ONAN Cooling
The simplest transformer cooling method using natural oil circulation and natural air convection, requiring no pumps or fans.
ONAN (Oil Natural Air Natural) is the most common cooling class for distribution transformers and smaller power transformers. Heat from core and copper losses warms oil, which rises naturally through winding ducts to radiators where it transfers heat to ambient air by natural convection.
Advantages: simplest design, fewest moving parts (highest reliability), lowest maintenance, quietest operation. Disadvantage: lowest cooling capacity per unit size.
Rating: typically up to ~60 MVA. Beyond this, ONAF (fans) or OFAF (pumps+fans) required.
QDTB uses ONAN as standard cooling for all distribution transformers up to 2,500 kVA and offers ONAN/ONAF dual-rating for larger units. See derating guide for high-altitude considerations.
Related Products
Related Terms
Cooling Class (ONAN/ONAF/ODAF)
Standardized codes describing transformer cooling methods: O=Oil, N=Natural, A=Air, F=Forced, D=Directed flow.
Temperature Rise
The temperature increase of transformer components above ambient at rated load, a key design parameter.
Distribution Transformer
A transformer stepping down voltage from the distribution network (11-33 kV) to utilization voltage (400/230V) for end consumers.
Hot-Spot Temperature
The highest temperature within transformer winding insulation, determining aging rate and loading capability.
More Terms Starting with “O”
Oil-Immersed Transformer
A transformer using mineral oil as insulation and cooling medium, most common for outdoor power and distribution.
On-Load Tap Changer (OLTC)
A device changing transformer tap positions under load without interrupting supply for automatic voltage regulation.
Overexcitation
A condition where core flux exceeds design level due to excessive voltage or reduced frequency.
Oil Preservation System
Equipment preventing moisture and oxygen from entering transformer oil, including conservators and sealed tanks.
More in “Design & Engineering”
Altitude Derating
The reduction in transformer capacity and insulation performance required at high altitudes where air density is lower.
Cooling Class (ONAN/ONAF/ODAF)
Standardized codes describing transformer cooling methods: O=Oil, N=Natural, A=Air, F=Forced, D=Directed flow.
Faraday Shield
An electrostatic shield between transformer windings blocking capacitive coupling of high-frequency noise.
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