Location of ground faults in DC grids

Abstract

The invention relates to a method for locating a ground fault in a DC grid (1) to which a plurality of load zones (3, 5) can be connected, wherein each load zone (3, 5) has two symmetrically grounded load zone lines (31, 32). In the method, a line voltage between a line potential at which the load zone line (31, 32) is located and a ground potential is continuously measured for each load zone line (31, 32). In the event of a ground fault being detected on a main line (7, 9) of the DC grid (1), the main line (7, 9) is separated from the two load zone lines (31, 32) of the load zone (3, 5), and if the difference in magnitude between the magnitudes of the line voltages of the two load zone lines (31, 32) of the load zone (3, 5) does not significantly decrease after the separation of the main line (7, 9) from the two load zone lines (31, 32), the ground fault is associated with the load zone (3, 5).

Description

Technical Field

[0001] The invention relates to a method for locating a ground fault in a DC network, to which a plurality of load areas can be connected. 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 case 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 connected to the DC grid in which the ground fault occurs. However, in order to shut down the faulty load area and 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] Therefore, the technical problem to be solved by the present invention is to specify an improved method for locating a ground fault in a DC network, to which a plurality of load areas can be connected.

[0004] According to the invention, the above-mentioned object is achieved by a method having the features according to the invention and a DC power supply system having the features according to the invention.

[0005] The method according to the invention is used for locating a ground fault in a DC grid, to which a plurality of load zones can be connected, wherein each load zone has two symmetrically grounded load zone lines, wherein a first load zone line can be connected to a first main line of the DC grid and a second load zone line can be connected to a second main line of the DC grid. In the method, a line voltage between a line potential at which the load zone line is located and a ground potential is continuously measured for each load zone line. In the case of a ground fault detected on a main line of the DC grid, if the magnitude difference between the magnitudes of the line voltages of the two load zone lines of one of the load zones does not significantly decrease after the main line is separated from the load zone line, the main line is separated from the two load zone lines of the load zone and the ground fault is associated with the load zone. Symmetrical grounding of the two load zone lines of a load zone is understood here as grounding of the same type of the two load zone lines, which in ground-fault-free operation maintains the line potential of the load zone lines symmetrically with respect to the ground potential, for example, ohmic-capacitive grounding of the two load zone lines is respectively realized by a parallel circuit formed by a grounding capacitor and a grounding resistor, the grounding capacitor having the same capacitance and the grounding resistor having the same resistance value. Furthermore, it is always assumed here and below that the DC grid is insulated from the ground potential or is grounded via a high impedance. The connection of the load area line to the main line of the DC grid can be direct or indirect, wherein an indirect connection can exist, for example, when the DC grid is constructed in layers, with the uppermost layer being directly connected to the main line and at least one further layer being connected to the main line via one or more layers located above it.

[0006] The method according to the invention makes use of the fact that a ground fault in a load area line involving a load area causes an asymmetry of the line potential of the load area line relative to the ground potential, so that the two line potentials are no longer symmetrical about the ground potential. If a ground fault occurs outside the load area, for example in another load area or directly on the main line, the symmetry of the line potential of the load area line relative to the ground potential is established again after the main line is separated from the load area line of the load area, because the ground fault no longer has an effect on the line potential. In contrast, when a ground fault occurs in the load area, the asymmetry of the line potential of the load area line relative to the ground potential does not change after the main line is separated from the load area line, or even increases slightly. In other words, when a ground fault occurs outside the load area, the difference in magnitude between the magnitudes of the line voltages of the two load area lines of the load area after the main line is separated from the load area line of the load area decreases, and when a ground fault occurs in the load area, the magnitude remains unchanged or increases. According to the invention, this is fully utilized in order to locate a ground fault detected on the main line.

[0007] In one embodiment of the method, a separation duration is predefined and, after a ground fault on the main line is detected, the main line is separated from the load area line of the load area for the separation duration. This embodiment of the method makes use of the fact that a short separation of the main line from the load area line of the load area is usually sufficient to determine and evaluate the magnitude difference between the magnitudes of the line voltages of two load area lines of the load area. Typically, a separation for a separation duration in the range of 1 ms to 1 s is sufficient, for example. The shortest possible separation of the main line from the load area line of the load area is advantageous so that the operation of the load area is impaired as little as possible by the separation.

