RU2365019C1 - Current phase-to-phase distributor - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to electrical engineering, and can be used for phase-to-phase distribution of current in magnetically unstable three-phase signals, for example for neutralisation of zero-sequence harmful effects on current feed circuit in three-phase AC-to-DC and DC-to-AC voltage transformers. Proposed device comprises the transformer with its two phase windings connected, each by one output terminal, to zero input terminal, while their other output terminal is connected to appropriate phase input terminal. The transformer comprises two magnetic cores with phase windings connected in series and furnished with central tap. The said windings have their central and extreme free terminals connected to zero and common phase input terminals, respectively. Note here that central output terminal divides the number of turns in each phase windings into unequal parts, smaller ones being connected to opposing phase input terminals.

EFFECT: possibility to use two single-phase transformers with various-shape magnetic cores instead of three-phase transformer.

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Description

The invention relates to electrical engineering and can be used for interphase current distribution in magnetically unbalanced three-phase devices, for example, to neutralize the harmful effects on the supply network of zero-sequence current in converters of three-phase AC to DC and / or AC.

Three-phase AC to DC converters are widely known (see A.A. Bulgakov. A New Theory of Controlled Rectifiers. M., 1970, p. 50, 51, fig. 33, fig. 36), made on magnetically unbalanced (in the case of connecting the primary winding into a star with an insulated neutral) three-phase transformer with a beam circuit for connecting its secondary windings and valves. Three-phase AC to DC converters are also known (see A.A. Abdulaev, A. G. Aslan-zade. Analysis of multi-pulse rectification - "Electricity", 1977, No. 8), performed on magnetically unbalanced (in the case of connecting the primary winding to a star with an insulated neutral) three-phase transformer with connection of the secondary windings through the valves to the zigzag according to the half-wave (Fig. 2), half-wave (Fig. 3 - ||) circuit or their serial connection according to the bridge circuit (Fig. 3 - |), providing in said two luperiodnom case, in particular, AC to DC conversion.

A feature of these converters is a magnetically unbalanced mode of operation of a three-phase transformer when the primary winding is connected to a star without zero output and there is no short-circuited additional winding connected to it in a triangle. The primary winding of such a transformer, in the absence of the mentioned additional winding, can only be connected in a triangle or in a star with a zero output. The connection of the primary winding of the transformer into a star without zero output leads to the appearance of a pulsating magnetic flux with a triple frequency in its magnetic circuit, which disrupts the operation of the converter (see I.L. Kaganov. Electronic and ion converters, M. - L., part 3. 1956 p. 99). The connection of the primary winding of the transformer into a triangle, due to, for example, increased requirements for its insulation, is not always acceptable. The connection of the primary winding of the transformer into a star with a zero output leads to pollution of the supply network with a zero-sequence current containing higher harmonics, the frequency of which is a multiple of three.

The severity of the problem increases in converters controlled from the primary side of the transformer, in which the presence of a neutral wire is a necessary condition for their operability.

It is known, for example, a three-phase AC to DC converter, containing a three-phase transformer, the primary windings of which together with the pairs of counter-parallel-connected controlled valves connected in series with them form a star connected to the phase input terminals, and the secondary windings are connected to the star and connected to the rectifier bridge, while additionally introduced a set of secondary windings, a rectifier bridge and controlled valves, the primary windings through additional e counter-parallel connected valves are connected to the zero input terminal, and an additional set of secondary windings and an additional rectifier bridge together with the main secondary windings and the main rectifier bridge are connected according to a twelve-pulse rectifier circuit (ed. St. No. 752681, class N02M 7/06, from 02/04/76).

The disadvantage of this converter is that the current flowing through the neutral wire pollutes the supply network with higher harmonics, the frequency of which is a multiple of three.

The set of reasons that impede the receipt of the required technical result is that an open circuit of this current makes the converter inoperative.

By additional author. St. No. 860238, cl. Н02М 7.12, from 02.08.79 to the main author. St. No. 752681, cl. Н02М 7/06, dated 04.02.76, the only device close to the proposed technical solution is known that contains an additionally introduced three-phase transformer, the primary windings of which are connected to a star, the common point of which is connected to the zero input terminal, and the group of additional windings is connected in a triangle.

The disadvantage of this device is the difficulty in that, in order to completely neutralize the harmful effects on the zero-sequence current network at close to zero unlocking thyristors, it is necessary to have an additional winding connected to a triangle on the transformer. In addition, the inability to use separate magnetic circuits within the framework of the known scheme limits the operational capabilities of the device.

