Dry type and oil immersed transformers do the same job with different physics, and the choice is usually made on fire risk, location and lifecycle cost rather than on efficiency. This comparison covers the factors that actually decide it.
Cooling medium and insulation
An oil immersed transformer uses mineral oil or a synthetic ester both to insulate and to carry heat from the windings to the tank surface and radiators. A dry type unit uses air for cooling and a solid insulation system for electrical insulation, which in practice means cast epoxy in a cast resin design or varnish impregnation in a VPI design. That single difference drives everything below.
Fire risk
This is the strongest argument for dry type. A dry type unit carries no flammable liquid, so it can be installed inside a building, close to the load, without a containment pit, a fire rated room or a suppression system. Oil immersed units installed indoors require containment for the oil volume and, in many jurisdictions, fire separation. In hospitals, tunnels, high rise buildings and offshore accommodation modules, this requirement alone decides the question.
Efficiency and rating
Oil is a better heat transfer medium than air, so oil immersed designs reach higher ratings in a given volume and generally run cooler. Dry type units are normally offered up to around 6,000 kVA in cast resin construction; above that, oil immersed becomes the practical choice. Within the range where both are available, efficiency differences are second order compared with the loss capitalisation the buyer applies.
Footprint and weight
For the same rating a dry type unit is usually larger and, in cast resin construction, heavier. It also needs clearance around it for cooling air. An oil immersed unit is more compact but needs space for radiators and, indoors, for containment. Which wins depends on whether the constraint is floor area or volume.
Maintenance
Dry type units need periodic cleaning, visual inspection, connection torque checks and verification that cooling air paths are clear. There is no liquid to sample or replace. Oil immersed units need oil level checks, periodic oil testing for moisture and dielectric strength, and eventual oil treatment or replacement. For sites without an electrical maintenance team, the dry type advantage is decisive; for sites with one, oil testing is routine and cheap.
Environment
Dry type suits indoor, clean and dry locations, and does well in humid, dusty or coastal conditions when cast resin encapsulated. Oil immersed suits outdoor installation, including pole and pad mounted units, and tolerates wide ambient ranges. In very humid or contaminated atmospheres, an oil immersed unit has an advantage because the oil seals the active part from the environment.
Noise
Oil immersed units are generally quieter because the oil and the tank damp core noise. Dry type units in an occupied building may need acoustic treatment or resilient mounting.
Lifecycle cost
Dry type units usually cost more to buy and less to operate and maintain. Oil immersed units cost less to buy and more to keep compliant, because oil testing, containment inspection and eventual oil disposal all fall on the operator. Over a 25 to 30 year life the totals converge, and the winner depends on the site: an inaccessible or unstaffed site favours dry type, a staffed outdoor substation favours oil.
Decision summary
- Choose dry type: indoor installation, occupied building, no maintenance team, fire risk is the binding constraint, rating up to about 6,000 kVA.
- Choose oil immersed: outdoor installation, rating above the dry type range, staffed site with routine oil testing, or a very humid and contaminated atmosphere.
- Either: ratings in the low MVA range where the site has both an indoor option and a maintenance capability, in which case lifecycle cost decides.
If you are still unsure, send the rating, the location, the ambient conditions and your maintenance capability and we will give you a recommendation with the reasoning.