3.31
Date Code 20110408 Instruction Manual SEL-551 Relay
Relay Elements and Logic
Reclosing Relay
After successful reclose initiation, open interval timing does not start until
allowed by the stall open interval timing setting 79STL. If 79STL = logical 1,
open interval timing is stalled. If 79STL = logical 0, open interval timing can
proceed.
If an open interval time has not yet started timing (79STL = logical 1 still), the
79SKP setting is still processed. In such conditions (open interval timing has
not yet started timing), if 79SKP = logical 1, the relay increments the shot
counter to the next shot and then loads the open interval time corresponding to
the new shot (see Table 3.6). If the new shot turns out to be the “last shot,” no
open interval timing takes place, and the relay goes to the Lockout State if the
circuit breaker is open (see preceding discussion on last shot and shot
counter).
If the relay is in the middle of timing on an open interval and 79STL changes
state to 79STL = logical 1, open interval timing stops where it is. If 79STL
changes state back to 79STL = logical 0, open interval timing resumes where
it left off.
Factory Settings Example
The skip shot factory setting is:
79SKP = 50P2 * SH0
If shot = 0 (Relay Word bit SH0 = logical 1) and phase current is above the
phase instantaneous overcurrent element 50P2 threshold (Relay Word bit
50P2 = logical 1), at the instant of successful reclose initiation, the shot
counter is incremented from shot = 0 to shot = 1. Then, the first open interval
time (setting 79OI1) is skipped, and the relay times on the second open
interval time (setting 79OI2) instead.
Note that the first open interval time (setting 79OI1) is a short time, while the
following second open interval time (setting 79OI2) is significantly longer.
For a high magnitude fault (greater than the phase instantaneous overcurrent
element 50P2 threshold), the first open interval time is skipped, and open
interval timing proceeds on the following second open interval time.
Once the shot is incremented to shot = 1, Relay Word bit SH0 = logical 0 and
then setting 79SKP = logical 0, regardless of Relay Word bit 50P2.
The stall open interval timing factory setting is:
79STL = TRIP
After successful reclose initiation, open interval timing does not start as long
as the trip condition is present (Relay Word bit TRIP = logical 1). As
discussed previously, if an open interval time has not yet started timing
(79STL = logical 1 still), the 79SKP setting is still processed. Once the trip
condition goes away (Relay Word bit TRIP = logical 0), open interval timing
can proceed.
Shot
Corresponding
Relay Word Bit
Corresponding
Open Interval
Open Interval
Factory Setting
0 SH0 79OI1 30 cycles
1 SH1 79OI2 600 cycles
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