Sel SEL-421 - Page 184

R.2.16
SEL-421 Relay Reference Manual Date Code 20111215
Auto-Reclosing and Synchronism Check
Two-Circuit-Breaker Auto-Reclosing
Circuit Breaker BK1 is the leader for the following conditions:
➤ BK1 is the only circuit breaker in service
➤ BK1 and BK2 are in service and BK1 is selected as the leader
(SLBK1 := 1)
➤ BK1 and BK2 are in service and the setting combination
SLBK := 0 and SLBK2 := 1 is not in effect
Circuit Breaker BK2 is the leader for the following conditions:
➤ BK2 is the only circuit breaker in service
➤ BK1 and BK2 are in service and BK2 is selected as the leader
(SLKBK1 := 0 and SLBK2 := 1)
➤ If neither circuit breaker is in service, there is no leader.
The following three Relay Word bits describe which circuit breaker is the
leader:
➤ LEADBK0, No Breaker In Service
➤ LEADBK1, Leader Breaker = Breaker 1
➤ LEADBK2, Leader Breaker = Breaker 2
The SEL-421 loads the corresponding circuit breaker settings into the leader
Relay Word bits (LEADBK0, LEADBK1, and LEADBK2). If there is no
leader (no circuit breaker is active), the relay loads a logical 0 into LEADBK1
and LEADBK2, and a logical 1 into LEADBK0.
Follower Logic
The FBKCEN SELOGIC control equation, Follower Breaker Closing Enable,
defines the conditions necessary for the follower breaker to reclose.
The SEL-421 selects the follower as follows:
➤ If Circuit Breaker BK1 is the leader and Circuit Breaker BK2 is
not locked out, then Circuit Breaker BK2 is the follower.
➤ If Circuit Breaker BK2 is the leader and Circuit Breaker BK1 is
not locked out, then Circuit Breaker BK1 is the follower.
➤ If fewer than two circuit breakers are in service (NBK0 or
NBK1 is asserted), then there is no follower.
The following three Relay Word bits describe which circuit breaker is the
follower:
➤ FOLBK0, No Follower Breaker
➤ FOLBK1, Follower Breaker = Breaker 1
➤ FOLBK2, Follower Breaker = Breaker 2
If there is no follower (in the case of only one circuit breaker, for example),
the relay loads a logical 0 into the follower SEL
OGIC control equation
FBKCEN.
Dynamic Selection of Leader and Follower Circuit Breakers
The SEL-421 dynamically selects the leader and follower circuit breakers
during the reclose cycle. The relay calculates the leader in the ready (reset)
state. At the start of the reclose cycle, the relay freezes this calculation and