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.
| 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:
| 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.