6-28
Close and Reclose Logic
Date Code 20001006
SEL-351 Instruction Manual
When the relay is in the Reset or Lockout States, Relay Word bit 79CY is deasserted to logical 0.
Thus, the 79BRS setting has no effect when the relay is in the Reset or Lockout States. When a
circuit breaker is closed from lockout, there could be cold load inrush current that momentarily
picks up a time-overcurrent element [e.g., phase time-overcurrent element 51PT pickup (51P)
asserts momentarily]. But, this assertion of pickup 51P has no effect on reset timing because the
relay is in the Lockout State (79CY = logical 0). The relay will time immediately on reset time
79RSLD and take the relay from the Lockout State to the Reset State with no additional delay
because 79BRS is deasserted to logical 0.
When the relay is in the Reclose Cycle State, Relay Word bit 79CY is asserted to logical 1. Thus,
the factory 79BRS setting can function to block reset timing if time-overcurrent pickup 51P or
51G is picked up while the relay is in the Reclose Cycle State. This helps prevent repetitive "trip-
reclose" cycling.
Additional Settings Example 2
If the block reset timing setting is:
79BRS
= 51P + 51G
then reset timing is blocked if time-overcurrent pickup 51P or 51G is picked up, regardless of the
reclosing relay state.
Sequence Coordination Setting (79SEQ)
The sequence coordination setting 79SEQ keeps the relay in step with a downstream line recloser
in a sequence coordination scheme, which prevents overreaching SEL-351 Relay 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 (SEL
OGIC
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 6.8) has
operated to clear a fault. Incrementing the shot counter keeps the SEL-351 Relay “in step” with
the downstream device, as is shown in the following
Additional Settings Examples 1
and
Additional Settings Examples 2
.
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 SEL
OGIC
Control Equation setting 79BRS [see
Block
Reset Timing (79BRS)
earlier in this subsection].
Содержание SEL-351
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