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.
|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|
|Fourth letter - Circulation mechanism for external cooling medium|
|F||forced circulation [fans (air cooling), pumps (water cooling)]|
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:
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.