RU2379810C1 - Method to protect three-phase electric circuit against asymmetric modes - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: proposed invention van be used for protecting whatever three-phase circuits against asymmetric modes. Proposed method consists in that phase wire currents are passed through three fuses fitted in electric circuits of said wires, and each fuse is additionally heated by current originating at outputs of filters and passing through heaters arranged nearby fuses.

EFFECT: higher safety.

2 dwg

Description

The invention relates to the electric power industry and can be used to protect three-phase electric networks from unbalanced modes, both with zero wires and without them.

A known method of protecting a three-phase electrical network [Guide de I'installation electrique. Troisieme edition: Janvier 1982. Diffusion Center de Documentatione et Prodyctivite 38050 GRENOBLE CEDEX. Imprimerle Generale du Center, Seint-Etienne. Paris - France] with four fuses installed in phase conductors and in neutral.

The closest in technical essence to the proposed method is a method of protection [RF patent No. 2130663 RF IPC ⁶ H01H 3/10, 85/045, published on 05/20/99 bul. 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, as this reduces electrical safety;

- the use of heating elements in the circuit of neutral 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;

- it is used only in a four-wire three-phase network and cannot be used in other networks.

The emergence of asymmetric operating modes in three-phase electric networks is accompanied by the appearance of voltages of the reverse and zero sequences, which can be used to trigger the protection of the network from asymmetric modes. The voltage filters of the reverse and zero sequences at their outputs generate signals whose values are proportional to the asymmetry that occurs in a three-phase electric network. The heating elements are connected to the outputs of these filters and, if significant asymmetry occurs, create additional heating of the fuse inserts, 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 unbalanced modes used in any three-phase network, regardless of the number of wires by connecting filters of zero and reverse sequences to a three-phase network, to the outputs of which an additional heating element is installed, installed near the fuse.

This problem is solved by the proposed method, which does not require the inclusion in the circuit of the zero conductor of heating elements that have a sufficiently large resistance, which ultimately provides sufficient currents for tripping current protection.

The essence of the method lies in the fact that the currents of the phase wires pass 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 current appearing at the outputs of the voltage and reverse sequence voltage filters passing through the heating elements, installed in fuses near their fuse links.

The invention is illustrated in figures 1 and 2.

Figure 1 presents the wiring diagram for fuses 3, voltage filters, reverse 1 and zero 2 sequences and heating elements 4.

The emergence of asymmetric modes is accompanied by the appearance of reverse and zero sequence voltages in a three-phase network. The voltage filters of the reverse 1 and zero 2 sequences are connected to the phase wires L1, L2 and L3. Power supply to the consumer is carried out in a symmetrical mode through three phase wires. In the absence of asymmetric modes, the voltage at the outputs of these filters is zero and the current does not flow through the heating elements. When an asymmetric mode occurs, the outputs of the filters 1 and 2 appear voltage and through the heating elements 4 begins to flow current reverse and zero sequences.

Under the influence of this current, the temperature of the heating elements 4 increases, which leads to additional heating of the fuse links 3. The first fuse blows the fuse through which the largest current flows, causing an increase in asymmetry and, consequently, an increase in the current flowing through the heating elements, and increased heating of the two remaining fuses. 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 and a slight increase in current in a separate phase wire by 10% and the corresponding appearance of current in the filters of zero and reverse sequences to melt the fusible insert due to its additional heating using a heating element (point 2 in figure 2). When the current in the phase conductor increases to 130% 1n and the corresponding appearance of current in the filters of the zero and reverse sequences I _f = 0.3I _{H, the} fuse-link melts in 15-18 minutes (point 3 in figure 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 any three-phase network from asymmetric modes with the highest sensitivity.

Claims (1)

A method of protecting a three-phase electric network from asymmetrical modes, namely, that the currents of three-phase wires are passed through the fuse-links of the three-fuses installed in the electrical circuits of these wires, characterized in that the fuse-link of each of the three fuses is additionally heated by currents of the reverse and zero sequences flowing in series connected heating elements connected to the outputs of the voltage filters of the reverse and zero sequences and set opposite the sections of a smaller cross section of the fusible insert in the immediate vicinity of it.

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