3.36
SEL-551 Relay Instruction Manual Date Code 20110408
Relay Elements and Logic
Reclosing Relay
If the SEL-551 is in the Reset State (79RS = logical 1) and then a permanent
fault beyond the line recloser occurs (fault current I
F
in Figure 3.18), the line
recloser fast curve operates to clear the fault. The SEL-551 also sees the fault.
The phase time-overcurrent pickup 51P1 asserts and then deasserts without
tripping, incrementing the SEL-551 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-551 also sees the fault again. The
phase time-overcurrent pickup 51P1 asserts and then deasserts without
tripping, incrementing the SEL-551 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 SEL-551 does not operate on its faster-set phase time-
overcurrent element 51P1T (51P1T is “below” the line recloser slow curve)
because the shot counter is now at shot = 2. For this sequence coordination
scheme, the SEL-551 SEL
OGIC control equation trip equation is:
TR = 51P1T * (SH0 + SH1) + 51P2T
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 51P1T phase time-
overcurrent element from tripping. The 51P1T phase time-overcurrent
element is still operative and its pickup (51P1) 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.
Additional Settings Example 2
Review preceding Additional Settings Example 1 on page 3.34.
Assume that the line recloser in Figure 3.18 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-551 is set to operate once on 51P1T and then twice on
51P2T for faults between the SEL-551 and the line recloser. This results in the
following trip setting:
TR = 51P1T * (SHO) + 51P2T
This requires that two open interval settings be made (see Figure 3.16). This
corresponds to the last shot being:
last shot = 2
If the sequence coordination setting is:
79SEQ = 79RS * 51P1
and there is a permanent fault beyond the line recloser, the shot counter of the
SEL-551 will increment all the way to shot = 4 (see Figure 3.19). If there is a
coincident fault between the SEL-551 and line recloser, the SEL-551 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.
NOTE: Sequence coordination can
increment the shot counter beyond
last shot (last shot = 2 in this factory
setting example), but no further than
shot = 4.
The following Example 2 limits
sequence coordination shot counter
incrementing.
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