Dry-Type or Oil-Immersed? Distribution Transformer Selection
dry-type transformer selection: A cast resin dry-type transformer can be installed indoors without additional fire measures and is simple to maintain; a hermetic oil-immersed unit of the same rating usually offers lower losses and better overload behaviour but requires a fire compartment and an oil containment pit.
Contents
The link between the medium voltage switchgear and the low voltage switchboard of a plant is the distribution transformer, and its selection does not end with a kVA figure. Choosing between two technologies of the same rating directly affects the fire scenario of the building, the annual cost of losses, harmonic loading and the maintenance organisation. This article compares dry-type and oil-immersed transformers together with their consequences on the panel and switchgear side.
The fundamental difference between the two technologies
In a dry-type transformer the windings are encapsulated in cast epoxy resin or wound with vacuum impregnated insulation systems, and cooling is natural or fan-assisted air. In an oil-immersed unit the windings operate in mineral oil or ester fluid, and the hermetic tank absorbs expansion through corrugated walls. Changing the insulating medium changes partial discharge behaviour, the thermal time constant and the post-fault behaviour at the same time.
Fire safety and indoor installation
Dry-type transformers are classified under IEC 60076-11 with F classes for fire behaviour, E classes for environment and C classes for climate; class F1 means self-extinguishing behaviour and low smoke emission once the ignition source is removed. They are therefore preferred where the transformer must sit next to the load, such as underground car parks, hospitals or data centres. If an oil-immersed unit is installed indoors, a fire-rated room, an oil containment pit and where required a suppression system are added to the design; ester filled units with a high fire point, classified as K classes, relax these requirements.
Losses, efficiency and load profile
Transformer cost is judged not by the purchase price but by the no-load and load losses paid over its lifetime. No-load loss is paid twenty-four hours a day, so in a facility that is permanently energised but lightly loaded, amorphous core or low P0 designs stand out. Load loss grows with the square of the current, so in production plants with a high demand factor the Pk figure becomes decisive. The second tier of the European ecodesign requirements raised the minimum efficiency limits, and selection follows that table.
Cooling, overload capability and harmonic loading
| Criterion | Dry-type (cast resin) | Oil-immersed (hermetic) |
|---|---|---|
| Typical rating range | 100 – 3150 kVA | 50 – 2500 kVA |
| Fire behaviour | Class F1, self-extinguishing | Oil fire risk, extra measures needed |
| Indoor installation | Can sit directly next to the load | Fire room and oil pit required |
| Overload behaviour | Limited increase with fans | Large thermal time constant, flexible |
| Maintenance | Dust cleaning and partial discharge checks | Oil analysis and leak inspection |
Because the thermal time constant of an oil-immersed unit is large, it handles short-term overloads more comfortably; on dry-type units forced ventilation typically raises the rating by around forty per cent. In installations dominated by drives, UPS systems and LED drivers, current harmonics create additional losses and the transformer is resized according to its k-factor or factor K. The table below compares both technologies against the selection criteria.
Substation layout, switchgear and panel connection
The transformer is supplied on the medium voltage side by a fuse switch cubicle or a circuit breaker cubicle, and on the low voltage side it is connected to the incoming breaker of the main distribution board by busbar or cable. The layout resolves ventilation cross-sections, maintenance access distances, temperature monitoring sensors and the routing of protection signals together. Since the short-circuit study is based on the transformer rating and impedance voltage, the withstand rating of the switchboard is verified with those figures.
Key takeaways: dry-type transformer selection
- Layout decision — Where the transformer sits beside the load indoors, class F1 dry-type units lead.
- Cost of losses — P0 dominates on permanently energised light loads, Pk at high demand factors.
- Harmonic loading — In drive and UPS dominated plants the transformer is resized with its k-factor.
- Panel compatibility — Impedance voltage and rating are verified against LV board short-circuit withstand.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is a dry-type transformer always safer?
In terms of fire load, a class F1 dry-type unit offers a clear indoor advantage. However, in dusty or condensing environments surface contamination can lead to partial discharge, so the environmental class and degree of protection must also be part of the selection.
Which load figure should the transformer rating follow?
The calculated demand is increased by a margin for future growth and by the harmonic effect. Keeping continuous load between roughly sixty and eighty per cent of the rating gives a balanced compromise between loss cost and overload headroom.
Does an ester filled transformer count as oil-immersed?
Technically it is a liquid-immersed transformer, but the fire point of natural or synthetic ester fluids is far higher than that of mineral oil. Classified as K classes, these units require fewer fire precautions indoors than mineral oil designs.
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Let us clarify transformer technology, rating and loss class against your load profile and building fire scenario; Devpan engineers the switchgear and panel side of the substation end to end. Related Devpan solution: medium voltage switchgear.