[0008] In a further embodiment of the method, an absolute or relative minimum reduction is predefined, and if the reduction in the magnitude difference between the magnitudes of the line voltages of the two load area lines after the main line is separated from the load area lines exceeds the minimum reduction, the reduction in the magnitude difference is classified as significant. For example, the minimum reduction is specified as 2% to 30%, in particular about 10%, of the DC voltage between the main lines in normal operation of the DC grid. By specifying the minimum reduction in the magnitude difference between the magnitudes of the line voltages, it is possible to quantify when the reduction in the magnitude difference after the main line is separated from the load area lines of the load area is considered significant. As a result, incorrect positioning of ground faults due to only a small reduction in the magnitude difference after the main line is separated from the load area lines of the load area can be advantageously reduced or completely avoided.

[0009] In a further embodiment of the method, after the ground fault on the main line is detected, the main line is successively separated from two load area lines of different load areas until the ground fault is associated with a load area. As a result, a systematic search can be conducted for load areas in which the ground fault has occurred. If the ground fault is not associated with any load area after the search is completed, it can also be concluded that the ground fault has occurred directly on the main line in which the ground fault has also been detected.

[0010] The DC power grid according to the present invention comprises:

[0011] - Two main lines,

[0012] a plurality of load zones, each of which has a first load zone line and a second load zone line, wherein the first load zone line can be connected to a first main line of the DC grid via a first switch, and the second load zone line can be connected to a second main line of the DC grid via a second switch,

[0013] a measuring unit for each load region line, the measuring unit being designed to continuously measure the line voltage between the line potential at which the load region line is located and ground potential,

[0014] - a ground fault monitoring unit designed to detect ground faults on the main line,

[0015] a control unit for controlling the switches, the control unit being designed to separate the main line from two load area lines of the load area by opening the switches connecting the load area lines to the main line in the event of a ground fault detected by the ground fault monitoring unit on the main line of the DC network,

[0016] - and an evaluation unit for evaluating the measurement signal of the measuring unit, the evaluation unit being designed to associate a ground fault detected by the ground fault monitoring unit with the load area having a load area line separated from the main line, if the difference in magnitude between the magnitudes of the line voltages of the two load area lines has not significantly decreased after the separation of the main line from the two load area lines.

[0017] For example, the first switch or the second switch of each load zone is an electronic switch, or both switches are electronic switches.

[0018] Furthermore, the control unit can be designed to disconnect the main line from the load area line of the load area only for a predetermined disconnection duration, for example a disconnection duration in the range of 1 ms to 1 s, after detection of a ground fault on the main line.

[0019] Furthermore, the evaluation unit may be designed to classify a reduction in the magnitude difference between the magnitudes of the line voltages of two load region lines of the load region as significant if the reduction in the magnitude difference between the magnitudes of the line voltages of the two load region lines after the main line is separated exceeds a predetermined minimum reduction.

[0020] The DC power supply system according to the invention makes it possible to carry out the method according to the invention with the advantages described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above-described characteristics, features and advantages of the present invention and the manner of achieving them become clearer and easier to understand in conjunction with the following description of an embodiment, which is described in more detail in conjunction with the accompanying drawings. In the accompanying drawings:

[0022] Figure 1 shows a block diagram of a DC grid to which multiple load areas can be connected,

[0023] Figure 2The block diagram shows a load region and a connection unit, by means of which the load region can be connected to a DC grid.

[0024] Parts which correspond to one another are provided with the same reference numerals in the figures. DETAILED DESCRIPTION

[0025] Figure 1 The schematic diagram shows an exemplary embodiment of a DC grid 1 to which a plurality of load areas 3, 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 electrical mains 7 , 9 , a connection unit 10 , 12 for each load area 3 , 5 , as well as a control unit 13 , a ground fault monitoring unit 15 and an evaluation unit 17 .

[0027] exist Figure 1 2 shows by way of example two load areas 3, 5. Each load area 3, 5 has an electrical load 19, 21, for example an electrical machine and a converter connected upstream of the electrical machine on the grid side.

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

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

[0030] The control unit 13 is connected to the switching units 10, 12, the ground fault monitoring unit 15 and the evaluation unit 17. The switching of the load areas 3, 5 is controlled by the control unit 13.

[0031] The ground fault monitoring unit 15 is designed to detect ground faults on the main lines 7, 9. For this purpose, the ground fault monitoring unit 15 is connected to each main line 7, 9 and is designed to measure a voltage between a ground potential and a DC voltage potential at which the main lines 7, 9 are located. Furthermore, the ground fault monitoring unit 15 is designed to report detected ground faults to the control unit 13.

