Consider a power system shown below:

Given that:

The positive sequence impedances are

 and

3-phase Base MVA = 100

Voltage base = 400 kV (Line to Line)

Nominal system frequency = 50 Hz

The reference voltage for phase ‘a’ is defined as

A symmetrical three phase fault occurs at centre of the line. i.e. point 'F’ at time . The positive sequence impedance from source  to point 'F' equals 0.004+ j0.04pu. The waveform correspond phase 'a' fault current from bus X reveals that decaying dc offset current is negative and in magnitude at its maximum initial value. Assume that the negative sequence impedances are equal to positive sequence impedances, and the zero sequence impedances are three times positive sequence impedances.

The instantof the fault will be