Generator transformers are employed in generating stations to connect the power station to the transmission system. Generator transformers step up the generator output at low voltage to the voltage at which the transmission system operates.
The rating of the generator transformers is equal to the rated output of the generator. Generator Circuit Breaker (GCB) is generally located between the generator and generator transformer.
Virtually all the transformers connected to the generators up to and including 500 MW are of three phase construction. But for generators rating above 500MW output, the generator transformers comprise of banks of three single phase units. All the generator transformers will have delta connected Low Voltage (LV) winding system and star connected High Voltage (HV) windings. With single phase banks, the delta connection of the LV winding system is obtained either by an oil filled busbar box extending across all the three phases which includes the necessary LV line terminals for the connection to the generator bars or by a system of air insulated phase isolated busbar trunking.Generally these generator transformers are connected directly to the generating units. The connections between generator and generator transformer is made through Isolated Phase Bus Duct (IPBD).
In order to cater the voltage drop across generator transformer while carrying full output from the connected generator, generator transformers will have the turns ratio giving an enhanced no load voltage on principal tapping eg: 22kV/432kV for supplying 400kV at full load. This contrasts with the bilk supply point or super grid interconnection transformer, where the no load voltage ratio of the transformer on principal tapping is simply the ratio of the two system voltage between which the transformer is connected eg: 400/132kV or 275/33kV. For both the examples above, the tapping range is adjusted to cater for the anticipated varying voltage regulation.
To facilitate the adjustment of the output voltage to mach the transmission system operating requirements which vary from time to time (depending on the demand for both real power MW and reactive power MVAr), generator transformers are fitted with on-load tap changers to adjust the voltage variations in the HV side of the generator transformers. These tap changers are connected to the tap windings located at the HV neutral end. For example, on 800 MVA transformer, a typical range of voltage variations would be +2% to -18% of the transformers with 18 discrete steps of 1.11% to provide 19 tapping positions.
Another important distinction between the generator transformer and transmission transformer practice concerns security of the supply. For transmission transformers, particularly with bulk supply point transformers, the security of the supply is achieved by employing two or more transformers operating in parallel. If a generator transformer fails in service, the output of the associated generator is lost to the system. Therefore excellent reliability of generator transformers is required for the availability of the generated output of the power station.