Application Guide - NP900 Series
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A996A
relay and for that the vector group selection of the transformer setup has either “N” or “n”
representing either HV side or LV side grounding.
What this selection actually does is that it deducts the calculated zero sequence current
from the per-unitized currents before differential calculation thus negating the outside
earth fault effect.
Figure 1.1.1.6-160 Correctly selected transformer setting prevents the differential function
operation in out of zone earth faults.
This selection basically does for the “N” or “n” selected side or both of them the zero
sequence elimination by applying the before mentioned correction.
IL1
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Corr
= IL1
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-
IL1
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+ IL2
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+ IL3
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3
