In the main substations of urban rail transit power supply systems, the main transformer typically uses a three-phase oil-immersed transformer. An oil-immersed transformer primarily consists of an iron core, windings, oil tank, voltage regulator, radiator, oil pillow, gas relay, insulating bushing, and explosion-proof pipe.
1. The iron core is made of stacked silicon steel sheets with excellent magnetic conductivity, forming a closed magnetic flux circuit. The transformer's primary and secondary windings are wound around this core.
Transformer cores come in two structures: core-type and shell-type. The core-type is the most widely used transformer. A core-type core consists of an iron core leg and an iron yoke. Oil-immersed transformers have internal cooling channels for oil circulation, facilitating oil circulation and enhancing heat dissipation.
2. The winding, also known as the coil, is the transformer's conductive circuit. It is wound in a multi-layer cylindrical shape using copper or aluminum wire. The primary and secondary windings are concentrically wrapped around the core leg. For insulation, the low-voltage winding is generally placed inside and the high-voltage winding is placed outside. Insulating material is wrapped around the conductors to ensure insulation between them and from ground.
3. The oil tank is the outer casing of an oil-immersed transformer. Besides containing the oil, it also serves as a mounting point for other components.
4. The voltage regulator is installed to ensure the stability of the transformer's secondary voltage. When the power supply voltage fluctuates, the voltage regulator adjusts the transformer's tap changer to ensure a stable secondary output voltage. Voltage regulators are categorized as on-load and off-load.
5. The radiator is mounted on the wall of the oil tank, with its upper and lower sections connected to the tank via pipes. When there is a temperature difference between the oil in the upper and lower parts of the transformer, convection of the oil occurs through the radiator, cooling the oil and returning it to the tank, thereby lowering the transformer's oil temperature. To improve cooling efficiency, natural cooling, forced air cooling, and forced water cooling can be employed.
6. The oil pillow is also called the oil cabinet. Transformer oil expands and contracts with temperature fluctuations, and the oil level will rise or fall with temperature fluctuations. The oil pillow provides a buffer for the thermal expansion and contraction of the oil, keeping the tank constantly full. Furthermore, the pillow reduces the contact area between the oil and air, slowing oil oxidation.
7. The gas relay, also known as the gas relay, is the primary protection device for internal transformer faults. It is installed in the middle of the oil pipe connecting the oil tank and the oil pillow. In the event of a serious internal transformer fault, the gas relay switches on the circuit breaker tripping circuit; in the event of a less serious internal transformer fault, the gas relay switches on the fault signal circuit.
8. The high and low voltage insulation bushings are located on the top cover of the transformer's oil tank. Oil-immersed transformers generally use porcelain insulation bushings. These bushings ensure good insulation between the high and low voltage winding leads and the oil tank, and also secure the leads.
9. The explosion-proof pipe, also known as the safety air duct, is installed on the transformer's oil tank, with its outlet sealed with glass explosion-proof film. When a serious fault occurs inside the transformer and the gas relay fails, the gas inside the oil tank will break through the glass explosion-proof membrane and spray out from the safety air duct to prevent the transformer from exploding.