Answer Posted / mehedi islam aneek
The first letter designates the internal cooling medium in
contact with the windings.
O= mineral oil or synthetic insulation fluid with a fire
point ≤ 300°C
K = insulating fluid with a fire point > 300°C
L = insulating liquid with no me3asurable fire point.
The second letter designates the circulation mechanism for
internal cooling medium
N = Natural convection flow through cooling equipment
and in windings
F = Forced circulation through cooling equipment and
natural convention flow in the windings (also called
"directed flow")
D = Forced circulation through cooling equipment,
directed from the cooling equipment into at least the main
windings
The third letter designates external cooling medium
A = Air
W = Water
The fourth letter designates the circulation mechanism for
the external cooling medium.
N = Natural convection
F = Forced circulation (Fans (air cooling) , pumps
(water cooling))
For example: ONAN designates an oil filled unit that has
natural convection flow in the tank and utilizes natural air
convection cooling externally.
If this transformer has fans added for forced air
externally, the designation would be ONAF.
A transformer that has natural convection cooling as a base
rating and an elevated rating when fans were added later,
would be designated as ONAN/ONAF.
High fire point fluids use the designation of "K" for fluid
type. Thus a naturally cooled high fire point fluid would be
KNAN and the same unit with fans would be KNAF.
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