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Figure 2-28
Current flow monitoring
Each of the phase currents and an additional plausibility current (see below) are filtered by numerical filter al-
gorithms so that only the fundamental component is used for further evaluation.
Special features recognize the instant of current interruption. In case of sinusoidal currents the current interrup-
tion 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 plausibility check. If configured correspondingly, a separate
threshold value can be used for this plausibility check (see Figure 2-29).
As plausibility current, the earth current (residual current I
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
= I
+ I
0
L1
Additionally, the value calculated by 7VK61 of three times the negative sequence current 3·I
sibility check. This is calculated according to the equation:
3·I
= I
+ a
2
L1
where
j120°.
a = e
These plausibility currents do not have any direct influence on the basic functionality of the breaker failure pro-
tection 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.
SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009
Simplified function diagram of circuit breaker failure protection controlled by circuit breaker aux-
iliary contact
+ I
L2
L3
2
·I
+ a·I
L2
L3
2.6 Circuit breaker failure protection (optional)
(3·I
)) is preferably used. If the residual current from
E
0
Functions
is used for plau-
2
115
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