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 194: Time sequence
t2MPh: Time delay of the back-up trip at multi-phase initiate. The critical fault
clearance time is often shorter in case of multi-phase faults, compared to single phase-
to-ground faults. Therefore there is a possibility to reduce the back-up trip delay for
multi-phase faults. Typical setting is 90 – 150 ms.
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. After the set
time an alarm is given, so that actions can be done to repair the circuit breaker. The
time delay for back-up trip is bypassed when the 52FAIL is active. Typical setting is
2.0 seconds.
Section 9 1MRK 502 071-UUS A
Current protection
402 Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
Application manual
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