Three 6.6 kV, 3-$\varphi$, 10 MVA alternators are connected to a common bus. Each alternator has a positive negative and zero sequence reactances as 0.15 p.u., 0.1 p.u.,0.05 p.u. respectively. A single line to ground fault occurs at point F, as shown in figure below.



$I_f$ is the fault current when all the alternator neutrals solidy grouned $I_f^{\prime }$ is the fault current when only one neutral is solidly grounded and other two neutrals are isolated. Then the value of $|I_f-I_f^{\prime }|=$________kA.

1. 6.512