Do we need zero sequence current elimination for a YNd11 transformer ? 1

[usercan]

Should we activate zero sequence current elimination in differential relays for a YnD11 power transformer with a zig-zag grounding transformer on the delta side and grounded on the star side ?

 

[cranky108]

So you have a hopner connected transformer? Last one I saw had delta connected CT's on both sides.

 

[stevenal]

Yes. Zero sequence on the delta side circulates within the delta where it is not measured by the CTs.

In the past we would connect the CTs on the wye side in delta to filter the zero sequence while correcting for phase shift. With microprocessor relays, we connect CTs in wye on both sides, and perform the filtering and phase shift in the relay.

 

[RRaghunath]

It is YNd11 Step-down transformer, with a Zigzag transformer on delta side providing earth reference.
Everyone agrees Zero sequence filtering is to be selected on Y-side.
With regard to Delta side, it depends on how the Zigzag transformer is connected.
# If Zigzag transformer is connected at transformer terminals and the CTs are located in the switchgear incomer panel, zero sequence filtering is required.
# If Zigzag transformer is connected to the switchgear busbar, the incomer CT doesn't see zero sequence currents and hence no filtering is required.

 

[usercan]

It is YNd11 Step-down transformer, with a Zigzag transformer on delta side providing earth reference.
Everyone agrees Zero sequence filtering is to be selected on Y-side.
With regard to Delta side, it depends on how the Zigzag transformer is connected.
# If Zigzag transformer is connected at transformer terminals and the CTs are located in the switchgear incomer panel, zero sequence filtering is required.
# If Zigzag transformer is connected to the switchgear busbar, the incomer CT doesn't see zero sequence currents and hence no filtering is required.

Thanks for great answer. This might be slightly off topic but how does relay sense the zero sequence current for an external fault ? Does relay always calculate the symmetrical components?