What cooling options are available for your transformers? — Transformer FAQ

Transformer cooling is one of the most critical design parameters, directly affecting capacity rating, physical size, operational life, and total cost of ownership. The cooling designation follows a four-letter code defined by IEC 60076-2:

ONAN (Oil Natural Air Natural) — The simplest and most reliable configuration. Oil circulates by natural convection (thermal buoyancy), and heat is dissipated through radiators to ambient air without any fans or pumps. ONAN is the base rating for most distribution and smaller power transformers. No moving parts means minimal maintenance and zero noise from cooling equipment.

ONAF (Oil Natural Air Forced) — Adds electric fans to the radiator banks, increasing cooling capacity by 25–40% over ONAN. Most power transformers are dual-rated ONAN/ONAF: they operate in quiet ONAN mode at base load and automatically switch to ONAF during peak demand via temperature-controlled fan starters.

KNAN (Ester Natural Air Natural) — Uses natural or synthetic ester fluid instead of mineral oil. Ester provides a dramatically higher fire point (360°C vs 170°C for mineral oil), is biodegradable, and actively absorbs moisture from cellulose insulation — extending paper life by 5–8×. KNAN is increasingly specified for indoor, urban, environmentally sensitive, and fire-critical installations.

AN/AF (Air Natural / Air Forced) — For dry-type (cast resin) transformers. AN relies on natural air convection; AF adds enclosure fans for higher ratings. Cast resin units are inherently fire-safe (F1 class) and are the standard choice for data centres, hospitals, high-rise buildings, and underground installations.

At ETS, we help clients select the optimal cooling configuration based on ambient temperature (critical in Gulf climates where 50°C+ is common), altitude derating, load profile (base load vs cyclic), noise requirements, and fire safety classification.

Transformer cooling is one of the most critical design parameters, directly affecting capacity rating, physical size, operational life, and total cost of ownership. The cooling designation follows a four-letter code defined by IEC 60076-2:

ONAN (Oil Natural Air Natural) — The simplest and most reliable configuration. Oil circulates by natural convection (thermal buoyancy), and heat is dissipated through radiators to ambient air without any fans or pumps. ONAN is the base rating for most distribution and smaller power transformers. No moving parts means minimal maintenance and zero noise from cooling equipment.

ONAF (Oil Natural Air Forced) — Adds electric fans to the radiator banks, increasing cooling capacity by 25–40% over ONAN. Most power transformers are dual-rated ONAN/ONAF: they operate in quiet ONAN mode at base load and automatically switch to ONAF during peak demand via temperature-controlled fan starters.

KNAN (Ester Natural Air Natural) — Uses natural or synthetic ester fluid instead of mineral oil. Ester provides a dramatically higher fire point (360°C vs 170°C for mineral oil), is biodegradable, and actively absorbs moisture from cellulose insulation — extending paper life by 5–8×. KNAN is increasingly specified for indoor, urban, environmentally sensitive, and fire-critical installations.

AN/AF (Air Natural / Air Forced) — For dry-type (cast resin) transformers. AN relies on natural air convection; AF adds enclosure fans for higher ratings. Cast resin units are inherently fire-safe (F1 class) and are the standard choice for data centres, hospitals, high-rise buildings, and underground installations.

At ETS, we help clients select the optimal cooling configuration based on ambient temperature (critical in Gulf climates where 50°C+ is common), altitude derating, load profile (base load vs cyclic), noise requirements, and fire safety classification.