Specifications

7SR242 Duobias Applications Guide

6.2 Protection of Star/Star Transformer With Tertiary

Winding

Figure 6-4:

The provision of the tertiary winding in star/star transformers both stabilises 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.

The transformer has a phase shift of zero. To prevent undesired tripping for external faults a zero sequence shunt

is required, this is implemented by selecting star/delta interposing CT settings. The Interposing CT Connection

setting on all sets of current inputs must be set to the same Yd setting e.g. all Yd1, -30° or all Yd11, 30°.