A power transformer is a static machine used for transforming power from one circuit to another without changing frequency. This is a very basic definition of transformer. Generally 10 MVA and 36 kV and above can be named but this can be shown differences from a manufacturer to another. There are many types and classifications according to core, structure, functionality etc.
The manufacturing process of power transformer averagely takes 4-6 months therefore all manufacturers have to consider with the highest discipline for all electrical and mechanical design parameters.