Basler BE1-CDS220, Instruction Manual, Page: 95 / 476

BE1-CDS220 Protection and Control 4-29
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 4-25.
The phase-to-phase fault is made up of both positive and negative-sequence components as shown in
Figure 4-26 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 4-25. Phase-to-Phase Fault Magnitude
Figure 4-26. Sequence Components for an A-B Fault