2.8 Circuit Breaker Failure Protection
107
7ST6 Manual
E50417-G1176-C251-A3
Two-stage Breaker
Failure Protection
With two-stage operation, the trip command is repeated after a time delay
tBF1
(ad-
dress
3904
) to the local feeder breaker, normally to a different set of trip coils of this
breaker.
If the circuit breaker does not respond to the repeated trip command, the protection
trips after a second delay time
tBF2
(address
3905
) the adjacent circuit breakers, i.e.
those of the busbar or the affected busbar section and, if necessary, also the circuit
breaker at the remote end, if the fault is not yet eliminated.
The delay times of the two stages can be set separately
• For trip repetition to the local feeder circuit breaker after a trip of the feeder protec-
tion
tBF1
(address
3904
),
• For trip of the adjacent circuit breakers (busbar zone)
tBF2
(address
3905
).
The delay times are determined from the maximum operating time of the feeder circuit
breaker, the reset time of the current detectors of the breaker failure protection, plus a
safety margin which allows for any tolerance of the delay timers. The time sequence
is illustrated in Figure 2-29. For sinusoidal currents, the dropout time can be assumed
to be less than 3/4 the cycle duration at rated frequency. If current transformer satura-
tion is anticipated, the time should be set to 1.5 ms.
Figure 2-29
Time sequence example for two-stage breaker failure protection
www
. ElectricalPartManuals
. com