SU1465832A1 - Method of measuring leak current - Google Patents

Abstract

The invention relates to a technique for measuring and controlling insulation parameters of electrical installations. The purpose of the invention is to increase the sensitivity of the determination of leakage current. An auxiliary circuit is periodically switched, equipped with an auxiliary current source 5 and a conductance 7, previously connected between the midpoint of the operating current source 1 and the ground. The leakage current is determined by the formula 1, YEJE -JJ) ("hL) / (), where U and the voltage between the midpoint of the operating current source and ground before a leakage occurs, respectively, with the auxiliary circuit turned on and off; iTjgy is the voltage between the midpoint of the operating current source and the ground after a leak occurs when the auxiliary circuit is turned off; E is the electromotive force of the auxiliary current source; YP - auxiliary circuit conductivity. The method provides a determination of the leakage current increment that occurs when there are sudden closures in the network. 1 silt, 3 $. (L C 4 and SP 00 00 to

Description

This invention relates to electricalxHHKe, namely to the counter-insulation technique of electrical installations,

The purpose of the invention is to increase

 The sensitivity of determining the leakage current due to a periodic commutation of the auxiliary circuit t4UHH containing the auxiliary source of the current I The drawing shows the scheme using this method,

Source 1 operating current connection; the consumer 2 of electricity using line 3, and having conductive W 4 isolation of the phases relative to ze1 1 auxiliary current source 5 with ground through switch 6 I; through conduction 7 with a midpoint (neutral of working source 1; its current. Phase-sensitive volt meter 8 is connected between ground and midpoint of source 1 operating current and 9 meters between the midpoint of operating source 1 and its phase output.

The essence of the method is that between the midpoint of the netsheet 1 of the operating current and the ground, an auxiliary source 5 1 of the same frequency as the operating one is connected, then the specified source is switched on and off periodically, and the Neutral bias voltage is measured both with the ijtpH turned on and the auxiliary current source turned off, and about the leakage current, the iio trial measures.

The bias voltage II of an electrical installation, both AC and DC, is determined by the formula:

Y; ,

and

AND

the conductivity of the L-th phase relative to the earth; the number of phases of the system; general conductivity of current-carrying parts relative to earth.

 Y -,.

In this case, the voltage, currents and conductivities with alternating current are described by complex numbers, and at constant, real numbers. Since the real number is a special case of the comlex, everywhere, except for the case of DC only, Uj I and Y are used to denote the voltage and conductivity

If an auxiliary current source is connected between the medium (zero) point of the operating current source and the ground, but the neutral bias voltage is changed and is determined by the formula:

Eyf

U; Y (2)

where is e

- EMF of auxiliary power source;

Y B

the conductance of the auxiliary circuit (i.e., the circuit containing the auxiliary current source | is connected between the neutral and the ground. If the auxiliary current source is disconnected from the lg phase to the earth, there is a current leakage, then the bias voltage Ungijj is determined by the formula :

de and.

50

 - l.45

voltage of the 1st phase relative to the midpoint of the operating current source, t, e. relative to the neutral, and for the midpoint i is zero (hereinafter in a similar

context, speaking of phase, implies both the pole and the battery terminal, and other similar elements; presses, interconnected, are treated as one clip);

silt

th-u, y:

. Y

(3)

and - voltage of the m-th phase relate G

Where

 h

neutral;

leakage conductivity. If the leakage current to earth occurs when the auxiliary current source is switched on, the neutral bias voltage is determined by the formula

.t

-t-ey,

Y + Y t E

+ Y

W

Using Ohm's law and the fact that the voltage of the phase relative to the earth is equal to the corresponding phase voltage, reduced by the neutral voltage, with a joint solution (1), (2) and (3), a formula is obtained for the leakage current Ti ,. when the auxiliary current source is off:

R

Y Cf-lI liyjfl -I-Hl) UNE - UH

Similarly, from (1), (2) and 4), a formula is obtained for the leakage current with the auxiliary current source on:

Г - Y (l-: iiilb§ibi f§).

E - EUNE - iT

In order to implement the proposed method, an auxiliary current source 5 of the same frequency as the operating current is introduced between the medium (zero) point of the operating current source 1 and the ground, and then periodically switched on and off using a switch. In addition, the voltage between the zero point and the ground, with the auxiliary current source turned on and off, using a phase-sensitive voltmeter 8. If there is no output from the midpoint, an artificial zero point is used, which is obtained by means of a star from the same conductivities. The measurements themselves are carried out with a high-resistance voltmeter, which includes between the zero point, natural or artificial, and ground. The value measured is the displacement voltage of the neutral. In ac networks dp the indicated measurements use a phase-sensitive device dn to increase the accuracy of the measurements, an artificial zero point is created separately for the auxiliary current source and for the voltmeter.

Instead of the above, an indirect measurement method can be used, which at least in AC networks allows the use of simpler, more convenient and accurate instruments and does not require a zero point for a voltmeter.

For example, in the case of symmetric three-phase systems with a voltmeter with

very large resistance (compared with the total resistance of the current-carrying parts to the ground) measure the voltage modules Od, Ug, and between phases A, B, C and earth and the module phase voltage and.One of the possible options for measuring the voltage Uq , is that

measure the voltage modulus between any two phases, i.e. line voltage, and the value obtained is divided by 4h. This allows you to find the displacement voltage vector of the neutral

 . . p ,, „2„ and

  I. : d3,

about -511-j 9

zi

9

, w., -uc

20

zi

f

five

In the case of direct current installation, a high-voltage voltmeter measures the voltage U between the positive pole and ground and U between the negative pole and, which allows to find the ground voltage relative to the midpoint of the operating current source:

0,

and, (and-and.

The description of 1e periodically repeated measurements of the bias voltage of the neutral allows first of all to establish the time of leakage, for which each time the current value of the bias voltage of the neutral is compared with that obtained earlier and, if they do not match, the occurrence of a leak is recorded. Naturally, both of the compared voltages should be related to the same states of the auxiliary current source, i.e. in both cases, it must be either on or off. In this case, it is assumed that in the time between two identical states of the auxiliary current source no more than one leakage occurs, i.e. it is assumed that during the time between two leaks they manage to perform at least two measurements.

If the leakage current found does not reach values that require special measures, such as disconnecting the network, the measurements continue as described.

The proposed method can be used in scientific research, as well as in the development of devices for protection against electric shock. The use of this method makes it possible to increase the sensitivity of leakage current detection, which I leads to an increase in the reliability of power supply to consumers and the safety of operation of electrical installations.

Claims (1)

Invention Formula The method of determining the leakage current, which consists in measuring the parameters of the auxiliary circuit connected to earth, characterized in that, in order to increase the sensitivity, an auxiliary circuit is periodically switched, equipped with an auxiliary current source having the same frequency as the operating current, previously connected between the average point of the operating current source and ground, measure the voltage between point source working 1465832 current and ground as when turned on, so and when the auxiliary circuit is off, and the leakage current is 1 ,. determined by the formula R (E.1 UniUk2G GUN1  NE - UN where y and. ii, Conductivity of the auxiliary circuit; electromotive force of auxiliary current source; the voltage is equal to the average point of the operating current source and the ground before the occurrence of a leakage, respectively, when the auxiliary circuit is turned on and off; and. - voltage between the midpoint of the pafso4ero current source and ground after a leakage occurs with the auxiliary circuit switched on.