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126
Breaker failure protection (BFP)
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Trip Logic
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The connectable inputs are connectable by configuration to the binary inputs of the ter-
minal or to other internal functions' outputs. The outputs are connectable by configura-
tion to the binary output relays. "Connectables" and "outputs" can be connected to the
free-logic functions of the unit, OR gates, and in that way add connection links
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The breaker-failure protection can be started either internally or externally. The start
pulse is sealed-in as long as the current exceeds the preset current level, to prevent a
restart of the BFP timers in case of a chattering starting contact. The preset current level
may be set to (0.05 - 2.0) · Ir where Ir is 1 or 5 A.
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The current is filtered through a specially designed high-pass filter to obtain the re-
quired suppression of the dc components.
High-pass filtering is performed basically for two reasons, i.e. to remove the:
•
dc component caused by saturated current transformers with a decaying current due
to de-energizing of the secondary circuit. This is done to achieve a more correct rep-
resentation of the real current in the line.
External start
Breaker-failure
protection
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START
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TRRET
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