Electric circuit arrangement and method for coupling an insulation monitoring device to an ungrounded power supply system

Abstract

The invention relates to an electric circuit arrangement and a method for coupling an insulation monitoring device to an ungrounded power supply system via a coupling impedance, which is realized to be operant for each active conductor of the power supply system and which is formed as an ohmic resistance circuit, the ohmic resistance circuit having a settable resistance value which is changeable and a switching-off function for decoupling the insulation monitoring device from the network and being realized as a bidirectional cascade comprising a series circuit of two transistors provided in a mirror-inverted manner, each having a diode connected in parallel, a controlled change in resistance of the transistors for setting the changeable resistance value being effected by a control circuit and the switching-off function for decoupling from the grid being realized by setting a maximum resistance value.

Claims

1. An electric circuit arrangement (10) for coupling an insulation monitoring device (4) to an ungrounded power supply system (2) having a coupling impedance, which is realized to be operant for each active conductor of the power supply system (2) and which is formed as an ohmic resistance circuit (12), wherein the ohmic resistance circuit (12) is designed to have a settable resistance value which is changeable and to have a switching-off function for decoupling the insulation monitoring device (4) from the network and the ohmic resistance circuit (12) is realized as a bidirectional cascade (14) comprising a series circuit of two transistors (16a, 16b) provided in a mirror-inverted manner, each having a diode (18a, 18b) connected in parallel, wherein a controlled change in resistance of the transistors (16a, 16b) for setting the changeable resistance value is effected by a control circuit (30) and the switching-off function for decoupling from a grid is realized by setting a maximum resistance value.

2. The electric circuit arrangement (10) according to claim 1, wherein the control circuit (30) comprises: a transformation block (32), which transforms a settable target resistance value (R0) to a target voltage (U0) as a reference variable by means of an actual current (Ix); a current measuring instrument (35), which measures a transistor current (IT) flowing through the transistor cascade (14) and which supplies said transistor current (IT), scaled as an actual current (Ix), back into the transformation block (32); a comparison element (34), which compares the target voltage (U0) with an actual voltage (Ux) and forms a differential voltage (Ud) as a control deviation; a voltage measuring instrument (36), which measures a transistor voltage (UT) dropping across the transistor cascade (14) and which supplies said transistor voltage (UT), scaled as an actual voltage (Ux), back into the comparison element (34); a controller (40), which generates a manipulated variable (W) from the differential voltage (Ud) for controlling a controlled section, said manipulated variable (W) being formed by means of the bidirectional transistor cascade (14), having the changeable resistance value as a controlled variable.

3. The electric circuit arrangement (10) according to claim 2, wherein the controller (40) is designed as a PI controller.

4. The electric circuit arrangement (10) according to claim 1, wherein a protective resistor (Rs) for each active conductor (L1, L2), which is connected in series to the respective ohmic resistance circuit (12).

5. The electric circuit arrangement (10) according to claim 1, wherein an embodiment as an extension module for the insulation monitoring device (4) or as a separate structural unit.

6. A method for coupling an insulation monitoring device (4) to an ungrounded power supply system (2) via a coupling impedance, which is realized to be operant for each active conductor of the power supply system (2) and which is formed as an ohmic resistance circuit (12), wherein setting of a changeable resistance value of the ohmic resistance circuit (12), wherein a switching-off function for decoupling the insulation monitoring device from the network is effected by setting the resistance value and the ohmic resistance circuit (12) is realized as a bidirectional cascade (14) comprising a series circuit of two transistors (16a, 16b) provided in a mirror-inverted manner, each having a diode (18a, 18b) connected in parallel, the setting of the changeable resistance value is effected by means of a controlled change in resistance of the transistors (16a, 16b) within a control circuit (30) and the switching-off function for decoupling from the network is realized by setting a maximum resistance value.

7. The method according to claim 6, wherein in the control circuit (30), a settable target resistance value (R0) is transformed to a target voltage (U0) as a reference variable through a transformation block (32) by means of an actual current (Ix); a transistor current (IT) flowing through the transistor cascade (14) is measured and supplied to the transformation block (32) scaled as an actual current (Ix) by means of a current measuring instrument (35); the target voltage (U0) is compared with an actual voltage (Ux) in a comparison element (34) and a differential voltage (Ud) is formed as a control deviation; a transistor voltage (UT) dropping across the transistor cascade (14) is measured and supplied to the comparison element (34) scaled as an actual voltage (Ux) by means of a voltage measuring instrument (36); and a manipulated variable is generated from the differential voltage (Ud) in a controller, said manipulated variable controlling a controlled section, which is formed from the bidirectional transistor cascade (14), having the changeable resistance value as a controlled variable.

8. The method according to claim 7, wherein a PI controller is implemented as the controller (40).

9. The method according to claim 1, wherein a protective resistor (Rs) for each active conductor (L1, L2) is connected in series to the respective ohmic resistance circuit (12).

