R.1.120
SEL-421 Relay Reference Manual Date Code 20111215
Protection Functions
Circuit Breaker Failure Protection
Relay Word bit NBF1 (Breaker 1 Low Current Breaker Failure) asserts when
timer NPU1 (Low Current Breaker Failure Time Delay on Pickup) expires and
one of the following conditions exists:
➤ Circuit Breaker 1 residual overcurrent element 50R1 is asserted
and the relay does not detect an open pole in any of the three
phases for Circuit Breaker 1 (i.e., NOT B1OPHA, NOT
B1OPHB, or NOT B1OPHC)
➤ Relay Word bit BFIN1 (No Current Breaker Failure Initiation)
is asserted
For no current applications, such as a digital signal indicating a loss-of-field
from a generator, use inputs BFI3P1 and BFINn. Circuit breaker failure
clearing can occur after timer NPU1 times out. For no current/residual current
breaker failure trips, insert NBF1 in the circuit breaker failure trip SEL
OGIC
control equation BFTR1 (see Circuit Breaker Failure Trip Logic on
page R.1.121).
Failure to Interrupt
Load Current
Protection Logic
The circuit breaker failure protection used during load conditions is
independent from circuit breaker failure protection that you use during fault
conditions. Use circuit breaker failure protection for load conditions either
alone or in addition to circuit breaker failure protection for fault conditions as
a second level of breaker failure protection. Figure 1.79 shows that the output
of the load current protection is Relay Word bit LCBF1 (Load Current Breaker
Failure). Use this output to activate an external alarm, retrip the circuit
breaker, or energize a lockout relay.
Load Current Detection: 50LP1
This scheme detects failures of the circuit breaker to open when circuit
breaker current is greater than the 50LP1 setting. The 50LP1 element should
pick up when the protected circuit breaker is closed.
If the protected circuit breaker is in a ring-bus or circuit breaker-and-a-half
arrangement, set 50LP1 to pick up for the line-charging current of the shortest
line that circuit breaker services. Use the following equation to calculate the
charging current for a given line:
Equation 1.15
Time Delay on Pickup: LCPU1
The time delay setting for this protection scheme is typically longer than fault
current conditions because of lower current duties associated with this type of
circuit breaker failure operation. Extending the time delay allows more time
for a slow but operative circuit breaker to clear a low-current fault. A
disadvantage with the extended time delay is that a fault continues if the
circuit breaker fails. Weigh these considerations when selecting time delays
for this scheme. Please note that some circuit breakers take more time than
other circuit breakers to break low amounts of current; consult the
manufacturer of the protected circuit breaker for details.
where:
V
g
= Line-to-ground voltage
B
c
= Total line capacitive susceptance
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