GE L60 Instructions Manual page 659

CHAPTER 9: THEORY OF OPERATION
It is obvious from the Sequence Network Outputs figure, that a similar fault on a different phase yields different results. This
is illustrated in the following right figure where a phase-A-to-ground fault at the same location is analyzed. As noted
earlier, the integrator timers in phase comparison schemes are generally set for about 3 ms. This permits tripping on
internal faults with as much as 115° between the phase angles of the currents entering both ends of the lines. On this
basis, only excitation by I_2 – (0.20) ´ I_1 proves satisfactory for the two cases studied in the two figures that follow.
Figure 9-7: Vector relationship in a two-terminal faulted
line (B-to-G)
Actually, only two simple faults were investigated. Different results would have been obtained for these same kind of faults
if the relative magnitudes of load current, positive sequence fault current, and zero sequence fault current had been
assumed differently. Also, for the values of currents assumed, different results would be obtained for other types of faults.
In addition, if different combinations and weighting factors of the sequence components had been investigated still
different answers would have resulted. In the proper selection of sequence components and weighting factors for Mixed
Excitation phase comparison, the following points must be considered:
• Whatever combination and weighting factors are employed, the application rules need to be simple enough to make
the application practical
• Use the fewest number of sequence components
• The effects of load current must be minimized. Thus, weight negative and/or zero sequence components over the
positive sequence components
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
Figure 9-8: Vector relationship in a two-terminal faulted
line (A-toG)
OVERVIEW
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