Siemens SIPROTEC 4 7UT6 Series Manual page 105

[diff-trafo-schaltgranpass-ynd5-020926-rei, 1, en_GB]
Figure 2-30
Figure 2-31 shows an example of an earth fault on the delta side outside the protected zone if an earthed
starpoint former (zigzag winding) is installed within the protected zone. Here, a zero sequence current occurs
on the right side but not on the left, as above. If the starpoint former were outside the protected zone (i.e. CTs
between power transformer and starpoint former), the zero sequence current would not pass through the
measuring point (CTs) and would not have any harmful effect.
[diff-trafo-erdkurzschuss-beisp2-020926-rei, 1, en_GB]
Figure 2-31
The disadvantage of elimination of the zero sequence current is that the protection becomes less sensitive
(factor 2 / because the zero sequence current amounts to
3
Therefore, elimination is suppressed in case the starpoint is not earthed (see
current can be included
one should do without the advantage of that option in order to avoid recognition of a breakdown of the surge
voltage arrester as an internal fault. For this purpose, the starpoint of the respective side must be set to
Earthed (addresses 313STARPNT SIDE 1, 323STARPNT SIDE 2, 333STARPNT SIDE 3, 343STARPNT
SIDE 4, 353STARPNT SIDE 5).
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
Matching the transformer vector group, example YNd5 (magnitudes not considered)
Example of an earth fault outside the protected transformer with a neutral earthing reactor
within the protected zone
(Figure 2-29). If, for example, a surge voltage arrester is connected to the starpoint,
1 / in case of an earth fault in the protected area.
3
Figure 2-28), or the starpoint
Functions
2.2 Differential Protection
105