Sel SEL-421 - Failure to Interrupt; Circuit Breaker Failure Protection Logic; Retrip Logic

R.1.116
SEL-421 Relay Reference Manual Date Code 20111215
Protection Functions
Circuit Breaker Failure Protection
you to have different breaker failure times to differentiate between single-pole
and three-pole tripping conditions. The failure-to-trip-load-current logic uses
the circuit breaker failure initiation input for three-pole trips only. The
flashover protection logic does not need voltage information.
Subsidence current results from the energy trapped in the CT magnetizing
branch after the circuit breaker opens to clear a fault or interrupt load.
Subsidence current exponentially decays and delays resetting of instantaneous
overcurrent elements. However, the open phase detection logic causes the
SEL-421 50Fn elements to reset in less that one cycle during subsidence
current conditions see Figure 1.78, and Figure 1.79 and Figure 1.80). The
open phase detection logic output is BnOPH (see Table 1.19).
Failure to Interrupt
Fault Current:
Scheme 1
Circuit Breaker Failure Protection Logic
The logic shown in Figure 1.72 applies to single circuit breaker
configurations. Fault current causes 50FA1 (Breaker 1 A-Phase Instantaneous
Overcurrent Element) to assert immediately following fault inception and just
prior to the assertion of Relay Word bit BFI3P1 (Breaker 1 Three-Pole Circuit
Breaker Failure Initiation). At circuit breaker failure initiation, timer BFPU1
(Breaker 1 Circuit Breaker Failure Time Delay on Pickup Timer) starts timing.
If 50FA1 remains asserted when the BFPU1 timer expires, Relay Word bit
FBF1 asserts. Use this Relay Word bit in the circuit breaker failure tripping
logic to cause a circuit breaker failure trip (see Circuit Breaker Failure Trip
Logic on page R.1.121). If the protected circuit breaker opens successfully,
50FA1 drops out before the BFPU1 timer expires and FBF1 does not assert.
Figure 1.72 Scheme 1 Logic Diagram
Retrip Logic
Some three-pole circuit breakers have two separate trip coils. If one trip coil
fails, the local protection can attempt to energize the second trip coil to
prevent an impending circuit breaker failure operation. Configure your
protection system to always attempt a local retrip using the second trip coil
before the circuit breaker failure pickup time delay timer expires.
RTPU1 (Retrip Time Delay on Pickup Timer) begins timing when BFI3P1
asserts. Relay Word bit RT1 (Breaker 1 Retrip) asserts immediately after
RTPU1 times out. Assign a control output to trip the circuit breaker when
Relay Word bit RT1 asserts.
BFPU1
0
RTPU1
0
FBF1
RT1
FBFB1
FBFA1
FBFC1
50FA1
BFI3P1
BFIA1
RTB1
RTC1
RTA1
Relay
Word
Bits
Relay
Word
Bits
Relay
Word
Bits
Relay
Word
Bits
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