Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

Page: 267 / 706

6.29
Date Code 20070117
Instruction Manual SEL-351-5, -6, -7 Relay
Close and Reclose Logic
Reclosing Relay
Assume that the line recloser in Figure 6.8 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-351 is set to operate once on 51PT and then twice on
51AT, 51BT, or 51CT for faults between the SEL-351 and the line recloser.
This results in the following trip setting:
TR = 51PT * (SHO) + 51AT + 51BT + 51CT
This requires that two open interval settings be made (see Table 6.2 and
Figure 6.6 ). This corresponds to the last shot being:
last shot = 2
If the sequence coordination setting is:
79SEQ = 79RS * 51P
and there is a permanent fault beyond the line recloser, the shot counter of the
SEL-351 will increment all the way to shot = 4 (see
Figure 6.9 ). If there is a
coincident fault between the SEL-351 and the line recloser, the SEL-351 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 * 51P * SH0
Refer to Figure 6.10 .
If the SEL-351 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 6.8 ), the line recloser fast curve
operates to clear the fault. The SEL-351 also sees the fault. The phase time-
overcurrent pickup 51P asserts and then deasserts without tripping,
incrementing the relay shot counter from:
shot = 0 to shot = 1
Now the SEL-351 cannot operate on its faster-set phase time-overcurrent
element 51PT because the shot counter is at shot = 1 (SH0 = logical 0):
TR =
51PT * (SHO) + 51AT + 51BT + 51CT
= 51PT * (logical 0) + 51AT +
51BT + 51CT = 51AT + 51BT + 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 6.10 Operation of SEL-351 Shot Counter for Sequence Coordination With Line Recloser (Additional
Settings Example 2)
79SEQ = 79RS * 51P1 * SHO
51P1
Shot Counter
SH0
0 1
q w e w e r e
r