Oil-immersed transformer cooling methods are consistent on 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.
|First letter – Internal cooling medium in contact with the windings
|mineral oil or synthetic insulating liquid with fire point ≤300Ã‚Â°C
|insulating liquid with fire point >300Ã‚Â°C
|insulating liquid with no measurable fire point
|Second letter – Circulation mechanism for internal cooling medium
|natural convection flow through cooling equipment and in windings
|forced circulation through cooling equipment (i.e., coolant pumps), natural convection flow in windings (also called nondirected flow)
|forced circulation through cooling equipment, directed from the cooling equipment into at least the main windings
|Third letter – External cooling medium
|Fourth letter – Circulation mechanism for external cooling medium
|forced circulation [fans (air cooling), pumps (water cooling)]
ONAN/ONAF. The transformer has a set of fans which may be put in service as desired at high loading. The coolant circulation is by natural convection only.
ONAN/OFAF. The coolant circulation is by natural convection only at base loading. However, the transformer has cooling equipment with pumps and fans to increase the power-carrying capacity at high loading.
In IEEE Std C57.12.00 – 1993, method of cooling was only represented by a two-letter or three-letter code. Equivalent old – new cooling designations are given below:
1. IEEE Std C57.12.00-2000 Liquid Immersed Distribution, Power and Regulating Transformers
2. IEC 60076-2: 1993, Power transformersÃ¢â‚¬â€Part 2: Temperature rise.