9-6 L60 Line Phase Comparison System GE Multilin9.1 OVERVIEW 9 THEORY OF OPERATION9Of course, the output from the receiver will depend on the keying of the remote transmitter. The transmitters at all line termi-nals are keyed in the same manner. They are keyed ON by an output from FDL (Fault Detector Low-set) and keyed OFF bythe squaring amplifier via AND2 during the positive half cycles of current. The FDL function is required at all terminals in allphase comparison blocking schemes to initiate a blocking signal from the associated transmitter. This is received at theremote receiver and blocks tripping via the comparer during external faults. FDL has a more sensitive setting and thereforeoperates faster than the remote FDH function. It is obvious from Figure 9–4 that if an external fault occurred, and FDL didnot operate at least as fast as the remote FDH, false tripping could occur because of the lack of receiver output. In generalFDL is set so as not to pick up on load current but still with a lower pick up than FDH so that it will operate before FDH. Foran internal fault, the currents entering both ends of the line are in phase with each other. Thus, during the half cycle that theSQ AMP is providing an input to AND1, the associated receiver is producing no output, and so tripping will take place atboth ends of the line.For an external fault, the current entering one terminal is 180° out of phase with the current entering the other terminal.Under these conditions, during the half cycles when the SQ AMP is producing outputs, the associated receiver is also pro-viding an output thus preventing an AND1 output. No tripping will take place.9.1.3 VARIATIONS IN PHASE COMPARISON SCHEMESThere are a number of different phase comparison schemes in general use today and while all of these employ the samebasic means of comparison described above, significant differences do exist. These differences relate to the following:• Phase comparison excitation (component or current to be compared).• Pure phase comparison vs. combined phase and directional comparison.• Blocking vs. tripping schemes.• Single vs. dual phase comparison.9.1.4 PHASE COMPARISON EXCITATIONa) DESCRIPTIONBefore discussing this subject, it is well to consider what takes place in terms of the currents that are available for compari-son when a fault occurs on a power system. The table below lists the sequence components of fault current that are pres-ent during the various different kinds of faults while Figure 9–6 illustrates the relative phase positions of the sequencecomponents of fault current for the different kinds of faults and the different phases involved.Figure 9–6 shows the relative phase positions of the outputs of a positive sequence network, a negative sequence network,and a zero sequence network all referenced to phase A. The transfer functions of these three networks are given by the fol-lowing equations.(EQ 9.1)Table 9–1: FAULT TYPESTYPE OF FAULT SEQUENCE COMPONENTSPOSITIVE NEGATIVE ZEROSingle-Phase-to-Ground yes yes yesPhase-to-Phase yes yes noDouble-Phase-to-Ground yes yes yesThree-Phase yes no noI113--- I a I b 120°∠ I c 120°–∠+ +( )=I213--- I a I b 120– °∠ I c 120°∠+ +( )=I013--- I a I b I c+ +( )=
