1. The structure of transformer body this paper mainly introduces the structure of transformer body from three aspects: core, winding and lead wire. A transformer iron core structure 01. The function transformer of iron core is made according to the principle of electromagnetic induction, and the magnetic circuit is the medium of electric energy conversion. The iron core is the main magnetic circuit of the transformer, whose main function is magnetic conduction. It transforms the electrical energy of the primary circuit into the magnetic energy, and from the magnetic energy into the electrical energy of the secondary circuit. At the same time, the iron core is also the mechanical framework of the transformer. The clamping device of the iron core not only makes the magnetic conductor a mechanically complete structure, but also covers it with an insulated coil to support the lead wire, and installed almost all the components inside the transformer. The weight of the iron core is the biggest among the parts of the transformer, accounting for about 60% of the total weight in the dry-type transformer, and about 40% in the oil-immersed transformer.

02. The iron core is composed of two parts: the core column and the Iron Yoke. The iron core column is sleeved with a winding, and the Iron Yoke connects the iron core column to form a closed magnetic circuit. The plane diagram of the core of the transformer is shown in Fig. 1. Fig. 1A is a single-phase transformer, Fig. 1B is a three-phase transformer. The core structure can be divided into two parts. Y is the part used to close the magnetic circuit, called the iron yoke. A single-phase transformer has two core columns, and a three-phase transformer has three core columns.

Because the magnetic flux in the transformer core is an alternating magnetic flux, in order to reduce the eddy current loss, the transformer core is generally made of silicon steel with high resistivity into a certain size of iron chip, the silicon steel sheets that make up the iron core are first cut into the desired shape and size, i. e. the sheet stamping, and then the sheet stamping is combined in an overlapping manner. Fig. 2A is the iron core of the single-phase transformer. Each layer consists of 4 laminations. Fig. 2B shows the iron core of the three-phase transformer. Each layer consists of 6 plates. The combination of the two layers of plates has been arranged in such a way that the joints of the magnetic circuits in each layer are staggered. This type of assembly is called Overlapping Assembly, this assembly avoids Eddy current flow between the steel sheets. And because each layer of laminations staggered inlay, in the core compression, can use fewer fasteners to make the structure simple. When assembling, the sheet metal is first folded and assembled into an integral iron core, then the lower iron yoke is clamped, the upper iron yoke sheet metal is drawn out to expose the iron core pillar, the prefabricated winding is sheathed on the iron core pillar, and finally the upper iron yoke sheet metal is inserted into the upper iron core pillar.

According to the winding arrangement in the core, the transformer is divided into two core and Shell. The difference is mainly in the distribution of the magnetic circuit, the yoke of the shell-type transformer core surrounds the coil, most of the core of the core transformer is in the coil, only part of the coil is outside the yoke, which is used to form the magnetic circuit.

03. In the normal operation of the iron-core heat sink transformer, the heat will be generated due to the iron loss in the iron core, and the larger the weight and volume of the iron core, the more heat will be generated. Transformer oil temperature above 95 degrees is easy to aging, so the surface temperature of the core should be controlled below this temperature as far as possible. The heat dissipation structure is mainly for increasing the heat dissipation surface of the iron core. The cooling of iron core mainly includes the cooling of iron core oil duct and the cooling of iron core air duct. In the large capacity oil-immersed transformer, oil grooves are often arranged between the laminations of the iron core to enhance the heat dissipation effect. The oil groove is divided into two kinds, one is parallel to the silicon steel sheet, one is perpendicular to the steel sheet, as shown in figure 4, the latter arrangement has better heat dissipation effect, but the structure is more complex.

In dry-type transformers, the core is air-cooled. In order to ensure that the core temperature does not exceed the allowable value, the air duct is usually installed in the core column and the Iron Yoke. 04. The noise of iron-core transformer will produce noise in operation. The noise of the transformer body is caused by the magnetostriction of the silicon steel, or the noise of the transformer core is basically caused by magnetostriction. The so-called magnetostriction is that the size of silicon steel sheet increases along the magnetic induction line while the size of silicon steel sheet decreases perpendicular to the magnetic induction line, which is called magnetostriction. In addition, the structure and geometry of the iron core, the technology of the iron core processing and manufacturing will have a certain impact on the noise level. The noise level of iron core can be reduced by the following technical measures: (1) using high quality silicon steel sheet with small magnetostriction ε value. (2) reduce the magnetic flux density of the iron core. (3) improving the structure of the iron core. (4) choose reasonable size of iron core. (5) adopt advanced processing technology.

