Oil-Immersed vs Dry-Type Transformer — Basic Comparison
The first and most critical decision when selecting a distribution transformer is: oil-immersed or dry-type? Oil-immersed transformers are cooled with mineral or vegetable oil, while dry-type transformers are insulated with air or cast resin. The two technologies differ significantly in terms of fire safety, service life, cost, installation location, and maintenance requirements. In this article, we examine in detail the pros/cons of both technologies and which one is more suitable for which facility.
Quick Comparison Table
| Criteria | Oil-Immersed Transformer | Dry-Type Transformer |
|---|---|---|
| Cooling | Mineral oil (ONAN/ONAF) | Air (AN/AF) or resin |
| Fire class | E2 / F1 (oil is flammable) | F1 — non-flammable (cast resin) |
| Service life (average) | 35–45 years | 25–35 years |
| Initial investment | 100 (reference) | 140–180 (40–80% more expensive) |
| Capacity range | 50 kVA – 100 MVA | 100 kVA – 25 MVA |
| No-load loss | Low | Medium–High |
| Load loss | Low | High |
| Efficiency | 98.5–99.5% | 97.5–98.8% |
| Quietness | Quiet (≤65 dB) | Noisier (60–75 dB) |
| Maintenance | Oil analysis required | Periodic cleaning |
| Indoor suitability | Limited (fire cell required) | Excellent |
| Outdoor suitability | Excellent | Additional enclosure required |
1. Oil-Immersed Transformer
Operating Principle
Inside the tank, mineral or vegetable oil serves both insulation and cooling. Cooling types:
- ONAN: Oil Natural Air Natural (natural circulation)
- ONAF: Oil Natural Air Forced (forced air)
- OFAF: Oil Forced Air Forced (forced oil + air)
- ODAF: Oil Directed Air Forced (directed oil)
Advantages
- Low initial investment (40–80% cheaper than dry type)
- High efficiency (98.5%+) — especially at large capacities
- Long service life (40+ years) — can be extended by oil replacement
- High overload capability (short-term 150–200%)
- Wide capacity range (up to 100 MVA)
Disadvantages
- Fire risk — mineral oil ignites at 145°C; an extinguishing system such as FM200/Inergen may be required
- Leakage risk — a concrete oil pit is mandatory
- Annual oil analysis (DGA, BDV, acidity)
- Limited indoor use (a separate fire-resistant room is mandatory)
- Environmental risk (oil spills, soil contamination)
Application Areas
- Outdoor transformer kiosks (compact concrete cabin)
- MV distribution in industrial facilities
- Renewable energy (solar/wind) grid connection transformer
- Network distribution transformer (neighborhoods, organized industrial zones)
2. Dry-Type Transformer
Operating Principle
Windings are coated with epoxy resin (cast resin) or vacuum pressure impregnated (VPI). Cooling is by air:
- AN: Air Natural (natural air)
- AF: Air Forced (forced fan)
Advantages
- Fire-resistant (F1 class, self-extinguishing)
- Contains no mercury or oil (environmentally friendly)
- No oil pit required (simpler installation)
- Indoor compatible (inside buildings, car parks, attic floors)
- Low maintenance (no oil analysis)
- No explosion risk (no oil vapor)
- Modern project requirements (LEED, BREEAM green building certification)
Disadvantages
- High initial investment (40–80% more expensive)
- Lower efficiency (97.5–98.8%)
- Difficult outdoor use (additional enclosure for IP/HVAC solutions)
- Noise (70–75 dB in fan-cooled models)
- Limited capacity (generally below 25 MVA)
- Spare-part lead times may be long
Application Areas
- Hospitals, shopping malls, hotels (fire safety mandatory)
- Data centers (24/7 critical infrastructure)
- Underground transformer substations (no oil leakage risk)
- Petrochemical and chemical plants (explosion-risk environments)
- Green building certified projects (LEED, BREEAM)
- On-site transformer rooms within residential complexes
3. Regulations and Legal Requirements
In Türkiye, pursuant to Article 51 of the Regulation on the Fire Protection of Buildings (BYKY):
- In residential buildings higher than 51 m, if the transformer is to be installed inside the building, dry-type is MANDATORY
- In shopping malls, hospitals, hotels, and high-rise buildings, dry-type is MANDATORY for an in-building transformer substation
- In industrial facilities, if there is a separate transformer building, an oil-immersed type may be used
- In underground transformer substations, dry-type is PREFERRED
4. Total Cost of Ownership (TCO) Comparison
1000 kVA transformer, 20-year projection:
| Item | Oil-Immersed | Dry-Type |
|---|---|---|
| Initial investment | 580,000 TL | 950,000 TL |
| Oil pit construction | 95,000 TL | 0 |
| Annual maintenance (20 years × ) | 12,000 TL/year | 6,000 TL/year |
| Oil analysis (20 years) | 60,000 TL | 0 |
| 20-year energy loss (efficiency difference) | 280,000 TL | 380,000 TL |
| TOTAL TCO | 1,255,000 TL | 1,450,000 TL |
Result: Oil-immersed is about 15% more economical in the long term. However, if fire safety and space constraints are critical, dry-type is unrivaled.
5. Brand Recommendations (TEDAŞ Approved)
- Oil-immersed: BEST Transformatör, ELİN Trafo, ABB, BALIKESİR ELEKTROMEKANİK, Astor
- Dry-type: ABB Resibloc, Schneider Trihal, Siemens Geafol, Çetinkaya CET, ELG
DOA Enerji Transformer Service
DOA Enerji Aydınlatma, as a TEDAŞ/BEDAŞ-approved MV systems specialist, provides oil-immersed and dry-type transformer supply, switchgear cell installation, turnkey transformer substation setup, and commissioning services. For the correct type selection, we perform a free site survey and TCO analysis.
Frequently Asked Questions
Can I use an oil-immersed type in our hospital project?
No. Pursuant to BYKY Article 51, installing an oil-immersed transformer inside a hospital building is prohibited. An oil-immersed type can be used in a separate transformer building; dry-type is mandatory inside the building.
Why is the service life of a dry-type transformer shorter?
Due to thermal expansion, the resin coating forms microcracks over time. This allows moisture and dust ingress, weakening insulation. In oil-immersed types, this issue is eliminated by oil replacement; in dry-type, winding replacement is required.
Will fan noise be a problem in a dry-type transformer?
It can reach up to 75 dB; this is at the level of a kitchen hood. In quiet environments such as offices and hospitals, it can be reduced to 65 dB with acoustic panels. Oil-immersed types rarely exceed the 65 dB limit.
What is the difference between a cast resin transformer and VPI?
In cast resin, the winding is completely embedded in resin (highest durability). In VPI, the winding is dipped in resin under vacuum (cheaper, shorter service life). For fire safety, cast resin is more reliable.
Does TEDAŞ accept dry-type transformers?
Yes. Cast resin transformers such as Trihal (Schneider), Resibloc (ABB), and Geafol (Siemens) are included in the TEDAŞ type-test list. Domestic brands such as ELG and Çetinkaya are also approved.