Oil-Immersed Transformers: Mature Technology for Large-Capacity Power Transmission

Oil-immersed transformers are a widely applied and technically mature type of transformer in power systems. Their core characteristic is that windings and cores are fully immersed in specialized insulating oil and sealed within a robust oil tank. Boasting excellent insulation and heat dissipation performance, they serve as the core equipment for large-capacity, high-voltage power transmission.

Their key components include the core, windings, insulating oil, oil tank, oil conservator, bushings, and other critical parts:

The core is constructed by laminating high-permeability silicon steel sheets, forming the magnetic circuit and transmitting magnetic flux.

The windings, wound with insulated copper or aluminum wire, are categorized into high-voltage and low-voltage windings, acting as the core pathway for power transfer.

Insulating oil performs four major functions: insulation, heat dissipation, arc extinction, and protection. It electrically isolates the windings from the core, absorbs heat generated during operation and dissipates it into the air, extinguishes arcs in internal arc faults, and slows the aging of insulating materials.

The oil tank houses the core components and insulating oil, equipped with accessories such as oil level gauges, thermometers, and gas relays. The oil conservator compensates for the thermal expansion and contraction of insulating oil, reduces oil contact with air, and extends service life.

Bushings lead the winding leads out of the oil tank, withstand internal oil pressure and external line voltage, and ensure safe wiring connections.

Oil-immersed transformers present distinct advantages and limitations:

Advantages:

Exceptional insulation and heat dissipation performance, coupled with mature technology and stable, reliable operation.

Capable of being manufactured in ultra-large capacities (ranging from tens of MVA to thousands of MVA) and high voltage classes (500 kV and above), a feat unattainable by dry-type transformers.

Strong overload capacity; with proper maintenance, service life can reach 20–30 years or longer.

Relatively low manufacturing cost per unit capacity in the large-capacity segment.

Limitations:

Flammable insulating oil poses fire risks, requiring stringent fire protection for installation environments and necessitating accident oil sumps.

Insulating oil leakage may cause environmental pollution, and waste oil must be disposed of professionally.

High maintenance requirements, including regular inspections of oil level and temperature, oil sample testing, and oil filtration or replacement when necessary.

Large size and weight, imposing high bearing requirements on installation foundations; unsuitable for sites with stringent fire and explosion protection demands or confined spaces.

In terms of application scenarios, oil-immersed transformers are primarily deployed for large-capacity, high-voltage power transmission and transformation. They are widely used as step-up transformers in power plants, step-down transformers in substations, hub transformers in high-voltage transmission networks, as well as in main step-down stations of industrial and mining enterprises and power supply for large-scale power equipment. In distribution networks, large-capacity pole-mounted transformers or those installed in box-type substations also commonly adopt the oil-immersed design, making them the core supporting equipment for long-distance, large-capacity power transmission in power systems.