Power transformer engineering reference
Power Transformer Cooling: ONAN, ONAF, OFAF, ODAF and Control Logic
Power transformer cooling guide covering ONAN, ONAF, OFAF and ODAF, radiators, fans, pumps, oil-flow feedback, auxiliary supplies, redundancy, FAT and operation.
Meaning of ONAN, ONAF, OFAF and ODAF
ONAN normally means natural oil circulation and natural air cooling. ONAF adds forced air while oil circulation remains natural. OFAF uses forced oil and forced air. ODAF uses forced, directed oil flow through designated winding paths together with forced air. The nameplate and approved drawings define the actual rating stages.
The complete heat path
Heat is generated in windings and the core, transferred to oil, transported to radiators or coolers, and rejected to the outside medium. Dirty surfaces, incorrect fan direction, insufficient pump flow, closed valves, failed oil-flow indication, unsuitable cooling-water conditions or loss of auxiliary supply can break the chain even when a motor is shown as running.
Control and redundancy
Forced systems should define duty and standby groups, staged starts, automatic and manual modes, alarm logic, auxiliary supplies and changeover. The required redundancy depends on project reliability criteria. Each group should have an identifiable state, command, feedback and failure indication.
Oil flow and pump start
A running pump does not prove correct circulation. Check valves, flow feedback, pressure or flow indications, cooler temperature differences and equal loading of cooling banks. Pump-start sequence can also influence oil movement near the Buchholz relay and should follow the manufacturer logic.
FAT and site verification
FAT should verify fans, pumps, oil-flow devices, auxiliary-supply changeover, automatic starts, alarms, remote indications and each cooling group. Site inspection should also compare cooler temperatures, airflow, vibration, noise, leakage and contamination.
Operating boundary after cooling loss
Permissible loading and time following partial or total cooling loss depend on transformer design, temperature and operating rules. A generic duration should not be applied. Use the manufacturer curves and the approved site procedure.
Procurement and FAT hand-in-hand matrix
| Buyer / engineer should define | Supplier should submit | FAT / commissioning should verify |
|---|---|---|
| Cooling stages | Ratings at each ONAN/ONAF/OFAF/ODAF stage | Nameplate and staged functional test |
| Auxiliaries | Fan/pump groups, supplies and standby philosophy | Each group, supply changeover and failure alarm |
| Feedback | Oil-flow, motor status and remote signals | Command-to-feedback consistency |
| Operating data | Temperature instruments and loss-of-cooling limits | Alarm, start and documented response |
Frequently asked questions
Is ONAF acceptable if the fans are rotating?
Not by itself. Oil path, airflow, feedback, control and actual temperature response must also be verified.
How long may a transformer operate after cooler failure?
Only the equipment-specific manufacturer limits and approved operating procedure can answer that question.
Technical references
- Qin Ganghua, Wang Zhanhong, Wang Xin, et al. 《火力发电企业绝缘技术监督手册》 [Insulation Technical Supervision Manual for Thermal Power Enterprises]. Zhejiang University Press. Used for transformer components, tap changers, oil systems, cooling, grounding and operating inspection.
- Sun Kejun, Jing Chenghao, Wang Zhongjie, et al. 《电工手册》 [Electrical Engineering Handbook]. China Machine Press. Used for oil-immersed construction, pre-energization checks, breathers, cooling equipment and routine inspection.
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- Transformer Conservator, Bladder, Breather and Silica Gel: Oil Preservation and Level Guide
- Power Transformer Cooling: ONAN, ONAF, OFAF, ODAF and Control Logic
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Final engineering values, setpoints and operating decisions require written confirmation from the project engineer, owner or utility and manufacturer.