RU2536332C1 - Method for insulation resistance measurement at connections and search of connections with damaged insulation in direct-current mains with insulated neutral - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method for insulation resistance measurement at connections and search of connections with damaged insulation in direct-current mains with insulated neutral is based on measurement of currents passing through the mains connections upon coupling of a resistive element to poles. Simultaneously voltage is balanced at the poles by coupling two in-series identical resistors, which common point is connected to the ground through the third resistor. Values of insulation equivalent resistance for connections are calculated according to the formula that uses only measured differential currents as a variable.

EFFECT: simplifying measurement of connections resistance and search of connections with damaged insulation.

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Description

A method for measuring insulation resistance of connections and searching for connections with damaged insulation in a DC network with an isolated neutral is based on connecting between the poles of the network two identical resistors connected in series, the common point of which is connected to ground through a third resistor, and measuring currents flowing through the network connections in two cases of connecting resistive elements to each of the poles. The values of the total (equivalent) insulation resistance of the connections are determined by the difference in the steady-state differential currents flowing through the connections caused by the connection of the resistive element separately to the positive and negative pole of the network. The invention is aimed at simplifying the determination of insulation resistance of joints and reducing the search time for connections with damaged insulation.

The invention relates to electrical engineering and can be used to create portable search devices for connections with damaged insulation of direct current networks.

There is a method of searching for an element with reduced insulation resistance in a branched electric network of direct operating current, based on the application of a control alternating current and short pulses to the network and the selection of the active component of the alternating current in the controlled object (Patent of the Russian Federation RU 2180124, IPC ⁷ G01R 31/11, date of publication 02.27.2002 Authors: Vainshtein R.A., Shestakova V.V.). The disadvantage of this method is that the determination of the location of insulation damage is associated with the determination of the beginning of a short current pulse, the value of which depends on the capacity of the controlled connection, which reduces accuracy and complicates the design of connection search devices with poor insulation.

There is a method in which a control alternating voltage of a given frequency is applied to the network, different from the network frequency (Patent of the Russian Federation RU 2275645, IPC G01R 27/18 (2006.01), G01R 31/02 (2006.01), publication date April 27, 2006. Authors : Kislov E.A., Leontiev I.V., Levichev Yu.D., Kudrin I.A.). A distinctive feature of the method is the connection to a controlled connection of the calibration resistive-capacitive impedance. Based on the results of two measurements of the leakage currents when the impedance is connected and not connected, the insulation resistance of the controlled connection is determined. The disadvantage of this method is the presence of galvanic communication between the current meter of the control connection and the operating current network itself, which complicates its use in portable devices for monitoring the isolation of outgoing connections, and also reduces electrical safety.

Closest to the invention is a method for determining the insulation resistance of the connections in the DC network with an isolated neutral, based on measuring the voltage between the "ground" and the poles before and after connecting the resistive element to the poles, as well as on measuring the currents flowing through the network connections before and after connecting a resistive element (Patent of the Russian Federation RU 2381513, IPC G01R 27/18, publication date 02/10/2010 Authors: Alimov Yu.N., Galkin I.A., Shavarin N.I.). The value of the insulation resistance of the connections is calculated based on the values of the voltage at the poles of the network and the voltage between the poles, as well as the increments of the currents of the connections caused by the connection of an electrically controlled resistive element to the poles of the network. The disadvantage of this method is the need to measure the voltage of the poles relative to the ground, which complicates the design - especially of portable devices for searching for connections with damaged insulation, and also increases the time to search for connections with damaged insulation.

The objective of the invention is to simplify the method of determining the resistance of the connections and search for connections with damaged insulation, which allows to simplify the design of portable devices for searching for connections with damaged insulation.

The essence of the invention lies in the fact that in a method for measuring insulation resistances and searching for connections with damaged insulation in a DC network with an isolated neutral, based on the measurement of currents flowing through the network connections after connecting a resistive element to the poles, the poles are aligned at the same time by switching in series connected by two identical resistors, the common point of which through the third resistor is connected to the "ground", characterized in that the values of equivalent res the resistivity insulation of connections calculated by the formula

R from _equ. = A / | I ₊ -I _- |,

where R _iz.ekv - total (equivalent) insulation resistance of the controlled connection;

I ₊ - steady-state differential current of controlled connection, caused by connecting a resistive element to the positive pole;

I _- is the steady-state differential current of the controlled connection caused by the connection of a resistive element to the negative pole;

A is a coefficient depending on the current increment ΔI = | I ₊ -I _- |, caused by connecting a resistive element to the positive and negative pole of the network. It is set in a tabular way or graphically.

The invention is illustrated by diagrams ( _figure 1, 2) and the dependencies ΔI = F (R _from.Eq ) (figure 3, 4, 5), obtained by calculation and confirmed experimentally on the equipment of NPP "ECRA". The diagram of Fig. 1 shows a power supply of a DC network with isolated neutral 1, loads of connections 6, 8, 10, capacitances and active resistances of their insulation, for example, connection 7 has good insulation relative to the “+” bus, connection 9 has good insulation with respect to tires “-”, connection 5 has damaged insulation with respect to tires “+” and “-”. In addition, the diagram shows resistive elements 4 and 3 connected via keys K1 and K2 to the “+” and “-” buses, respectively, devices 14, 15, 16 for measuring currents flowing through connections 5, 7, 9, resistors 11, 12, connected in series and connected in parallel to the buses of the source 1, a resistor 13 connected between the common point of the resistors 11, 12 and ground.

