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Monitoring the Current Flow
Each of the phase currents and an additional plausibility current (see below) are filtered by numerical filter
algorithms so that only the fundamental component is used for further evaluation.
Special measures are taken in order to detect a current interruption. In case of sinusoidal currents the current
interruption is detected after approximately 10 ms. With aperiodic DC current components in the fault current
and/or in the current transformer secondary circuit after interruption (e.g. current transformers with linearized
core), or saturation of the current transformers caused by the DC component in the fault current, it can take
one AC cycle before the interruption of the primary current is reliably detected.
The currents are monitored and compared with the set limit value. Besides the three phase currents, two addi-
tional current thresholds are provided in order to allow a plausibility check. For this plausibility check, a sepa-
rate threshold value can be used if the configuration is made accordingly (see
As plausibility current, the ground current (residual current Ι
from the neutral of the current transformer set is connected to the device it is used. If the residual current is
not available, the device calculates it with the formula:
3·Ι
= Ι
+ Ι
+ Ι
0
A
B
Additionally, the value calculated by 7SD80 of three times the negative sequence current 3·Ι
sibility check. This is calculated according to the equation:
3·Ι
= Ι
+ a
2
·Ι
2
A
where
a = e
j120°.
These plausibility currents do not have any direct influence on the basic function of the breaker failure protec-
tion but they allow a plausibility check that at least two current thresholds must be exceeded before any of the
time delays are started.
In case of high-resistance ground faults, it can happen that the ground current exceeds the sensitive threshold
value 50NBF PICKUP (address 3912) whereas the phase current involved in the short circuit does not exceed
the threshold value 50BF PICKUP (address 3902). The plausibility check would prevent the start of the
breaker failure protection. In this case, the pickup threshold of the phase current monitoring 50BF PICKUP
can be switched to the threshold value 50NBF PICKUP. Use the binary input 1404 >50BF 3I0> for this
purpose. This binary input is linked to an external signal that is suggestive of a high-resistance fault, e.g.
ground fault or displacement voltage detected. The ground current threshold that is set more sensitive is thus
also used for monitoring the phase currents
SIPROTEC 4, 7SD80, Manual
E50417-G1100-C474-A2, Edition 02.2018
C
+ a·Ι
B
C
(3·Ι
) is preferably used. If the residual current
E
0
(Figure
2-36).
2.6 Circuit Breaker Failure Protection 50BF
Figure
2-36).
is used for plau-
2
Functions
95
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