CHAPTER 9: THEORY OF OPERATION OVERVIEWL60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-99Figure 9-6: Sequence network outputsThe phase positions of the sequence network outputs differ depending on the phase or phases that are faulted as well asthe 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-groundfaults.Observe from the table presented that positive sequence currents are available for all kinds of faults, negative sequencecurrents are available for all but three-phase faults, and zero sequence currents are available only for faults involvingground. Thus, it appears that if one single sequence component of current were to be selected for use to make the phasecomparison, then the positive sequence component suffices. Actually, this is not the case in many if not most of theapplications 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 oneend is in-phase with that entering the other end. This is a tripping situation for the phase comparison scheme. However,any load flow across the line during the fault produces a positive sequence component of load current entering one end ofthe line that is 180° out of phase with that entering the other end (that is, the positive sequence component of load currententering one end is in phase with that leaving the other end). This is a non-tripping situation for the phase comparisonscheme. The phase position of the load component relative to the fault component depends on such factors as thedirection of the load flow, power factor of the load flow, and the phase angles of the system impedances. The phaseposition of the “net” (load plus fault) positive sequence current entering one end of the line relative to that entering theother end depends on these same factors plus the relative magnitude of the fault and load components of current.
