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[boby]

hi friends,
we have one 21 MW, 11KV,50HZ,3000RPM BHEL make static excitation type alternator. for that we have 2 rotor earth fault relays. they are stage 1 and stage2.
stage 1 of type VAEM 21 MK 2
stage 2 of type CAEM 33
there are small brushes mounted on the rotor for rotor earth fault protection.
my doubt is how rotor earth fault protection can be done..............or any other different methods for rotor earthfault protection......... explain briefly