Siemens SIPROTEC 7SD610 - 2.13 Circuit Breaker Failure Protection (optional)

2 Functions
178
7SD610 Manual
C53000-G1176-C145-4
Figure 2-62 Simplified function diagram of circuit breaker failure protection controlled by
circuit breaker auxiliary contact
Current flow
monitoring
Each of the phase currents and an additional plausibility current (see below) are fil-
tered by numerical filter algorithms so that only the fundamental component is used
for further evaluation.
Special features recognize the instant of current interruption. In case of sinusoidal cur-
rents 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 further current thresholds are provided in order to allow a plausi-
bility check. If configured correspondingly, a separate threshold value can be used for
this plausibility check (see Figure 2-63).
As plausibility current, the earth current (residual current I
E
(3·I
0
)) is preferably used.
If the residual current from the starpoint 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· I
0
= I
L1
+ I
L2
+ I
L3
Additionally, the value calculated by 7SD610 of three times the negative sequence
current 3·I
2
is used for plausibility check. This is calculated according to the equation:
3· I
2
= I
L1
+ a
2
·I
L2
+ a·I
L3
where
a
= e
j120°.
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.
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