Sel 551 - Page 72

3.34
SEL-551 Relay Instruction Manual Date Code 20110408
Relay Elements and Logic
Reclosing Relay
Additional Settings Example 2
If the block reset timing setting is:
79BRS = 51P1 + 51G1
then reset timing is blocked if time-overcurrent pickup 51P1 or 51G1 is picked
up, regardless of the reclosing relay state.
Sequence
Coordination (79SEQ)
The sequence coordination setting 79SEQ keeps the SEL-551 in step with a
downstream line recloser in a sequence coordination scheme, which prevents
overreaching SEL-551 overcurrent elements from tripping for faults beyond
the line recloser. This is accomplished by incrementing the shot counter and
supervising overcurrent elements with resultant shot counter elements.
In order for the sequence coordination setting 79SEQ to increment the shot
counter, both the following conditions must be true:
➤ No trip present (Relay Word bit TRIP = logical 0)
➤ Circuit breaker closed (SELOGIC control equation setting 52A
= logical 1, effectively)
The sequence coordination setting 79SEQ is usually set with some overcurrent
element pickups. If the above two conditions are both true and a set
overcurrent element pickup asserts for at least 1.25 cycles and then deasserts,
the shot counter increments by one count. This assertion/ deassertion indicates
that a downstream device (e.g., line recloser—see Figure 3.18) has operated to
clear a fault. Incrementing the shot counter keeps the SEL-551 “in step” with
the downstream device, as is shown in the following Additional Settings
Example 1 and Additional Settings Example 2 on page 3.36.
Every time a sequence coordination operation occurs, the shot counter is
incremented, and the reset timer is loaded up with reset time 79RSD.
Sequence coordination can increment the shot counter beyond last shot, but no
further than shot = 4. The shot counter returns to shot = 0 after the reset timer
times out. Reset timing is subject to the previously discussed SEL
OGIC control
equation setting 79BRS.
Sequence coordination operation does not change the reclosing relay state. For
example, if the relay is in the Reset State and there is a sequence coordination
operation, it remains in the Reset State.
Factory Settings Example
Sequence coordination is not enabled in the factory settings:
79SEQ = 0
Additional Settings Example 1
With sequence coordination setting:
79SEQ = 79RS * 51P1
sequence coordination is operable only when the relay is in the Reset State
(79RS = logical 1).
Refer to Figure 3.18.