Circuit arrangement having an active measuring voltage for determining an insulation resistance against ground potential in an ungrounded power supply system

Abstract

A circuit arrangement (20) having an active measuring voltage (U.sub.G) for determining an insulation resistance (R.sub.F) or a complex-valued insulation impedance (Z.sub.F) of an ungrounded power supply system (12) against ground potential (PE), the circuit arrangement (20) having a measuring path (24) which runs between an active conductor (L1, L2) of the power supply system (12) and the ground potential (PE) and includes a measuring-voltage generator (V.sub.G) for generating the measuring voltage (U.sub.G), a measuring resistance (R.sub.M) for capturing a measured voltage (U.sub.M) and a coupling resistance (R.sub.A), the circuit arrangement (20) having a signal evaluation circuit (26) which includes a signal input for evaluating the measured voltage (U.sub.M) and a ground connection (GND). The ground connection (GND) is connected to a ground potential (PE).

Claims

1. A circuit arrangement (20, 40) having an active measuring voltage (U.sub.G) for determining an insulation resistance (R.sub.F) or a complex-valued insulation impedance (Z.sub.F) of an ungrounded power supply system (12) against ground potential (PE), the circuit arrangement (20, 40) having a measuring path (24, 44) which runs between an active conductor (L1, L2) of the power supply system (12) and the ground potential (PE) and comprises a measuring-voltage generator (V.sub.G) for generating the measuring voltage (U.sub.G), a measuring resistance (R.sub.M) for capturing a measured voltage (U.sub.M) and a coupling resistance (R.sub.A), the circuit arrangement (20, 40) comprising a signal evaluation circuit (26, 46) which comprises a signal input for evaluating the measured voltage (U.sub.M) and a ground connection (GND), wherein the ground connection (GND) is connected to a ground potential (PE).

2. The circuit arrangement according to claim 1, wherein the ground potential (PE), the measuring-voltage generator (V.sub.G), the measuring resistance (R.sub.M) and the coupling resistance (R.sub.A) make up a series connection in the measuring path (24) in a first order.

3. The circuit arrangement according to claim 2, further including a multi-pole coupling in an ungrounded power supply system (12) having at least two active conductors (L1, L2), at least two of the active conductors (L1, L2) each being connected to the ground potential (PE) via a measuring path (24).

4. The circuit arrangement according to claim 3, wherein the measuring paths (24) are realized individually.

5. The circuit arrangement according to claim 3, wherein at least two measuring paths (24) comprise a shared measuring-voltage generator (V.sub.G).

6. The circuit arrangement according to claim 1, wherein the ground potential (PE), the measuring resistance (R.sub.M), the measuring-voltage generator (V.sub.G) and the coupling resistance (R.sub.A) make up a series connection in the measuring path (44) in a second order.

7. The circuit arrangement according to claim 6, further including a multi-pole coupling in an ungrounded power supply system (12) having at least two active conductors (L1, L2), at least two of the active conductors (L1, L2) each being individually connected to the ground potential (PE) via a measuring path (44).

8. A method of using the circuit arrangement (20, 40) having a multi-pole coupling via individual measuring paths (24, 44) according to claim 4, comprising causing the corresponding measuring-voltage generators (V.sub.G) to generate different measuring voltages (U.sub.G) to test the functionality of the couplings.

9. A method of using the circuit arrangement (20, 40) comprising a multi-pole coupling via individual measuring paths (24, 44) according to claim 4, comprising simultaneous active measuring of a corresponding insulation resistance (R.sub.F) or a complex-valued insulation impedance (Z.sub.F) against ground potential (PE) in several ungrounded power supply systems (12), active conductors (L1, L2) of different power supply systems (12) being assigned to the measuring paths (24, 44).

10. The method according to claim 9, where different measuring voltages (U.sub.G) are applied to the corresponding power supply systems (12) for identifying a low-impedance connection between the power supply systems (12).

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Further advantageous embodiment features are derived from the following description and the drawings which describe preferred embodiments of the invention using examples.

