Date Code 20000421
Applications
5-13
SEL-251, -2, -3 Instruction Manual
The other negative-sequence overcurrent elements and settings are:
50Q
Negative-sequence definite-time overcurrent element pickup
50QT
Negative-sequence definite-time overcurrent element
Settings:
50Q
pickup (secondary amps)
50QT
time delay
The SEL-251 Relay phase and ground overcurrent elements have similar element types and
settings.
If you coordinate the negative-sequence time-overcurrent element (
51QT
) with the phase time-
overcurrent element of a line recloser, settings can be as sensitive as the line recloser phase time-
overcurrent element pickup for LL faults. This is possible because
51QT
does not respond to
balanced load. The relay requires proper coordination time between
51QT
and the line recloser
phase time-overcurrent element.
If you coordinate
51QT
with a phase time-overcurrent element for LL faults, it should have no
coordination problems with ground time-overcurrent elements. Ground time-overcurrent
elements usually have pickup values one-third or one-fourth the magnitude of phase time-
overcurrent relay pickups in their respective protection zones. Ground time-overcurrent relays
also have shorter time dials than phase time-overcurrent relays in their respective protection
zones. For an LG fault,
51QT
sees the same current magnitude as a ground time-overcurrent
relay:
I
I
for a LG fault
x I
x I
2
0
2
0
3
3
=
=
,
Negative-Sequence Overcurrent Elements and Delta-Wye Transformers
Phase-to-ground and phase-to-phase faults on the secondary side of delta-wye transformers can
generate negative-sequence currents on the primary system. This should be considered when
entering negative-sequence element settings, especially if the delta-wye transformer bank is a
large kVA transformer (small impedance). Ground/residual elements are not affected by such
phase-to-ground faults because the delta winding does not allow zero-sequence generation in the
primary system. See Figure 5.8 and Figure 5.9.
Summary of Figure 5.8
In general, for a phase-to-ground fault on the secondary side of a delta-wye transformer, the
following primary current relationship is true:
I
x I
x I
x I
P
P
=
=
173
173
3
2
2
.
.
Thus, for this case, the negative-sequence overcurrent elements have the same sensitivity as a
primary phase overcurrent element (e.g., the fuses for the delta-wye transformer) if the negative-
sequence overcurrent element pickup value is a factor of 1.73 times the primary phase
overcurrent element pickup value.