Sel 751A - Page 167

4.75
Date Code 20100129 Instruction Manual SEL-751A Relay
Protection and Logic Functions
Reclose Logic
TR := 51P1T AND SHO OR 51AT OR 51BT OR 51CT
This requires that two open interval settings be made (see Tab le 4.2 7
and Figure 4.35). This corresponds to the last shot being:
last shot = 2
If the sequence coordination setting is:
79SEQ := 79RS AND 51P1P
and there is a permanent fault beyond the line recloser, the shot
counter of the SEL-751A will increment all the way to shot = 4 (see
Figure 4.38). If there is a coincident fault
between the SEL-751A and
the line recloser, the SEL-751A will trip and go to the Lockout State.
Any time the shot counter is at a value equal to or greater than last
shot and the relay trips, it goes to the Lockout State.
To avoid this problem, make the following sequence coordination
setting:
79SEQ := 79RS AND 51P1P AND SH0
Refer to Figure 4.39.
If the SEL-751A is in the Reset State (79RS = logical 0) with the shot
counter reset (shot = 0; SH0 = logical 1) and then a permanent fault
beyond the line recloser occurs (fault current I
F
in Figure 4.37), the
line recloser fast curve operates to clear the fault. The SEL-751A also
sees the fault. The phase time-overcurrent pickup 51P1P asserts and
then deasserts without tripping, incrementing the relay shot counter
from:
shot = 0 to shot = 1
Now the SEL-751A cannot operate on its faster-set phase time-
overcurrent element 51P1T because the shot counter is at shot = 1
(SH0 = logical 0):
TR := 51P1T AND SHO OR 51AT OR 51BT OR 51CT = (logical 0) OR 51AT OR
51BT OR 51CT
q Fault occurs beyond line recloser; w fault cleared by line recloser fast curve; e line recloser recloses into fault; r fault
cleared by line recloser slow curve.
Figure 4.39 Operation of SEL-751A Shot Counter for Sequence Coordination With Line Recloser (Additional
Settings Example 2)
The line recloser continues to operate for the permanent fault beyond
it, but the SEL-751A shot counter does not continue to increment.
Sequence coordination setting 79SEQ is effectively disabled by the
shot counter incrementing from shot = 0 to shot = 1.
79SEQ = 79RS * 51P1 * SHO
51P1
Shot Counter
SH0
01
qw ew e r e
r