Modern production and life cannot do without electricity, and power transmission and obtaining various levels of voltage cannot do without transformers. A transformer is a static electrical appliance that can convert AC power of one voltage level into AC power of another voltage level of the same frequency through electromagnetic induction between coils.

Transformers can be divided into: power transformers for power transmission and distribution, including step-up and step-down transformers, etc.; special transformers for special power supplies, including welding transformers, rectifier transformers, electric furnace transformers, medium-frequency transformers, etc.; instrument transformers for measurement, including current transformers, voltage transformers, autotransformers (voltage regulators), etc.; small power transformers for automatic control systems; impedance converters for communication systems, etc.

1.1.1 Basic working principle and structure of transformers

The transformer works on the principle of electromagnetic induction. Figure 1.1 is a schematic diagram of its working principle. The main components of the transformer are an iron core and two windings on the iron core. The two windings have different numbers of turns and are insulated from each other. There is only magnetic coupling but no electrical connection between the two windings. Among them, the winding connected to the power supply side is called the primary winding or primary winding; the winding used to connect the load is called the secondary winding or secondary winding.

If winding 1 is connected to an AC power supply, an AC current i1 will flow through the winding, generating an alternating magnetic flux Φ with the same frequency as the external voltage u1 and simultaneously interlinked with the primary and secondary windings in the iron core. According to the principle of electromagnetic induction, electromotive forces e1 and e2 of the same frequency are induced in the two windings respectively.

If the load is connected to winding 2, under the action of the electromotive force e2, electric energy can be output to the load, that is, the current i2 will flow through the load to realize the transfer of electric energy.

From equations (1.1) and (1.2), it can be seen that the magnitude of the induced electromotive force of the primary and secondary windings is proportional to the number of turns of each winding, and the induced electromotive force of the winding is approximately the voltage of each winding. Therefore, as long as the turns ratio of the winding is changed, the purpose of changing the voltage can be achieved. This is the transformer principle of the transformer. That is

1.1.2 Basic structure of transformer

The main components of the transformer are iron core, winding, oil tank, cooling device, insulating bushing and protective device. Figure 1.2 is a schematic diagram of the structure of an oil-immersed power transformer.

It has the function of heat dissipation and protection of the body; the transformer oil acts as an insulator. The iron core and winding are the parts of the transformer that transmit energy through electromagnetic induction, which is called the body of the transformer. The oil tank is used to hold oil, and at the same time it serves as mechanical support and cooling; the role of the bushing is to insulate the transformer lead from the oil tank; the protective device plays the role of protecting the transformer.

(1) Iron core The iron core is the main magnetic circuit of the transformer and its mechanical skeleton. The iron core consists of two parts: the iron core column and the iron yoke. The winding is mounted on the iron core column, and the role of the iron yoke is to close the entire magnetic circuit. The laminated iron core is divided into two types according to its structural form: core type and shell type. The core type transformer has a simple structure, and the assembly and insulation of the winding are also relatively easy. The iron core of domestic power transformers mainly adopts the core type structure.

(2) Windings Transformer windings are of two types: core-type and overlapping types. The power transformers produced in my country basically have only one structural type, namely the core transformer, so the windings all adopt the core-type structure, as shown in Figure 1.3.

The so-called core-type winding means that on any cross section of the core column, the windings are all wrapped around the outside of the core column with the same cylindrical wire. Generally, the low-voltage winding is always placed inside close to the core, and the high-voltage winding is placed outside. A certain insulation gap and heat dissipation channel (oil channel) must be left between the high-voltage winding and the low-voltage winding, as well as between the low-voltage winding and the core column, and separated by an insulating cardboard tube. The size of the insulation distance depends on the voltage level of the winding and the gap 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.

(3) Oil tank and cooling device The body of the oil-immersed transformer is immersed in an oil tank filled with transformer oil. Transformer oil is both an insulating medium and a cooling medium. Through convection after being heated, it brings the heat of the core and winding to the box wall and cooling device, and then dissipates it into the surrounding air.

(4) Insulating bushing The transformer bushing is an insulating device that leads the high and low voltage leads of the coil to the outside of the box. It insulates the leads from the ground (shell) and also serves to fix the leads. Most bushings are installed on the box cover, with a conductive rod passing through the middle. The lower end of the bushing extends into the oil tank and connects to the winding lead. The upper part of the bushing is exposed outside the box and connected to the external circuit.

(5) Protective device Oil conservator (also known as oil pillow) The oil conservator is an oil protection device installed on the transformer oil tank cover and connected to the oil tank with a curved connecting pipe. The function of the oil conservator is to ensure that the transformer oil tank is full of oil, reduce the contact area between oil and air, and thus reduce the speed of moisture absorption and aging of the transformer oil. Desiccant (also known as respirator) Through it, the atmosphere is connected to the oil pillow. When the transformer oil changes its oil level due to thermal expansion and contraction, the gas will enter and exit through the desiccant. The desiccant contains silica gel or activated alumina to absorb the moisture in the air entering the oil pillow. Safety airway (also known as explosion-proof cylinder) The safety airway is installed on the top of the oil tank. It is a long steel cylinder with a glass plate or phenolic paperboard of a certain thickness installed on the upper end, and the lower end is connected to the oil tank. Its function is to allow the oil and gas flow to break through the glass plate or phenolic paperboard and release when the pressure inside the transformer suddenly increases due to a fault, so as to avoid the box wall from bursting. Oil purifier (also known as thermal siphon oil purifier) ​​The oil purifier uses the natural circulation of oil to filter the oil through an adsorbent to improve the performance of the transformer oil in operation. Gas relay (also known as gas relay) The gas relay is installed between the connecting pipe of the oil pillow and the oil tank. When a fault occurs inside the transformer (such as insulation breakdown, inter-turn short circuit, iron core accident, etc.) and gas is generated, or when the oil tank leaks and the oil level drops, the gas relay will operate and send a signal for the operator to deal with it in time; if the accident is serious, the circuit breaker can automatically trip to protect the transformer.

(6) Tap changer When the transformer is running under load, the secondary terminal voltage changes with the load size and power factor. If the voltage changes too much, it will have an adverse effect on the user. In order to ensure that the change of the secondary terminal voltage is within the allowable range, a tap is usually set on the high-voltage side of the transformer, and a tap changer is installed to adjust the working turns of the high-voltage winding, thereby adjusting the secondary terminal voltage. The reason why the tap is set on the high-voltage side is that the high-voltage winding is on the outermost side, which is convenient for leading out the tap; in addition, the current on the high-voltage side is relatively small, the conductor cross-sectional area of ​​the tap lead and the current-carrying part of the tap changer is also small, and the switch contacts are also easy to manufacture. Small and medium-sized power transformers generally have three taps, recorded as UN ±5%. Large power transformers use five or more taps, for example, UN ±2×2.5% or UN±8×1.5%. There are two types of tap changers: one is that it can only be adjusted when the power is off, called an off-load tap changer; the other is that it can be adjusted under load, called an on-load tap changer.