[0032] The evaluation unit 17 is connected to the control unit 13 and the access units 10 , 12 and is designed to evaluate the line voltage of the load area 3 , 5 in a manner described in more detail below.

[0033] Figure 2 By way of example, a block diagram of a load region 3 and a connection unit 10 is shown, with the aid of which the load region 3 can be connected to the DC grid 1 .

[0034] 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. The load region 3 also 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 two load region lines 31, 32 are grounded symmetrically, wherein the grounding capacitor 33 has the same capacitance and the grounding resistors 35, 36 have the same resistance value, so that in fault-free operation the load region potentials at which the load region lines 31, 32 are located remain symmetrical with respect to the ground potential.

[0035] The access unit 10 has two grid-side connections 41 , 42 , two load zone-side connections 43 , 44 , two switches 45 , 51 and two measuring units 53 , 54 .

[0036] 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 .

[0037] The first switch 45 is connected between the first grid-side connection 41 and the first load-area-side connection 43. The first switch 45 is an electronic switch having 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, i.e. the semiconductor switches 55, 56 are connected in series but have opposite conduction directions to each other. Each diode 57, 58 is connected in anti-parallel with one of the semiconductor switches 55, 56. The first switch 45 is designed as a fast switch that can disconnect a short-circuit current.

[0038] The second switch 51 is connected between the second grid-side connection 42 and the second load area-side connection 44. The second switch 51 is, for example, a mechanical load isolating switch. Alternatively, the second switch 51 can also be designed as an electronic switch like the first switch 45. The first switch 45 (or its semiconductor switches 55, 56) and the second switch 51 can be controlled by the control unit 13.

[0039] The first measuring unit 53 is designed to continuously measure a first line voltage associated with the first load range line 31 between a line potential at the first load range line 31 and a ground potential. The second measuring unit 54 is designed to continuously measure a second line voltage associated with the second load range line 32 between a line potential at the second load range line 32 and a ground potential. The measuring units 53, 54 transmit the measurement signals detected by them to the evaluation unit 17.

[0040] The second load zone 5 (and every other load zone connectable to the DC grid 1 ) likewise has two symmetrically grounded load zone lines 31 , 32 and can be connected to the DC grid 1 via a connection unit 12 designed like the connection unit 10 .

[0041] According to the present invention, in the event of a ground fault being detected on the main lines 7, 9 of the DC grid 1, the two main lines 7, 9 are separated from the load area lines 31, 32 of the load areas 3, 5 connected thereto by the control unit 13 causing the switches 45, 51 connecting the load area lines 31, 32 to the main lines 3, 5 to be disconnected for a predetermined short separation duration, for example a separation duration in the range of 1 ms to 1 s.

[0042] After the main line 7, 9 is separated from the load area line 31, 32 of the load area 3, 5, it is checked by the evaluation unit 17 whether the magnitude difference between the magnitudes of the line voltages of the two load area lines 31, 32 of the load area 3, 5 has significantly decreased or whether the magnitudes of the line voltages of the two load area lines 31, 32 are equal to each other. If the magnitude difference has not significantly decreased, the evaluation unit 17 associates a ground fault with this load area 3, 5.

[0043] Furthermore, it can be provided that after detecting a ground fault on a main line 7 , 9 , the main line 7 , 9 is successively disconnected from two load range lines 31 , 32 of different load ranges 3 , 5 for a predetermined disconnection duration in each case until the ground fault is associated with the load range 3 , 5 .

[0044] The separation of the main line 7, 9 from the load area line 31 of the load area 3, 5 is preferably carried out at a time point at which the loads 19, 21 of the load area 3, 5 do not require energy or the required energy can be buffered for the separation duration, for example by a storage capacitor (not shown) which is connected between the load area lines 31, 32 of the load area 3, 5.

[0045] The embodiments of the method according to the invention and the DC grid 1 according to the invention described with reference to the figures can be modified in different ways to other embodiments. For example, it can be provided that the evaluation unit 17 is integrated into the control unit 13 or into the ground fault monitoring unit 15. Alternatively, it can be provided that, instead of the superior evaluation unit 17, a local evaluation unit 17 is provided for each access unit 10, 12, which evaluates the line voltage of the load area line 31, 32 of the access unit 10, 12. In addition, it can be provided that each access unit 10, 12 has a local control unit 13, which is designed to disconnect the switch 45, 51 of the access unit 10, 12 for a disconnection time in the event of a ground fault detected on the main line 7, 9 of the DC grid 1. Here, the local evaluation unit 17 can also be integrated into the local control unit 13.

