[dw_7SR5_function87Tsettingsexample8, 1, en_US]
Figure 5-60
7SR24 Applied to Yd Transformer with an In Zone Earthing Transformer
Protection of Star/Star Transformer With Tertiary Winding
[dw_7SR5_function87Tsettingsexample9, 1, en_US]
Figure 5-61 Protection of Star/Star Transformer with Tertiary
The provision of the tertiary winding in star/star transformers both stabilizes the neutral potential and can
allow earth fault current to flow in the secondary connections i.e. reduces the zero sequence impedance. An
earth fault on the LV side of the transformer external to the differential protection zone will cause zero
sequence currents to flow in the CTs on the LV side without corresponding current to flow in the line CTs on
the HV side. If these zero sequence currents are allowed to flow through the differential elements they may
cause undesired tripping.
Protection and Automation Functions
5.22 87T Transformer Differential Protection
324 Reyrolle 7SR5, Transformer Protection Device, Device Manual
C53000-G7040-C015-1, Edition 11.2019
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