3 Mounting and Commissioning
420
7SA522 Manual
C53000-G1176-C155-3
Figure 3-26 Polarity check for the current input I
4
, example for 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.
DANGER!
Energized equipment of the power system! Capacitive coupled voltages at discon-
nected equipment of the power system !
Non-observance of the following measure will result in death, severe personal injury
or substantial property damage.
Primary measurements must only be carried out on disconnected and earthed equip-
ment of the power system!
The configuration shown in Figure 3-27 corresponds to an earth current flowing
through the line, in other words an earth fault in the forward direction.
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