[0046] Although the present invention has been illustrated and described in detail through the 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 locating a ground fault in a DC grid (1) to which a plurality of load areas (3, 5) can be connected, wherein: Each load zone (3, 5) has two symmetrically grounded load zone lines (31, 32), wherein a first load zone line (31) can be connected to a first main line (7) of the DC grid (1) and a second load zone line (32) can be connected to a second main line (9) of the DC grid (1), wherein - continuously measuring for each load zone line (31, 32) the line voltage between the line potential at which the load zone line (31, 32) is located and ground potential, and - in the event of a ground fault being detected on a main line (7, 9) of the DC grid (1), the main line (7, 9) is disconnected from two load area lines (31, 32) of a load area (3, 5), and if the magnitude difference between the magnitudes of the line voltages of the two load area lines (31, 32) does not significantly decrease after the main line (7, 9) is disconnected from the two load area lines (31, 32) of the load area (3, 5), the ground fault is associated with the load area (3, 5), wherein an absolute or relative minimum decrease is predefined, and if the magnitude difference between the magnitudes of the line voltages of the two load area lines (31, 32) after the main line (7, 9) is disconnected from the two load area lines (31, 32) of the load area (3, 5) exceeds the minimum decrease, the decrease in the magnitude difference is classified as significant.

2. The method according to claim 1, wherein: A separation time is predetermined and, after a ground fault is detected on a main line (7, 9), the main line (7, 9) is separated from a load range line (31, 32) of a load range (3, 5) for the separation time.

3. The method according to claim 2, wherein: A separation time in the range from 1 ms to 1 s is predefined.

4. The method according to any one of claims 1 to 3, wherein: By opening a first switch (45) arranged between the first main line (7) and the first load area line (31), the first main line (7) is separated from the first load area line (31) connected thereto; by opening a second switch (51) arranged between the second main line (9) and the second load area line (32), each second main line (9) is separated from the second load area line (32) connected thereto.

5. The method according to any one of claims 1 to 3, wherein: After detecting a ground fault on a main line (7, 9), the main line (7, 9) is sequentially separated from two load area lines (31, 32) of different load areas (3, 5) until the ground fault is associated with the load area (3, 5).

6. A DC power grid (1), comprising: - two main lines (7, 9), - a plurality of load zones (3, 5), each of which has a first load zone line (31) and a second load zone line (32), the first load zone line being connectable to a first main line (7) of the DC grid (1) via a first switch (45), and the second load zone line being connectable to a second main line (9) of the DC grid (1) via a second switch (51), a measuring unit (53, 54) for each load region line (31, 32), the measuring unit being designed to continuously measure the line voltage between the line potential at which the load region line (31, 32) is located and ground potential, - a ground fault monitoring unit (15) designed to detect a ground fault on the main line (7, 9), a control unit (13) for controlling the first switch (45) and the second switch (51), the control unit being designed to, in the event of a ground fault detected by the ground fault monitoring unit (15) on the main line (7, 9) of the DC grid (1), disconnect the two load area lines (31, 32) of the load area (3, 5) from the main line (7, 9) by opening the switches (45, 51) connecting the two load area lines (31, 32) of the load area (3, 5) to the main line (7, 9), - and an evaluation unit (17) for evaluating the measurement signal of the measuring unit (53, 54), the evaluation unit being designed to associate a ground fault detected by the ground fault monitoring unit (15) with the load region (3, 5) having a load region line (31, 32) separated from the main line (7, 9) if the magnitude difference between the magnitudes of the line voltages of the two load region lines (31, 32) of the load region (3, 5) has not significantly decreased after the main line (7, 9) has been separated from the load region (3, 5), wherein the evaluation unit (17) is designed to classify a decrease in the magnitude difference between the magnitudes of the line voltages of the two load region lines (31, 32) of the load region (3, 5) as significant if the decrease in the magnitude difference between the magnitudes of the line voltages of the two load region lines (31, 32) after the main line (7, 9) has been separated from the load region (3, 5) exceeds a predetermined minimum decrease.

7. The DC grid (1) according to claim 6, wherein: The first switch (45) and / or the second switch (51) of each load zone (3, 5) is an electronic switch.

8. The DC grid (1) according to claim 6 or 7, wherein: The control unit (13) is designed to disconnect the main line (7, 9) from a load range line (31, 32) of a load range (3, 5) only for a predetermined disconnection duration after a ground fault on the main line (7, 9) has been detected.

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