Function of Unit Auxiliary Transformer (UAT):
Unit Auxiliary Transformer is nothing but a step down transformer, which is connected parallel to the generator before the generator circuit breaker. Unit Auxiliary Transformer Is used to deliver the power to the motor, fans, pumps, lightings etc.
Purpose of Unit Auxiliary Transformer (UAT):
Consider above circuit diagram of a generator, circuit breaker, distribution transformer and other auxiliary loads. In this, you have a small generator rating of 1 MW and a transformer of 1.5 MVA rating. Now, You have started a generator and built upto rated voltage. Then you are going to charge the transformer using the small generator. After closing transformer breaker using the generator voltage, the voltage across the generator gets drop down due to the transformer’s starting high inrush current. This high inrush current causes severe voltage drops across the generator terminal and significantly damage the generator. Therefore, to avoid such condition the transformer is connected in parallel with the generator before the generator circuit breaker. Whenever, you excite the generator, the transformer also gets charge parallelly. Such Transformers are called Unit Auxiliary transformer. It is just used for delivering power to the auxiliary loads.
Note: All the protection would be paralleled with the generator. The UAT generator Differential protection is the popular protection used to protect both transformer and generator winding.
- Home Island Power plants, where the grid supply is not available.
- Charging a transformer with a small Diesel generator