With the increasing of power system capacity， the capacity of single main equipment is often much less than that of whole system. The parameter selection of protective current transformer（CT） becomes difficult faced by relay protection engineer. If CT is selected according to rated capacity of main equipments， CT may saturate deeply when serious internal fault happens. In this case the performance of relay protection should be studied carefully. The performance of relay protection has been studied with methods of experiment and theoretic analysis when internal fault causes CT deeply saturated. According to the experimental results， various factors influencing the relay protection actions have been analyzed， from the aspects of CT parameters， the secondary circuit and the relay protection. To ensure reliable relay protection， CT must meet the basic requirements. As long as these basic requirements satisfied， it is feasible to choose current transformer according to the main equipment capacity.