RU2263364C2 - Cascade current transformer - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: connected to low-voltage metering winding of in-service current transformer is primary winding of additional current transformer with transformer ratio equal to unity so that cascade current transformer is formed, its measurement error being equal to sum of errors of in-service transformer and additional transformer whose error throughout entire range of loads connected to its secondary winding is approximately equal in absolute value to that of in-service current transformer and its polarity is reverse to that of error of in-service current transformer. Additional current transformer has magnetic circuit, primary winding, and secondary winding; its magnetic circuit is assembled of several cores wound of ferromagnetic ribbon and inserted into each other or placed one on top of other and separated by nonmagnetic gaskets. Secondary winding is wound on magnetic circuit assembled of several cores so that part of secondary winding turns passed through holes in nonmagnetic gaskets enclose at least one core.

EFFECT: enhanced measurement accuracy of in-service current transformer.

3 cl, 1 dwg

Description

The invention relates to electrical engineering and can be used to improve the accuracy of measurements of current transformers in operation.

Currently, there are known methods of reducing the error of current transformers by unwinding a certain number of turns from the secondary winding of the transformer or adding a certain number of turns to the secondary winding (the so-called winding correction), as well as methods of reducing the errors of the transformer by magnetizing the core, which is achieved by installing additional magnetic circuits inside the transformer and winding on them additional parts of the secondary winding [1]. However, this can only be done during the manufacturing process of the transformer at the factory.

The closest in technical essence and the achieved result is a cascade current transformer in which the errors of the transformer are composed of the errors of its stages, and the measurement windings of the cascade transformer with errors that have different signs, allows to obtain the error of the cascade transformer is very small in absolute value [2].

However, to reduce the absolute value of the error in the cascade current transformer is also possible only in the process of its manufacture in the factory.

This fact does not allow to obtain the required accuracy class of the current transformer in operation.

The objective of the invention is to reduce the error of current transformers in operation.

This problem is solved in such a way that the primary winding of the additional low-voltage transformer is connected to the low-voltage terminals of the measuring winding of the current transformer in operation, the transformation coefficient of which is unity, and the magnetic circuit and the secondary winding are made in such a way that in the entire range of measured currents and throughout range of loads connected to the secondary winding of an additional transformer, the absolute value of the current error is approximately equal to the absolute Achen transformer in operation current error and the sign opposite to that of its current error transformer in service.

This is achieved by the fact that the magnetic circuit of an additional transformer made according to this invention contains two or more cores wound from a ferromagnetic tape, superimposed on one another by end surfaces and separated from each other by spacers of magnetized material, and the secondary winding is wound on a magnetic circuit assembled from individual cores, so that some part of the turns passing through the through hole in the gaskets of non-magnetic material covers only part of the nitoprovoda (one or more cores).

This is also achieved if the magnetic circuit of the auxiliary transformer contains two or more cores wound from ferromagnetic tape, inserted into each other and separated from each other by spacers of non-magnetic materials, and the secondary winding is wound on a magnetic circuit assembled from separate cores so that some part of the turns passing through holes in gaskets of non-magnetic material, covers only part of the cross section of the magnetic circuit (one or more cores).

This is also achieved by the fact that two or more magnetic cores of an additional transformer, containing 3 or more cores, wound from a ferromagnetic tape, inserted into each other and separated from each other by spacers of non-magnetic material, are superimposed on each other by end surfaces and separated from each other gaskets of non-magnetic material, and the secondary winding is wound on these magnetic cores, so that some of its turns passing through the through holes in gaskets of non-magnetic material and covers only a part sectional cores (one or more cores).

Figures 1 and 2 show additional current transformers made according to this invention, the magnetic cores of which contain two or more cores 1.1, 1.2 ... 1n, wound from a ferromagnetic tape, and the secondary windings 2 with several turns 3.1 ... 3n, covering only part of the cross section of the magnetic cores (one or more cores) passing through the through holes 4.1 ... 4n in the gaskets of non-magnetic material 5.1 ... 5n, and are led out by taps 6 and 7.

The primary winding is not indicated in the drawing.

Figure 3 also presents an additional current transformer made according to this invention, which consists of a magnetic circuit containing 3 or more cores 1.1, 1.2 ... 1n, wound with a ferromagnetic tape, as inserted into each other and separated by spacers of non-magnetic material 2.1 2.2 ... 2n, and superimposed on each other by end surfaces and separated from each other by spacers of non-magnetic material 3.1 ... 3n, from the secondary winding 4, wound on the magnetic circuit, with several turns spanning only part of the cross section of the magnetic circuit (one or more cores) 5.1 ... 5n passing through the through holes 6.1 ... 6n in gaskets of non-magnetic material, with taps from the secondary winding 7 and 8.

