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Why Oil-Immersed?
Oil-immersed transformers use insulating oil for both cooling and dielectric strength, allowing higher power density and lower cost at large ratings. They dominate substations, outdoor networks, and heavy industry.
Selection Parameters
| Parameter | What to specify |
|---|---|
| Rated power (MVA/kVA) | Based on load + growth margin |
| Voltage ratio | e.g., 33/0.4 kV, 110/11 kV |
| Cooling class | ONAN / ONAF / OFAF |
| Tap changer | Off-load or on-load (OLTC) |
| Insulating fluid | Mineral oil or ester (fire-retardant) |
Maintenance Essentials
- Monitor oil temperature and level continuously.
- Test oil (DGA, moisture, breakdown voltage) on schedule.
- Inspect bushings and gaskets for leaks.
- Service the tap changer and cooling system.
Safety Notes
Oil is combustible, so site fire protection, bund walls, and rapid fault clearance matter. In fire-sensitive indoor locations, consider a dry-type alternative — compare in our guide.
Frequently Asked Questions
Mineral oil or ester?
Ester fluids are more fire-resistant and biodegradable — preferred where safety or environmental rules are strict.
How long do they last?
25–40 years with proper oil care; DGA trends predict remaining life.