ONAF (Oil Natural Air Forced) — Transformer Glossary

Cooling method where oil circulates by natural convection but fans force air over the radiators, increasing cooling capacity by 25–40%.

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ONAF (Oil Natural Air Forced)

Cooling method where oil circulates by natural convection but fans force air over the radiators, increasing cooling capacity by 25–40%.

ONAF fans are typically thermostatically controlled, activating when oil temperature exceeds a setpoint (usually 65–75°C). This provides automatic load management: the transformer operates silently in ONAN at base load and activates fans only during peak demand. Dual-rated transformers (e.g., 80/100 MVA ONAN/ONAF) are common in utility applications where load varies significantly between day and night.

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transformersMedium Power TransformersMedium power transformers for sub-transmission, industrial, and generation applications.

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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.