What are potential transformer?

potential transformer is that which transforms one range of voltage (potential difference ) to another range without changing frequency.

A 'potential transformer' is the North American term for what, elsewhere, is called a 'voltage transformer'. It, together with a current transformer, belongs to a class of Transformers known as 'instrument transformers'.

Potential transformers are used to (1) reduce the value of a high-voltage supply to a lower value (often 120 V) so that it may be read by ordinary voltmeters, and (2) to electrically-isolate the voltmeter from the high-voltage source for the purpose of safety.

In electricity substations, potential transformers are also used to provide an input to certain types of protective relay.