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.

Author

  • A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia, and Professional Electrical Engineer (PEE 2574 - 1st Place April 1991) - Philippines with extensive experience in concept selection, front-end engineering, HV & LV detail design, construction, and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and wastewater pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).

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