The transformer is a device used to transform either the voltage or current in a circuit from one value to another. There are power, distribution and instrument transformers. Power and distribution transformers are installed in power generating and distribution centers for supplying electricity to different types of users. They are usually large.

An instrument transformer on the other hand, is used for measuring various parameters of the power supply in distribution. They come as potential and current transformers. The primaries of these transformers are connected to supply sources and the secondary to various instruments or protective devices in the installation. The secondary feed the meters for measuring voltage, current, power, energy and power factor and system protective relays.