Generally, a single line-to-ground fault on a transmission line occurs when one conductor drops to the ground or comes in contact with the neutral conductor. Such types of failures may occur in power system due to many reasons like high-speed wind, falling off a tree, lightning, etc.
Suppose the phase a is connected to ground at the fault point F as shown in a figure below. Ia, Ib and Ic are the current and Va, Vb and Vc are the voltage across the three phase line a, b and c respectively. The fault impedance of the line is Zf.
Since only phase a is connected to ground at the fault, phase b and c are open circuited and carries no current; i.e fault current is Ia and Ib = 0, Ic = 0. The voltage at the fault point F is Va = Zf Ia.
The symmetrical component of the fault current in phase “a” at the fault point can be written as