Functions
2.38 Breaker Failure Protection (ANSI 50BF)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
264
Criteria
The two pickup criteria (current criterion, circuit breaker auxiliary contact) are OR-combined. In case of a trip-
ping without short circuit current, e.g. for voltage protection on light load, the current is not a safe criterion for
circuit breaker response. For this reason pickup is also made possible using the auxiliary contact criterion.
The current criterion is fulfilled if at least one of the three phase currents exceeds a parametrized threshold
value (CIRC. BR. I>). The dropout is performed if all three phase currents fall below 95 % of the pickup
threshold value.
In the operating condition 0 the current criterion is inactive. In that case, the breaker failure protection will be
activated only by the breaker auxiliary contacts.
If the binary input of the circuit breaker auxiliary contact is inactive, only the current criterion is effective and the
breaker failure protection cannot become active with a tripping signal if the current is below the CIRC. BR.
I> threshold.
Two-Channel Feature
To increase security and to protect against possible disturbance impulses the binary input for an external trip
signal is stabilized. This external signal must be present during the entire period of the delay time. Otherwise,
the timer is reset and no tripping signal is issued. A redundant binary input „>ext.start2 B/F“ is linked to
further enhance the security against unwanted operation. This means that no initiation is possible unless both
binary inputs are activated. The two-channel feature is also effective for an “internal” initiation.
Logic
If the breaker failure protection has picked up, a corresponding message is transmitted and a parameterized
time delay starts. If the pickup criteria are still fulfilled on expiration of this time, a redundant source evaluation
before fault clearing is initiated via a further AND combination through a higher level circuit breaker.
A pickup drops off and no trip command is produced by the breaker failure protection if
• one of the internal start conditions (Output relay BO3 or via CFC) or „>ext.start1 B/F“ or
„>ext.start2 B/F“ causing the pickup drop off.
• a tripping signal of the protective functions still exists, whereas the current criterion and the auxiliary contact
criterion drops off.
The following figure shows the logic diagram for the breaker failure protection function. The overall breaker
failure protection can be enabled or disabled via parameters and also blocked dynamically via binary input
„>BLOCK BkrFail“ (e.g. during a machine protection check).
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