Answer Posted / guest

whats the basic concept is to provide accurate protection
against fault for the system!
for alternator:
to do so we need to consider the extreme part from top of
the phase to neutral (as potential near neutral and earth is
low the most chances of such fault is to occur near the
neutral). now if the fault occurs near the neutral point,
the covered winding between the fault point and neutral
produce small magnitude of the fault current;
to measure that small signal current we require extreme
sensitive relay system!
if we use such then it ll also detect the unnecessary faults
such as inaccuracy of C.T or saturation of C.T etc. to avoid
malfunction: this is the reason why we protect only 80 to 85
% of winding and left 15% of winding from neutral side
unprotected!
so only the differential protection is not suffice!!
REF: to protect this remaining 15% we need other type of
earth fault protection scheme(neutral connected through
resistor or neutral connected via transformer)! (restricted
earth fault protection is designed to 15% of rated phase
current)

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