(2) FIG. 1 shows a circuit arrangement for determining an insulation resistance according to the state of the art,

(3) FIG. 2 shows a circuit arrangement according to the invention having measuring-path elements in a first order,

(4) FIG. 3a shows a measuring path with galvanic isolation in a differential amplifier,

(5) FIG. 3b shows a measuring path with galvanic isolation in the signal processing of the measured voltage,

(6) FIG. 4 shows a circuit arrangement according to the invention having the measuring-path elements in a second order,

(7) FIGS. 5a, 5b show how the measuring voltage is determined without its own signal processing,

(8) FIGS. 6a, 6b show a measuring-voltage generator with a galvanically isolated supply in two embodiments, and

(9) FIG. 7 shows a use of the circuit arrangement according to the invention in independent power supply systems.

DETAILED DESCRIPTION

(10) FIG. 1 shows the general set-up of a circuit arrangement 10 for determining the insulation resistance (insulation monitoring device) of an ungrounded power supply system 12. A network voltage source V.sub.N supplies a load R.sub.L via two active conductors L1, L2.

(11) Insulation monitoring device 10 is connected between both active conductors L1, L2 and ground (ground potential) PE, insulation monitoring device 10 being coupled with each active conductor L1, L2 via a branched measuring path 14 so that a closed measuring-current circuit can be formed via active conductors L1, L2 and insulation resistance R.sub.F. Power supply system 12 is additionally described by a leakage capacitance C.sub.E which is modeled in the same manner as insulation resistance R.sub.F and forms a complex-valued insulation impedance Z.sub.F in conjunction with insulation resistance R.sub.F. In a simplified illustration, this complex-valued Z.sub.F is only illustrated for active conductor L2; generally, however, an insulation impedance Z.sub.F is applied to each active conductor L1, L2.

(12) Starting from ground potential PE, measuring path 14 comprises a measuring-voltage generator V.sub.G, a measuring resistance R.sub.M and a coupling resistance R.sub.A (V.sub.G, R.sub.M, R.sub.A and S.sub.A are to be described here and in the following for all corresponding measuring-path elements V.sub.Gi, R.sub.Mi, R.sub.Ai and S.sub.Ai which all function in the same manner). Optionally, a switch S.sub.A is provided in each measuring path 14 for disconnecting insulation monitoring device 10 from ungrounded power supply system 12.

(13) Measuring-voltage generator V.sub.G generates a measuring voltage U.sub.G which drives a current in the measuring circuit closing via insulation resistance R.sub.F, the current leading to a measurable voltage drop U.sub.M (measured voltage) at measuring resistance R.sub.M. Measuring voltage U.sub.G and measured voltage U.sub.M are supplied to a signal evaluation circuit 16 to determine insulation resistance R.sub.F. For digitally processing input signals, signal evaluation device 16 comprises analog-digital converters ADC and a microcontroller μC along with an intersection 18, e.g., for outputting an alarm signal.

(14) FIG. 2 shows a circuit arrangement 20 according to the invention; starting from ground potential PE, measuring-voltage generator V.sub.G, measuring resistance R.sub.M and coupling resistance R.sub.A make up a series connection in branched measuring path 24 in a first order. According to the invention, ground connection GND of signal evaluation circuit 26 is connected to ground potential PE via a galvanic connection 22.

(15) When supplying an alternating-current signal (measuring voltage U.sub.G) by means of measuring-voltage generator V.sub.G, complex-valued insulation impedance Z.sub.F can be calculated with its components R.sub.F (real part) and C.sub.E (imaginary part) in signal evaluation circuit 26 via digital signal processing algorithms, such as the discrete Fourier Transformation (DFT).

(16) In the illustrated multi-pole coupling, two measuring paths 24 dispose over a shared measuring-voltage generator V.sub.G. Alternatively, each measuring path 24 can comprise its own measuring-current generator V.sub.G.

(17) In the configuration in this first order, a high common mode suppression for determining measured voltage U.sub.M, which was detected via a differential amplifier 25 (instrumentation amplifier), is required since (low) measured voltage U.sub.M is superposed by a (high) common mode measuring voltage U.sub.G. Faced with the present voltage conditions—measured voltage U.sub.M is in the range of ±2 Volts with respect to measuring voltage U.sub.G which is in the range of ±20 Volts—, an instrumentation amplifier would be required which has a common mode suppression of at least 120 dB for correctly determining measured voltage U.sub.M.

(18) As an alternative to an instrumentation amplifier 25 of this high quality, reaching the required high common mode suppression via galvanic isolation in the signal processing path is therefore ideally suited for measured voltage U.sub.M.

