Electrical protective device and method for selective disconnection of a subsystem in case of a second fault in an IT power supply system

Abstract

An electrical protective device (20) and method are for selective disconnection of a subsystem (6a, 6b) in the event of a second fault in an ungrounded power supply system (2) with a main system (4) and at least one subsystem (6a, 6b), the subsystem (6a, 6b) having a differential current measuring device (12a, 12b) and a switching device (14a, 14b) for separating the subsystem (6a, 6b). The invention is based on generating and applying a measuring signal voltage (U.sub.m) between one or more phase(s) of the main system (4) or from a neutral point of the main system (4) against ground (9) using a resonant coupling circuit (22) that has a measuring signal generator (24) and a series resonant circuit (26) connected in series to the measuring signal generator (24), a resonant frequency (f.sub.0AK) of the series resonant circuit (26) being set to correspond to the measuring signal frequency.

Claims

1. An electrical protective device (20) for selecting disconnection of a subsystem (6a, 6b) in an event of a second fault in an ungrounded power supply system (2) that consists of a main system (4) and at least one subsystem (6a, 6b), the subsystem (6a, 6b) having a differential current measuring device (12a, 12b) and a switching device (14a, 14b) for separating the subsystem (6a, 6b), the electrical protective device (20) comprising a resonant coupling circuit (22) that is connected against a ground (9) from one or more phase(s) of the main system (4) or from a neutral point of the main system (4) and has a measuring signal generator (24) for generating a measuring signal voltage (U.sub.m) that has a measuring frequency signal and a series resonant circuit (26) that is connected in series to the measuring signal generator (24) and has a capacitance (C.sub.AK), an inductivity (L.sub.AK) and an ohmic resistance (R.sub.AK).

2. The electrical protective device (20) according to claim 1, characterized in that the capacitance (C.sub.AK) and the inductivity (L.sub.AK) of the series resonant circuit (26) are realized in such a manner that a resonant frequency (f.sub.0AK) of the series resonant circuit (26) corresponds to the measuring signal frequency of the measuring signal voltage (U.sub.m).

3. The electrical protective device (20) according to claim 1, characterized in that the capacitance (C.sub.AK) and the inductivity (L.sub.AK) of the series resonant circuit (26) are realized in such a manner that the resonant frequency (f.sub.0AK) of the series resonant circuit (26) is sufficiently different from an insulation resistance measuring frequency of an insulation monitoring device (16) that is arranged in the power supply system (2) and sufficiently different from a supply frequency of the power supply system (2).

4. The electrical protective device (20) according to claim 1, characterized in that the capacitance (C.sub.AK) of the series resonant circuit (26) is several times smaller than the sum of the system leakage capacitances (C.sub.e) present in the power supply system (2).

5. The electrical protective device (20) according to claim 1, characterized in that the ohmic resistance (R.sub.AK) of the series resonant circuit (26) is realized in such a manner that it is low enough for the resonant coupling circuit (22) to approximately have a characteristic of an ideal voltage source.

6. The electrical protective device (20) according to claim 1, characterized in that the resonant coupling circuit (22) has a control circuit that changes the measuring signal frequency in such a manner that an amplitude of a measuring current that is driven by the measuring signal voltage (U.sub.m) is at a maximum.

7. The electrical protective device (20) according to claim 1, characterized by a phase comparator (28a, 28b) that is associated with the differential current measuring device (12a, 12b) of the subsystem (6a, 6b) and ascertains a phase of a differential current registered in the differential current measuring device (12a, 12b) in relation to a phase of the measuring signal voltage (U.sub.m).

8. The electrical protective device (20) according to claim 7, characterized by a communication channel (30) between the resonant coupling circuit (22) and the phase comparator (28a, 28b) for transmitting the phase of the measuring signal voltage (U.sub.m).

9. The electrical protective device (20) according to claim 1, characterized in that a secondary coil of a differential current transformer of the differential current measuring device (12a, 12b) is terminated with a capacitive burden impedance so that in connection with the differential current transformer, which is realized as a Rogowski coil, a parallel resonant circuit is realized that is tuned to the resonant frequency (f.sub.0AK) of the series resonant circuit (26).

10. The electrical protective device (20) according to claim 1, characterized by a separating device for separating the subsystem as a functional component of the electrical protective device.

11. A method for selecting disconnection of a subsystem (6a, 6b) in an event of a second fault in an ungrounded power supply system (2) that consists of a main system (4) and at least one subsystem (6a, 6b), the subsystem (6a, 6b) having a differential current measuring device (12a, 12b) and a switching device (14a, 14b) for separating the subsystem (6a, 6b), the method steps comprising of: generating the measuring voltage signal (U.sub.m) that has a measuring frequency signal, applying the measuring voltage signal (U.sub.m) between one or more phase(s) of the main system (4) or from a neutral point of the main system (4) against a ground (9) by means of a resonant coupling circuit (22) that has a measuring signal generator (24) and a series resonant circuit (26) that is connected in series to the measuring signal generator (24), wherein a resonant frequency (f.sub.0AK) of the series resonant circuit (26) is set in such a manner that it corresponds to the measuring signal frequency.

