The term power transformer is generally used to refer to transformers with a rating of 500kVA or greater.
Power transformers are used in distribution systems wherever there is a need to interface between different voltage levels i.e. to step up and step down voltages.
Power transformers are generally of the liquid-immersed type. However, Power transformers used for indoor applications may be air-cooled.
Based on the size ranges, transformers can be classified into three types.
Small Power Transformers : 500 to 7500kVA
Medium power transformers : 7500 to 100MVA
Large power transformers : 100MVA and beyond
The average life of a transformer is around 30 years.