The primary winding is not indicated in the drawing.

From consideration of figure 1, which shows the design of an additional transformer, it follows that gaskets of non-magnetic material 5.1 ... 5n divide the cross section of the magnetic circuit into parallel sections (cores 1.1, 1.2 ... 1.n) and turns 3.1 ... 3.n, passing in the holes 4.1 ... 4n, cover only part of the cross section of the magnetic circuit, for example, core 1.1.

Due to the fact that the winding direction of these turns coincides with the direction of winding the remaining turns 2 of the secondary winding, the direction of the flow created by turns 3.1 ... 3.n, in the cores 1.2 ... 1.n coincides with the direction of the flow created by turns 2 , and the magnetic fluxes in the cores 1.2 ... 1.n are summed, and in the core 1.1 the flux created by the coils 3.1 ... 3.n is directed towards the flux created by the coils 2, and therefore the magnetic flux created by the coils 3.1 ... 3.n is subtracted from the magnetic flux created by the turns 2. Thus, the values of induction and magnetic pro the tightness in the areas near the turns 3.1 ... 3.n in the core 1.1 and in the cores 1.2 ... 1.n are different.

With insignificant primary currents (up to 10-20% of the nominal), a large fraction of the flux created by turns 2 of the secondary winding passes through the cores 1.2 ... 1.n in areas near the turns 3.1 ... 3.n. Due to the magnetizing action of the coils 3.1 ... 3.n in the sections of the cores 1.2 ... 1.n located near the coils 3.1 ... 3.n, the induction and magnetic permeability are significantly increased, and in the core 1.1 in the areas located near the coils 3.1 ... 3.n, the induction value and magnetic permeability are insignificant due to the demagnetizing effect of the flux created by the coils 3.1 ... 3.n in this core and directed against the flux created by the coils 2 in the same core. In this case, the coils 3.1 ... 3.n almost do not adhere to the flow created by the coils 2 in the entire magnetic circuit. This increases the transformation coefficient of the current transformer, and the working number of turns of the secondary winding is equal to the number of turns of pos.2, which reduces the absolute value of the negative current error and can even lead to the current error becoming positive, since the current in the secondary winding increases.

With an increase in the primary current, a redistribution of magnetic flux occurs between the cores of the magnetic circuit in the place of winding coils 3.1 ... 3.n. In cores 1.2 ... 1.n, induction approaches the induction of saturation due to the magnetizing action of the flux created by turns 3.1 ... 3.n in these cores. The magnetic permeability in these cores is reduced. And in core 1.1, induction increases, but does not yet reach saturation induction due to the demagnetizing effect of the flux created by turns 3.1 ... 3.n, and the magnetic permeability is maintained significant.

Most of the magnetic flux passing through the core 1.1 in the place of winding the coils 3.1 ... 3.n now coheses with the coils 3.1 ... 3.n, and the number of coils in the secondary winding, which now consists of coils 2 and turns 3.1 ... 3.n. In this case, the transformation coefficient decreases, and the absolute value of the positive current error decreases.

Thus, in the entire range of measured currents and in the entire range of loads connected to the secondary winding of an additional transformer, with a certain number of turns 3.1 ... 3.n and a certain number of cores 1.2 ... 1.n covered by turns 3.1 ... 3.n, the absolute value of the current error of the additional transformer is selected such that it turns out to be approximately equal to the absolute value of the current error of the current transformer in operation, and its sign is opposite to the sign of the current error in operation of the transformer.

Sources of information

1. Handbook of high voltage electrical apparatus. Leningrad, Energoatomizdat, Leningrad branch p. 338-339.

2. Current transformers. V.V. Afanasyev et al., Leningrad, ENERGY, Leningrad Branch, 1980, pp. 48-50.

Claims (1)

A cascade current transformer, including a current transformer of the first stage of the cascade, the low-voltage leads of the measuring winding of which are connected to the terminals of the primary winding of the additional low-voltage current transformer with a transformation coefficient equal to unity, which is the second stage of the cascade and containing the primary winding, a magnetic circuit consisting of at least two cores wound with a tape of ferromagnetic material, said cores being inserted into each other or a torus superimposed evymi surfaces and are separated from each other by spacers of non-magnetic material, and a secondary winding wound on the magnetic circuit so that a part of turns passes through the through holes in the pads of a nonmagnetic material, covering at least one of said cores.

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