BE1-951
Protection and Control
4-23
I
B 2
I
A 2
= |
I
2
| =
√
3
I
B 1
I
A 1
I
C 1
I
C 2
I
A P h a s e
I
B P h a s e
N e g . S e q .
(I
2
)
P o s . S e q .
(I
1
)
P h a s e C u r r e n t
I
C P h a s e
=
0
D2843-05.vsd
02-08-00
I
A P h a s e
Figure 4-11. Sequence Components for an A-B Fault
D2843-04.vsd
02-08-99
3 P h F A U L T
=
V
3 P h F A U L T
V
I
P h - P h F A U L T
=
V*
√
3
=
3 P h F A U L T
*
√
3
2
P h - P h F A U L T
V
I
I
Z
Z
Z
Z
Z
I
I
Z
*2
Z
Figure 4-12. Phase-to-Phase Fault Magnitude
Negative Sequence Pickup Settings
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
4
3/2 (87%) of the three-phase fault at the
same location. This is illustrated in Figure 4-11.
The phase-to-phase fault is made up of both
positive and negative sequence components as
shown in Figure 4-12. For a phase-to-phase
fault, the magnitude of the negative sequence
component is 1/
4
3(58%) of the magnitude of the
total phase current. When these two factors
(
4
3/2 and 1/
4
3) are combined, the
4
3 factors
cancel, which leaves the one-half factor.
Negative Sequence Coordination Settings
The 51Q Settings should be checked for coordination with phase-only sensing devices such as downstream
fuses and reclosers and/or ground relays. To plot the negative sequence time current characteristics on the
same plot for the phase devices, you need to multiply the negative sequence element pickup value by the
correct multiplier. The multiplier is the ratio of phase current to negative sequence current for the fault type
