RU2379818C1 - Device for interphase current distribution - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention can be used for interphase current distribution in magnetically unbalanced three-phase devices. Proposed device comprises zero output terminal, transformer with three phase windings connected, each by one terminal, to appropriate phase input terminal. Note here that transformer incorporates three magnetic cores with phase windings connected in series, each comprising intermediate output terminal to divide the winding number of turns into equal parts that is connected to zero input terminal.

EFFECT: possibility to use three single-phase transformers with various-configuration magnetic cores instead of one-phase transformer, absence of extra winding.

1 dwg

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-II) circuit or their serial connection according to the bridge circuit (Fig. 3-I), providing in the mentioned bisexual In the periodic case, in particular, the conversion of alternating voltage to direct.

A feature of these converters is the 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 a 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 08/02/79 to the main bus. St. No. 752681, cl. Н02М 7/06, dated 04.02.76, the only device close to the proposed technical solution for interphase current distribution 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 connected in a triangle.

The disadvantage of this device is the difficulty in that to achieve complete neutralization of the harmful effects on the zero-sequence current network at close to zero unlocking angles of the thyristors, an additional winding connected to a triangle is required on the transformer. In addition, the inability to use separate magnetic circuits within the framework of the known circuit 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, the phase windings of which are connected in a three-phase group and each connected with one output to the zero input terminal, and the other to the corresponding phase input terminal, the transformer is made on three magnetic cores with series connected by phase windings and an intermediate terminal in each, connected by intermediate terminals to a zero input terminal, while the intermediate terminal divides the number of turns of each phase winding into equal parts.

The technical result achieved in the proposed one consists in the possibility of using three 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. In addition, the estimated power of the windings of such a transformer group is 15% less than in the prototype.

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 three single-phase transformers 1, 2 and 3 with windings 4, 5, 6, 7, and 8, 9, respectively. These windings are connected in series, and their number of turns is equal. The end (beginning) of winding 4 (8) of transformer 1 (3) is connected to the input terminal of phase A. The end (beginning) of winding 6 (5) of transformer 2 (1) is connected to the input terminal of phase B. The end (beginning) of winding 9 (7) ) transformer 3 (2) is connected to the input terminal of phase C. Common points of opposite terminals of pairs of windings 4, 5; 6, 7, and 8, 9, each pair constituting a corresponding phase winding, are connected to a zero input terminal 0, to which a wire is connected for a zero sequence current flow of the aforementioned three-phase AC voltage transformer converter.

The interconnection of the primary phase windings of different transformers with beginnings or ends, in connection with the absence of secondary phase windings, does not matter. The sum of the number of turns of the phase windings of each transformer is the nominal design number of turns for connecting these windings to the mains voltage U _l .

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 three equal parts, each of which flows to the intermediate terminal of the phase winding of the corresponding transformer 1, 2, and 3. Further, each of the three current components branches into two equal parts along the phase windings of the transformers 1, 2 and 3 flows 1/6 of the total current. The branching of the current into parts proportional to the equal number of turns of the sections of the phase winding of each transformer having a common intermediate output is due to the presence of autotransformer communication between these sections, providing the necessary balance of ampere turns. On the other hand, transformers 1, 2 and 3 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 at each nodal connection point of the phase windings of different transformers is one third of the total current, i.e. 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 the characteristic feature of the operation of the converter in question is the alternate simultaneous loading 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 true 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.

раз больше числа витков фазной обмотки прототипа, т.к. к первой прикладывается линейное питающее напряжение, а ко второй - фазное. С другой стороны, сечение фазной обмотки каждого трансформатора предлагаемого в 2 раза меньше сечения фазной обмотки прототипа, т.к. ток в фазной обмотке первого равен 1/6 от общего тока, а ток в фазной обмотке второго - 1/3 от общего тока. Поэтому расчетная мощность фазных обмоток трансформаторов предлагаемого в

раз меньше расчетной мощности фазных обмоток трансформатора прототипа.The number of turns of the phase winding of each transformer proposed in

times the number of turns of the phase winding of the prototype, because a linear supply voltage is applied to the first, and phase voltage to the second. On the other hand, the cross section of the phase winding of each transformer proposed is 2 times smaller than the cross section of the phase winding of the prototype, because the current in the phase winding of the first is 1/6 of the total current, and the current in the phase winding of the second is 1/3 of the total current. Therefore, the rated power of the phase windings of the transformers proposed in

times less than the rated power of the phase windings of the prototype transformer.

Claims (1)

A device for interphase current distribution containing a zero input terminal, a transformer with three phase windings, each connected to one terminal to a corresponding phase input terminal, characterized in that the transformer is made on three magnetic cores with series-connected phase windings, each of which contains an intermediate terminal dividing its number of turns into equal parts and connected to the zero input pin.