What is the purpose of unit transformer, unit auxillary transformer and station transformer in power plants?

auxillary transformers are those which are used for the auxillary machines which are used inside the plant to run the plant.Station transformers are that transformers in which input is from the generator and output goes to the bus bar

Unit transformer for step low voltage to Medium voltage

Auxillary transformer for Give auxillary supply to generator 

Station transformer for step medium voltage to high voltage for Network line voltage .