Question:
Natural air cooling is used in transformers with ratings up to
Natural air cooling is used in transformers with ratings up to
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Dry-type transformers with Air Natural (AN) cooling are primarily installed indoors, underground, or in residential high-rises where oil-immersed units are prohibited due to fire hazards.
Updated On: Jun 18, 2026
- 20 MVA
- 60 MVA
- 3 MVA
- 30 MVA
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The Correct Option is C
Solution and Explanation
Step 1: Understanding Transformer Cooling Principles:
Transformers generate heat during operation due to copper losses ($I^2R$) in the windings and core losses (hysteresis and eddy current) in the steel laminations. To prevent insulation degradation, this heat must be continuously dissipated.
Step 2: Classifying Cooling Methods:
For dry-type transformers, cooling is achieved directly through the surrounding air:
- Air Natural (AN) Cooling: Heat is transferred from the core/windings to the surrounding air via natural convection currents without any mechanical assistance.
- Air Forced (AF) Cooling: Heat is dissipated rapidly by forcing air currents over the core and windings using fans or blowers.
Step 3: Correlating Power Ratings and Cooling:
Natural Air (AN) cooling is only effective when the total heat generated is low. In power engineering standards:
- Air Natural (AN): Used for smaller distribution or dry-type indoor transformers with power ratings up to 3 MVA (or 1.5 MVA in highly confined spaces).
- Air Forced (AF): Employed for dry-type ratings from $3\text{ MVA}$ up to $15\text{ MVA}$.
- Oil-Immersed systems (ONAN, ONAF): Mandated for ratings from $15\text{ MVA}$ up to several hundred MVA.
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