What is the difference between ONAN and ONAF cooling? — Transformer FAQ
ONAN (Oil Natural Air Natural) relies entirely on natural convection — oil circulates by thermal buoyancy, and radiators dissipate heat to ambient air. ONA
What is the difference between ONAN and ONAF cooling?
ONAN (Oil Natural Air Natural) relies entirely on natural convection — oil circulates by thermal buoyancy, and radiators dissipate heat to ambient air. ONAF (Oil Natural Air Forced) adds fans to the radiators, increasing cooling capacity by 25–40%. Most power transformers are dual-rated ONAN/ONAF, operating in ONAN at base load and switching to ONAF during peak demand.
Understanding the difference between ONAN and ONAF cooling is essential for proper transformer specification, particularly in hot-climate regions like the Middle East where ambient temperatures regularly exceed 45°C.
ONAN — Oil Natural Air Natural
The transformer oil circulates solely by natural convection: hot oil rises from the windings, flows through the header pipe to the radiators, cools as it descends through the radiator panels, and returns to the bottom of the tank. No pumps or fans are involved. This is the simplest, most reliable, and quietest cooling method.
Advantages: Zero auxiliary power consumption, no moving parts (highest reliability), silent operation, minimal maintenance.
Limitations: Limited cooling capacity — the transformer must be physically larger for a given kVA rating compared to forced-cooling options.
ONAF — Oil Natural Air Forced
Same oil circulation as ONAN, but fans are mounted on the radiator banks to force air across the cooling surfaces. This increases heat dissipation by 25–40%, allowing higher loading from the same physical transformer size.
Dual Rating (ONAN/ONAF)
Most power transformers are specified with dual ratings — for example, 40/50 MVA ONAN/ONAF. The transformer operates in quiet ONAN mode at loads up to 40 MVA. When winding temperature rises above a setpoint (typically 85°C), the fan contactors energise automatically, enabling the full 50 MVA ONAF rating. This provides operational flexibility and energy savings during off-peak hours.
Hot Climate Considerations
In the Gulf region, standard IEC 60076 assumes 40°C maximum ambient temperature. When ambient regularly exceeds this (e.g., 50°C in summer), the transformer must be derated or specified with enhanced cooling. At ETS, all transformers are designed for 50°C ambient as standard — a critical specification that some manufacturers overlook.
Understanding the difference between ONAN and ONAF cooling is essential for proper transformer specification, particularly in hot-climate regions like the Middle East where ambient temperatures regularly exceed 45°C.
ONAN — Oil Natural Air Natural
The transformer oil circulates solely by natural convection: hot oil rises from the windings, flows through the header pipe to the radiators, cools as it descends through the radiator panels, and returns to the bottom of the tank. No pumps or fans are involved. This is the simplest, most reliable, and quietest cooling method.
Advantages: Zero auxiliary power consumption, no moving parts (highest reliability), silent operation, minimal maintenance.
Limitations: Limited cooling capacity — the transformer must be physically larger for a given kVA rating compared to forced-cooling options.
ONAF — Oil Natural Air Forced
Same oil circulation as ONAN, but fans are mounted on the radiator banks to force air across the cooling surfaces. This increases heat dissipation by 25–40%, allowing higher loading from the same physical transformer size.
Dual Rating (ONAN/ONAF)
Most power transformers are specified with dual ratings — for example, 40/50 MVA ONAN/ONAF. The transformer operates in quiet ONAN mode at loads up to 40 MVA. When winding temperature rises above a setpoint (typically 85°C), the fan contactors energise automatically, enabling the full 50 MVA ONAF rating. This provides operational flexibility and energy savings during off-peak hours.
Hot Climate Considerations
In the Gulf region, standard IEC 60076 assumes 40°C maximum ambient temperature. When ambient regularly exceeds this (e.g., 50°C in summer), the transformer must be derated or specified with enhanced cooling. At ETS, all transformers are designed for 50°C ambient as standard — a critical specification that some manufacturers overlook.
Specify the Right Product
Three ETS units this question typically applies to.
Get a Quote Tailored to This Specification
Send us your application context — we'll come back with a sized solution, certified to IEC, IEEE/ANSI or UL, and a delivery slot from our Dubai factory.
Can't Find What You Need?
Our engineering team is happy to answer technical questions and provide custom documentation.