In Thermal and Nuclear Power Plants different types of power transformers are employed:
Types of Transformers:
This is the main power transformer employed in the power plant. It steps the voltage from 21kV to 230 or 400kV and delivers the power. Stepping up the voltage reduces the transmission losses which occur during the power transmission to long distances. The rating of this transformer (MVA rating) will be almost equal to the alternator or generator rating.
Unit Auxiliary Transformers:
These transformers are connected to the Generator Transformer bus. These transformers steps down the voltage from 230kV or 400kV to 6.6kV (230/6.6kV or 400kV/6.6kV) and supply the power to the electrical auxiliaries present in the plant (motors, drives, lighting and other plant loads).
Station Transformer or Startup Transformer:
This transformer provides electrical power to the plant during start up when no supply is available to the plant (generator is not operating). It also steps down the voltage like unit auxiliary transformers and supply power the lant auxiliaries.
Station Transformer and Unit Auxiliary Transformers are connected to the grid, so that they can get power when Turbo-Generator is not in operation and supply power to the plant auxiliaries.
These are small distribution transformers supply power to plant electrical auxiliaries rated at 415V by stepping down the voltage (6.6kV/415V).