A Transformer is a static apparatus, with no moving parts, which transforms electrical power from one circuit to another with changes in voltage and current and no change in frequency. There are two types of transformers classified by their function: Step up Transformer and Step down Transformer.

A Step up Transformer is a device which converts the low primary voltage to a high secondary voltage i.e. it steps up the input voltage. A Step down Transformer on the other hand, steps down the input voltage i.e. the secondary voltage is less than the primary voltage.

The voltage from the Power Plant or Generation Station is around 20kV. In order to transmit this voltage over long distances, it is stepped up to 440kV using a Step up Transformer. This voltage with increased levels is then transmitted to a distribution station. A transformer in which the output (secondary) voltage is greater than its input (primary) voltage is called a step-up transformer. The step-up transformer decreases the output current for keeping the input and output power of the system equal.

Step-down transformer is made up of two or more coil wound on the iron core of the transformer. It works on the principle of magnetic induction between the coils. The voltage applied to the primary of the coil magnetise the iron core which induces the secondary windings of the transformer. Thus the voltage transforms from primary to the secondary winding of the transformer.