Zero sequence compensation for external earth faults
This example did present only one type of transformer and its properties. Very common variation in this
kind of transformer is when the star side, no matter HV or LV or both, is grounded thus forming a route
outside of the “Differential zone”.
Figure. 5.4.18. - 83. Transformer grounding and external fault.
The grounding needs to be known in the differential current calculation since if it is not compensated
any low impedance earthfault outside of the differential zone shall cause differential current measured
and possible tripping of the differential protection. For this purpose the calculated zero sequence
compensation is used. This has to be told to the 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.
AQ-T257
Instruction manual
Version: 2.00
163
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