What is an Oil-Immersed Transformer?

What is an Oil-Immersed Transformer?

An oil-immersed transformer is a power transformer that uses insulating oil (mineral oil or synthetic ester) for both cooling and electrical insulation. The core and windings are submerged in oil, which dissipates heat and prevents electrical breakdown.

                          

Key Components

1. Core: Laminated silicon steel to reduce eddy current      losses.

2. Windings: Copper or aluminum conductors (HV/LV).

3. Tank: Sealed steel container filled with oil.

4. Insulating      Oil:

• Mineral oil (most common).

• Bio-based or synthetic ester (eco-friendly       alternatives).

5. Cooling      System:

• Radiators/fins (ONAN – Oil Natural Air Natural).

• Fans (ONAF – Oil Natural Air Forced).

6. Protection      Devices:

• Buchholz relay (gas detection for internal faults).

• Pressure relief valve.

• Temperature sensors (oil/winding).

Types of Oil-Immersed Transformers

1. Hermetically      Sealed

• Fully sealed tank (no oil exposure to air).

• No conservator; low maintenance.

2. Conservator-Type

• Expansion tank (conservator) with breather to       absorb moisture.

• Common in large transformers.

3. Step-Lap      Core Design

• Reduced no-load losses and noise.

4. Specialty      Types:

• Fire-resistant (less-flammable oil).

• Underground (compact design).

Advantages

✔ High Efficiency: Oil provides excellent heat dissipation.
✔ Long Lifespan: Properly maintained, lasts 25-30 years.
✔ Cost-Effective: Lower initial cost than dry-type transformers.
✔ Overload Capacity: Handles temporary overloads better.

Disadvantages

✖ Fire Risk: Oil is flammable (requires fireproof barriers).
✖ Maintenance: Regular oil testing and leak checks needed.
✖ Environmental Concerns: Oil disposal and spill risks.

Applications

• Power Distribution: Step-down 11kV/33kV to 400V.

• Industrial Plants: Steel mills, chemical factories.

• Renewable Energy: Wind/solar farm substations.

• Railways: Traction power supply.

Maintenance Requirements

1. Oil      Testing:

• Dielectric strength (>30 kV for mineral oil).

• Acidity (<0.1 mg KOH/g).

• Moisture content (<20 ppm).

2. Leak Inspection: Check gaskets, welds, and valves.

3. Sludge Removal: Clean oil ducts periodically.

Standards & Certifications

• IEC 60076: International standards.

• IEEE C57.12.00: North American standards.

• GB/T 6451: Chinese standards.

Oil-Immersed vs. Dry-Type Transformers

Feature	Oil-Immersed	Dry-Type
Cooling	Oil	Air/Solid Insulation
Fire Risk	Higher	Lower
Maintenance	More frequent	Minimal
Efficiency	Better at high loads	Slightly lower
Cost	Lower initial cost	Higher

Purchasing Considerations

1. Capacity (kVA): Match to load requirements.

2. Voltage Ratio: Input/output voltage levels.

3. Impedance: Typically 4–10% for distribution transformers.

4. Environment: Indoor/outdoor, temperature, humidity.

5. Certifications: Ensure compliance with local regulations.

FAQ

Q: Can oil-immersed transformers be used indoors?
A: Yes, but require fireproof vaults and ventilation (NFPA 70 rules).

Q: How often should oil be tested?
A: Annually for routine checks; after overloads or faults.

Q: Are biodegradable oils reliable?
A: Yes, synthetic esters offer similar performance with lower fire risk.

For a customized selection, provide:

• Load details (kVA, voltage).

• Installation environment.

• Special requirements (e.g., low noise, high      efficiency).