3 Mounting and Commissioning
404
7UT613/63x Manual
C53000-G1176-C160-2
Figure 3-28 Differential and Restraint Currents - Example of Plausible Measurements
3.3.9 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 starpoint current is not avail-
able then the zero sequence current tests serve for verification of the correct process-
ing 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 three-
phase measuring location of the protected object where the starpoint is earthed, on
auto-transformers from the high-voltage side. In transformers 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. Figures 3-29 to 3-36 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 3-29 with figure 3-30).
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