where:
t
cbopen
is the maximum opening time for the circuit breaker
t
BFP_reset
is the maximum time for breaker failure protection to detect correct breaker function (the
current criteria reset)
t
margin
is a safety margin
It is often required that the total fault clearance time shall be less than a given critical
time. This time is often dependent of the ability to maintain transient stability in case
of a fault close to a power plant.
Time
The fault
occurs
Protection
operate time
Trip and Pickup
CCRBRF
(50BF)
Normal t
cbopen
Margin
Retrip delay t1 t
cbopen
after re-trip
t
BFPreset
Minimum back-up trip delay t2
Critical fault clearance time for stability
ANSI05000479_3_en.vsd
ANSI05000479 V3 EN-US
Figure 101: Time sequence
t3: Additional time delay to t2 for a second back-up trip TRBU2. In some applications
there might be a requirement to have separated back-up trip functions, tripping
different back-up circuit breakers.
tCBAlarm: Time delay for alarm in case of indication of faulty circuit breaker. There is
a binary input 52FAIL from the circuit breaker. This signal is activated when internal
supervision in the circuit breaker detect that the circuit breaker is unable to clear fault.
This could be the case when gas pressure is low in a SF6 circuit breaker, of others.
After the set time an alarm is given, so that actions can be done to repair the circuit
1MRK 505 370-UUS A Section 7
Current protection
Busbar protection REB670 2.2 ANSI 221
Application manual
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