Oil Immersed vs Dry Type Distribution Transformer

The single biggest construction decision when buying a distribution transformer is oil immersed or dry type. Both do the same electrical job; they differ in cooling, fire behaviour, maintenance, noise, cost and where they can be installed.

This comparison covers the decision criteria that matter for industrial and commercial projects.

How each one is cooled

Oil immersed: the core and windings sit in insulating oil, which carries heat to the tank and radiators. Cooling classes are ONAN (natural) and ONAF (fans).

Dry type: the windings are encapsulated in epoxy resin (cast resin) or vacuum pressure impregnated (VPI), and heat is removed by air. Cooling classes are AN (natural) and AF (fans).

Fire safety

This is usually the deciding factor. Dry type units are self-extinguishing, halogen free and have a very low fire load, so they can be installed inside occupied buildings. Oil immersed units contain flammable liquid and normally require an outdoor or dedicated, bunded, fire-separated substation.

Maintenance

Oil immersed units need periodic inspection, oil level checks and occasional oil testing for moisture and dissolved gas. Sealed hermetic designs need very little. Dry type units need essentially no liquid maintenance, mainly cleaning and inspection of the winding temperature monitoring.

Noise, size and cost

Dry type units are typically slightly larger and more audible at a given rating, and cost more at higher ratings. Oil immersed units are more compact per kVA and usually cheaper above roughly 1,000–1,500 kVA.

Comparison summary

Fire safety. Dry type wins, no oil, self-extinguishing

Indoor installation. Dry type preferred inside occupied buildings

Overload capability. Oil immersed generally better

Capital cost at high ratings. Oil immersed generally lower

Maintenance. Dry type requires less

Environmental risk. Dry type has no liquid leak risk; ester fluid reduces risk for oil units

Frequently Asked Questions

Can a dry type transformer be used outdoors?

Yes, in a ventilated enclosure with the right IP rating and protection from direct water and solar gain. Many projects still prefer oil immersed outdoors.

Can an oil transformer be installed indoors?

It can, but it requires a dedicated room with fire separation, oil containment and ventilation, and local regulations often restrict it.

Which lasts longer?

Both have long service lives when correctly loaded and maintained. Insulation ageing is driven mainly by operating temperature, so ventilation and loading discipline matter more than construction type.

Related Products, Applications and Support

Not sure which rating fits your project? Send us your load list and we will size it for you, or email info@xsdfftransformer.com, call +86 158 6789 7761 or WhatsApp +852 5416 2620.