ONAN (Oil Natural Air Natural) — Transformer Glossary

Cooling method where oil circulates by natural convection and heat is dissipated to air through radiators without fans or pumps.

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ONAN (Oil Natural Air Natural)

Cooling method where oil circulates by natural convection and heat is dissipated to air through radiators without fans or pumps.

ONAN is the simplest and most reliable cooling configuration — no moving parts, no auxiliary power required. The oil rises as it heats (thermal buoyancy), flows through radiator tubes where it cools, and descends back into the tank. ONAN rating is the base rating of the transformer. Most power transformers are dual-rated ONAN/ONAF, where ONAF adds fans to increase capacity by 25–40%. The choice between ONAN and ONAF depends on ambient temperature, load profile, and space constraints.

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Where this term matters in the ETS portfolio.

transformersDistribution Liquid TransformersReliable liquid-filled distribution transformers for utility and commercial applications.

transformersPackage Substation TransformersCompact, factory-assembled package substation transformers for utility and industrial distribution networks.

transformersAuxiliary TransformersCompact auxiliary and station service transformers for power plants, substations, and industrial facilities.

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KNAN (Ester Natural Air Natural)Cooling method using natural or synthetic ester fluid instead of mineral oil, with natural convection cooling. The 'K' designates ester fluid per IEC naming conventions.

Temperature RiseThe increase in winding and oil temperature above ambient when the transformer operates at rated load. A key specification for transformer longevity.