Sel 751A, Instruction Manual

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4.74
SEL-751A Relay Instruction Manual Date Code 20100129
Protection and Logic Functions
Reclose Logic
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 4.38 Operation of SEL-751A Shot Counter for Sequence Coordination With Line Recloser (Additional
Settings Example 1)
If the SEL-751A is in the Reset State (79RS = 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
When the line recloser recloses its circuit breaker, the line recloser
fast curve operates again to clear the fault. The SEL-751A also sees
the fault again. The phase time-overcurrent pickup 51P1P asserts and
then deasserts without tripping, incrementing the relay shot counter
from:
shot = 1 to shot = 2
The line recloser fast curve is now disabled after operating twice.
When the line recloser recloses its circuit breaker, the line recloser
slow curve operates to clear the fault. The relay does not operate on
its faster-set phase time-overcurrent element 51P1 (51P1T is “below”
the line recloser slow curve) because the shot counter is now at shot
= 2. For this sequence coordination scheme, the SEL
OGIC
control
equation trip equation is:
TR := 51P1T AND (SH0 OR SH1) OR 51AT OR 51BT OR 51CT
With the shot counter at shot = 2, Relay Word bits SH0 (shot = 0) and
SH1 (shot = 1) are both deasserted to logical 0. This keeps the 51PT
phase time-overcurrent element from tripping. The 51P1T phase time-
overcurrent element is still operative, and its pickup (51P1P) can still
assert and then deassert, thus continuing the sequencing of the shot
counter to shot = 3, etc. The 51P1T phase time-overcurrent element
cannot cause a trip because shot
≥
2, and SH0 and SH1 both are
deasserted to logical 0.
The shot counter returns to shot = 0 after the reset timer (loaded
with reset time 79RSD) times out.
EXAMPLE 4.20 Additional Settings Example 2
Review preceding Example 1.
Assume that the line recloser in Figure 4.37 is set to operate twice on
the fast curve and then twice on the slow curve for faults beyond the
line recloser.
Assume that the SEL-751A is set to operate once on 51P1T and then
twice on 51AT, 51BT, or 51CT for faults between the SEL-751A and the
line recloser. This results in the following trip setting:
79SEQ := 79RS AND 51P1P
SH0 OR SH1
SH1
SH0
Shot Counter 0 1 2 3 4
q w e w e r e r
NOTE:
Sequence coordination can
increment the shot counter beyond
last shot in this example (last shot = 2
in this factory setting example) but no
further than shot = 4.
The following Example 2 limits
sequence coordination shot counter
incrementing.