Negative-Sequence Overcurrent Protection
For years, protection engineers have enjoyed increased sensitivity to phase-to-ground unbalances with
the application of ground relays. Ground relays can be set more sensitively than phase relays because a
balanced load has no ground (3
I0
) current component. The negative-sequence elements can provide
similar increased sensitivity to phase-to-phase faults because a balanced load has no negative-sequence
(I
2
) current component.
Pickup Settings for Negative-Sequence Overcurrent
A typical setting for the negative-sequence elements might be one-half the phase pickup setting in order
to achieve equal sensitivity to phase-to-phase faults as three-phase faults. This number comes from the
fact that the magnitude of the current for a phase-to-phase fault is
3/2 (87%) of the three-phase fault at
the same location. This is illustrated in Figure 4-28.
The phase-to-phase fault is made up of both positive and negative-sequence components as shown in
Figure 4-29 or a phase-to-phase fault, the magnitude of the negative-sequence component is 1/ 3 (58%)
of the magnitude of the total phase current. When these two factors (
3/2 and 1/
3) are combined, the
3
factors cancel which leaves the one-half factor.
Figure 4-28
. Phase-to-Phase Fault Magnitude
Figure 4-29. Sequence Components for an A-B Fault
4-34
BE1-CDS240 Protection and Control
9365200990 Rev F
Summary of Contents for BE1-CDS240
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