3.3 Commissioning
503
7SD5 Manual
C53000-G1176-C169-1
Figure 3-31 Polarity check of I
4
, example with earth current of a parallel line
I
4
from a Power
Transformer Star-
point
If I
4
is the earth current measured in the starpoint of a power transformer and intended
for the earth fault protection direction determination (for earthed networks), then the
polarity check can only be carried out with a zero sequence current flowing through
the transformer. A test voltage source is required for this purpose (single-phase low
voltage source).
Caution!
Feeding of zero sequence currents via a transformer without broken delta winding.
Inadmissible heating of the transformer is possible!
Zero sequence current should only be routed via a transformer if it has a delta winding,
therefore e.g. Yd, Dy or Yy with a compensating winding.
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