What is an oil immersed transformer?
Oil immersed transformers are the standard choice for high voltage and large power applications. This guide covers how they are built, how they are cooled and where they are used.
An oil immersed transformer uses mineral oil or another insulating fluid as both the insulating and the cooling medium. The oil surrounds the core and windings, insulating them and carrying heat away. That dual role is the reason the type dominates at high voltage: an insulating fluid that also transports heat lets a compact core and winding assembly handle very large ratings.
Learn more about our oil immersed transformer technology.
Construction
Core
- Grain-oriented silicon steel laminations
- Cooled, mitred joints for low no-load loss
- Epoxy coating for corrosion protection
Windings
- Copper or aluminium conductors
- Paper insulation
- Cylindrical or disc winding design
Winding type follows voltage and rating. Disc windings with interleaved or shielded construction are used at higher voltages because they distribute the impulse voltage more evenly across the winding, which matters for lightning impulse withstand. Where short-circuit duty is severe — close to large generators, for instance — interleaved or continuously transposed conductors reduce circulating current loss and improve mechanical strength.
Tank and accessories
- Welded steel tank
- Radiators for cooling
- Conservator tank for oil expansion
- Buchholz relay for gas protection
- Oil and winding temperature indicators
- Pressure relief device
Each accessory has a job. The conservator keeps the insulating oil in contact with the atmosphere through a breather rather than exposing the main tank surface, which limits oxidation and moisture ingress. The Buchholz relay detects the gas produced by incipient faults and, on severe faults, the oil surge. The pressure relief device protects the tank if internal pressure rises faster than the tank can withstand.
Cooling methods
ONAN (oil natural air natural)
Oil and air both move by natural convection. It is the simplest arrangement and is used on small to medium transformers.
ONAF (oil natural air forced)
Oil still circulates naturally, but fans force air across the radiators, raising capacity by 30 to 50%.
OFAF (oil forced air forced)
Pumps drive the oil and fans handle the air side, which is the usual arrangement on large units.
Read our detailed guide on cooling methods.
Advantages
- Efficient heat dissipation
- High dielectric strength
- Suitable for high voltage and large ratings
- Long service life with proper maintenance
- Good overload capability thanks to the thermal mass of the oil
- Proven technology with decades of operating data
Disadvantages and where dry-type wins
- Fire load — mineral oil is combustible, so indoor installation is restricted
- Oil containment and fire separation are required at site
- Periodic oil testing is part of the maintenance routine
- Oil handling and disposal at end of life
Where the unit must sit inside an occupied building, in a basement or in a tunnel, these constraints often push the design to cast resin. Our comparison of oil-type and dry-type transformers sets out the decision.
Applications
- Utility transmission substations at 110 kV and above
- Generation — GSU and station service transformers
- Industrial plants — process, furnace and rectifier supply
- Renewable energy — collector and step-up transformers for wind and solar
- Outdoor distribution — pole-mounted and pad-mounted units
Key specification parameters
| Parameter | Typical value / range |
|---|---|
| Rated voltage class | Up to 500 kV and above |
| Rated power | Up to several hundred MVA |
| Impedance voltage | Typically 8% to 18% for power transformers |
| Temperature rise | 55 K oil / 65 K winding (standard) |
| Cooling class | ONAN / ONAF / OFAF / OFWF |
| Vector group | YNd11, YNyn0, Dyn11 and others |
| Tap changer | Off-circuit or on-load |
Installation and maintenance considerations
- Provide bunding or an oil pit where oil could reach drainage or watercourses
- Keep clearances for cooling air and for servicing radiators and coolers
- Test oil before commissioning and at the intervals in our maintenance guide
- Record baseline readings at commissioning for later comparison
- Plan for oil handling at site and for disposal at end of life
Alternatives to mineral oil
Ester-based insulating fluids — natural and synthetic — have higher flash and fire points and are more readily biodegradable than mineral oil. They are used where the fire risk or the environmental exposure justifies the higher cost, and they also tolerate higher temperatures, which can allow a smaller unit for the same rating. Availability and price vary by market, so they are usually specified explicitly rather than assumed.
Browse our oil-immersed transformers and power transformers, or contact us with your rating and site conditions.
How oil-immersed and dry-type transformers compare in practice
| Factor | Oil-Immersed | Dry-Type |
|---|---|---|
| Insulation medium | Mineral or ester oil | Cast resin or air |
| Installation preference | Outdoor | Indoor |
| Fire load | Combustible oil | Low |
| Site requirements | Bunding, fire separation, oil pit | Ventilation, fire-rated enclosure |
| Overload capability | High thermal mass | More limited |
| Maintenance | Oil testing programme | Cleaning and connection checks |
| Practical rating range | Up to the largest transmission units | Generally medium ratings |
| End of life | Oil disposal required | Straightforward recycling |
Where the rating is in the medium range and the site is indoor, the choice is usually settled by fire rules. Where the unit is outdoor and the rating is high, oil is the standard answer, because no dry-type design can remove heat as effectively at that scale.