the negative sequence current of fault currents; On the
other hand, when a phase-to-phase fault occurs out of the
protection zone, sequence currents depend on the
sequence impedance of the motor itself. To a motor in
operation, its positive sequence impedance ZID is bigger
than its negative sequence impedance Z2~ due to the slip
frequency; as a result, 12> II. Considering the reliable
coefficient, we will lock the protection as 12> 1 .211. With
this method, we can ensure the sensitivity to the slight
unbalanced faults of motors and distinguish the fault in
or out of protection zone.