GE UR series Instruction Manual page 445

8 THEORY OF OPERATION 8.1 OVERVIEW
Figure 8–6: SEQUENCE NETWORK OUTPUTS
It is interesting to note that 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.
It will be observed from Table 8–1 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, the positive sequence component would suffice. Actually, this is not the case in many if not most of the appli-
cations 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
will be 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 will produce 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 cur-
rent 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 will
depend on these same factors plus the relative magnitude of the fault and load components of current.
In general, the heavier the fault current and the lighter the load current, the more suitable is the use of pure positive
sequence for phase comparison. Heavier line loadings and lower fault currents will tend to make the scheme less apt to
function properly for internal faults. Thus, pure positive sequence phase comparison appears practical only in a minority of
the cases and so is not suitable for a scheme that is to be generally applicable.
Significant negative sequence currents are present only during faults, they are present in all but balanced three phase
faults, and there is no significant negative sequence component of load current. All this combines to make pure negative
sequence ideal for phase comparison except that it will not operate for balanced three phase faults. Similar comments may
GE Multilin L60 Line Phase Comparison System 8-7