The set of reasons that prevent obtaining the required technical result is that the device is made on one common magnetic circuit of an additionally introduced three-phase transformer.

The problem to which the proposed technical solution is directed is to simplify and expand operational capabilities.

This problem is solved in that in the proposed device for interphase current distribution containing a transformer, two phase windings of which are connected each with one output to the zero input terminal, and the other to the corresponding phase input terminal, the transformer is made on two magnetic circuits with series-connected phase windings and intermediate output in each, connected intermediate and free extreme conclusions respectively to the zero and common phase input output, while the intermediate output divides the number of turns of each phase winding into unequal parts, half of which are connected to unlike phase input terminals.

The technical result achieved in the proposed one consists in the possibility of using two single-phase transformers with magnetic cores of various configurations, including twisted ring-shaped tape, instead of one three-phase and there is no need for an additional winding connected in a triangle, with the same quality of neutralizing the harmful effects on the network of higher harmonics of the current in the neutral wire.

The drawing shows a schematic diagram of a device for interfacial distribution of zero-sequence current of a three-phase AC voltage converter, the transformer of which is magnetically balanced by connecting its primary winding to a star with zero output, where the arrows show the directions of the flow of the zero-sequence current fraction I ₀ to its positive half-period of triple frequency.

The device contains two single-phase transformers 1 and 2 with windings respectively 3, 4 and 5, 6. The number of turns of the winding 3 (5) is two times less than the number of turns of the winding 4 (6). The end (beginning) of the winding 3 (5) of the transformer 1 (2) is connected to the input terminal of phase A (C). The beginning (end) of winding 4 (6) is connected to the input terminal of phase B. The common points of the opposite terminals of the pairs of windings 3, 4 and 5, 6 are connected to the zero input terminal 0, to which a wire is connected for the zero-sequence current of the above-mentioned three-phase AC transformer voltage.

Suppose that the primary winding of a transformer of a three-phase voltage converter connected to a star with a zero output is connected to the input terminals A, B, C and 0, while the converter operates in such a way that an alternating current of triple frequency flows through the zero wire, and this in a positive half-period of this It flows in the direction from input terminal 0 to input terminals A, B, C, and in the negative half-cycle in the opposite direction.

In the positive half-period of this current, it branches into two equal parts, the first (second) of which at the common point of windings 3 and 4 (5 and 6) branches into two unequal parts, and the twice as large one flows from the beginning (end) to the end ( the beginning) of winding 3 (5) with half the number of turns and then to the input terminal of phase A (C), and half of them flows from the end (beginning) to the beginning (end) of winding 4 (6) with twice as many turns and then to the input terminal of phase B. The current branching into parts inversely proportional to the unequal number of turns winding of each transformer having a common intermediate conclusion, due to the existence autotransformer connections between these portions, which provides necessary balance of ampere-turns. On the other hand, transformers 1 and 2 do not carry an independent load and, relative to the zero-sequence current, function as current dividers that self-sufficient ensure an ideal distribution of currents. Thus, the fraction of current in the triple frequency half-cycle under consideration in each winding with a smaller (large) number of turns is one third (one sixth) of the total current. As a result, the total current flowing through input terminal 0 is divided into three equal parts, each flowing through one of the phase input terminals in the direction of the supply network.

Since a characteristic feature of the operation of the converter in question is the alternate simultaneous load of the zero sequence current of only one of the phases of the supply network, this current in each discrete interval is distributed between the phases of the network in the ratio 2/3, 1/3, 1/3 instead of the ratio 1, 0 , 0, which is valid in the absence of an interphase current distributor.

In the negative half-cycle of the zero sequence current flow, the inverse process of summing the current shares in the common neutral wire, similar to that described, occurs.

As a result, during each period of the supply voltage in the linear wires currents are formed, which do not contain higher harmonics that are multiples of three.

Claims (1)

A device for interphase current distribution containing a transformer, the two phase windings of which are each connected with one terminal to the zero input terminal, and the other to the corresponding phase input terminal, characterized in that the transformer is made on two magnetic circuits with series-connected phase windings and an intermediate terminal in each connected by intermediate and free extreme terminals respectively to the zero and common phase input terminal, while the intermediate terminal divides the number of turns of each phase winding into unequal parts, half of which are connected to unlike phase input terminals.