ABSTRACT
Pilot relaying is characterized in that it cooperates with a communication channel to identify the conditions that exist locally for a remote transmission line terminal. It assures the ability to trip both terminals at high speed for all faults on the protected circuit. Information on the phase angle of the single-phase filter output voltage is accomplished with an on–off system, or with a frequency-shift system. Current differential schemes extend the phase-comparison principle by including magnitude as well as phase relationship in the comparison of the quantities at the two transmission line terminals. The directional-comparison type of relaying uses the fundamental concept that directional units at all terminals of the protected line must agree that a fault is internal for tripping of circuit breakers to be initiated. The segregated-phase comparison system is ideal for single-pole tripping because of its unerring ability to identify a faulted phase.
