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Sudden current change function (SCC)
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The sudden current change function SCC measures the current increase per time unit
(di/dt) in all phases (one or two). In order to distinguish between heavy traction loads
and faults, it is used to enable the instantaneous trip from the measuring zone 2 and/or
the measuring zone 3 on the distance protection. The function can be activated or deac-
tivated via a binary input.
The sudden current change function's operating modes may be selected from the fol-
lowing:
•
Off.
•
As start for zones 2 and 3 (Normal).
•
As start for zones 2 and 3 and as backup function when the primary protection is
down due to different reasons, e. g. blocking from the fuse failure supervision func-
tion (Backup).
The output signals from the sudden change current function should be configured to the
logical element for release of tripping from the impedance function, see figure in sec-
tion )XQFWLRQDOLW\.
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The current measuring element in one of the integrated signal processors continuously
measures the phase currents (IL1 and IL2) in two-phase systems and current IL1 in sin-
gle phase systems. The current signal is filtered by a low pass filter. The difference be-
tween the filtered value and the actual current value is calculated and compared with
the set operate value. The output signal SCC-START is activated instantaneously if the
measured current difference exceeds the set operate value. A timer delays the resetting
of the signal by the set time t.
The simplified logic diagram for the SCC function is shown in figure 45.
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