2.2 Differential Protection
117
7UT613/63x Manual
C53000-G1176-C160-2
1
/
3
· (2 I
L1
– 1 I
L2
– 1 I
L3
) =
1
/
3
· (3 I
L1
– I
L1
– I
L2
– I
L3
) =
1
/
3
· (3 I
L1
– 3 I
0
) = (I
L1
– I
0
).
Zero sequence current elimination achieves that fault currents which flow via the
transformer during earth faults in the network in case of an earth point in the protected
zone (transformer starpoint or starpoint former by neutral earth reactor) are rendered
harmless without any special external measures. Refer e.g. to Figure 2-30: Because
of the earthed starpoint, a zero sequence current occurs on the right side during a
network fault but not on the left side. Comparison of the phase currents, without zero
sequence current elimination and without inclusion of the starpoint current, would
cause a wrong result (current difference in spite of an external fault).
Figure 2-30 Matching the transformer vector group, example YNd5 (magnitudes not consid-
ered)
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.
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