Method and device for determining the division of a total insulation resistance and the division of a total system leakage capacitance in an ungrounded power supply system

Abstract

A method and device for determining a division of a total insulation resistance and of a total system leakage capacitance in an ungrounded power supply system. The basic idea is to determine how the total insulation resistance is divided into partial insulation resistances and how the total system leakage capacitance is divided into partial system leakage capacitances between the active conductors of the ungrounded power supply system from displacement voltages measured between each of the active conductors of the ungrounded power supply system based on values determined in advance for the total insulation resistance and for the total system leakage capacitance of the ungrounded power supply system. By evaluating the displacement voltages in terms of their changes in amplitude, their frequency and their phasing, conclusions can be drawn as to the division of the total insulation resistance and of the total system leakage capacitance between the individual active conductors.

Claims

1. A method for determining a division of a total insulation resistance (R.sub.iso) in an ungrounded power supply system (2, 12) comprising active conductors (L.sub.+, L.sub.−, L.sub.×) between which a conductor-conductor voltage (U.sub.dc, U.sub.ac, U.sub.acxy) occurs, the method comprising the method steps of: determining the total insulation resistance (R.sub.iso) of the ungrounded power supply system (2, 12), characterized by measuring displacement voltages (U.sub.L+_E, U.sub.L−_E, U.sub.L×_E) between each of the active conductors (L.sub.+, L.sub.−, L.sub.×) and ground (E), determining a resistance division factor (r.sub.R), which describes the division of the total insulation resistance (R.sub.iso) into partial insulation resistances (R.sub.iso+, R.sub.iso− R.sub.iso×) related to the active conductors (L.sub.+, L.sub.−, L.sub.×), as a function of at least one of the parameters amplitude, frequency, phase of the displacement voltages (U.sub.L+_E, U.sub.L−_E, U.sub.L×_E) measured, wherein the resistance division factor (r.sub.R) is determined from the relation of the amplitudes of the displacement voltages (U.sub.L×_E), taking into account the frequency and the phasing of the displacement voltages (U.sub.L×_E).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantageous embodiment features are apparent from the following description and from the drawing, which illustrates preferred embodiments of the invention using examples.

(2) FIG. 1 shows an application of the method according to the invention in an ungrounded direct-voltage power supply system;

(3) FIG. 2 shows measurements of the displacement voltages in the ungrounded direct-voltage power supply system illustrated in FIG. 1;

(4) FIG. 3 shows an application of the method according to the invention in an ungrounded single-phase alternating-voltage power supply system; and

(5) FIG. 4 shows measurements of the displacement voltages in the single-phase alternating-voltage power supply system illustrated in FIG. 3.

DETAILED DESCRIPTION

(6) FIG. 1 shows an ungrounded DC power supply system 2 comprising two active conductors L.sub.+ and L.sub.−. DC power supply system 2 is fed by a voltage source U.sub.dc, which supplies a load 4 connected between active conductors L.sub.+ and L.sub.− with energy.

(7) For monitoring the total insulation resistance R.sub.iso of ungrounded DC power supply system 2, an insulation monitoring device IMD which superimposes a measuring voltage U.sub.m on power supply system 2 is connected between conductors L.sub.+, L.sub.− on one side and ground (ground potential) E on the other side. Via partial insulation resistances R.sub.iso+, R.sub.iso−, which are assigned to respective active conductors L.sub.+, L.sub.− and act as leakage resistances, and via partial system leakage capacitances C.sub.e+, C.sub.e− assigned to respective active conductors L.sub.+, L.sub.−, a measuring current I.sub.m occurs, which causes a corresponding voltage drop at a measuring resistor R.sub.m of insulation monitoring device IMD, said voltage drop being evaluated by insulation monitoring device IMD. Thus determined total insulation resistance R.sub.iso is the result of the parallel connection of partial insulation resistances R.sub.iso+, R.sub.iso−.

(8) In order to be able to draw a conclusion as to the division of total insulation resistance R.sub.iso into partial insulation resistances R.sub.iso+, R.sub.iso− assigned to respective active conductors L.sub.+ and L.sub.−, displacement voltages U.sub.L+_E and U.sub.L−_E, which occur between active conductor L.sub.+ and ground and between active conductor L.sub.− and ground E, respectively, are additionally measured.

(9) Furthermore, a total system leakage capacitance C.sub.e is measured, which is the result of the parallel connection of partial leakage capacitances C.sub.e+ and C.sub.e− assigned to respective active conductors L.sub.+, L.sub.−.

(10) FIG. 2 shows measurements of displacement voltages U.sub.L+_E and U.sub.L−_E, which occur in response to an abrupt change of conductor-conductor voltage (input voltage) U.sub.dc.

