Transformer Cooling Systems According to Rating

 

Transformer Cooling Systems According to Rating

Transformers generate heat due to core losses (hysteresis and eddy currents) and copper losses (I²R losses in windings). Efficient cooling is essential to maintain performance, extend lifespan, and prevent insulation failure. The choice of cooling system largely depends on transformer rating (kVA/MVA) and application.

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1. Classification of Transformer Cooling Systems

Cooling systems in transformers are classified mainly into Oil-immersed (liquid-cooled) and Dry-type (air-cooled).

Type	Full Form / Description	Typical Ratings	Remarks
ONAN	Oil Natural Air Natural	Up to 5–10 MVA	Oil circulates naturally, cooled by air; simplest system.
ONAF	Oil Natural Air Forced	5–50 MVA	Oil circulates naturally; additional air fans increase cooling.
OFAN	Oil Forced Air Natural	20–100 MVA	Oil is pumped mechanically; air cooling is natural.
OFAF	Oil Forced Air Forced	50–250 MVA	Both oil and air are circulated by pumps/fans; high cooling efficiency.
OFWF	Oil Forced Water Forced	>100 MVA	Oil is circulated and cooled via water-cooled heat exchangers; used in large power transformers.
Dry-type AN	Air Natural	Up to 2–3 MVA	Air-cooled; used indoors or environmentally sensitive areas.
Dry-type AF	Air Forced	3–10 MVA	Fans improve cooling; no oil required, fire-safe.

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2. Cooling Systems in Detail

2.1 Oil-Immersed Transformers

a) ONAN (Oil Natural Air Natural)

• Rating: Up to 5–10 MVA
• Mechanism: Oil circulates due to natural convection; heat dissipates to the tank walls and air.
• Figure:

·         Windings → Hot Oil → Tank Walls → Air → Ambient

• Applications: Distribution transformers, small power transformers.
• Advantages: Simple, reliable, low maintenance.
• Limitations: Limited cooling capacity; unsuitable for high ratings.

b) ONAF (Oil Natural Air Forced)

• Rating: 5–50 MVA
• Mechanism: Oil rises naturally; air fans increase heat dissipation.
• Advantages: 1.5–2× better cooling than ONAN.
• Applications: Industrial transformers, moderate rating power transformers.

c) OFAF / OFWF (Oil Forced Air/Water Forced)

• Rating: 50–250+ MVA
• Mechanism: Oil is pumped; heat is removed by air or water.
• Applications: Large power transformers, grid substations.
• Advantages: Highest cooling efficiency; can operate at full load continuously.
• Figures:
• OFAF: Up to 2–3× ONAN cooling
• OFWF: Up to 5× ONAN cooling

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2.2 Dry-Type Transformers (Air-Cooled)

a) AN (Air Natural)

• Rating: Up to 2–3 MVA
• Mechanism: Heat is transferred directly to ambient air.
• Applications: Indoor installations, hazardous areas (no oil risk).

b) AF (Air Forced)

• Rating: 3–10 MVA
• Mechanism: Fans improve heat removal; allows higher loading.
• Advantages: Fire-safe, low maintenance; suitable for indoor use.

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3. Typical Transformer Rating vs Cooling System Table

Transformer Rating	Cooling System Used	Remarks
< 5 MVA	ONAN / Dry-type AN	Small distribution transformers
5–50 MVA	ONAF / Dry-type AF	Industrial & medium power transformers
50–250 MVA	OFAF / OFWF	Large power transformers
> 250 MVA	OFWF	Ultra-high rating, grid-level transformers

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4. Best Cooling System

• Best cooling system: Oil Forced Water Forced (OFWF)
• Reason: Maximum heat dissipation efficiency; suitable for continuous full-load operation.
• Typical Use: Ultra-high rating power transformers (≥100 MVA) in substations.
• Trade-offs: Higher cost, complex maintenance, water availability required.

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5. Summary

• Cooling depends primarily on transformer rating.
• Small transformers → natural oil/air cooling.
• Medium transformers → forced air or oil circulation.
• Large transformers → oil/water forced systems.
• Dry-type transformers are safer for indoor and hazardous environments but limited to lower ratings.
• Efficiency & reliability increase with forced oil/water systems.