5.153
Date Code 20171021 Instruction Manual SEL-421 Relay
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 5.154).
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 5.115 shows that the output of the load current protec-
tion 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 5.31
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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