Comprehensive Guide to Oil-Immersed Transformers: Design, Standards, and Applications

Oil-immersed transformers use insulating oil as both the dielectric and cooling medium. The transformer core and windings are sealed inside a tank filled with transformer oil, typically mineral oil or synthetic ester oil.

Transformer oil functions: It provides insulation by preventing internal breakdown and ensures cooling by carrying heat away from the windings and core through natural or forced convection.

Design Features of Oil-Immersed Transformers

Insulation Design

Combines solid insulation such as insulating paper with liquid insulation (transformer oil). The oil duct layout is optimized to ensure smooth circulation and effective impregnation.

Core Design

Constructed with cold-rolled grain-oriented silicon steel sheets to minimize eddy current and hysteresis losses. Typically adopts a core-type structure with windings mounted on core limbs. Single-point grounding prevents discharge and enhances safety.

Winding Design

Winding structures vary with capacity and voltage. Small-capacity transformers often use double-layer cylindrical low-voltage windings, while high-voltage windings are placed outside for better insulation arrangements.

Tank and Cooling

The transformer tank provides both oil containment and mechanical support. Small-capacity units rely on radiators, while large-capacity transformers employ forced oil circulation with air or water cooling systems.

Protective Devices

Equipped with conservators, dehydrating breathers, and other protective components. Conservators adjust oil volume, while breathers remove moisture. Modern systems include online monitoring for early fault detection.

Tap Changer

Two types are used: off-load tap changers (operated when de-energized) and on-load tap changers (operated under load). Both adjust the turns ratio to meet voltage requirements.

Standards and Regulations for Oil-Immersed Transformers

International Standards

IEC 60076 series: The key international standard defining requirements and testing methods.

IEEE C57.12.00: U.S. standard covering general requirements for oil-immersed transformers.

IEEE C57.91: Transformer loading guide with operating conditions for different loads.

Chinese Standards

GB 1094 series: Equivalent to IEC 60076, the fundamental Chinese transformer standard.

GB/T 6451: Defines technical parameters for oil-immersed power transformers.

GB 20052: Establishes mandatory energy efficiency limits and grades for transformers.

Applications of Oil-Immersed Transformers

Power Generation

Step-up transformers increase generator output voltage (e.g., 10.5kV, 20kV) to transmission voltages (110kV, 220kV, 500kV and above), reducing long-distance transmission losses.

Transmission and Distribution Networks

Transmission: Serve as main transformers in ultra-high voltage (UHV) and extra-high voltage (EHV) substations, forming grid hubs.

Distribution: Step down voltages from 110kV to 10kV, and further to 400V/230V for urban and rural users.

Industrial and Commercial Applications

Rectifier transformers supply DC power for electrolysis, chemical processes, and rail transit.

Furnace transformers support electric arc furnaces and induction furnaces with low voltage and high current.

Critical facilities such as data centers, hospitals, and large commercial complexes rely on oil-immersed transformers for primary and backup power.

Renewable Energy

Wind farms use transformers to step up turbine output voltage to collector line voltage.

Photovoltaic (PV) power plants increase inverter output voltage for grid connection.

Advantages and Limitations of Oil-Immersed Transformers

Advantages

Excellent insulation and cooling performance, suitable for high voltage and large capacity applications.

Proven technology with high reliability and long service life, often exceeding 25 years.

Cost-effective for medium and large-scale transformers.

Effective fault detection via dissolved gas analysis and oil chromatography.

Limitations

Fire risk due to mineral oil flammability; synthetic ester oil mitigates this risk with higher flash point and biodegradability.

Environmental concerns: oil leakage may contaminate soil and water sources.

Regular maintenance is required, including oil quality checks and silica gel replacement.

Larger size and heavier weight compared to dry-type transformers.

With over a century of development, oil-immersed transformer technology is mature, reliable, and remains the backbone of global power systems. The integration of new insulating materials, intelligent monitoring systems, and eco-friendly oils is driving the industry toward higher efficiency, enhanced environmental protection, and smarter grid integration.