it is used for transmission line.its are connected to the 
parallel with line.

Yes both are same. They are used to measure the voltage in 
the electric circuits. Their main role is to condition 
(step down) the voltage to be measured to levels suitable 
for the measuring instrument.

Yes Potential and Voltage transformer are same and their 
use are not only resticted to measuring purpose only but it 
is further supplied to relaying panel or to trip circuit so 
that the protection purpose and measuring are 
simultaneously get satisfied accordingly.

Yes VT and PT are same.

it is mainly used for Protection and metering.

the Tx.line have high voltage. if you want to measure the 
voltage it should also designed with HV. it leads more cost.
therfore we are steping down the voltage to a low level. it 
is usally 110V.

also it is used for distance protection. the distance 
protection is based on Impedance. Impedance can be 
calculated by using the formula Z= Voltage/Current.

VT and PT are the same thing. they step down high voltages 
(132kV,33kv, 11kV etc) to a nominal value usually 110v 
which is used to energize substation instruments such as 
protective relays.importance of this is not to have bulky 
relays which can withstand high voltages.

vt's are voltage transformers, based on inductive 
principle,pt's are potential transformer's and covers vt & 
cvt (capacitive voltage transformers).applications are same 
but working is different.

yes it is same but cant use for same application

Yes both r same. They r used to step down the high voltages 
to a lower voltages that can be measured by instruments 
like ammeter, voltmeter,wattmeter & relays which r used for 
protective purposes.