SU1734162A1 - Method of protection of instrumentation voltage transformer against overload - Google Patents

Abstract

Summary of the Invention: By applying a low voltage to a protected transformer when the industrial frequency in the neutral of the protected transformer exceeds the second setpoint, the transformer is overloaded when the supply voltage in the network is asymmetrical, which increases its service life. 4 il.

Description

The invention relates to electrical engineering, in particular, to relay protection of voltage measuring transformers against overload in networks with insulated neutral under normal operating conditions, as well as with a circuit to earth.

Methods are known for protecting measuring voltage transformers (IOLs) based on fixing the overload current either in series with the primary winding of the active resistance or creating a parallel current path through the auxiliary transformer.

The main disadvantage of these methods is the very low efficiency in increasing the service life of voltage transformers. This is due to the fact that all methods do not take into account the parameters of the insulation of the electrical network relative to the ground in combination with the impedance of the voltage transformer, and the protection does not work in the normal mode of the electrical network with a slight asymmetry of the phase voltages. With certain isolation parameters, data

methods either do not significantly reduce the overload current, or, conversely, increase it.

The closest to the proposed method is an overvoltage protection of intr, in which the overload current is measured by the level of higher harmonics in a zero-sequence voltage during an arc fault to earth or ferroresonant processes and compared with a permissible value. voltage transformer.

The main disadvantage of the known method of protection against overload lies in the fact that in the normal operating mode of the electrical network, i.e. when there are almost no phase-to-ground arc faults and no higher harmonics, but phase voltage unbalances are insignificant, the resistance is not included in the transformer neutral, which leads to transformer overload under certain parameters of the insulation, (/)

WITH

Xi

with

Ј

Th

thereby, its service life and measurement accuracy.

The aim of the invention is to increase the service life of the transformer by reducing overloads when the supply voltage in the network is asymmetrical.

The goal is achieved by the fact that in a known method of protecting an instrument voltage transformer against overload, based on measuring the monitored quantity and comparing it with the first setpoint, above which the active resistance is applied to the neutral of the protected transformer, the current of industrial frequency is used as the monitored quantity. in the neutral of the protected transformer, in this case, if the monitored current exceeds the second predetermined setpoint after the indicated switching on of the active resistance, the protection aemy transformer fed low voltage.

Figure 1 shows a replacement circuit for an electrical network with an insulated neutral and a protected instrument transformer; figure 2 - diagram of the device that implements this method of protection; Fig. 3 shows the dependence of the current I of the active and capacitive resistances of the insulation of the electrical network relative to the earth; Fig. 4 is a timing diagram of the operation of the device, where RH is the active resistance in the neutral of a voltage transformer; U is the value of the voltage supplied to the transformer; 1Cp.1, Icp2 currents of operation of the first and second threshold elements, respectively.

The proposed method is based on the condition of transferring power from the energy source to the receiver. The voltage transformer is hereby considered as a receiver. Consider a replacement circuit for an electrical network with insulation parameters relative to the earth and a voltage transformer (Fig. 1). In Figure 1, the following conventions are accepted; U is the mains voltage; xc of, giz capacitive and active resistance of isolation of the phase of the network relative to the earth; Rrp, htr is the active and inductive phase resistance of a voltage transformer. A replacement circuit for an electrical network can be converted from parallel to serial. Then the current flowing through the transformer, taking into account the parameters of the insulation of the network relative to the earth, is determined;

kH -UH

from - V to kz, D -.2, (, b from 2

(- + RTP) + (x Tr + -)

U out

U out

where UH is the nominal linear voltage applied to the voltage transformer;

VEZ, UZ. UIZ-- measured active, am-

bone and full conduction of the insulation of the network phase relative to the earth;

KH - coefficient taking into account an increase in phase voltage higher than nominal as a result of phase asymmetry

stress

In addition, a constant current component Pr flows through the transformer, caused by the power consumption connected to the voltage transformer.

Thus, the current flowing through the transformer consists of two components: the variable O, depending on the magnitude of the active and capacitive insulation resistances and their ratio, and a constant 1P:

tr of + Inp.

The current Tp through the transformer is determined from the condition Tp dop, where 1Dp is an acceptable amount of current for a certain

accuracy class. If the values are measured from, then the transformer overload current can be monitored. In order that the voltage transformer does not introduce an error in the measurements above the required one, and the service life remains within the limits specified, it is given by the manufacturer the permissible power for a certain accuracy class, i.e. the permissible current value is dop, due to the condition of the symmetry of the phase voltages. So, for example, for an NTMI-6 type transformer in the first accuracy class of 5 OD.1 150 VA (I add.1 14.4 mA, and in the accuracy class 0.5-75 VA Odop.o, 5 7.2 mA). With an insignificant asymmetry of the phase voltages, the magnitude of the current from is determined by the parameters of the insulation of the phases relative to the earth Gyz and Sys. Thus, when limiting overcurrent, it is necessary to take into account the insulation parameters

electrical network relative to the earth.

