ABB Transformer Types: Dry-Type vs. Oil-Immersed
Transformers are the silent workhorses of industrial power distribution. ABB manufactures both dry-type and oil-immersed transformers across a broad power and voltage range. Choosing between the two technologies involves trade-offs across safety, environment, total cost of ownership, and installation constraints.
How Each Technology Works
Dry-Type Transformers
In dry-type transformers, the core and windings are cooled by air circulation — either natural convection (AN cooling) or forced air with fans (AF cooling). ABB's RESIBLOC® and EcoDry™ ranges encapsulate windings in resin or use open-wound designs with vacuum-pressure impregnation (VPI).
Oil-Immersed Transformers
Oil-immersed (also called liquid-filled) transformers submerge the core and windings in insulating mineral oil or ester fluid. The oil simultaneously insulates and conducts heat to the tank walls and radiators. ABB's oil transformer range includes distribution, power, and special-purpose units from a few kVA into the hundreds of MVA range.
Key Comparison Factors
Safety and Fire Risk
Dry-type transformers present minimal fire risk and produce no flammable liquid spill hazard, making them the preferred choice for:
- Indoor installations in occupied buildings (hospitals, offices, shopping centers)
- Locations near flammable materials or processes
- Underground substations where drainage is difficult
Oil-immersed transformers require oil containment bunds and, in some jurisdictions, fire suppression systems when installed indoors. However, modern bio-ester filled units (ABB's BIOTEMP® or MIDEL-filled variants) offer significantly improved fire safety compared to mineral oil.
Efficiency and Losses
Oil-immersed transformers generally achieve slightly lower no-load (core) losses than equivalent dry-type units, particularly at larger power ratings. For transformers energized continuously at partial load — a common industrial scenario — this difference in no-load losses can be significant over the transformer's 25–40 year service life.
ABB's EcoDry™ uses amorphous core material to dramatically reduce no-load losses, narrowing the efficiency gap with oil units.
Power Range and Voltage
| Parameter | Dry-Type (ABB) | Oil-Immersed (ABB) |
|---|---|---|
| Power range | Typically up to 40 MVA | kVA to hundreds of MVA |
| Voltage (HV side) | Up to 36 kV (MV class) | Up to ultra-high voltage (UHV) |
| Cooling options | AN, AF | ONAN, ONAF, OFAF, ODAF |
| Overload capability | Limited without derating | Excellent with oil cooling |
Installation and Maintenance
Dry-type transformers are generally easier to install and maintain:
- No oil sampling or filtration required
- Lighter weight in smaller ratings
- No containment pit or spill response planning needed
- Suitable for compact indoor MV/LV substations
Oil-immersed units require periodic oil dielectric testing, dissolved gas analysis (DGA), and filtration. However, their robust thermal mass means they handle overloads and fault conditions exceptionally well.
Cost Considerations
Dry-type transformers typically carry a higher capital cost than oil-immersed equivalents at the same power rating, especially above 1 MVA. However, when you factor in containment bund construction, oil maintenance, and potential environmental liability, the total cost of ownership can be comparable or even favor dry-type in certain scenarios.
Which Should You Choose?
- Choose dry-type when: indoor installation, fire safety is paramount, environmental regulations are strict, or the rating is below 10 MVA.
- Choose oil-immersed when: outdoor installation, ratings exceed 10 MVA, overload capability is critical, or lowest lifecycle cost is the priority for utility-scale projects.
Conclusion
ABB's transformer portfolio covers both technologies with mature, field-proven designs. The "best" transformer is always the one matched to your specific installation environment, power rating, and operational requirements. Engage ABB or an authorized engineering partner early in the design phase to optimize your substation layout and transformer specification together.