In short
Oil-immersed transformers suit outdoor, utility and higher-rating applications and generally offer better overload capability. Dry-type transformers suit indoor sites close to people, such as buildings, hospitals and data centres, because they contain no liquid. The installation location, fire codes, rating and lifetime cost usually decide the choice.
The core difference
An oil-immersed transformer uses an insulating liquid, usually mineral oil or an ester fluid, to insulate the windings and carry heat away to radiators or a cooled tank. A dry-type transformer uses solid insulation, commonly cast resin or vacuum-impregnated systems, and is cooled by air. That single difference shapes where each type can be installed and what it needs over its life.
Where each type fits
Oil-immersed transformers are the usual choice outdoors, in utility networks, at substations and for higher ratings. They tolerate harsh outdoor environments well and generally offer good overload capability thanks to the thermal capacity of the liquid.
Dry-type transformers are widely used indoors and close to people, such as in commercial buildings, hospitals, data centres, tunnels and industrial plants where fire risk and liquid spills are a concern. They can often be installed nearer the load, which shortens LV cable runs.
Fire safety and environment
Mineral oil is combustible, so oil-filled installations typically need measures such as oil containment, fire separation or suppression, depending on local codes. Ester fluids have much higher fire points and are biodegradable, which can relax some of these requirements.
Dry-type transformers avoid liquid altogether. IEC 60076-11 classifies them by environmental (E), climatic (C) and fire behaviour (F) classes, so you can specify the class appropriate to the location.
Maintenance
Oil-immersed units benefit from periodic oil testing, for example dissolved gas analysis, moisture content and breakdown voltage, which also gives early warning of developing faults. Dry-type units need little routine maintenance beyond keeping windings clean, checking connections and ensuring ventilation, but they offer fewer condition-monitoring indicators.
Size, noise and cost
For the same rating, dry-type transformers are usually larger and more expensive to buy, and they are more common in the lower and medium distribution ratings. They need clean, ventilated rooms and may be noisier unless specified with enclosures. Oil-immersed units are more compact for a given rating but bring the cost of containment and fire measures where required.
Side-by-side comparison
| Consideration | Oil-immersed | Dry-type |
|---|---|---|
| Insulation and cooling | Mineral oil or ester fluid | Solid insulation, air-cooled |
| Typical location | Outdoor, substations, utility networks | Indoor, buildings, near the load |
| Fire and spill measures | Often required, depending on fluid and code | No liquid to contain |
| Maintenance | Periodic oil testing | Cleaning and inspection |
| Overload capability | Generally good | More limited |
| Size and purchase cost | More compact, lower cost | Larger, higher cost |
| Main standard | IEC 60076 series | IEC 60076-11 |
Making the decision
Start from the installation: indoor or outdoor, proximity to people, local fire codes and available space. Then consider rating, load profile, maintenance capability and lifetime cost. If you share the site conditions and rating with your enquiry, suitable options of either type can be assessed. Our transformer specification guide lists what to include.