Thank you for your replies and help. Also I managed to find some useful papers on the subject on the internet and I think I understand the P.U system now but still I have to apply it to a real life case to determine the short circuit current in a system.
However I would like to call on your help again and ask if anyone can tell me the imp/Km of standard 11kV, 33kV,66kV and 132kV. overhead transmission lines. Thank you.