What's different between CT and VT,where we can use this devices in electrical network?

These are Instrument Transformers, CT measures & Protect Current passing through electrical system and VT measures & protects Voltage across the system. Both Devices are used at Primary(Main) Voltage Network and through secondary current & voltage (Relays), we control & protect the abnormality of electrical network.

CT: current transformer, PT: potential transformer. Both are instrument transformers and used for metering as well as protection purposes.

CT IS A CURRENT TRANSFORMER IS ALSO KNOWNS AS INSTRUMENT TRANSFORMER.

CT USED IN METERING AND PROTECTION OF EQUIPMENT MAINLY IN SUBSTATION....

VT OR PT . POTENCIAL TRANSFORMER IS ALSO USED IN METERING PURPOSE AND PROTECTION OF EQUIPMENT..

These devices used in power systems in order to protect system . If there is change of current In input bus side to ouput bus side then relay sens this abnormal condition from ct and trips system through cb.

CT and VT are Instrument Transformer..CT is used for measuring Current across the circuit..VT is used for measuring Voltage across circuit... and We use it for measuring and Protection..

CT is for Current transformer and VT is for voltage transformer . CT is used to measure the current drawn by a device or system . VT is used to measure the voltage across system or device . As the voltage is high value like220V so we transform it to3v or5v and then convert it to DC voltage n measure it using Controllers like Microcontroller , PLC or any other device which accept DC volt . After measuring Current and voltage we can easily control and monitor a system for protection purpose and safety or any other application. 

Current transformers and potential transformers both are used for metering purposes 

more over they also provide a protection for the system

CT means Current transformer. CT measures only Curent

VT means Voltage Transformer. VT measures only Voltage

It is used in Power Transformer, Watt hour meter.