In the power transmission and distribution network, the oil-immersed transformer serves as the core equipment for voltage conversion and electric energy transmission. It undertakes the vital task of connecting power grids with different voltage levels and ensuring stable power supply. From step-up power transmission in power plants to terminal power distribution for urban and rural residents, as well as power supply for large industrial enterprises and grid connection of new energy power stations, oil-immersed transformers feature mature technology, excellent insulation and heat dissipation performance, making them indispensable energy hubs in the power system. Since transformer oil was first applied as insulation and cooling medium in the late 19th century, oil-immersed transformers have undergone over a century of technological upgrading and formed a complete product series, widely used in various power scenarios. This paper comprehensively analyzes the working principle, structural composition, classification characteristics, application scenarios and operation maintenance essentials of oil-immersed transformers, combining the latest industrial standards and practical application experience to fully introduce this key power equipment.

1. Definition and Working Principle

An oil-immersed transformer is a power equipment that realizes voltage rise and drop based on electromagnetic induction principle, adopting insulating oil as insulation and cooling medium. Its iron core and windings are completely immersed in the sealed fuel tank filled with transformer oil, and the dual functions of insulating oil ensure stable equipment operation. Its working principle can be summarized as electromagnetic induction for voltage transformation and insulating oil for heat dissipation and insulation.

Firstly, alternating current is supplied to the primary side coil to generate alternating magnetic field. Secondly, the alternating magnetic field is coupled through the iron core and transmitted to the secondary side coil. Finally, alternating voltage is induced on the secondary side coil. The voltage ratio is proportional to the coil turns ratio, realizing conversion between high voltage and low voltage to meet power demand of different scenarios.

During operation, heat is generated by iron core and windings due to current loss. Transformer oil transfers heat to the fuel tank and radiator through natural convection or forced circulation for heat dissipation. Meanwhile, it isolates windings and iron core from air to prevent internal discharge and short circuit and slow down aging of insulating materials. In case of internal electric arc fault, insulating oil can also extinguish electric arc to improve operation safety.

2. Main Structural Components

The structural design of oil-immersed transformers centers on insulation, heat dissipation and power conversion, consisting of core components and auxiliary components.

2.1 Core Components

1. Iron Core: Made of high-permeability cold-rolled grain-oriented silicon steel sheets, it forms the magnetic circuit, reduces eddy current loss and hysteresis loss, and improves magnetic energy transmission efficiency. Its performance directly affects energy consumption and operation stability of transformers.

2. Winding: Divided into high-voltage winding and low-voltage winding, made of insulated copper or aluminum wires. High-voltage winding has more turns and thinner wire diameter, while low-voltage winding has fewer turns and thicker wire diameter. The turns ratio determines voltage conversion ratio.

3. Insulating Oil: The key medium with outstanding insulation strength, thermal conductivity and chemical stability. It functions in insulation, heat dissipation, arc extinction and equipment protection.

4. Fuel Tank: A sealed container welded by steel plates to hold iron core, windings and insulating oil, providing mechanical protection. Radiators are equipped on the tank surface to enhance heat dissipation.

2.2 Auxiliary Components

1. Oil Conservator: Connected to the fuel tank to compensate volume expansion and contraction of insulating oil caused by temperature change, keep the tank full of oil and slow down oil aging.

2. Bushing: Installed on the tank cover, serving as insulation device leading internal windings to external circuits, which must withstand internal oil pressure and external circuit voltage.

3. Breathing Apparatus: Filled with silica gel desiccant to absorb moisture in air, preventing water from invading insulating oil and deteriorating oil quality.

4. Other Accessories: Including thermometer, oil level gauge, gas relay, pressure relief valve and other monitoring and protection parts to realize real-time operation monitoring and fault protection.

3. Classification and Application Scenarios

3.1 Classification by Sealing Type

1. Non-sealed Oil-immersed Transformer: Simple structure and good heat dissipation, widely used in industrial and mining enterprises, agriculture and civil buildings.

2. Closed Oil-immersed Transformer: Good sealing performance, suitable for petroleum, chemical industry and other harsh environments with oil pollution and chemical corrosion.

3. Fully-sealed Oil-immersed Transformer: Excellent waterproof and dustproof performance, applicable to outdoor power distribution and poor working conditions with low maintenance demand.

3.2 Classification by Cooling Mode

1. ONAN Oil Natural Cooling: Heat dissipation by natural oil convection, simple structure and easy maintenance, suitable for small-capacity distribution transformers.

2. ONAF Oil Forced Air Cooling: Equipped with cooling fans to improve heat dissipation efficiency, applied in medium-capacity transformers in industrial parks and urban power grids.

3. OFAF Forced Oil Circulation Cooling: Accelerate oil circulation via oil pump with high heat dissipation efficiency, mainly used in large-capacity high-voltage transformers in power plants and substations.

3.3 Classification by Purpose

1. Distribution Transformer: Convert high grid voltage into low voltage for daily production and living use, the most widely used type in urban and rural power distribution networks.

2. Step-up Transformer: Applied in power plants to boost generator voltage for long-distance power transmission and reduce electric energy loss.

3. Special-purpose Transformer: Including electric furnace transformer, welding transformer, high-voltage test transformer and other customized equipment for industrial special production.

3.4 Main Application Fields

Oil-immersed transformers are widely used in power system substations, power plants, steel plants, mines, wind farms, photovoltaic power stations, urban power distribution, airports, subways and other large infrastructure power supply systems. They are the preferred equipment for high-voltage and large-capacity power transmission projects.