What is meant by ONAN, ONAF and OFAF capacities of a Power Transformer?

i hope you reffering to Transmissioin level 40MVA and above Power transformer (few cases 25MVA  and above) Oil cooled Transformers.Transformer cooling system becomes of high importance in recent infrastructre design ,where Power Transformers can be INDOOR (underground ,inhoused in other development or partially underground ) or ENCLOSED due to explossioin hazard in dense public areas.Other on ground standalone design doesnt matter much to conventional design.

ONAN,ONFA,OFAF is quite basics.Transforemr windings are cooled by Oil and in turn Oil is cooled by air and as per arrangement you can have Fans to force the air and pump to circulate the oil.OFAF is globally adopted design unless special cases.

The design matters how to assure enough air at raditor FANS or RADIATOR surface.

some of the utilities are opting for GIT gas insulated Transformer instead of Oil cooled , or we need additional ventilation or cooling system for Transformer vault .i cannot design the system here but its a basic idea to you .

refer NEC 450 if you can for transformer vault.

ONAN means Oil Natural Air Natural (When the oil is circulated in oil fins without any forced measure, it is called oil natural and air on the fins goes by nature.

ONAF means Oil Natural Air Forced (The oil will be circulated without any forced measure but the air will be blown through fans). This increase the capacity of power transformer by 33% of the base capacity.

OFAF means Oil Forced Air Forced (The oil and air will both be circulated through fans). This will increase the capacity of power transformer by 66% of the base capacity

ONAN means Oil Natural Air Natural Capacity 

ONAF means Oil Natural Air Forced Capacity 

OFAF means Oil Forced Air Forced Capacity 

ONAN<ONAF<OFAF

ONAN,ONFA,OFAF is quite basics.Transforemr windings are cooled by Oil and in turn Oil is cooled by air and as per arrangement you can have Fans to force the air and pump to circulate the oil.OFAF is globally adopted design unless special cases.