[diff-strom-ausloesekennlinie, 1, en_GB]
Figure 3-34 Differential and Restraint Currents - Example of Plausible Measurements
Zero Sequence Current Tests on the Protected Object
The zero sequence current tests are only necessary if the starpoint of a three-phase object or a single-phase
transformer is earthed on a side or winding. If more than one starpoint is earthed, then the zero sequence
current test has to be performed for each earthed winding.
If the current between starpoint and earth is available and fed to one of the 1-phase current inputs of the
device the polarity of the earth current (starpoint current) at a 1-phase current input is essential for zero
sequence current inclusion of the differential protection and the restricted earth fault protection. If the star-
point current is not available then the zero sequence current tests serve for verification of the correct
processing of the zero sequence currents in the differential protection.
NOTE
It must be taken into consideration that tripping may occur if connections were made wrong.
Preparation of Zero Sequence Current Tests
Zero sequence current measurements are always performed from that side or 3-phase measuring location of
the protected object where the starpoint is earthed, on auto-transformers from the high-voltage side. In trans-
formers there must be a delta winding (d-winding or compensating winding). The sides which are not
included in the tests remain open as the delta winding ensures low-ohmic termination of the earth current
path.
The test arrangement varies with the application. Figure 3-35 to Figure 3-42 show schematic examples of the
test arrangement on a star-delta power transformer. The starpoint current is included into the tests. If it is not
available the relevant connection is omitted (compare Figure 3-35 with Figure 3-36).
3.3.9
Mounting and Commissioning
3.3 Commissioning
378 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
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