05. In the normal operation of the earthing transformer with iron core, the electric field between the live windings and leads and the fuel tank is non-uniform, and the iron core and its metal parts are all in this electric field. Because the potential of electrostatic induction is different, the levitation potential of iron core and its metal parts is also different. When the potential difference between two points is enough to break through the insulation between them, a spark discharge occurs. This discharge can break down transformer oil and damage solid insulation. In order to avoid this situation, the iron core and its metal parts must be reliably grounded. The iron core must be grounded bit by bit. When the iron core or other metal components are grounded at two or more points, a closed circuit will be formed between the grounding points, forming a circulating current. The current can be as high as tens of Amperes, causing local overheating, leading to oil decomposition, and possibly causing the ground piece to fuse, burn out the core. None of this is allowed. Therefore, the iron core must be grounded, and it must be grounded bit by bit.

Transformer winding structure 01. The function of winding winding winding is the most basic part of transformer, is to establish the magnetic field and transmission electric energy circuit part, usually with insulated paper wrapped copper wire or aluminum wire wound, and set on the transformer iron core column. The iron core of the transformer shall have sufficient insulation strength, mechanical strength and heat resistance.

02. The winding structure of winding transformer can be divided into two kinds: layer structure and pie structure. The layer structure means that the windings are sequentially wound along its axis, which is generally used in the S8 and S9 series low-loss power transformers. The cake structure means that the windings are continuously wound along its radial direction to form a cake (segment) , then by a number of cakes along the axial configuration, generally in 110 kv and above high-voltage large, large-scale transformers used.

Most of the windings of power transformers produced in our country adopt concentric structure. The so-called concentric winding, is in any cross-section of the core column, the winding is the same cylindrical line set in the core column outside. High and low voltage windings, as well as low voltage windings and iron core between the column must be left with a certain insulation gap and heat channel (oil channel) , and separated by insulated cardboard tube. The insulation distance depends on the voltage level of the windings and the gap required for the cooling channel. When the low-voltage winding is placed inside near the core post, the distance between it and the core post is relatively small, so the size of the winding can be reduced and the overall size of the transformer can also be reduced.

The three-winding transformer is most commonly used in power system. Using A three-winding transformer to connect three different voltage transmission system is more economical, less land occupation and more convenient for maintenance and management than using two ordinary transformers. Three-phase three-winding transformers usually use y-y-△ connection, that is, the primary and secondary winding are Y-connection, and the third winding is Delta, as shown in fig. The delta connection is itself a closed loop, permitting the passage of the third harmonic current in the same phase so that the third harmonic voltage does not appear in the primary and secondary windings of the Y connection. In this way it can provide a neutral point for both the primary and secondary edges. In long distance transmission systems, the third winding can also be connected to the Synchronous condenser to increase the line’s power factor.

Transformer lead structure 01. Lead material and classification transformer windings external connection of the lead-out terminals of the winding known as lead, through the lead to the external power supply power into the transformer, but also through the lead transmission of electricity from the transformer output to the outside. The main types of leads are as follows: the lead wire connecting the end of the winding wire to the sleeve; (2) the lead wire connecting the end of the winding; (3) the materials of the lead wire connecting the winding part to the switch are generally as follows: (1) the Bare Copper Rod, application: 10kV Class 6300kVA and below transformers; (2) paper-wrapped Copper Bar, application: 10 ~ 35kV small capacity transformer; (3) Bare Copper Bar, application: 10kV and below low-voltage winding lead; (4) copper stranded wire, application: All voltage levels, especially 110kV and above lead; (5) Copper Tube, application: 220KV and above transformer lead. In order to ensure adequate insulation distance, leads through laminated wood, cardboard insulation, must meet the electrical performance, mechanical strength, temperature rise of these three requirements. The selection of lead wire is also based on the electric field strength and mechanical strength, as well as short-circuit when the temperature rise and long-term load when the temperature rise to choose these aspects.

02. The connection of the lead transformer leads the connection form has: braze welding, gas welding, cold pressure welding and bolt connection. Brazing electrodes shall be made of phosphor-copper Alloy for the connection of winding leads to leads and between leads. Gas Welding is used for the welding of Copper Strip leads and the welding of through-cable type casing joint. Cold pressure welding is to lead the connection of the two terminals inserted into a metal tube, and then use the mold on the metal tube extrusion, the two terminals pressed together. Cold pressure welding does not need heating, welding is safer, there is no false welding, and burn leads and other parts of insulation, extrusion quality, good tensile strength. Therefore, cold pressure welding is the main lead connection mode of large transformers at present. The bolt connection is mainly used for the lead wire connected with the guide rod type bushing. The lead wire can be disassembled to compensate for the deviation of the lead wire length. 03. In order to ensure the insulation distance of the lead and withstand the vibration and shock of electric power when running or short-circuit without displacement and deformation, a clamping piece must be used to fasten the lead. In order to improve the mechanical strength and electrical strength of the lead clamp, the structure of the lead clamp generally adopts the structure of wooden bracket. When the clamp is fixed with the metal parts of the transformer body, the metal bolts can be used to improve the mechanical strength, however, when fixing between the clamping pieces, epoxy bolts must be used and anti-loosening device must be provided. In the clamping lead should be increased insulation cardboard as an additional insulation to prevent blocking lead.

 Source: Artisan Repair

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