Generator Step Up Transformers-An Overview

Generator Step-up Transformer units are used to increase the voltage of a generator and connect the supply to a bus bar.  For instance, if the generator has a voltage of 3300V and the busbar a voltage of 6600V, the Step Up Transformer will have a ratio of 3300/6600.
Generator Transformers are also used to limit the fault level of the generator in case of a fault.
Transformers used for these applications are called Generator Step-up Units.  These Transformers are designed to operate at near full load. 
Every time a fault occurs in the grid, these transformers are subjected to stress.
As these transformers are connected to the grid, they are subjected to constant voltage fluctuations.     Voltage fluctuations cause stress to the transformer windings.  These windings have to be specially designed to withstand these stresses.  Localized heating is another issue which is a result of over excitation by the generator during voltage fluctuations.  Localized heating can damage the metallic accessories of the transformer. 
Hence, the magnetic circuit is specially constructed to have very low leakage flux. 
Generator Step Up Transformers are available in both single and three phase units.  They can also be designed to withstand wide temperature variations.