switched on, and the line voltage is evaluated before the interrupter switch 51 and the protective switch are switched on. If the first line voltage does not change significantly after the pre-charging switch 47 is switched on, it is inferred that there is a ground fault in the load area 3.

[0048] Although the present invention has been illustrated and described in detail through preferred embodiments, the present invention is not limited to the disclosed examples and those skilled in the art may derive other variations therefrom without departing from the scope of protection of the present invention.

Claims

1. A method for detecting a ground fault in a load area (3) that can be connected to a DC grid (1), the load area having a first load area line (31), the first load area line (31) being grounded via a parallel circuit formed by a first grounding capacitor (33) and a first grounding resistor (35), and being connectable to a first main line (7) of the DC grid (1) via a protection switch (45), and the load area having a second load area line (32), the second load area line being grounded via a parallel circuit formed by a second grounding capacitor (34) and a second grounding resistor (36), and being connectable to a second main line (9) of the DC grid (1) via an interrupter switch (51), wherein - continuously measuring at least one line voltage associated with the first and second load region lines (31, 32) between the line potential at which the first and second load region lines (31, 32) are located and ground potential, and - before the load area (3) is connected, when no ground fault is detected on the first main line (7) and the second main line (9) of the DC grid (1), the interrupter switch (51) is switched on when the protection switch (45) is switched off, - and if at least one line voltage does not change significantly after the interrupter switch (51) is switched on, it is concluded that a ground fault is present in the load region (3).

2. The method according to claim 1, wherein: The associated line voltage is respectively measured and evaluated continuously for two load region lines (31, 32).

3. The method according to claim 1 or 2, wherein: A first delay duration and a first absolute or relative minimum change are predefined, and a change in the line voltage after the interrupter (51) is switched on is classified as significant if the change in the line voltage after the first delay duration exceeds the first minimum change after the interrupter (51) is switched on.

4. The method according to claim 3, wherein: The first delay period is predefined in the range of 0.1 ms to 100 ms.

5. The method according to claim 3, wherein: The first minimum change is predefined in the range of 5% to 50% of the rated voltage of the DC network (1).

6. The method according to claim 1 or 2, wherein: In addition to being connectable to the first main line (7) via the protection switch (45), the first load area line (31) can also be connected to the first main line (7) via a series circuit formed by a pre-charging switch (47) and a pre-charging resistor (49), the series circuit being connected in parallel with the protection switch (45), and wherein, in the event of a ground fault being detected on the second main line (9) of the DC grid (1) before the load area (3) is connected, the pre-charging switch (47) is turned on when the protection switch (45) is opened and the interrupter switch (51) is opened, and if at least one line voltage does not change significantly after the pre-charging switch (47) is turned on, it is inferred that there is a ground fault in the load area (3).

7. The method according to claim 6, wherein: Two load range lines (31, 32) are connected to one another via a storage capacitor (37) and the charge state of the storage capacitor (37) is detected, wherein if the storage capacitor (37) is not significantly charged after the pre-charging switch (47) is switched on, it is inferred that a ground fault is present on the first load range line (31).

8. The method according to claim 7, wherein: If the storage capacitor (37) is significantly charged after the pre-charging switch (47) is switched on, it is concluded that a ground fault is present on the second load range line (32).

9. The method according to claim 6, wherein: A second delay duration and an absolute or relative second minimum change are predetermined, and a change in the line voltage after the pre-charging switch (47) is turned on is classified as significant if the change in the line voltage over the second delay duration after the pre-charging switch (47) is turned on exceeds the second minimum change.

10. An access unit (10) for accessing a load area (3) to a DC grid (1), the DC grid having two main lines (7, 9) and a ground fault monitoring unit (15), the ground fault monitoring unit being designed to detect a ground fault on the main line (7, 9) for each main line (7, 9), the access unit (10) comprising: a protection switch (45) connected between the first main line (7) and a first load zone line (31) of the load zone (3), - an interrupter switch (51) connected between a second main line (9) and a second load zone line (32) of the load zone (3), a measuring unit (53, 54) for at least one load region line (31, 32), the measuring unit being designed to continuously measure a line voltage associated with a first or second load region line (31, 32) between a line potential at which the first or second load region line (31, 32) is located and a ground potential, and - a control unit (52), which is designed to control the protection switch (45) and to identify the switching state of the interrupter switch (51); The invention relates to a method for detecting a ground fault in a first main line (7) and a second main line (9) of a DC power grid (1) and determining whether a ground fault exists in the load area (3). The method comprises: receiving measurement signals of at least one measurement unit (53, 54) and the ground fault monitoring unit (15); and in a case where no ground fault is detected on a first main line (7) and a second main line (9) of the DC power grid (1) before the load area (3) is connected, and if at least one line voltage does not change significantly when the protection switch (45) is disconnected after the interrupter switch (51) is turned on, it is inferred that a ground fault exists in the load area (3).

11. The access unit (10) according to claim 10, comprising a series circuit formed by a pre-charging switch (47) and a pre-charging resistor (49), the series circuit being connected in parallel with the protection switch (45), wherein the control unit is designed to identify the switching state of the pre-charging switch (47) and, in the case where a ground fault is detected on the second main line (9) of the DC grid (1) before the load area (3) is connected, infer that there is a ground fault in the load area (3) if at least one line voltage does not change significantly after the pre-charging switch (47) is turned on when the protection switch (45) is turned off and the interrupter switch (51) is turned off.

Images