[foschwe2-041012-01.tif, 1, en_US]
The recommended setting value of 0.2 I/Irated,S lies above it.
•
Recommended setting value (_:105) Slope = 0.07
You can stabilize the protection function (less sensitive setting) with external multiphase short-circuits to
ground with the parameter Slope. To determine the setting value, no pickup value increase can occur up to
the rated current. After this, the gradient must be active. To derive the gradient, it is assumed that continua-
tion of the straight line must go through the coordinate origin (see Figure 6-278). Determine the intersection
from the threshold value and the restraint current at rated current. Calculate the gradient as follows:
[fostbref-231012-01.tif, 1, en_US]
[dwsteiga-221012-01.tif, 2, en_US]
Figure 6-278 Derivation of the Setting Value for the Gradient
If you have several measuring points on an outgoing side (see Figure 6-270), Siemens recommends using the
value 3 I/Irated,S in the intersection calculation for the restraint current. In the load case, the maximum trans-
former rated current flows on one side.
If, however, several measuring points are on the supply side (for example, breaker-and-a-half layout), Siemens
recommends including all phase currents in the intersection calculation, in order to avoid too strong a stabili-
zation. With 2 measuring points, a value of 6 I/Irated,S results as intersection with the threshold value. The
slope becomes flatter (0.2 I/Irated,S/6 I/Irated,S = 0.03).
Protection and Automation Functions
6.43 Restricted Ground-Fault Protection
SIPROTEC 5, Overcurrent Protection, Manual 873
C53000-G5040-C017-8, Edition 07.2017
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