IEEE / IEC Oil-immersed Transformers – Methods of Cooling

Oil-immersed transformer cooling methods are consistent in both the IEC and IEEE. This designation is one of the few that is the same for both standards.

Oil-immersed transformer cooling method is expressed by a four-letter code described below.

Oil-immersed Transformer Methods of Cooling
Letters Description
First letter - Internal cooling medium in contact with the windings
O mineral oil or synthetic insulating liquid with fire point ≤300°C
K insulating liquid with fire point >300°C
L insulating liquid with no measurable fire point
Second letter - Circulation mechanism for internal cooling medium
N natural convection flow through cooling equipment and in windings
F forced circulation through cooling equipment (i.e., coolant pumps), natural convection flow in windings (also called nondirected flow)
D forced circulation through cooling equipment, directed from the cooling equipment into at least the main windings
Third letter - External cooling medium
A Air
W Water
Fourth letter - Circulation mechanism for external cooling medium
N natural convection
F forced circulation [fans (air cooling), pumps (water cooling)]

Examples:
ONAN/ONAF. The transformer has a set of fans that may be put in service as desired at high transformer loading. The coolant circulation is by natural convection only.

ONAN/OFAF. The coolant circulation is by natural convection only at base transformer loading. However, the transformer has cooling equipment with pumps and fans to increase the power-carrying capacity at high transformer loading.

In IEEE Std C57.12.00 - 1993, the method of cooling was only represented by a two-letter or three-letter code. Equivalent old - new cooling designations are given below:

Equivalent Method of Cooling
C57.12-2000 C57.12-1993
ONAN OA
ONAF FA
ONAN/ONAF/ONAF OA/FA/FA
ONAN/ONAF/OFAF OA/FA/FOA
ONAN/ODAF OA/FOA
ONAN/ODAF/ODAF OA/FOA/FOA
OFAF FOA
OFWF FOW
ODAF FOA
ODWF FOW

References:
1. IEEE Std C57.12.00-2000 Liquid Immersed Distribution, Power and Regulating Transformers
2. IEC 60076-2: 1993, Power transformers - Part 2: Temperature rise.