Power Transformer Body Structure

1. Transformer core structure

01 The role of the iron core

The transformer is manufactured 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, and its main function is to conduct magnetism. It converts the electrical energy of the primary circuit into magnetic energy, and from the magnetic energy into the electrical energy of the secondary circuit.

At the same time, the iron core is the mechanical skeleton of the transformer. The clamping device of the iron core not only makes the magnetic conductor a mechanically complete structure, but also has an insulated coil on it, which supports the lead wire and almost installs the inside of the transformer. of all parts. The weight of the iron core is the largest among all the components of the transformer, accounting for about 60% of the total weight in dry-type transformers, and about 40% in oil-immersed transformers.

02 Form of iron core

The iron core is composed of two parts, an iron core column and an iron yoke. The iron core column is covered with windings, and the iron core column is connected by the iron yoke to form a closed magnetic circuit.

The plan view of the iron core of the transformer is shown in Figure 1. Figure 1a is a single-phase transformer, and Figure 1b is a three-phase transformer. The core structure can be divided into two parts, C is the part of the coil, called the core column. Y is the part used to close the magnetic circuit, called the iron yoke. A single-phase transformer has two legs, and a three-phase transformer has three legs.

Since 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 a silicon steel sheet with a large resistivity into a certain size of iron chip, and the silicon steel sheet that constitutes the iron core is cut first. It is punched into the desired shape and size, and then the punched pieces are combined in an overlapping manner. Figure 2a shows the iron core of a single-phase transformer, each layer is composed of 4 punched sheets. Figure 2b shows the iron core of a three-phase transformer. Each layer consists of 6 pieces. The punching pieces of each two-layer combination are arranged in different ways to make the joints of the magnetic circuits of each layer staggered from each other. This assembly method is called overlapping. assembly, which can avoid eddy currents circulating between the steel sheets. And because each layer of punching sheets is staggered and inlaid, less fasteners can be used to make the structure simple when pressing the iron core. When assembling, first stack the punched sheets to form a whole iron core, then clamp the lower iron yoke, remove the upper iron yoke punching sheet to expose the iron core column, put the prefabricated winding on the iron core column, and finally Insert the punched piece of the upper yoke that was pulled out.

According to the arrangement of the windings in the core, the transformer is divided into two types: core type and shell type. The difference is mainly in the distribution of the magnetic circuit. The yoke of the shell type transformer core surrounds the coil, and most of the core type transformer core In the coil, only part is outside the coil, that is, the iron yoke, which is used to form the magnetic circuit.

03 Heat dissipation of iron core

When the transformer is in normal operation, the iron core will generate heat due to iron loss, 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 age, so the temperature of the iron core surface should be controlled below this temperature as much as possible, which requires the heat dissipation structure of the iron core to quickly dissipate the heat of the iron core. The heat dissipation structure is mainly to increase the heat dissipation surface of the iron core. The heat dissipation of the iron core mainly includes the heat dissipation of the iron core oil passages and the air passage heat dissipation of the iron core.

In oil-immersed transformers with larger capacity, oil grooves are often set between the laminations of the iron core to enhance the heat dissipation effect. The oil tank is divided into two types, one is parallel to the silicon steel sheet, and the other is perpendicular to the steel sheet, as shown in Figure 4. The latter arrangement has better heat dissipation effect, but the structure is more complicated.

The iron core of the dry-type transformer is air-cooled. In order to ensure that the iron core temperature does not exceed the allowable value, air passages are often installed in the iron core column and the iron yoke.

04 Iron core noise

Transformers generate noise during operation. The source of the noise of the transformer itself is the magnetostriction of the iron core silicon steel sheet, or the noise of the transformer core is basically caused by the magnetostriction. The so-called magnetostriction means that when the iron core is excited, the size of the silicon steel sheet increases along the direction of the magnetic field line; while the size of the silicon steel sheet decreases in the direction perpendicular to the magnetic field line, this size change is called magnetostriction. In addition, the structure and geometric size of the iron core, and the processing and manufacturing process of the iron core will have a certain degree of influence on its noise level.

The noise level of the iron core can be reduced by the following technical measures:

(1) Use high-quality silicon steel sheets with a small magnetostriction ε value.

(2) Reduce the magnetic flux density of the iron core.

(3) Improve the structure of the iron core.

(4) Choose a reasonable core size.

(5) Adopt advanced processing technology.

05 Grounding of the iron core

In the normal operation of the transformer, the electric field formed between the live windings, leads and the oil tank is an uneven electric field, and the iron core and its metal parts are in the electric field. Because the potential of electrostatic induction is different, the floating potential of the iron core and its metal parts is also different. When the potential difference between the two points reaches the ability to break down the insulation between them, a spark discharge occurs. This discharge can break down the transformer oil and damage the solid insulation. In order to avoid this situation, the iron core and its metal parts must be grounded reliably.

