What Is an Oil-Immersed Transformer: Construction, Oil Function and Standards

Definition and Operating Principle

An oil-immersed transformer is a transformer whose active part, the core and the windings, is submerged in insulating oil inside a sealed tank. The oil performs two functions simultaneously: it removes heat from the windings and the core, and it provides dielectric insulation between live parts and between live parts and earth. It also protects the solid insulation from moisture and oxygen, which is the reason oil-filled designs achieve very long service life in distribution networks.

Operation follows the same principle as any transformer. Alternating current in the primary winding sets up a varying magnetic flux in the laminated core, and that flux induces a voltage in the secondary winding in proportion to its number of turns. The oil does not participate in the energy conversion, but it determines how much current the machine can carry continuously and how much dielectric stress the insulation system can withstand.

Main Components

The tank construction determines how the oil is managed. A sealed corrugated tank absorbs thermal expansion through the deformation of its own walls and keeps the oil out of contact with air. A conservator design allows the oil volume to change in a separate expansion vessel, with a breather controlling the air that enters and leaves. Sealed designs are common in distribution ratings, while conservator designs remain usual for larger units where the oil volume change is significant.

The Dual Role of Insulating Oil

Cooling occurs by natural convection in the ONAN mode: heated oil rises from the windings, transfers heat to the tank walls or radiators, and returns downwards. Where greater capacity is needed, pumps and fans are added, giving the ONAF or OFAF designations defined in IEC 60076-1. The cooling designation is part of the transformer identity because it directly limits the permissible continuous load.

Dielectric performance is monitored through oil tests. New mineral oil supplied to IEC 60296 must reach a breakdown voltage of at least 30 kV across a 2.5 mm gap when tested by IEC 60156. During service, condition monitoring follows IEC 60422, which recommends keeping water content low, typically below about 20 mg/kg, and maintaining breakdown strength; for equipment rated below 72.5 kV a minimum of 30 kV is a widely applied reference. Acidity and dissolved gas content are tracked on the same sampling programme.

Ratings, Standard Framework and Test Regime

The general requirements, including rating plates, tolerances and routine tests, are set out in IEC 60076-1. Temperature rise limits of 60 K for top oil and 65 K for average winding are specified in IEC 60076-2. Insulation levels and dielectric test procedures, including lightning impulse withstand, are defined in IEC 60076-3, and short-circuit withstand capability in IEC 60076-5. Sound level determination follows IEC 60076-10, and winding frequency response analysis IEC 60076-18.

Routine tests applied to every unit therefore include winding resistance, ratio and phase relationship verification, short-circuit impedance and load loss, no-load loss and no-load current, and dielectric routine tests. The routine test report, together with the oil test certificate, is the document that allows the purchaser to accept the unit and later compare its condition against a known baseline.

Oil-Immersed and Dry-Type Designs Compared

Oil-immersed transformers generally offer lower cost per kilovolt-ampere, better overload capability and better heat removal for a given size, which makes them the standard choice for outdoor and distribution duty. Dry-type transformers remove the oil, eliminating the fire load and the containment requirement, which is valuable indoors, in tunnels and inside buildings where a liquid spill is unacceptable.

Selection therefore follows the installation environment more than the electrical rating. Where outdoor space is available and fire separation can be provided, an oil-immersed unit is normally the more economical solution. Where the transformer must sit inside an occupied building, the fire and containment considerations often point to a dry-type design despite the higher cost.

Service Life, Loading and Inspection

Insulation ageing is a function of temperature and time, and moisture accelerates it. In a sealed tank the oil is protected from atmosphere, so the rate of moisture ingress is low, but water can still be generated slowly by paper decomposition. Periodic oil sampling, together with dissolved gas analysis, is the practical way to detect thermal or electrical faults before failure.

Loading should respect the thermal limits in IEC 60076-2, with overload evaluated against the loading guide for oil-immersed transformers. Visual inspection covers oil leaks at weld seams and gaskets, bushing condition, corrosion of the tank finish, the operation of pressure relief devices, and the earthing connection. Because the unit is sealed, a leak is more significant than it would be on a conservator design, since oil can only be lost and not replaced without intervention.

Frequently Asked Questions

Q: What is an oil-immersed transformer?
A: It is a transformer whose core and windings are immersed in insulating oil inside a tank. The oil cools the active part and provides dielectric insulation, while also protecting the solid insulation from moisture and oxygen.

Q: Why is oil used instead of air?
A: Oil has a much higher dielectric strength and heat transfer capability than air, so a given rating can be built in a smaller volume, and the insulation withstands higher voltages without excessive clearance. The trade-off is a contained liquid that must be managed and tested.

Q: How often should the oil be tested?
A: It should be sampled periodically, with the interval set by criticality and observed trend. Interpretation follows IEC 60422, and dissolved gas results should be read as a trend rather than as single values, because rate of change carries the diagnostic information.

Q: What is the difference between ONAN and ONAF cooling?
A: ONAN denotes oil natural and air natural cooling, relying on convection alone. ONAF adds forced air through fans over the radiators. The cooling designation is defined in IEC 60076-1 and determines the permissible continuous loading.

Q: Can an oil-immersed transformer be installed indoors?
A: It can, but the installation must satisfy fire protection and oil containment requirements. Where those requirements cannot be met economically, a dry-type transformer is usually preferred even though its cost per kilovolt-ampere is higher.

Q: What determines the service life of the unit?
A: The condition of the solid insulation, mainly the paper wrapping the conductors, which ages as a function of temperature, moisture and oxygen. Keeping the oil dry and the hot-spot temperature within the limits of IEC 60076-2 is the most effective way to extend service life.