Siemens 7SR242 Duobias, Technical Manual

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©2015 Siemens Protection Devices Limited Chapter 7 Page 51 of 64
Section 6: Application Considerations and Examples
6.1 The Effects of An In Zone Earthing Transformer
The in zone earthing transformer is a source of zero-sequence fault current. An earth fault on the delta side of the
transformer external to the differential protection zone will cause zero sequence currents to flow in the CTs on the
delta side of the transformer without corresponding current to flow in the line CTs on the star side of the
transformer. If these zero sequence currents are allowed to flow through the differential elements they may cause
undesired tripping.
Figure 6-1: Relay Currents – External Earth Fault with In Zone Earthing Transformer
To prevent undesired tripping the ICT connections should be such as to cause the zero sequence currents to flow
in a closed delta CT secondary connection of low impedance instead of in the differential relay operating coil. As
we have already corrected for the transformer vector group on the star side a Ydy0 ICT is used on the delta side
winding.