Transformer Core Engineering Notes · CF-N241
Transformer cooling codes describe the cooling media and how they circulate. They are not efficiency grades. In the common O-coded arrangements discussed here, the key differences are natural or forced oil circulation, natural or forced air movement, and whether oil flow is directed through the windings. Read the circuit and rating schedule alongside the letters.
Read each code as a flow description
ONAN indicates natural oil and natural air circulation. ONAF retains natural internal oil circulation while fans force external air movement. OFAF uses forced oil and forced air circulation; ODAF additionally identifies directed internal oil flow. These descriptions do not provide the actual flow rate, temperature rise or permitted loading. Other insulating-liquid categories can use a different first letter, so an ester design should use its approved code rather than automatically retaining an O designation.
Connect cooling stages to guaranteed ratings
A transformer may have more than one declared rating associated with different cooling equipment in service. Identify which fans, pumps and coolers are required for each stage. Do not assume that the highest nameplate rating remains available after one cooling group is lost. The OEM should state the available continuous duty and any time-dependent emergency conditions. A list of installed fans is not a complete rating schedule.
Verify the internal path, especially directed flow
A pump can move liquid through the external circuit without ensuring the intended share passes through every winding duct. Directed-flow designs rely on internal arrangements that guide the flow and limit bypass. Review the hydraulic circuit and its thermal model, including the relevant failure states. Motor status alone does not demonstrate liquid delivery to the intended path. A change to ducts or internal guides can require revalidation even if the cooling code is unchanged.
Use the code as the start of a specification
State ambient conditions, liquid type, allowable temperatures, auxiliary supplies and control logic with the selected code. Require temperature-rise evidence applicable to the delivered configuration. IEC 60076-2 provides the liquid-immersed temperature-rise framework; its public abstract does not supply every design limit. A useful tender asks for the actual performance in each cooling state, rather than choosing the most elaborate code and assuming it must produce the best transformer.
| Review item | Evidence to request | Boundary to preserve |
|---|---|---|
| Natural oil | ONAN or ONAF, with the stated air arrangement | Fans do not automatically make internal oil flow forced |
| Forced oil | OFAF or directed-flow ODAF as designed | A pump does not prove the intended duct flow |
| Rating stages | Required equipment and limits for each stage | Cooling code alone does not define capacity |
References and scope
- IEC 60076-2:2011 — Temperature rise for liquid-immersed transformersPublic standard abstract · Temperature-rise requirements, cooling identification and the distinction between test conditions and installed duty.
- IEEE C57.91-2025 — Loading Mineral-Oil-Immersed Transformers and Step-Voltage RegulatorsPublic standard abstract · Mineral-oil loading, thermal duty and loading beyond nameplate; do not transfer the scope automatically to ester or dry-type equipment.
- IEC 60076-7:2018 — Loading guide for mineral-oil-immersed power transformersPublic standard abstract · Mineral-oil transformer loading and thermal-ageing framework, not a universal asset-life prediction.