The iron core must be grounded at one point. 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 grounding piece is blown and the iron core is burned out, these are not allowed. Therefore, the iron core must be grounded, and it must be grounded at one point.

2. Transformer winding structure

01 The role of winding

The winding is the most basic part of the transformer, and it is the part of the circuit that establishes the magnetic field and transmits electric energy. The iron core of the transformer should have sufficient dielectric strength, mechanical strength and heat resistance.

02 Type of winding

Transformer winding structure can generally be divided into two categories: layer structure and pie structure. The layered structure means that the turns of the winding are arranged and wound continuously along its axial direction, which is generally used in S8 and S9 series low-loss power transformers; the pie structure means that the turns of the winding are continuous along its radial direction. It is wound into a cake (segment), and then consists of many cakes arranged in the axial direction. It is generally used in large and extra-large transformers with high voltage of 110kV and above.

The power transformers produced in my country basically use concentric windings. The so-called concentric winding means that on any cross section of the iron core column, the windings are sheathed on the outside of the iron core column with the same cylindrical wire. A certain insulation gap and heat dissipation channel (oil channel) must be left between the high and low voltage windings, as well as between the low voltage winding and the iron core column, and are separated by insulating cardboard tubes. The size of the insulation distance depends on the voltage level of the winding and the clearance required for the heat dissipation channel. When the low-voltage winding is placed inside close to the core column, the insulation distance required between it and the core column is relatively small, so the size of the winding can be reduced, and the overall size of the entire transformer is also reduced at the same time.

The most commonly used in power systems is the three-winding transformer. Using one three-winding transformer to connect power transmission systems with three different voltages is more economical than using two ordinary transformers, occupies less space, and is more convenient for maintenance and management. The three-phase three-winding transformer usually adopts the Y-Y-△ connection method, that is, the primary and secondary windings are Y-connected, and the third winding is connected into a △, as shown in Figure XX. The △ connection itself is a closed loop, allowing the third harmonic current to pass through the same phase, so that the third harmonic voltage does not appear in the primary and secondary windings of the Y connection. This way it can provide a neutral point for both the primary and secondary sides. In the long-distance power transmission system, the tertiary winding can also be connected to the synchronous modulator to improve the power factor of the line.

3. Transformer lead structure

01 Lead material and classification

The wires that connect the outer ends of the windings of the transformer windings are called leads. The external power supply energy is input into the transformer through the leads, and the incoming power is also output from the transformer to the outside through the leads.

Leads mainly have the following categories:

(1) The lead wire connecting the winding wire end and the sleeve;

(2) Connecting leads between winding ends;

(3) The winding tap is connected to the tap lead of the switch

The materials of the leads are generally:

(1) Bare copper rod, scope of application: 10kV class 6300kVA and below transformers;

(2) Paper-coated round copper rod, applicable range: 10~35kV small capacity transformer;

(3) Bare copper bar, scope of application: 10kV and below low-voltage winding leads;

(4) Copper stranded wire, scope of application: all voltage levels, especially lead wires of 110kV and above;

(5) Copper tube, applicable scope: 220kV and above transformer lead.

In order to ensure sufficient insulation distance, the leads are insulated by laminated wood and cardboard, which must meet the requirements of electrical performance, mechanical strength, and temperature rise. The selection of the lead is also based on the electric field strength and mechanical strength, as well as the temperature rise during short circuit and the temperature rise during long-term load.

02 Lead connection

The connection forms of transformer leads are: brazing, gas welding, cold pressure welding and bolt connection.

The brazing electrode should be made of phosphor copper alloy for the connection between the winding outlet and the lead and between the leads.

Gas welding is used for the welding of copper bar leads and the welding of cable-type casing joints.

Cold pressure welding is to insert the two terminals connected by the lead into a metal tube, and then squeeze the metal tube with a die to press the two terminals together. Cold pressure welding does not require heating, welding is relatively safe, there is no virtual welding, and there is no insulation and extrusion quality of burned leads and other parts, and the tensile strength is good. Therefore, cold pressure welding is the main lead connection method for large transformers at present.

The bolt connection is mainly used for the lead wire connected with the guide rod type casing. The lead wire can be disassembled and can compensate for the deviation of the lead length. Usually, a curved arc lead structure that can be freely retracted is also called a soft connection.

03 Fastening of lead wires

In order to ensure the insulation distance of the lead wire, and to withstand the vibration and impact of the electromotive force during operation and short circuit without displacement and deformation, a clamp must be used to fasten the lead wire.

The lead clamp should have sufficient mechanical strength and electrical strength. For this reason, the structure of the lead clamp generally adopts a wooden bracket structure. When the clamp is fixed with the metal parts of the transformer body, metal bolts can be used to improve the mechanical strength. When fixing between the clamps, epoxy bolts must be used, and there are anti-loosening devices. Insulating cardboard should be added to the clamped lead as additional insulation to prevent the lead from being stuck.