Transformer is an electrical device and the job is transferring electrical energy from one to another electrical circuit, without changing the frequency. The attitude is to change voltage and current. Transformers either increases or decreases AC voltage.
Those are could satisfy variety of needs There are so different types of transformers in use today.
Distribution Transformer, Power transformers
Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200 MVA.
Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.
An autotransformer has one winding that is tapped at some point along the winding. Voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. The equivalent power rating of the autotransformer is lower than the actual load power rating. These type are suitable for:
•Power transmission and distributions
Capacitor voltage transformers
Capacitor Voltage Transformers (CVT), are used in power systems to step down extra high voltage signals and provide a low voltage signal, for metering or operating a relay.