(19) FIGS. 3a and 3b show two options with regard thereto. Hence, a galvanic isolation 32 can be intended within, for example, differential amplifier circuit 34 (FIG. 3a) or in signal processing circuit 36 (FIG. 3b) of measured voltage U.sub.M.

(20) FIG. 4 shows a circuit arrangement 40 according to the invention for determining insulation resistance R.sub.F in an ungrounded power supply system 12 having a multi-pole coupling. In contrast to the first order illustrated in FIG. 2, the measuring-path elements are disposed in measuring path 44 in a second order starting from ground potential PE, beginning with measuring resistance R.sub.M, followed by measuring-voltage generator V.sub.G and culminating in coupling resistance R.sub.A. Measured voltages U.sub.M are detected as a voltage drop via the corresponding measuring resistance R.sub.M directly against ground PE and supplied to signal evaluation unit 46 in conjunction with measuring voltages U.sub.G detected differentially via a differential amplifier 45.

(21) According to the invention, signal evaluation unit 46 is connected to ground potential PE via a galvanic connection 42.

(22) Far less requirements need to be fulfilled for the common mode suppression in the signal processing path of measuring voltage U.sub.G since measuring voltage U.sub.G is considerably larger than measured voltage U.sub.M.

(23) In FIGS. 5a and 5b, two circuit options for determining measuring voltage U.sub.G are illustrated. On principle, measuring voltage U.sub.G is required for determining insulation resistance R.sub.F. However, measuring voltage U.sub.G does not necessarily have to be known; via reference measurement with R.sub.F=0 Ohms, measuring voltage U.sub.G can refer to this reference measurement value. Measuring voltage U.sub.G does not have to be detected in a synchronous and continuous manner for capturing measured voltage U.sub.M. Hence, measuring voltage U.sub.G can be determined for measured voltage U.sub.M by means of signal evaluation circuit 26, 46 by closing a switch S.sub.G disposed parallel to the series connection consisting of measuring-voltage generator V.sub.G and measuring resistance R.sub.M, as illustrated in FIG. 5a.

(24) In extension of the circuit from FIG. 5a, (separation) switch S.sub.A can be opened according to FIG. 5b, and measured voltage U.sub.M can be adjusted to the tolerance range of measured voltage U.sub.M by means of a resistance R.sub.G additionally disposed in series to switch S.sub.G while determining measuring voltage U.sub.G.

(25) FIGS. 6a and 6b show a measuring-voltage generator V.sub.G having a galvanically isolated supply. To avoid cross flow bypassing measuring resistance R.sub.M against ground connection GND via measuring-voltage generator V.sub.G, which would falsify measurements, measuring-voltage generator V.sub.G must comprise a galvanically isolated supply and high-impedance inputs for control. If measuring-voltage generator V.sub.G is to be controlled via an analog signal and if measuring voltage U.sub.G is to be bipolar, as illustrated in FIG. 6a, measuring-voltage source V.sub.G can be set up having a differential amplifier supplied bipolarly via voltages V.sub.CC or V.sub.DD. In this context, supply voltage U.sub.GS supplies an internal and galvanically isolated voltage source 62. A control voltage U.sub.GE controls bipolar measuring voltage U.sub.G.

(26) The complexity of internal and galvanically isolated voltage source 62 can be reduced by using a differential amplifier 66 having a differential output. As illustrated in FIG. 6b, this amplifier type generates bipolar measuring voltage U.sub.G even when a supply is unipolar.

(27) FIG. 7 shows a use of circuit arrangement 40 according to the invention in the second order according to FIG. 4 in two independent power supply systems 12.

(28) An active conductor L1 from both independent power supply systems 12 are assigned to each of the two measuring paths 44 so that corresponding insulation resistances R.sub.F of power supply systems 12 can be determined individually from each other in signal evaluation unit 46. This circuit configuration thus enables simultaneously determining corresponding insulation resistance R.sub.F for several independent power supply systems 12 using only one circuit arrangement 40 of the invention, though by all means more than the two power supply systems 12 illustrated in an exemplary manner can be monitored simultaneously. Moreover, a multi-pole coupling to corresponding power supply system 12 also becomes possible with this configuration.

(29) Furthermore, a current flow occurring when measuring voltages U.sub.G are of different magnitudes and a low-impedance connection 72 (cross fault) is present between power supply systems 12 can be detected in measuring paths 44. A cross fault, even a complex-valued one, occurring between power supply systems 12 is thus identified.