Schweitzer Engineering Laboratories SEL-251 - Page 116

5-12 Applications Date Code 20000421
SEL-251, -2, -3 Instruction Manual
Table 5.1: System Conditions and Currents Generated
System
Condition
Phase
Current (I
P
)
Negative-Sequence
Current (I
2
)
Zero-Sequence
Current (I
0
)
LG Fault X X X
LLG Fault X X X
LL Fault X X
3-Phase Fault X
Balanced Load X
In traditional distribution feeder protection, residually connected ground overcurrent relays
operate on 3xI
0
current and cover LG and LLG faults. Phase overcurrent relays operate on I
P
current and cover LL faults and three-phase faults. Phase overcurrent relays are sensitive to load
current; their pickup settings must exceed load current levels.
Negative-sequence overcurrent relay elements in the SEL-251 Relay respond to 3xI
2
current.
These elements can be set to cover LL faults more sensitively than phase overcurrent elements.
This is because negative-sequence overcurrent elements do not respond to balanced load current.
For an LL fault (current magnitudes only):
( )
I x I
xI x I
x I
P
P
P
2
2
3 3
3 3
173
=
=
=
/
.
Thus, a negative-sequence overcurrent element has the same sensitivity for LL faults as a phase
overcurrent element if negative-sequence overcurrent element pickup value is a factor of 1.73
times the phase overcurrent element pickup value.
The following negative-sequence overcurrent elements (with respective settings) are available in
the SEL-251 Relay:
51QP Negative-sequence time-overcurrent element pickup
51QT Negative-sequence time-overcurrent element
Settings: 51QP pickup (secondary amps)
51QTD time dial
51QC curve
51QRS time delay or one cycle delay reset
The 51QRS setting activates time delay reset. During the setting procedure, the relay sends the
message: "Dly Reset Y/N?" If you press Y <ENTER>, the negative-sequence time-overcurrent
element emulates an induction disc, rotating back to reset after the 3xI
2
current drops below
51QP (time delay reset). If you press N <ENTER>, the negative-sequence time-overcurrent
element resets completely when the 3xI
2
current drops below 51QP for one cycle (one cycle
delay reset).