The real difference is the insulating and cooling medium, and everything else follows from it. An oil-filled transformer sits in a tank of mineral oil or ester; a dry-type transformer has its windings cast in epoxy resin, or wound as air-insulated coils, and is cooled by air. That one choice drives efficiency, fire behaviour, weight, installation cost and maintenance.
Efficiency and losses
Oil-filled units generally have lower losses than cast-resin units of the same rating, because oil removes heat more effectively than air and allows a more compact, lower-resistance winding. Both technologies must meet EU EcoDesign Tier 2, but the oil-filled unit typically gets there with less core and conductor material.
Over thirty years that difference adds up. On a continuously energised 1,000 kVA unit, a few hundred watts of extra no-load loss is in the order of a hundred megawatt-hours across the life of the asset. Put losses into the comparison, not just the purchase price.
Fire, environment and where you may install
This is usually the deciding factor. Cast-resin transformers are self-extinguishing and carry a fire behaviour classification — F1 in the common European scheme — so they can be installed inside buildings, in basements, on upper floors and in tunnels without a separate fire compartment or an oil containment pit.
Oil-filled units contain a flammable liquid, so they need a bund sized for the full oil volume and either an outdoor location or a fire-rated room. Ester fluids raise the fire point above 300 degrees Celsius and reduce those requirements considerably, without going as far as a dry unit.
Size, weight and site logistics
For the same rating a cast-resin unit is usually lighter, but not necessarily smaller in footprint — air needs more clearance than oil, and the protective enclosure adds volume. Oil-filled units are heavier because of the fluid and the tank, which matters for crane access and floor loading inside a building.
Dry-type units also need real ventilation. An enclosed room without adequate air exchange will derate a cast-resin transformer quickly, while a hermetically sealed oil-filled unit tolerates a poorly ventilated space far better.
Maintenance and expected life
- Oil-filled: periodic sampling for breakdown voltage and dissolved gas; gaskets and breather to check; the oil can be reconditioned. Sealed hermetic designs need very little routine attention.
- Dry-type: no fluid to manage, but the windings must be kept clean and dry, because dust and humidity degrade surface creepage. Cleaning intervals depend entirely on the environment.
- Both: sustained overload and thermal cycling are what actually shorten life, in either technology.
As a rule of thumb: choose cast resin when the transformer must stand inside a building, close to people, or where an oil leak would be unacceptable — and accept the higher purchase price and slightly higher losses. Choose oil-filled where you have an outdoor location or a suitable room, and you want the lowest losses and the lowest cost per kVA. Where the constraint is fire risk rather than the oil itself, an ester-filled unit is often the better middle path, and worth pricing before you rule oil out.
