A PT, Potential Transformer, can be thought of as a pure transformer with primary and secondary windings; PT’s are sometimes referred to as magnetic transformers due to the fact that their mode of operation is purely magnetic. It is used to step-down the input voltage from a power line to a voltage level that can be processed by metering devices and protection relays in a substation.
A CVT or CCVT, Capacitor Coupled Voltage Transformer, is made with two capacitor sets acting as a voltage divider that brings the line, actually the phase, voltage down to around 12-20 kV then this voltage is fed to a relatively small transformer for the voltage signal to be processed.
Also the CVT’s can be tuned to the fundamental frequency of the line, and the capacitance prevents the inductive fire-back of the coils in the transformer when a breaker trips. PT’s can’t provide such advantage.
Some CVT’s are also used to tune to PLCC, Power Line Carrier Communication, which is a signal transmitted over power lines providing carrier signal for Distance protection and communication between the connecting Substations.
Thus CVT shall be used when voltage level is high and we need Power Line Carrier Communication. It may be that CVT is just used for Metering and Protection Purpose but when a CVT is used in the Transmission Line then it also serves the purpose of PLCC.