When selecting a power transformer, one question arises in almost every project: dry-type or oil-immersed?
Both types perform the same function, but in different ways, and the price difference is significant enough to require proper evaluation rather than a decision based on habit.
In an oil-immersed transformer, the windings are submerged in oil, which provides both insulation and cooling. In a dry-type transformer, the insulation is made of epoxy resin and cooling is provided by air.
Almost every other difference between the two designs stems from this basic construction principle: fire behaviour, installation location, maintenance requirements and cost.
An oil-immersed transformer is generally less expensive for the same rated power, has lower losses and handles overloads better because the oil dissipates heat more efficiently.
For stand-alone outdoor transformer substations, conventional industrial applications and higher power ratings, the oil-immersed transformer is the standard choice.
The disadvantage is the oil itself. An oil containment bund or collection pit is required in the event of leakage, the condition of the oil must be checked periodically, and the design must account for the fact that transformer oil is flammable and therefore introduces additional fire-protection requirements.
A dry-type transformer contains no oil, which eliminates the risk of oil leakage and the additional fire load associated with it.
For this reason, it is usually the preferred choice for indoor installations, including basements of office buildings, shopping centres, hospitals, data centres and industrial halls where the transformer substation is located within the production facility itself.
Maintenance is simpler and mainly consists of cleaning and inspections, while resistance to moisture and dust depends on the selected protection and environmental class.
The purchase price is higher, losses are usually slightly greater, and sensitivity to prolonged overloads is more pronounced than with oil-immersed transformers.
In practice, the decision primarily comes down to location and risk.
For an outdoor substation located away from buildings, an oil-immersed transformer usually provides better value for money.
For a substation installed inside a building or immediately adjacent to areas occupied by people, a dry-type transformer is generally the more appropriate choice because fire safety and simpler maintenance outweigh the difference in purchase price.
Between these two clear situations lies a grey area in which the decision should be based on a total cost analysis, including procurement, lifetime energy losses, maintenance and fire-protection measures.
When all costs are assessed over a thirty-year operating life, the transformer with the lower purchase price is not always the transformer with the lower total cost of ownership.
