In the main substations of urban rail transit power supply systems, three-phase oil-immersed transformers are typically used. Oil-immersed transformers mainly consist of a core, windings, oil tank, voltage regulating device, radiator, oil conservator, gas relay, insulating bushings, and explosion-proof pipes.
1. Core The core is composed of stacked silicon steel sheets with good magnetic permeability, forming a closed magnetic flux circuit. Both the primary and secondary windings of the transformer are wound on the core.
Transformer cores are divided into two structures: core-type and shell-type. Currently, the most widely used transformers are core-type. A core-type core consists of a core column and a yoke. The core of an oil-immersed transformer has internal oil channels for cooling the core, facilitating oil circulation and enhancing heat dissipation.
2. Windings
Windings, also known as coils, are the conductive circuit of a transformer, made of copper or aluminum wire wound into multiple layers of cylindrical shape. The primary and secondary windings are concentrically wound on the iron core. For insulation, the low-voltage winding is generally inside and the high-voltage winding is outside. Insulating material wraps around the conductors to ensure insulation between conductors and between conductors and ground.
3. Oil Tank
The oil tank is the outer shell of an oil-immersed transformer. Its function, besides holding oil, is to house other components.
4. Voltage Regulating Device
The voltage regulating device is set up to ensure the stability of the transformer's secondary voltage. When the power supply voltage fluctuates, the voltage regulating device adjusts the transformer's tap changer to ensure a stable secondary output voltage. Voltage regulating devices are divided into on-load tap changers and off-load tap changers.
5. Radiator
The radiator is mounted on the tank wall, connected to the tank via pipes at the top and bottom. When there is a temperature difference between the upper and lower oil temperatures, convection current is created through the radiator, cooling the oil before it flows back to the tank, thus lowering the transformer oil temperature. To improve cooling efficiency, self-cooling, forced air cooling, and forced water cooling can be used.
6. Oil Conservator
The oil conservator, also known as the oil tank, provides a buffer against thermal expansion and contraction of transformer oil, causing the oil level to rise or fall. It keeps the tank full of oil and reduces the contact area between the oil and air, slowing down oxidation.
7. Gas Relay
The gas relay, also known as a methane relay, is the main protection device for internal transformer faults. It is installed in the middle of the oil pipe connecting the tank and the oil conservator. When a serious internal fault occurs in the transformer, the gas relay activates the circuit breaker tripping circuit; when a minor internal fault occurs, the gas relay activates the fault signal circuit.
8. Insulating Bushings High-voltage and low-voltage insulating bushings are located on the top cover of the transformer tank. Oil-immersed transformers generally use porcelain insulating bushings. The function of the insulating bushings is to maintain good insulation between the high-voltage and low-voltage winding leads and the tank, and to secure the leads.
9. Explosion-proof Pipe The explosion-proof pipe, also known as a safety vent, is installed on the transformer tank, and its outlet is sealed with a glass explosion-proof membrane. When a serious internal fault occurs in the transformer and the gas relay malfunctions, the gas inside the tank breaks through the glass explosion-proof membrane and is ejected from the safety vent, preventing the transformer from exploding.
