Schweitzer Engineering Laboratories SEL-421-4 - Page 494

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SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
Circuit Breaker Failure Application Examples
Retrip Time Delay
If the circuit breaker is equipped with two trip coils, the relay should attempt to
retrip the protected circuit breaker before a circuit breaker failure trip asserts.
Wait 4 cycles for the retrip.
RTPU1 := 4.000. Retrip Time Delay—BK1 (0.000–6000 cycles)
Circuit Breaker Failure Protection Initiation
To initiate circuit breaker failure protection for Circuit Breaker BK1, assign the
protection elements to Relay Word bit BFI3P1 (Three-Pole Breaker Failure Initi-
ate—BK1). This protection example uses three-pole tripping only.
BFI3P1 := 3PT. Three-Pole Breaker Failure Initiate—BK1 (SELOGIC Equation)
BFIA1 := NA. A-Phase Breaker Failure Initiate—BK1 (SELOGIC Equation)
BFIB1 := NA. B-Phase Breaker Failure Initiate—BK1 (SELOGIC Equation)
BFIC1 := NA. C-Phase Breaker Failure Initiate—BK1 (SELOGIC Equation)
Circuit Breaker Failure Protection Initiation Dropout Delay
Set the circuit breaker failure initiate dropout time delay to zero. Disable this fea-
ture for this application example because this is not a dual circuit breaker
scheme.
BFIDO1 := 0.000. Breaker Fail Initiate Dropout Delay—BK1 (0.000–1000
cycles)
Circuit Breaker Failure Protection Initiation Seal-In Delay
Set the latch logic circuit breaker failure pickup time delay to zero. Disable this
feature for this application example. Relay Word bit 3PT internally initiates cir-
cuit breaker failure protection and has a minimum duration three-pole time delay
on dropout (that is, TDUR3D).
BFISP1 := 0.000. Breaker Fail Initiate Seal-In Delay—BK1 (0.000–1000
cycles)
Residual Current Circuit Breaker Failure Protection
Enable no current/residual circuit breaker failure protection for Circuit Breaker
BK1. Use this logic to detect a circuit breaker failure and take appropriate action
when a weak source drives the fault or if the protected circuit breaker fails to trip
during a high-resistance ground fault.
ENCBF1 := Y. No Current/Residual Current Logic—BK1 (Y, N)
Residual Current Pickup
Set the pickup of the residual current level detector greater than maximum sys-
tem unbalance; assume a 15 percent maximum unbalance.
50RP1 = 0.15 • I
load
= 0.15 • 4.95 A = 0.74 A
50RP1 := 0.74. Residual Current Pickup—BK1 (0.25–50 A secondary)