To determine the Slope, the restraint current of both sides (400 kV and 230 kV) must be used. The rated
current is used as starting value for the gradient. Since side 2 (230 kV) is the reference side for the normaliza-
tion, the value of the 400 kV side must be adapted to this rated current. The adaptation factor results from the
inverse ratio (230 kV/400 kV). The following restraint current goes into the calculation:
[fostbrst-231012-01.tif, 1, en_US]
Siemens recommends using the setting value 0.07.
•
Recommended setting value (_:105) Slope = 0.07
Protection of a Resistance-Grounded Star Winding (Y Side)
[dwrefspa-170712-01.tif, 1, en_US]
Figure 6-280
Application with Resistance-Grounded Neutral Point
To also capture faults in the resistance to ground, a current transformer must be installed at the neutral point
of the protected object (see Figure 6-280). The pickup values can be derived from the data in the example. As
side rated value, the following results:
140 MVA/(√3 · 20 kV) = 4042 A
The following threshold value results as lower limit:
Protection and Automation Functions
6.43 Restricted Ground-Fault Protection
SIPROTEC 5, Overcurrent Protection, Manual 875
C53000-G5040-C017-8, Edition 07.2017
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