Power transformers may have different cooling systems, depending on the customer's specific requirements. For example :
ONAN (Natural circulation of Oil / Natural circulation of Air)
ONAF (Natural circulation of Oil / Forced circulation of Air)
ODAF (Directed circulation of Oil / Forced circulation of Air)
ODWF Directed circulation of Oil / Forced circulation of water)

Different types of regulation are at disposal :
Off circuit tap changer
On-load tap changer