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 reactance.
The reactance diagram can be obtained from impedance diagram if all the resistive components are neglected.
So, from figure each of the component is represented by its reactance.
It is usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors.
Voltage sag is the short reduction in the RMS voltage between 0.1 to 0.9 pu for a duration of 0.5 cycle to 1 minute
The connection of CT on primary and secondary side is always reverse to the connection of transformer.
To protect the power transformer with star-delta against fault, the type of connection of CTs should be in the delta–star connection otherwise the relay will operate when an external fault occurs due to zero sequence currents
Z = √(L/C)
C = L/Z²
C = 50*10⁻⁹/(50*50)
C = 0.02 nF
C = 20 pF