GE Multilin L60 Line Phase Comparison System 9-59 THEORY OF OPERATION 9.1 OVERVIEW9Figure 9–4: THREE-TERMINAL LINE PHASE COMPARISONFigure 9–5: SINGLE-PHASE COMPARISON BLOCKING SCHEME PRINCIPLEIt will be noted from Figure 9–5 that AND1 (the comparer) at each end of the line compares the coincidence time of the pos-itive half cycle of current with the absence of receiver output. This is initiated only when a fault is present as indicated by anoutput from FDH (Fault Detector High-set). FDH is set so that it does not pick up on load current but does pick up for allfaults on the protected line section. Thus, when a fault occurs, FDH picks up, and if the receiver output is not present for 3milliseconds during the positive half cycle of current out of the mixing network, a trip output will be obtained.
