The following view can be used to derive the threshold value. The transformer is supplied, for example, via the
delta winding and a 1-pole ground fault occurs on the star side.
NOTE
For estimation of the short-circuit current, note that the inductance changes quadratically with the winding
and linearly with the voltage.
The right part in the following figure represents the fault current as a function of the fault location. The fault
current curve I
F1
shows that the longitudinal differential protection with faults near the neutral point has
sensitivity problems due to the sinking current. On the other hand, the neutral-point current I
F2
is sufficiently
large. There is therefore no need to set the Threshold (current through the neutral point transformer) to
sensitive.
[dwf1pole-170712-01.tif, 1, en_US]
Figure 6-277
Principal Fault Current Curves with a 1-Pole Ground Fault
•
Recommended setting value (_:103) Threshold = 0.2 I/Irated,S
The previously mentioned condition must be maintained during the setting.
[foschwe1-170712-01.tif, 1, en_US]
The following lower limiting value results from the data from Figure 6-271:
Protection and Automation Functions
6.43 Restricted Ground-Fault Protection
872 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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