It is difficult to size the current transformer at high voltage side of the excitation transformer,since the extreme short circuit current would take place at the large generator outlet of a generator transformer set.So that to decrease the accuracy limit factor an idea to take the protective duty of internal faults cooperating with unclosed differential protections of the generator transformer/main transformer and the relay protection special for the excitation transformer is presented.And some rated transformation ratios of the current transformer are proposed for reference.Meanwhile comparing the differential protection with the over current protection of the excitation transformer,the over current protection is suggested to simplify the relay protection.