Exciter Shorted

Our alternater have some probleum iam explining briefly,

while alternator with load of 6MW that time Rotary earth fault 1 activated, we triped the unit then checked exciter winding got burned,we changed the exciter with another one again we synchronised (Auto synchronisation)with load of 2mw after faction of seconds we got alarm of rotary earth fault 1 we triped the unit and checked again exciter coils got shorted,

we didnt recognised the fault from where........................i have dought about before burning the exciter why we did not get any alarm to avoid equipment damage, after damage only we are getting the alarm from rotar earth 1.....

Please kindly expline me proper reson for frequently shorted in exciter,after short only rotary earth fault 1 was activated