The proposed method allows to increase the service life of the voltage transformer, as well as to reduce the measurement error in normal operation.

electrical network. To increase the service life and reduce the error, it is necessary to reduce the current of the transformer by reducing the current from. With a certain ratio of isolation parameters, the reduction of current | from when introducing active resistance in series with a transformer may not occur. For example, when Ces 0.25 microfarads; The vis is 5000 ohms and the allowable current for the first accuracy class I additional 14.4 mA is 14.55 mA, and when switched on in series with a active resistance transformer of 5000 ohms, the current Of increases and becomes 15.13 mA ( the measurement error increases, the service life of the transformer decreases, which goes into the third accuracy class). In this case, it is necessary to choose another way to reduce the current — the introduction of an inductive impedance into the transformer’s neutral or a reduced voltage supply, which is removed from the voltage divider, and the reduced voltage is chosen so that the current through the transformer is close to the trigger current of the second threshold element see figure 4).

The device (FIG. 2) for protecting the voltage transformer 1 comprises a resistor 2 in the neutral circuit of the primary winding connected in series with the first threshold element 3, made in the form of a current relay, which has an opening contact 4; rectifying bridge 5, the second threshold element 6, made in the form of a voltage relay, which has a closing contact 7, an inductive voltage divider 8 with taps from the phase windings and a high voltage contact, consisting of a contact coil 9, closing 10, 12 and disconnecting 11 contacts.

At the same time, parallel to resistor 2, an opening 4 contact and a rectifying bridge 5 are connected, the output of which is connected to the second threshold element 6, and the voltage divider 8 is connected between the mains phases and ground via the closing contact 10 of the high-voltage contactor: the primary winding of the voltage transformer 1 through the disconnecting 11 contacts to the mains phases and through the closing 12 contacts of the high voltage contactor to the tapes from the windings of the voltage divider 8, and the contactor coil 9 is connected to the power supply through the closing e contacts 7 of the second threshold element 6.

The first threshold element 3 is designed to connect the resistor 2 to the transformer neutral, and the second element 6 is to turn on the voltage transformer 1 through the voltage divider 8 using a high-voltage contactor, with

this, the trigger current of the first threshold element is less than the second.

The capacitive and active resistances of the insulation of individual phases of the electrical network relative to the earth are indicated in Fig. 2 as Heath and Guise, respectively.

The device that implements the method works as follows.

In order to control the current

0 overload in the transformer Up (from + Inp), it is sufficient to measure the current I3, since the current Pr is a constant value and is determined by the permanently connected measuring instruments. The current from is measured with

5 using the first 3 and second 6 threshold elements. With known values of Siz, Gie (see dependencies 1 and 2 in Fig. 3), the current Of does not exceed the tolerance for a certain accuracy class and contacts 4 of the first threshold element 3 are bridged by resistor 2. In this case, contact 7 of the second threshold element 6 is open, and contacts 10 and 12 of the high-voltage contactor are open, and contacts 11 are closed.

5 Thus, the total nominal voltage UH of the network is supplied to transformer 1 via contacts 11. As the value of Vis decreases (see Fig. 3, relationship 1 and 2), through the threshold element 3, the current rises.

0 This leads to the triggering of the first threshold element 3 (see FIG. 4, time ti), which opens its contacts 4 and includes resistance in the neutral of the transformer 1, thereby reducing the current Kr to

5 magnitude 1sr. I cf 2 (see figure 4).

If Tp increases when the active resistance is switched on (see Fig. 3, dependence 3) and exceeds the response current of the second threshold element 6, then

0 triggers (see Fig. 4, time t2) and its closing contacts 7 closes the supply circuit of the contactor coil 9, which switches the contacts 10-12 of the high-voltage contactor, and an undervoltage is applied to the transformer 1 (see Fig. 4, time xs), which is selected from the most severe conditions of operation of a voltage transformer with known insulation parameters relative to the earth for the considered area of operation of electrical networks subject to the conditions I cf.2 I add.1

Thus, using the proposed method, it is possible to choose a more optimal way to reduce the overload current of the voltage transformer, taking into account the parameters of the insulation resistance of the electrical network relative to the earth.

WITH,

The use of the proposed method of protecting the transformer against overload compared to existing methods allows increasing its service life and reducing measurement errors.

Claims (1)

Claims method of protecting an overvoltage measuring transformer based on measuring the monitored current value and comparing it with the first predetermined setpoint, above which the active resistance in the neutron is switched on Rally protected transformer, characterized in that, in order to increase the service life of the transformer by reducing overloads with asymmetry of the supply voltage in the network, the power frequency in the neutral of the protected transformer is used as a monitored quantity; the current exceeds the second predetermined setpoint; an undervoltage is applied to the protected transformer. I / HSie 1S FIG. / rig. 2 1 of mA 10 15 20 Fig.Z