GE L60 Instructions Manual page 657

CHAPTER 9: THEORY OF OPERATION OVERVIEW
Figure 9-6: Sequence network outputs
The phase positions of the sequence network outputs differ depending on the phase or phases that are faulted as well as
the type of fault. For example, while the positive, negative, and zero sequence components are all in phase for a single-
phase-A-to-ground fault, they are 120° out of phase with each other for phase-B-to-ground, and phase-C-to-ground
faults.
Observe from the table presented that positive sequence currents are available for all kinds of faults, negative sequence
currents are available for all but three-phase faults, and zero sequence currents are available only for faults involving
ground. Thus, it appears that if one single sequence component of current were to be selected for use to make the phase
comparison, then the positive sequence component suffices. Actually, this is not the case in many if not most of the
applications because of the presence of through-load current during the fault.
For a single-phase-to-ground fault on the protected line, the positive sequence component of fault current entering one
end is in-phase with that entering the other end. This is a tripping situation for the phase comparison scheme. However,
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any load flow across the line during the fault produces a positive sequence component of load current entering one end of
the line that is 180° out of phase with that entering the other end (that is, the positive sequence component of load current
entering one end is in phase with that leaving the other end). This is a non-tripping situation for the phase comparison
scheme. The phase position of the load component relative to the fault component depends on such factors as the
direction of the load flow, power factor of the load flow, and the phase angles of the system impedances. The phase
position of the "net" (load plus fault) positive sequence current entering one end of the line relative to that entering the
other end depends on these same factors plus the relative magnitude of the fault and load components of current.
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-9