RU2395145C1 - Protection method of three-phase electrical network with neutral wire from non-symmetrical modes - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: currents of phase conductors are passed through fuse strips of three fuses installed in electrical circuits of these wires; in addition, fuse strip of each fuse is heated with currents appearing on secondary windings of three intermediate current transformers attached to outputs of secondary windings of zero-sequence current transformer.

EFFECT: improving reliability and quick action of protection.

2 dwg

Description

The invention relates to the electric power industry and can be used to protect three-phase electric networks from asymmetric modes with zero wires.

A known method of protecting a three-phase electric network [Guide de I′in-stallation electrique. Troisieme edition: Janvier 1982. Diffusion Center de Documentatione et Prodyctivite 38050 GRENOBLE CEDEX. Imprimerie Generate du Center, Seint-Etienne. Paris-France] with four fuses installed in phase conductors and in zero.

The method does not provide the necessary sensitivity of protection when a small asymmetry occurs, accompanied by a slight increase in current in the phase wire, and also when asymmetry occurs, the fuse fuse is installed in the neutral wire, which leads to an open circuit. The zero wire in domestic electrical installations is used to zero equipment.

Disconnecting or blowing out the neutral wire often does not result in the burnout of the fuse inserts installed in the phase wire circuits, and the three-phase network remains in operation. However, this mode can cause increased asymmetry of the voltage and, as a result, disruption of the power, their non-economical mode and even damage.

The closest in technical essence to the proposed method is a method of protection [RF Patent No. 2130663 RF IPC ⁶ H01N 3/10, 85/045, published 05/20/99 bull. No. 14] using additional heating of the fuse inserts of the fuses included in the phase chains of the three-phase circuit from the heating elements included in the neutral wire circuit.

This method has the following disadvantages:

- normative and technical documentation [Electrical Installation Rules] requires that the neutral conductor circuits do not have any additional electrical devices installed, which reduces electrical safety;

- the use of heating elements in the circuit of zero conductors creates an increase in the resistance of the phase-zero loop, which leads to a decrease in the values of single-phase short-circuit currents that occur during breakdown of insulation on the housing, and as a result to a decrease in the sensitivity of protection and violation of electrical safety conditions.

The emergence of asymmetric operating modes in three-phase electric networks is accompanied by the appearance of current in the neutral conductor, which can be used to trip the network protection from asymmetrical modes. The neutral conductor passes through a zero sequence current transformer, in which currents appear on the outputs of the three secondary windings, the values of which are proportional to the asymmetry that occurs in the three-phase electric network. The primary windings of three intermediate current transformers are connected to these secondary windings, and the fuse inserts are connected to the secondary windings of these current transformers, and when asymmetry occurs, additional heating of the fuse inserts by currents is created, which ensures their operation and disconnection of the section with the asymmetry from the rest of the electrical network .

An object of the invention is to increase the sensitivity of protection against asymmetrical modes used in a three-phase electric network with a zero wire by passing the neutral wire through a zero-sequence current transformer and the occurrence of currents on its three secondary windings, the primary windings of three intermediate current transformers are separately connected to the outputs of each , and the secondary windings of the latter are connected to fusible inserts.

This problem is solved by the proposed method, which does not require the use of heating elements with high resistance, which ultimately provides currents sufficient for tripping current protection.

The essence of the method lies in the fact that the currents of the phase wires are passed through the fuse-links of the three fuses installed in the electrical circuits of these wires, and the fuse-link of each of the three fuses is additionally heated by the currents appearing on the secondary windings of the three intermediate current transformers.

The invention is illustrated by drawings, where Fig. 1 shows a diagram of the inclusion of fuse links 3, a zero-sequence current transformer TA1 having three secondary windings with outputs I1-I2, I3-I4, I5-I6, and intermediate current transformers TA2, TA3 and TA4 converting current to voltage.

The power supply to the consumer is carried out in a symmetrical mode through three phase wires L1, L2 and L3, no current flows in the neutral conductor N, therefore the current in the three secondary windings of the current transformer TA1 will be zero. The emergence of asymmetric modes is accompanied by the appearance of N current in the neutral conductor. The neutral conductor passes through a zero sequence current transformer. When an asymmetrical mode occurs, currents appear on the outputs of the zero sequence current transformer. These currents flowing through the primary windings 1 of three intermediate current transformers create a voltage on their secondary windings 2, which causes currents to flow through the fuse inserts of the fuses, creating their additional heating.

The fuse fuse blows first, through which the largest current flows, causing an increase in asymmetry and, consequently, an increase in additional current, which leads to increased heating of the two fuses remaining in operation. The operation is illustrated by the family of protective characteristics shown in figure 2.

With existing protection methods, in the case of a small asymmetry of currents and a slight increase in current in a separate phase conductor, it exceeds the nominal value I _H up to 20%, the fuse-link does not burn out, it melts only when the current increases to 130% of the nominal one hour after the current increases [Electrical part of stations and substations: Textbook for high schools / A.A. Vasiliev, I.P. Kryuchkov, E.F. Nayashkova and others; Ed. A.A. Vasilyeva. M .: Energy, 1980. - 608 S., ill.] (Point 1 in figure 2). The proposed method allows for a small asymmetry of currents, a slight increase in current in a separate phase conductor by 10% and the appearance of current in the neutral conductor and, accordingly, in the secondary windings of the intermediate current transformers to melt the fuse links due to their additional heating (point 2 in FIG. 2 ) When the current in the phase conductor increases to 130% I _H and the corresponding current appears in the current transformer I _f = 0.3I _{H, the} fuse-link melts in 10-15 minutes (point 3 in FIG. 2). With significant asymmetry of the currents and a phase current not exceeding the nominal, the fuse will melt in a time determined by the family of protective characteristics (point 4 of figure 2). Therefore, the proposed method provides the best protection for a three-phase network with a zero wire from unbalanced modes with the highest sensitivity.

Claims (1)

A method of protecting a three-phase electric network with a zero wire from asymmetrical modes, namely, that the currents of three phase wires are passed through the fuse-links of three fuses installed in the electrical circuits of these wires, characterized in that the fuse-link of each fuse is additionally heated by the current of an intermediate current transformer, each of which is connected to the outputs of the individual secondary windings of the zero sequence current transformer.

Images