4.152
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
The SEL-751 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-751 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 4.32 Additional Settings Example 2
Review preceding Example 1.
Assume that the line recloser in Figure 4.87 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-751 is set to operate once on 51P1T and then twice on
51AT, 51BT, or 51CT for faults between the SEL-751 and the line recloser. This
results in the following trip setting:
TR := 51P1T AND SHO OR 51AT OR 51BT OR 51CT
This requires that two open interval settings be made (see Table 4.54 and
Figure 4.85). 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-751 increments all the way to shot = 4 (see Figure 4.88). If there is a
coincident fault
between
the SEL-751 and the line recloser, the SEL-751 trips
and goes 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 4.89.
If the SEL-751 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 4.87), the line recloser fast curve
operates to clear the fault. The SEL-751 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-751 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
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.
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