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) Further advantageous embodiments can be derived from the following description and the drawings, which illustrate a preferred exemplary embodiment of the invention. In the figures:

(2) FIG. 1 an insulation monitoring device according to the state of the art,

(3) FIG. 2 an electric circuit arrangement according to the invention having a coupling impedance comprising a settable resistance value and

(4) FIG. 3 a control circuit of the electric circuit arrangement according to the invention.

DETAILED DESCRIPTION

(5) FIG. 1 illustrates an ungrounded power supply system 2 having active conductors (L.sub.1, L.sub.2) to which an insulation monitoring device 4 is connected in series to a separating device 9 between the active conductors (L.sub.1, L.sub.2) and protective earth (PE). The ungrounded power supply system 2 is further characterized by the leakage capacitances Ce1, Ce2 in relation to PE and by an insulation resistance R.sub.f in relation to PE to be monitored by the insulation monitoring device 4.

(6) The insulation monitoring device 4 comprises a voltage measuring generator 5 which generates a measuring voltage U.sub.m and superposes it over the ungrounded power supply system 2. The insulation resistance R.sub.f closes the measuring circuit and generates a measuring current I.sub.m, which causes a voltage drop at a measuring resistance R.sub.m, from which the value of the insulation resistance R.sub.f can be determined by an evaluation device 6.

(7) For controlling the separating device 9, the evaluation device 6 additionally comprises a signal output 7, via which the insulation monitoring can be deactivated.

(8) Furthermore, the resistances R.sub.1 and R.sub.2 having fixed resistance values are provided as coupling impedances in the insulation monitoring device 4 in order to adjust the operating range of the insulation monitoring device 4 to a network voltage of the ungrounded power supply system 2.

(9) FIG. 2 illustrate an electric circuit arrangement 10 according to the invention having the coupling impedance as an ohmic resistance circuit 12 having a settable resistance value which is changeable.

(10) In contrast to the fixed coupling impedances R.sub.1 and R.sub.2 known from the state of the art illustrated in FIG. 1, the electric circuit arrangement 10 for coupling the insulation monitoring device 4 as a coupling impedance for each active conductor L.sub.1, L.sub.2 according to the invention comprises an ohmic resistance circuit 12, which is designed as a bidirectional transistor cascade 14.

(11) The bidirectional transistor cascade 14 comprises a series circuit of two transistors (16a, 16b) provided in a mirror-inverted manner, each having a diode (18a, 18b) connected in parallel. MOSFETs are preferably used as transistors (16a, 16b), a controlled operating point displacement being effected by a MOSFET driver circuit, resulting in the desired resistance value being set in a voltage-controlled manner.

(12) Protective resistors R.sub.s are provided as a protective circuit in case of a short-circuited transistor cascade 14 for current limitation.

(13) The transistor cascade 14 is controlled via a control circuit 30, which is integrated in the evaluation device 6 of the insulation monitoring device 4 in the present invention. In the illustrated embodiment, the electric circuit arrangement 10 for coupling the insulation monitoring device 4 according to the invention is designed as an extension module of the insulation monitoring device 4. An embodiment as a separate structural unit is also possible.

(14) In both embodiments, the electric circuit arrangement 10 according to the invention circuitry-wise represents a simple and cost-effective realization of a coupling for an insulation monitoring device 4 due to the settable resistance value which is changeable having an integrated switching-off function on the basis of semiconductor components.

(15) FIG. 3 illustrates a control circuit 30 of the circuit arrangement according to the invention for coupling the insulation monitoring device 4. Initially, the desired resistance value R.sub.0 is specified as an input variable of the control circuit, said resistance value R.sub.0 being transformed to a target voltage U.sub.0 via a transformation block 32. The transformation is effected by a current measurement 35 which measures the transistor current I.sub.T flowing through the transistor cascade 14 and which supplies said transistor current I.sub.T, scaled as an actual current I.sub.x, back into the transformation block 32.

(16) A differential voltage U.sub.d is formed from the target voltage U.sub.0 and an actual voltage U.sub.x in a comparison element 34, said actual voltage U.sub.x being obtained from the transistor voltage U.sub.T dropping across the transistor cascade 14 by means of a voltage measurement 36.

(17) The transistor current flowing through the transistor cascade is measured by a current measurement and supplied to the transformation block scaled as an actual current.

(18) The differential voltage U.sub.d is supplied to a controller 40, which forms a manipulated variable W for controlling the transistor cascade 14 from said differential voltage U.sub.d, said controller 40 preferably being realized as a PI controller.

(19) In addition to the actual transistor (series) circuit 46, the transistor cascade 14 comprises a driver circuit 44 for controlling the transistor circuit 46. The actual resistance value R.sub.x of the transistor cascade 14 approaches the settable target resistance value R.sub.0 as a controlled variable.