The neutral point of a generator is usually earthed to facilitate protection of the stator winding and associated system.
Earthing also prevents damaging transient overvoltages in the event of an arcing earth fault or ferro resonance.
For HV generators, impedance is usually inserted in the stator earthing connection to limit the magnitude of earth fault current.
Simple logic of reactance diagram is that simplified equivalent circuit of power system in which the various components of power system are represented by their reactances.
The reactance diagram can be obtained from impedance diagram if all the resistive components are neglected.
The main function of an interposing CT is to balance the currents supplied to the relay where there would be an imbalance due to the ratios of the main CTs.
Interposing CTs are equipment with a ratio that can be selected by the user to achieve the balance required.
An interposing CT is installed between the secondary winding of the main CT and relay.