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Up to 10 MVA rating, transformers used is
Naturally cooled oil immersed
Naturally cooled oil forced
Air forced oil immersed
Air force oil circulated
Naturally cooled oil immersed
Transformers rated up to 10 MVA typically employ ONAN (Oil Natural Air Natural) cooling, which is the simplest and most cost-effective method for this power range. In this setup, heat produced in the core and windings is transferred by natural convection of the transformer oil to the tank walls, from where it is dissipated into the ambient air by natural radiation and convection.
Transformers rated up to 10 MVA typically employ ONAN (Oil Natural Air Natural) cooling, which is the simplest and most cost-effective method for this power range. In this setup, heat produced in the core and windings is transferred by natural convection of the transformer oil to the tank walls, from where it is dissipated into the ambient air by natural radiation and convection.
PlossтАЛ=I2R
╬ФT=hтЛЕAQтАЛ
The cooling process relies on the thermosiphon effect. Heated oil near the windings becomes less dense and rises to the top, while cooler oil from the tank walls descends to the bottom to replace it. This continuous natural circulation of oil effectively transfers heat to the tank exterior, which is often finned or corrugated to increase the surface area for dissipation to the atmosphere.
ONAN stands for Oil Natural, Air Natural.
Suitable for transformers where the heat dissipation surface area of the tank is sufficient to keep temperature rise within limits.
Economical and maintenance-free compared to forced cooling methods.
Used commonly for distribution and smaller power transformers.
High reliability due to no moving parts like fans or pumps.
Lower initial cost and reduced maintenance requirements.
Larger radiator size required for a given capacity compared to forced cooling.
Limited capacity range; not suitable for very large power transformers.
Distribution transformers
Small-scale power substation transformers
Above 10тБ╗┬│тБ░ MVA, forced cooling methods (ONAF, OFAF) are usually required because natural heat dissipation becomes insufficient.
Option B (ONAF) is typically used for ratings higher than 10тБ╗┬╣тБ╡ MVA where fan-assisted cooling is necessary to increase efficiency.
A is correct тАФ For transformers rated up to 10 MVA, the naturally cooled oil-immersed (ONAN) method provides sufficient heat dissipation without requiring auxiliary fans or pumps.
Always check the cooling code (O-N-A-N, O-N-A-F, O-F-A-F) as it is a common area for numerical and theoretical questions in Power Systems.