Oil-Immersed vs Dry-Type Transformers: A Selection Guide

Construction: What Is Actually Inside the Unit

In an oil-immersed transformer the core and windings are immersed in mineral insulating oil inside a sealed tank, and the oil serves three functions at once: it insulates, it transfers heat from the active part to the tank walls and radiators, and it excludes moisture and oxygen from the insulation. The insulation system is therefore a combination of solid insulation, oil and the tank, and its performance depends on oil quality as much as on the paper and pressboard. Unused mineral insulating oil is specified to IEC 60296, with breakdown voltage tested to IEC 60156. A dry-type transformer has no liquid at all: the windings are either cast in solid resin or impregnated and cooled by air, so the active part is visible and the enclosure is mainly a matter of mechanical protection and environmental degree of protection.

Fire Behaviour, Environmental Classes and Siting

The decisive difference in most projects is fire and siting. Mineral oil is combustible, so an oil-immersed unit normally needs bunding, fire separation and either an outdoor location or a dedicated ventilated room; installation requirements for such equipment are covered by IEC 61936-1. A dry-type transformer removes that risk: the design requirements for dry-type transformers up to 36 kV are set out in IEC 60076-11, which also defines the climate, environmental and fire behaviour classes. Fire behaviour class F1 indicates a design with limited fire load and self-extinguishing characteristics, environmental class E2 covers condensation and dust, and climate class C2 covers operation from very low to very high ambient temperature. Those three letters belong in the specification, because a dry-type transformer sited in a basement and one installed on a roof in a hot climate do not face the same duty. Enclosure protection is separately defined by the IP code in IEC 60529.

Rating Range and Typical Duty

Oil-immersed designs scale far beyond dry-type units: they serve distribution and power duties up to the highest transmission voltages, and their cooling can be increased from natural oil circulation to forced oil and forced air within the same tank concept. Dry-type transformers are applied at medium voltage, up to 36 kV and typically at ratings from a few hundred kilovolt-amperes to a few megavolt-amperes, which covers building services, commercial complexes, tunnels and industrial plants. Where a transformer must sit inside an occupied building, on a floor slab or on a roof, a dry-type unit is usually the only practical option because there is no oil to contain; where the requirement is a large block of capacity in an outdoor substation, oil-immersed equipment is normally the cheaper answer.

Cost, Losses and Lifetime Maintenance

Initial cost favours the oil-immersed unit for the same rating, and the gap widens as capacity increases. Life-cycle cost can move the other way. A dry-type transformer usually has somewhat higher no-load and load loss for the same rating, and its temperature rise depends entirely on air movement, so it is more sensitive to restricted ventilation. Against that, it needs no oil testing, no oil replacement, no leak repair and no bunding, and it presents no oil fire load. Oil-immersed units require periodic sampling for dielectric strength, water content, acidity and dissolved gas analysis, and those programmes are the main continuing cost. Purchase specifications for both types are anchored in IEC 60076-1 for power transformers, with temperature rise limits in IEC 60076-2 and insulation levels and dielectric tests in IEC 60076-3.

Selection Checklist

Work through the installation in order. Establish the voltage and rating, then the siting constraint and the fire strategy, then the ambient and environmental conditions at that exact location. Set the required loss levels and the cooling arrangement, and check that ventilation, clearances and protection are achievable. Fix the climate, environmental and fire behaviour classes in the specification, and state the test regime. The table below summarises the usual trade-offs.

Frequently Asked Questions

Q: Which transformer is safer inside a building?
A: A dry-type transformer, because it contains no combustible liquid. A unit declared to fire behaviour class F1 under IEC 60076-11 limits the fire load and is the usual choice for basements and occupied buildings.

Q: What voltage limit applies to dry-type transformers?
A: IEC 60076-11 covers dry-type transformers up to 36 kV, which is the practical limit for the range of ratings used in building and industrial distribution.

Q: Is an oil-immersed transformer cheaper overall?
A: Initial cost is usually lower for the same rating, but oil testing and containment add continuing cost, so the comparison should be made on life-cycle cost including no-load and load loss.

Q: What do the climate, environmental and fire class letters mean?
A: They come from IEC 60076-11. Climate class C2 covers the full ambient temperature range including extremes, environmental class E2 covers condensation and dust, and class F1 denotes limited fire load and self-extinguishing behaviour.

Q: How are losses and temperature rise limited?
A: Loss guarantees are agreed in the purchase specification and verified by the tests of IEC 60076-1, while temperature rise is limited according to IEC 60076-2.

Q: Which type suits a rooftop installation?
A: A dry-type unit, provided the ventilation is adequate for its air-cooled rating and the enclosure carries the required IP protection, since no oil containment is possible on a roof slab.