The main structure of the oil-immersed transformer consists of the body, oil tank, cooling device, protection device and outlet device.
1. The device body includes iron core, windings, insulated leads and tap-changer.
2. The fuel tank includes the fuel tank body and fuel tank accessories (oil drain valve, ground plate, trolley, nameplate, etc.).
3. The cooling device includes radiator and cooler.
4. Protection devices include oil conservator, oil level gauge, safety airway, moisture absorber, thermometer and gas relay.
5. The outlet device includes high and low voltage bushings.
2. Structure of various components of oil-immersed transformer
1. Iron core
The iron core of the transformer is the magnetic circuit part and the skeleton of the transformer body. It consists of an iron core column, an iron yoke and a clamping device. The part surrounding the winding is the iron core post. The part that connects the iron core columns to form a closed magnetic circuit is called the iron yoke. The clamping device is used to clamp the iron core, clamp the winding and fix the lead wire. The coil is mounted on an iron post, and the iron yoke is used to close the entire magnetic circuit. The iron core is made of silicon steel sheets, which have good magnetic conductivity and small hysteresis loss. There are three-phase windings A, B, and C on the iron core. Each phase winding is divided into high-voltage winding and low-voltage winding. Generally, the low-voltage winding is wound on the inner layer and the high-voltage winding is wound on the outer layer. In order to reduce hysteresis and eddy current losses in the iron core, the iron core is often made of silicon steel sheets with a high silicon content and a thickness of 0.35~0.5mm, and the sheets are coated with insulating paint.
2. Winding
The winding is the circuit part of the transformer and is generally made of insulating enameled or paper-wrapped aluminum wire or copper wire. Depending on the arrangement of high and low voltage windings, the windings are divided into concentric and overlapping types. For concentric windings, in order to facilitate the insulation between the windings and the iron core, the low-voltage windings are usually placed close to the iron core columns; for overlapping windings, in order to reduce the insulation distance, the low-voltage windings are usually placed close to the iron yoke.
3. Insulating sleeve
The outlet device is an insulating sleeve, which consists of a central conductive rod and a porcelain sleeve. The insulating bushing is the connecting component between the high and low voltage coils of the power transformer and the external lines. To lead the leads of the high and low voltage coils of the transformer from inside the tank to outside the tank, and to insulate the leads from the grounded tank, insulating sleeves must be used. The bushing not only insulates the lead from the ground, but also plays a role in fixing the lead. Therefore, the bushing of the power transformer must have the specified voltage strength, sufficient mechanical strength and good thermal stability. There are many forms of bushings. According to different structures, they can be divided into pure porcelain, porcelain oil-filled type and capacitor type. In order to reduce leakage caused by dust and rain, the shape of the porcelain insulating bushing is a multi-level umbrella shape.
4. Oil conservator
The oil conservator is also called an oil pillow. Its function is to ensure that the oil inside the transformer is always full when the transformer is in operation and the oil expands due to heat. When the transformer stops operating or the temperature drops and the oil shrinks. At the same time, the contact surface between the transformer and the air is reduced to reduce the impact of oxidation and moisture on the transformer oil.
5. Heat pipe
For small-capacity transformers, the heat generated by the windings and iron core is naturally dissipated and cooled through the contact between the transformer oil and the inner wall of the oil tank, and the contact between the outer wall of the oil tank and the outside cold air, without any additional cooling device. If the transformer capacity is slightly larger, a heat pipe can be welded on the outer wall of the oil tank to increase the heat dissipation area. The heat energy generated by the running iron core and winding causes the oil temperature to rise, the volume expands and the density becomes smaller, and it rises into the heat pipe. Then the oil entering the heat pipe decreases in temperature, shrinks in volume and increases in density, and then falls back into the oil tank and the iron core. The heat from the windings is dissipated through the natural circulation of the oil.
6. Moisture absorber
There is a moisture absorber on the oil conservator, which is equipped with a glass container and contains a desiccant to absorb moisture from the air entering the oil conservator.
7. Safe airway
Safety airway, also called explosion-proof pipe, is a trumpet-shaped pipe installed on the top of the transformer, and the mouth of the pipe is sealed with a diaphragm. Its function is that when a short-circuit fault occurs inside the transformer and the transformer oil decomposes into a large amount of gas, causing high pressure in the oil pipe, the safety airway nozzle diaphragm is broken first, and the oil gas is ejected from there, reducing the pressure in the oil tank and preventing the oil tank from collapsing. Deformation or explosion due to sudden increase in pressure.
