Method and measuring device for detecting a leakage current in an ungrounded, single-phase alternating-current power supply system

Patent metadata

Field	Value
Publication number	US-11719759-B2
Application number	US-202217737195-A
Country	US
Kind code	B2
Filing date	May 5, 2022
Priority date	May 6, 2021
Publication date	Aug 8, 2023
Grant date	Aug 8, 2023

Abstract

A method and a measuring device for detecting a leakage current in an ungrounded, single-phase alternating-current power supply system. A variable test resistance is switched between one of the outer conductors and ground and starting from a minimally admissible test-resistance value, at least one of three support test-resistance values is determined as support locations. In an equivalent circuit of the modeled alternating-current power supply system, an equations system is set up for describing the dependency of currents and voltages. An extrapolation on the test-resistance value zero leads to a calculated test current which corresponds to the leakage current to be detected. Consequently, a ground fault situation may be simulated without actually causing a dangerous ground fault.

First claim

The invention claimed is:

1. A method for detecting a leakage current (I d ) in an ungrounded, single-phase alternating-current power supply system ( 2 ) having two outer conductors (L 1 , L 2 ), the method comprising the following steps:
a) switching a variable test resistance (R var ) between one of the outer conductors (L 1 , L 2 ) and ground (PE), a test-resistance value being supposed to be set infinitely,
b) measuring an operating voltage (U 0 ) of the alternating-current power supply system ( 2 ) between the outer conductors (L 1 , L 2 ) by means of line-voltage measuring equipment ( 24 ),
c) measuring an outer-conductor-to-ground voltage (U L,Pe ) between one of the outer conductors (L 1 , L 2 ) and ground (PE) by means of ground-voltage measuring equipment ( 26 ),
d) should the condition that the outer-conductor-to-ground voltage (U L,Pe ) be greater than or equal to half the operating voltage (U 0 ) not be fulfilled, switching the variable test resistance (R var ) between the other outer conductor (L 1 , L 2 ) and ground (PE),
e) determining a minimally admissible test-resistance value (R var,min ) for the variable test resistance (R var ),
f) determining at least three support test-resistance values (R var,i ) as support locations starting from the minimally admissible test-resistance value (R var,min ),
g) setting the support test-resistance value (R var,i ) and measuring a correspondingly resulting, measured test current (I R , I Ri ) via the variable test resistance (R var ) by means of current measuring equipment ( 28 ) for detecting a functional metrological dependency of the measured test current (I Ri ) of the support test-resistance value (R var,i ),
h) mapping the alternating-current power supply system ( 2 ) via an equivalent circuit having the test resistance (R var ) and having leakage impedances (Z L1 , Z L2 ) which have a capacitive portion of leakage capacitances (C e1 , C e2 ) of the alternating-current power supply system ( 2 ),
i) determining the leakage capacitances (C e1 , C e2 ) from an equations system, which describes the equivalent circuit, by means of a numerical approximation method such that the deviation of a calculated test current (I R,fit , I Ri,fit ) from the measured test current (I Ri ) is minimized to the support test-resistance values (R var,i ),
j) calculating the leakage current (I d ) as a calculated test current (I R,fit , I R0,fit ) from the equations system having the detected leakage capacitances (C e1 , C e2 ) and having the rest-resistance value (R var , R 0 ) zero,
k) outputting the leakage current (I d ) and the leakage capacitances (C e1 , C e2 ).

2. The method according to claim 1 , wherein the numerical approximation method is executed according to the least-squares function approximation.

3. The method according to claim 1 , wherein ohmic portions (R f1 , R f2 ) of the leakage impedances (Z L1 , Z L2 ) are neglected in the equivalent circuit in the sense of an infinitely large resistance value.

4. The method according to claim 1 , wherein the method sequence is executed automatically by a computing unit ( 30 ).

5. A measuring device ( 20 , 21 ) for detecting a leakage current (I d ) in an ungrounded, single-phase alternating-current power supply system ( 2 ) having two outer conductors (L 1 , L 2 ), comprising:
a variable test resistance (R var ) which is switched to one of the outer conductors (L 1 , L 2 ) and ground (PE),
line-voltage measuring equipment ( 24 ) for measuring an operating voltage (U 0 ) of the alternating-current power supply system ( 2 ) between the outer conductors (L 1 , L 2 ),
ground-voltage measuring equipment ( 26 ) for measuring an outer-conductor-to-ground voltage (U L,Pe ) between one of the outer conductor (L 1 , L 2 ) and ground (PE),
a current measuring device ( 28 ) for measuring a settable, measured test current (I R , I Ri ) via the variable test resistance (R var ), and having
a computing unit ( 30 ) which is configured for controlling and executing the method sequence claimed in claim 1 .

6. The measuring device ( 20 , 21 ) according to claim 5 , wherein the variable test resistance (R var ), the ground-voltage measuring equipment ( 26 ), the current measuring equipment ( 28 ) and the computing unit ( 30 ) are realized as a structural unit ( 20 ), or in that the variable test resistance (R var ), the ground-voltage measuring equipment ( 26 ), the current measuring equipment ( 28 ), the computing unit ( 30 ) and the line-voltage measuring equipment ( 24 ) are realized as an enhanced structural unit ( 21 ).

7. The measuring device ( 20 , 21 ) according to claim 5 , wherein the variable test resistance (R var ) is configured as a resistance network having discretely switchable resistance values or as an electronic resistance having semiconductor structural elements.

Assignees

• Bender Gmbh & Co Kg

Inventors

• Reitz Julian
• Broeckmann Eckhard
• Hackl Dieter
• Schepp Karl

Classifications

• G01R31/52 Primary

  Testing for short-circuits, leakage current or ground faults · CPC title

• G01R27/18 Primary

  Measuring resistance to earth {, i.e. line to ground} · CPC title

• G01R27/2605

  Measuring capacitance (capacitive sensors G01D5/24) · CPC title

• G01R27/26

  Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants {; Measuring impedance or related variables} · CPC title

• G01R31/00

  Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere ({measuring superconductive properties G01R33/1238;} testing line transmission systems H04B3/46; testing or measuring semiconductors or solid state devices during manufacture {H10P74/00}) · CPC title