Functions of various components of oil-immersed transformers

1. Iron core
The iron core not only constitutes the magnetic circuit of the transformer, but also serves as the mechanical skeleton of the transformer to support the winding. The iron core consists of three parts: the core column, the iron yoke and the clamping device.
2. Winding
The winding is the circuit part of the transformer, which is wound with insulated copper wire or aluminum wire. The winding is the carrier of the current, generating magnetic flux and induced electromotive force.
3. Accessories
The accessories include oil storage cabinet, tap changer, safety airway, insulating bushing, etc. Its function is to ensure the safe and reliable operation of the transformer.
4. Oil tank
The oil tank is the outer shell of the oil-immersed transformer, which is used for heat dissipation and protection of the body (the body of the transformer is placed in the oil tank). The tank contains transformer oil for insulation.
5. Oil storage cabinet
The oil storage cabinet is installed on the oil tank to isolate the inside of the oil tank from the outside world. The oil storage cabinet is shaped like a horizontally rotating cylinder, and its function is to reduce the contact area between the transformer oil and the air. The volume is generally 10%~130% of the total oil volume, of which half is oil and half is gas, so that the oil can be buffered when it expands due to heat. An oil level gauge is installed on the side to observe the oil level.
In order to prevent moisture from entering the oil storage cabinet and to take oil samples regularly for testing, desiccant and oil drain valve are installed on the oil storage cabinet and the oil tank respectively.
6. Safety airway
The safety airway is a protective device installed on the top cover of the oil tank. It is a long round steel cylinder, the upper end is sealed with phenolic paperboard, and the lower end is connected to the oil tank. When a transformer fails and a large amount of gas is generated, the pressure in the oil tank increases, and the gas and oil will break through the explosion-proof membrane and spray out to avoid the oil tank from bursting.
In recent years, domestic power transformers have widely adopted pressure relief valves to replace explosion-proof pipes. Its advantages are high action accuracy, short delay time, automatic opening and closing, and overcoming the shortcomings of replacing explosion-proof pipes during power outages.
7. Gas relay
The gas relay is also called gas relay or float relay. Gas relay is the main protection device of transformer. Transformers above 800kV·A are equipped with gas relays in the connecting pipes of oil tank and oil storage cabinet.
The protective function of gas relay is: when the gas generated by the fault in the transformer reaches a certain level, the circuit is connected to alarm; when the oil level drops sharply due to serious oil leakage, the circuit is quickly cut off; when a sudden fault occurs in the transformer and the oil flows to the oil pillow, the circuit is quickly cut off.
8. Tap changer
To adjust the secondary voltage, 3 (sometimes 5) taps are often left at the corresponding position of the end section of each phase high-voltage coil, and these taps are connected to a switch, which is called a tap changer.
By connecting the switch with different joints, the number of turns of the original winding can be changed to achieve the purpose of adjusting the voltage. The tap changer is divided into on-load voltage-regulating tap changer and off-load voltage-regulating tap changer. The voltage range that can be adjusted by the tap changer is within ±5% of the rated voltage. The off-load voltage-regulating tap changer can only be adjusted after the power is off, otherwise there is a risk of personal and equipment accidents.
Any transformer should be equipped with a tap changer, because when the applied voltage exceeds 10% of the rated voltage of the transformer winding, the transformer magnetic flux density will increase greatly, causing the iron core to saturate and heat up, increasing iron loss, and thus cannot guarantee safe operation. Therefore, the transformer should adjust the tap according to the changes in the voltage system to ensure that the voltage is not too high and burns the motor and electrical appliances; if the voltage is too low, it will cause the motor to overheat or other electrical appliances to not work properly.
9. Insulating bushing
The insulating bushing is installed on the oil tank cover of the transformer. Its function is to lead the coil lead end out of the oil tank and insulate the lead from the oil tank. For voltages below 1kV, porcelain insulating bushings are used, for voltages between 10~35kV, gas-filled or oil-filled bushings are used, and for voltages above 110kV, capacitive bushings are used.
10. Transformer oil
Transformer oil can enhance insulation and heat dissipation. The medium is required to have high insulation strength, low viscosity, high flash point, low freezing point, and no impurities such as acid, alkali, dust and moisture.
11. Temperature measuring device
The temperature measuring device is used to monitor the oil surface temperature of the transformer. Small oil-immersed transformers use mercury thermometers, and large oil-immersed transformers use pressure thermometers.