12. The method according to claim 11, characterized in that the resonant frequency (f.sub.0AK) of the series resonant circuit (26) is set in such a manner that it is sufficiently different from an insulation resistance measuring signal frequency of an insulation monitoring device (16) that is arranged in the power supply system (2) and sufficiently different from a supply frequency of the power supply system (2).

13. The method according to claim 11, characterized in that by means of control, the measuring signal frequency is changed in such a manner that an amplitude of a measuring current that is driven by the measuring signal voltage (U.sub.m) reaches a maximum.

14. The method according to claim 11, characterized in that performing a phase comparison in which a phase of a differential current to be registered in the differential current measuring device (12a, 12b) is ascertained in relation to a phase of the measuring voltage signal (U.sub.m).

15. The method according to claim 14, characterized in that transmitting by the resonant coupling circuit (22) the phase of the measuring voltage signal direct (U.sub.m) to a phase comparator (28a, 28b).

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) Other advantageous embodiment features become apparent from the following description and from the drawings, which illustrate preferred embodiments of the invention by way of examples. In the figures:

(2) FIG. 1: shows an ungrounded power supply system (IT power supply system) that is monitored according to the state of the art;

(3) FIG. 2: shows an IT power supply system comprising an electrical protective device according to the invention; and

(4) FIG. 3: shows a simplified illustration of the IT power supply system that is monitored according to the invention.

DETAILED DESCRIPTION

(5) FIG. 1 shows a 3-phase IT power supply system 2 that is monitored according to the state of the art and consists of a main system 4 and two subsystems 6a, 6b that branch off from the main system 4 as supply branches for connected loads 8a, 8b. Each of the loads 8a, 8b is connected to ground potential/ground 9 via a protective conductor, whose resistance is indicated as R.sub.PE.

(6) The individual phase conductors of the main system 4 have the conductor resistance R.sub.Cu. With respect to their (complex-valued) insulation resistances, the main system 4 and the subsystems 6a, 6b are characterized by the system leakage capacitances C.sub.e and by the ohmic insulation resistances R.sub.iso. A residual current device (RCD) 10a, 10b is provided in each subsystem 6a, 6b, each RCD consisting of a differential current measuring device 12a, 12b and a switching device 14a, 14b for separating the subsystem 6a, 6b. For monitoring of the total insulation resistance of the IT power supply system 2, an insulation monitoring device (IMD) 16 is connected between the main branch 4 and ground 9.

(7) FIG. 2 shows the IT power supply system 2 comprising an embodiment of an electrical protective device 20 according to the invention.

(8) The electrical protective device 20 is realized as a resonant coupling circuit 22, which is connected between a phase of the main system 4 and ground 9 by way of example in FIG. 2. Generally, the resonant coupling circuit 22, similarly to the known insulation monitoring device (IMD) 16, can have a connection between ground 9 and all phases or a neutral point of the IT power supply system 2 to be monitored.

(9) The resonant coupling circuit 22 consists of a measuring signal generator 24 for generating a measuring signal voltage U.sub.m and a series resonant circuit 26 that is connected in series to the measuring signal generator 24 and has an inductivity L.sub.AK, a capacitance C.sub.AK and an ohmic resistance R.sub.AK.

(10) Furthermore, the electrical protective device 20 comprises one or more phase comparators 28a, 28b that are arranged in the respective differential current measuring devices 12a, 12b of the subsystems 6a, 6b. The phase comparator 28a, 28b is connected to the resonant coupling circuit 22 via a communication channel 30 for transmitting a phase of the measuring signal voltage U.sub.m.

(11) In cooperation with the respective differential current measuring device 12a, 12b and a switching device 14a, 14b for separating the subsystem 6a, 6b, the subsystem 6a, 6b that is affected by the second fault can be specifically disconnected under evaluation of the information gathered by the phase comparator 28a, 28b.

(12) In FIG. 3, a simplified illustration of the IT power supply system 2 is shown for the state of a second fault in a subsystem 6a or 6b, visible from the fault resistance R.sub.f. The IT power supply system 2 is monitored by the electrical protective device 20 according to the invention, which is realized as a resonant coupling circuit 22.

(13) For the measuring-frequency fault current, i.e. in the resonant case, the series resonant circuit 26 represents a low-resistance true resistance as compared to the insulation resistances prevalent in the non-faulty parts of the IT power supply system 2, so that the fault current driven by the measuring voltage U.sub.m can form a closed circuit via the (fault) resistance R.sub.f and the resonant coupling circuit 22.

(14) As a numerical example, without taking into account the system leakage capacitances and the insulation fault, a selected capacitance C.sub.AK of 4.8 nF and an inductivity of 10 H results in a value of 730.7 Hz for the resonant frequency f.sub.0AK of the series resonant circuit. If the ohmic resistance R.sub.AK is set to 100 ohms, then the quality factor of this narrow-band series resonant circuit is Q=456.