(11) With a voltage jump of 10 V from 540 V to 540 V, a mesh equation U.sub.dc=U.sub.L+_E−U.sub.L−_E results in the curves shown in FIG. 2 for displacement voltages U.sub.L+_E and U.sub.L−_E. After termination of the transient effect, displacement voltages U.sub.L+_E and U.sub.L−_E each have the same absolute value of 275 V, which suggests a symmetrical division of determined total insulation resistance R.sub.iso into partial insulation resistances R.sub.iso+, R.sub.iso−.

(12) The relation of the two final values of displacement voltages U.sub.L+_E and U.sub.L−_E at the two active conductors L.sub.+, L.sub.− can be used directly to determine the relation of partial insulation resistance R.sub.iso+ of active conductor L.sub.+ to total insulation resistance R.sub.iso and thus determine resistance division factor r.sub.R.

(13) Generally, the following equations generally apply to an ungrounded DC power supply system comprising two active conductors:

(14) $R_{\mathrm{iso}+}=r_R\times R_{\mathrm{iso}}\mathrm{and}{R}_{\mathrm{iso}}=\left(\frac{1}{r_R-1}+1\right)\times R_{\mathrm{iso}}.$

(15) From the duration of a transient effect of respective displacement voltage U.sub.L+_E, U.sub.L−_E caused by the change in amplitude of conductor-conductor voltage U.sub.dc and from the time constants obtained from the curve of the transient effects, capacitance division factor r.sub.c of partial system leakage capacitance C.sub.e+ of active conductor L.sub.+ to total system leakage capacitance C.sub.e of the power supply system can be obtained.

(16) Consequently, the following applies to partial system leakage capacitances C.sub.e+, C.sub.e−: C.sub.8+−r.sub.C×C.sub.8 and C.sub.8−−(1−r.sub.C)×C.sub.8.

(17) For example, with a resistance division factor r.sub.R of 25 and a determined total insulation resistance R.sub.iso of 19.2 KΩ, the above formulas render the division R.sub.iso+=480 KΩ and R.sub.iso−=20 KΩ.

(18) In the same manner, a value of 0.25 for capacitance division factor r.sub.C and a determined total system leakage capacitance C.sub.e of 40 μF result in the division of C.sub.e+=10 μF and C.sub.e−=30 μF for the partial system leakage capacitances of active conductors L.sub.+ and L.sub.−.

(19) FIG. 3 shows a single-phase alternating-current power supply system which has two active conductors L.sub.1, L.sub.2 and to which a load 4 is connected. Like in direct voltage power supply system 12 described in FIG. 1, insulation monitoring including determination of total insulation resistance R.sub.iso by an insulation monitoring device IMD takes place. Displacement voltages U.sub.L1_E and U.sub.L2_E are measured to be able to determine a division of total insulation resistance R.sub.iso into partial insulation resistances R.sub.iso1, R.sub.iso2 assigned to active conductors L.sub.1, L.sub.2 as per the invention.

(20) In FIG. 4, the curves over time of conductor-conductor voltage U.sub.ac (input voltage) and of displacement voltages U.sub.L1_E and U.sub.L2_E measurable at respective active conductors L.sub.1, L.sub.2 are illustrated.

(21) Unlike in direct-voltage power supply system 2 described in FIG. 1, a change in conductor-conductor voltage U.sub.ac which is excited externally by a change in load, for example, is not required because conductor-conductor voltage U.sub.ac changes by nature (sinusoidally) in an alternating-voltage power supply system. Division factors r.sub.R and r.sub.C can be determined based on the changes in amplitude of measured displacement voltages U.sub.L1_E and U.sub.L2_E and based on the phase relation between displacement voltages U.sub.L1_E and U.sub.L2_E.

(22) Thus, the present invention advantageously exploits the fact that in addition to the common-mode measuring signal input by insulation monitoring device IMD to determine total insulation resistance R.sub.iso, conductor-conductor voltages U.sub.dc, U.sub.ac, U.sub.acxy (indices x, y represent active conductors x, y) exhibit changes over time—which are caused by external events, such as load changes, or are present implicitly, like in alternating-voltage power supply systems—and effect the changes over time of registered displacement voltages U.sub.L+_E, U.sub.L−_E, U.sub.L×_E.

(23) By evaluating displacement voltages U.sub.L+_E, U.sub.L−_E, U.sub.L×_E in terms of their changes in amplitude, their frequency and their phasing, conclusions can be drawn as to the division of total insulation resistance R.sub.iso and of total system leakage capacitance C.sub.e between individual active conductors L.sub.+, L.sub.−, L.sub.×.

(24) Advantageously, no other highly precise current measurements are required. Likewise, there is no need for additional voltage-proof switches between active conductors L.sub.+, L.sub.−, L.sub.× and ground E, which would be needed to change a coupling impedance of insulation monitoring device IMD in order to be able to determine the division of total insulation resistance R.sub.iso or of total system leakage capacitance C.sub.e.