What are the different transformer cooling methods?

Transformer cooling methods are designated by four-letter codes. ONAN (Oil Natural Air Natural): oil convection, natural air cooling - simplest, for small transformers. ONAF (Oil Natural Air Forced): adds fans for higher ratings. OFAF (Oil Forced Air Forced): oil pumps plus fans for large transformers. ODAF (Oil Directed Air Forced): directed oil flow for maximum efficiency. Dry-type transformers use AN or AF cooling. Cooling determines transformer rating - forced cooling increases capacity by 25-50%.