What is the basic working principle of a CT?

CT is no different than a usual transformer, which either steps up or steps down voltage and hence its principle of working is electro magnetic induction. CT is used to measure current of high magnitude. It is basically a step up transformer whose primary is connected in series with the line whose current is to be measured and an ammeter is connected across the secondary. The voltage is stepped up and the current is stepped down. The high magnitude current is calculated from Line1/Line2 = N2/N1 and the ammeter reading.

The current transformer consists of a primary winding, a magnetic core and a secondary winding. The alternating current in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary winding circuit. CT in my studies was used to protect load from high current.----------------------------------------N.B

CT can be Computed tomography which is completely different term. 

 

If your question is about "current  transformer" then it is used to measure the current consumed by the AC high load and it is always connected in series to the load as Ammeter.

 

it is same as the normal transformer which is used but the thing is it reduces the high current being produced in the secondary winding and provides convenient transmit of power and also an ammeter is placed to see the ampere reading