The voltage transformer or potential transformer is called PT or VT, is an instrument transformer that is used for metering and protection purpose. Its function to make a low voltage from the high voltage of a certain ratio, so that the large voltage can be easily measurable or sensed by using this low voltage by low voltage capacity costly equipments like relay, PLC.
The voltage transformer is a transformer that is connected to a very large impedance (use in virtual open circuit mode). A voltage transformer is designed to give the secondary a voltage which is proportional to that applied to its primary.
For a voltage transformer (VT) the ratio of voltage between the primary and the secondary is constant. The secondary voltage is independent of the load. If Z is the load impedance of a VT, then at the secondary, we have: