The maximum 3-phase short-circuit current I"
sc
flowing through is (at a source voltage of 1.1 V
N
):
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With a safety margin of 10 %, the following setting value results:
[foglnste-170309-01.tif, 1, en_US]
If short-circuit currents exceed 1496 A (primary) or 12.5 A (secondary), there is a short circuit on the line to be
protected. It can be disconnected immediately.
NOTE
The calculation was performed with absolute values, which is accurate enough for overhead lines. A
complex calculation is required only if the source impedance and the line impedance have extremely
different angles.
Parameter: Dropout ratio
•
Recommended setting value (_:3901:4) Dropout ratio = 0.90
The recommended setting value of 0.90 is sufficient for many applications. To obtain high-precision measure-
ments, the Dropout ratio can be reduced. If you expect highly fluctuating measurands at the pickup
threshold, you can increase the setting value of the parameter Dropout ratio. This avoids chattering of the
tripping stage.
Release Procedure via Protection Interface
This stage can be applied only if the device is equipped with a protection interface.
7.11.5
Protection and Automation Functions
7.11 Instantaneous High-Current Tripping
770 SIPROTEC 5, High-Voltage Bay Controller, Manual
C53000-G5040-C015-9, Edition 11.2017
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