Siemens siprotec 7SA6 User Manual page 410

Functions
Current Flow
Monitoring
6-228
Protection
trip
Feeder protection
(internal or external)
Figure 6-112 Simplified function diagram of circuit breaker failure protection controlled by
circuit breaker auxiliary contact
Each of the phase currents and an additional plausibility current (see below) are
filtered by numerical filter algorithms so that only the fundamental frequency is used
for further evaluation.
Special features recognise the instant of current interruption. With sinusoidal currents,
current interruption is detected after approx.10 ms. With d.c. transient current
components present in the fault current and/or in the current transformer secondary
circuit after interruption (e.g. current transformers with linearized core) or if the current
transformers are saturated by the d.c. component in the fault current, it can take one
a.c. cycle before the disappearance of the primary current is reliably detected.
The currents are monitored and compared with the set threshold. Besides the three
phase currents, two further current detectors are provided in order to allow a
plausibility check (see Figure 6-113).
The earth current (residual current I
If the residual current from the star-point of the current transformer set is connected to
the device this is used for 3· I
calculates it with the formula
3·I = I + I + I .
0 L1 L2 L3
Additionally, three times the negative sequence current 3·I
check. This is calculated by the 7SA6 according to the equation:
2
3·I = I + a · I + a · I
2 L1 L2
where
j120°
a = e .
These plausibility currents do not have any direct influence on the basic functionality
of the breaker failure protection but they allow a plausibility check in that at least two
current thresholds must have been exceeded before any of the breaker failure delay
times can be started, thus providing high security against false operation.
L+
Circuit breaker failure protection
Feeder
= 3·I ) is preferably used as plausibility current.
E 0
. If the residual current is not available the device
0
L3
Bus-bar
T–BF 0
&
Trip
bus-bar
is used for plausibility
2
7SA6 Manual
C53000-G1176-C133-1