What is Booster Transformer ?

When electric power is transmitted over long distance, voltage level falls due to the losses in the power line. Hence it is desirable to control the voltage of transmission line. In order to obtain constant voltage at the consumer’s end, voltage drop must be compensated to obtain a desired value. For that purpose, booster  transformer is used. As the name suggest, booster transformer compensate the voltage drop by  increasing the voltage to the desired value. It raises the voltage of a feeder at a point far away from the main transformer. The secondary winding of booster transformer is connected in series with the power line whose voltage is to be increase whereas its primary is connected from the secondary of regulating transformer (see the fig).

Booster Transformer

Let takes a look at the above diagram. The voltage at AA is maintained by tap changer in the main transformer. However, there may be a voltage drop between the feeder AA and BB terminals due to long feeder. Feeder voltage can be regulated by changing the taps on the regulating transformer. Regulating transformer with tap arrangement supply power to the booster transformer. Booster transformer is connected in such a way that the voltage generated in its secondary is in phase with the power line. In simple words, the output voltage of booster transformer (Vb) is in phase with the supply voltage (Vs). In this way, booster transformer raises the voltage of power line to the desired value.

Booster Transformers in Railway Traction system:

Booster transformers are also used in railway traction system. Let consider the schematic diagram of traction system having traction transformer. One side of traction transformer (TS) is connected to the catenary (CAT) wire while the other part which is the return is connected to the return conductor (RC) and running rails (RR). A booster transformer (BT) is also there whose one side is connected in series with the catenary wire while the other winding is connected in series with the return conductor.

Booster transformer in railway traction system

Let first consider the case without booster transformer (fig B). In that case, current flows from the traction transformer to the catenary wire and then goes to the rails through locomotive ( I shows the direction of current). When the energy is used by motors, some amount of returning current come back to the traction substation(its source) through return conductor while the other remaining current comes from the running rails. Now, the returning current (stray current) coming from the rails causes inductive interference in nearby cables of communication system and in signal equipment’s which should be avoided. In order to minimize the interference effects, booster transformers are used.

Now,  consider the railway traction system with booster transformer (Fig A). The ampere turn ratio of (BT) is 1:1 which simply means that the current in the primary winding is almost equal to the current in the secondary side. In that case, booster transformer causes the returning current from the rails to divert its path towards the return conductor through connecting wire. This means that the current in the return conductor is almost equal to the current in the catenary wire. This also means that very negligible amount of stray current flows in the earth connected rails due to which interference effects are minimized. Hence addition of booster transformer makes the returning current from the rails to move through the return conductor.

Advantages of Booster Transformer :

Booster transformer maintains the uniform voltage throughout the power line that have distributed load.

It is found where electric power is supplied to a very long distance.

Regulating equipment is independent of the main transformer. Hence any internal failure or damage of regulating equipment (booster and regulating transformer) will not affect the main transformer.


That’s all. Hope this will helps you.

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