R.1.121
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Circuit Breaker Failure Protection
The recommended setting for LCPU1 is the sum of the following:
➤ Nominal circuit breaker operate time
➤ 50LP1 dropout time
➤ Safety margin
Calculate the safety margin by subtracting all conditions required to isolate
the fault during a circuit breaker failure condition from the maximum
acceptable fault clearing time. The safety margin will be longer in this case
than for the fault current logic because the total acceptable time to clear the
fault at these lower fault duties is longer.
Load Current Circuit Breaker Failure Initiation: BFILC1
Program SELOGIC control equation BFILC1 (Load Current Breaker Failure
Initiation) to initiate this scheme. For example, use the auxiliary contacts from
the circuit breaker to detect when the circuit breaker is open. Relay Word bit
LCBF11 asserts if Relay Word bit BFILC1 remains asserted for time LCPU1
and the relay detects load current.
Circuit Breaker
Flashover Protection
Circuit breaker failure protection during flashover conditions is independent
of the other circuit breaker protection functions. Use this protection either
alone or in addition to the other protection.
Use current flow to detect when an open circuit breaker pole flashes over. Set
BLKFOA1 to TPA or CLS1 to block flashover protection for 6 cycles if an A-
phase single-pole trip occurs, or when circuit breaker BK1 closes.
Figure 1.80 shows the flashover circuit breaker failure logic. Flashover timer
FOPU1 (Flashover Time Delay—BK1) starts timing if the circuit breaker is
open and current exceeds setting 50FO1 (Flashover Current Pickup—BK1).
The relay uses pole-open logic BnOPH to determine whether the circuit
breaker is open.
The output of the flashover protection is Relay Word bit FOBF1. Use this
output to activate an external alarm, retrip the circuit breaker, or energize a
lockout relay.
Circuit Breaker
Failure Trip Logic
The SEL-421 has dedicated circuit breaker failure trip logic (see Figure 1.81).
Set SEL
OGIC control equation BFTR1 (Breaker Failure Trip—BK1) to assert
for circuit breaker failure trips from Relay Word bits FBF1, NBF1, LCBF1,
and FOBF1.
When this SEL
OGIC control equation asserts, the relay sets Relay Word bit
BFTRIP1 (Breaker Failure Trip for Circuit Breaker BK1) to logical 1 until
BFTR1 deasserts, timer TDUR3D times out, and an unlatch or reset condition
is active.
Unlatch Circuit Breaker Failure Trip Equation
Use SELOGIC control equation BFULTR1 (Breaker Failure Unlatch Trip—
BK1) to define the conditions that unlatch the control outputs that assert
during a circuit breaker failure trip. BFULTR1 unlatches the circuit breaker
trip condition BFTRIP1.
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