A Booster transformer is one that is often used toward the end of a power line to raise the voltage to the desired value. It is used for controlling the voltage of a feeder at a point far away from the main transformer.
The secondary of the booster transformer is connected in series with the line, and its primary is supplied from the secondary of the regulating transformer. The output winding of the regulating transformer is so connected to the primary of the booster transformer that the voltage injected in the line voltage is in phase with the supply Voltage.
By changing taps on the regulating transformer, the magnitude of line voltage can be changed and thus feeder voltage can be regulated. The rating of regulating transformer is only a fraction of that of the main transformer.
A Booster transformer is used in railways for eliminating the flow of stray current. The stray current disturbs the communication system and also damage the electronic devices of the trains passing through them.