Specifications

7SR242 Duobias Applications Guide

6.5 Protection of Auto Transformers

The transformer has a phase shift of zero. To prevent undesired tripping of the overall differential protection for

external faults a zero sequence shunt is required, this is implemented by selecting star/delta ICT Connection

settings. The ICT Connection setting on all both sets of CT inputs must be the same e.g. all Yd1, -30° or all

Yd11, 30°. The inrush inhibit (81HBL2) must be Enabled as the magnetising inrush currents in each phase will not

balance.

Figure 6-8: AC Connections for Auto-Transformer Overall Protection

REF protection can be applied if a neutral CT is available and all CTs have the same ratio:

Figure 6-9: AC Connections for Auto-Transformer Overall and REF Protection