Dry-Type or Oil-Immersed Transformer: A Selection Guide

The short answer, and why it is not enough

A dry-type transformer has no oil, so it needs no oil containment and no fire suppression arrangement. An oil-immersed transformer is more compact for a given rating, is cheaper at higher ratings, and copes better with a high ambient temperature if it has the right cooling class.

That is the trade-off in one sentence. The rest of this guide is about the details that decide which side of it your project falls on.

Efficiency and losses

Both families are built to the efficiency standard in force, and both can be supplied at level-1 efficiency. The difference in losses is small at the rating levels that matter for most industrial buyers, and it is not usually the deciding factor.

Where it does matter is where the transformer runs at a low load factor for long periods. No-load losses dominate in that case, and the comparison should be made on the total loss at your actual duty cycle rather than on the nameplate figures.

Fire safety and installation environment

This is usually where the decision is actually made. A dry-type transformer in a building with people in it removes an entire category of fire-protection requirement: no oil catchment, no fire suppression system, no separation distance from the combustible structure.

An oil-immersed transformer is equally safe when it is installed correctly with the containment volume and the separation distances respected — but that arrangement costs floor area, which in a dense plant or a building with a basement is often the constraint.

Footprint and where the transformer goes

For the same rating, a dry-type unit is physically larger. It needs more ventilation because it loses heat directly to the room rather than through a radiator and oil, and it needs an enclosure around it for the insulation class.

An oil-immersed unit has a smaller footprint and can be installed outdoors on a concrete base with the radiators facing free air. In an outdoor substation with a defined compound, that is usually the more efficient use of the space.

Ambient temperature, altitude and cooling

Dry-type units need an air-cooling class that matches the room. An AN class unit in a hot environment loses rating; an AF class unit with forced cooling is used instead.

Oil-immersed units have an oil class that governs the temperature rise. Oil-immersed units are also better suited to a site with a high ambient temperature, and the altitude derating applies to both families on the same basis.

When you send the site conditions with your enquiry — altitude, ambient temperature and the installation arrangement — the rating is fixed against those inputs rather than against a nominal indoor condition.

Maintenance and expected service life

An oil-immersed transformer needs periodic oil sampling and, in a sealed unit, very little else. A dry-type transformer needs no oil maintenance, but the resin surface should be inspected periodically for dust accumulation and partial discharge activity, particularly in a dusty or humid environment.

For a maintenance team that is not dedicated to electrical equipment, the dry-type unit usually wins on the total effort over a 20-year life.