In real-time operational direct current networks, the traditional insulation control circuit can serve as resistors 11, 12, 13 (Electrical reference book. In 4 vol. T.3. Production, transmission and distribution of electrical energy. 8th ed., Rev. And additional . - M .: Publishing House of MPEI, 2002. - 964 p., P. 609, Fig. 47.37), in which the resistors 11, 12 have 1 kOhm. As a resistor 13, the PH 51/32 relay coil is used, the resistance of which is 3.9 kOhm.

The introduction of resistors 11, 12, 13 made it possible to reduce the neutral bias voltage with asymmetric deterioration of the insulation of the mains current network, which reduces the likelihood of false triggering of protective devices in the operating current circuits.

Resistances R5 = R4 = 30 kOhm are used in the EKRA-SKI insulation monitoring system, which has found application at many electric power facilities in Russia.

To determine the equivalent insulation resistance of current conductors relative to the ground, it is necessary to know not only the values of the differential connection currents for the two cases of connecting resistive elements to the poles of the source, but also the values of the pole voltages relative to the ground and the voltage of the DC voltage source.

Generally for equivalent insulation resistance

R _{i.s. equ} = (EU ₊ -U _- ) / | I _{i +} -I _i- |,

where I _{i +} is the differential current of i-connection caused by the connection to the positive pole of the resistive element,

I _i- - differential current of i-connection, caused by connecting a resistive element to the negative pole,

U ₊ is the voltage at the positive pole when a resistive element is connected to it,

U _- is the voltage at the negative pole when a resistive element is connected to it,

E is the voltage between the poles of the source.

Figure 2 shows a network equivalent circuit of the operational DC on the example of joining one where I ₁ and I ₂ - currents on the positive and negative wire connection controlled, R _{of +} - insulation resistance controlled positive wire connection, R _iz - Resistance insulation of the negative wire of the controlled connection, R from the _{network +} is the insulation resistance of the positive pole of the network, R from the _network is the insulation resistance of the negative pole of the network.

Differential currents I ₊ = I ₁ -I ₂ and I _- = I ₁ -I _{2 of the} controlled connection when the keys K1 and K2 are closed, respectively, depend on the insulation resistance of the positive and negative poles of the controlled connection and the insulation resistance of the network.

The mathematical modeling of the operational direct current network with a voltage of 220 V for the case of R1 = R2 = 1 kOhm, R3 = 3.9 kOhm, and R5 = R4 = 30 kOhm made it possible to obtain the dependences of the current increment ΔI = | I ₊ -I _- | from the total (equivalent) insulation resistance of the controlled connection R _from.eq = R _{from +} // R _{from -} . Figure 3 shows the calculated dependences of the increment of the connection currents with asymmetric deterioration of the insulation of the connection R _{from +} = 10 MΩ, R _from - = 0 ... 100 kΩ, obtained for the case of R from the _{network +} = R from the _network = 10 _MΩ .

Figure 4 shows the calculated dependences of the increment of the connection currents with asymmetric deterioration of the insulation of the connection obtained for the case R from the _{network +} = R from the _network - = = 100 kOhm, R _{from +} = 10 MOhm, R _from _ = 0 ... 100 kOhm.

Figure 5 shows the calculated dependences of the increment of the connection currents with a symmetric deterioration of the insulation of the connection R _{from +} = R _from- = 5 ... 50 kOhm obtained for the case of R _{network +} = R from the _net- 100 kOhm.

As can be seen from the figures of figure 3, figure 4 and figure 5, the increment of the differential currents of the connection ΔI = | I ₊ -I _- | both with asymmetric and symmetric deterioration of the insulation of the connection, it depends only on the equivalent insulation resistance of the controlled connection and practically does not depend (with an accuracy of 10%) on the insulation resistance of the _auxiliary current network in the range R _iseq = 5 ... 100 kOhm at R _{network +} = R _Iset- = 100 ... 1000 kOhm. This circumstance made it possible to obtain the dependence A = R from _eq · _ΔI , presented in Fig.6 and in table 1.

Table 1 ΔI, mA 0 0.5 1,0 2.0 3.0 5,0 7.5 A, kOhm / mA 25 24 22 eighteen fifteen 7.5 0

An example of a specific implementation of this method for measuring insulation resistances of connections and searching for connections with damaged insulation of an operational DC network can be a portable device for searching for connections with damaged insulation developed by EKRA EKRA-PKI LLC, which allows finding connections with damaged insulation with equivalent insulation resistance less than 100 kOhm.

Claims (1)

A method for measuring insulation resistance of connections and searching for connections with damaged insulation in a DC network with an isolated neutral, in which two identical resistors are connected in series between the poles of the network, the common point of which is grounded through the third resistor, and the differential currents flowing through the network connections are measured at In two cases of connecting resistive elements to each of the poles relative to the earth, in accordance with the method of the invention, the value is full Foot) insulation resistance of connection is determined from the expression
R out of _equ. = A / ΔI,
where ΔI = I _{i +} -1 _i- ,
I _{i +} is the current of the i-th connection caused by connecting a resistive element to the positive pole;
I _i- is the current of the i-th connection caused by the connection of a resistive element to the negative pole;
A is a coefficient depending on ΔI and specified in a tabular manner or graphically.

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