Auto TXFMR and Wye-Gndd/Wye-Gndd TXFMR zero sequence network 2

[Uquresh1]

The zero sequence network for an Auto TX and for a Yg/Yg transformer both show that there is no connection to the ground bus (No), which means there will be no 0seq current flowing through that circuit (unless there is a Delta tertiary which does connect to the ground bus in the 0seq network).

My question is: for transformer differential application, for the differential CTs (connected Wye) from high side and low side of transformer, do we need to account for any 0seq current showing up on, say, lowside CT for a through fault and causing a 87 trip since high side CT didn't see any 3Io. Digital relays have the ability to select an option to filter 0seq current from these CT readings, but do I really have to filter this 0seq current in the case of Auto or Yg/Yg transformers?

Thanks for your input.

 

[prc]

If the auto star neutral is connected to earth(ground),then zerosequence current will flow. In fact thsre are two parallel paths in zerosequence circuit, N &T. See 1954 December IEEE paper- Auto Transformers for Power System- O T Farry

 

[stevenal]

There is no connection to the zero sequence reference bus at the transformer, but there is no open circuit in the zero sequence network either. The transformer you describe is represented as an impedance between the two terminals in this network. No filtering is needed since zero sequence will show up in the same proportion on both sides for unbalanced load and through fault conditions. It is when there is a connection to the reference bus that filtering needs to be considered.

 

[DaveDuncan]

As there is no connection from either neutal to earth / ground, no earth fault current can flow through the transformer, or be sourced from the transformer at HV or MV. This is shown in the table from the Schneider NPAG book, where the top diagram represents the condition described. All the links are open, thus no flow to earth is possible.

As no earth fault current can be sourced from the transformer, the differential protection (87) does not have to cater for a zero sequence shunt.

 

[stevenal]

Dave,
The OP spoke of an auto and a Yg/Yg transformer. You need to look at the third diagram.

 

[Uquresh1]

@Steven , Now if I do have a delta tertiary on the Auto txfmr, and also on the Yg-Yg txfmr, would I then need the zero seq. filtering? Would I need the filtering on both HS and LS windings? Also does it matter that there is load connected to the Delta tertiary, or if it's there but not being used (what I am getting at is would the delta tertiray still connnect the Zero seq. network to the Gnd bus whether its has load connected or its unused).

Thanks.

 

[stevenal]

NPAG and Westinghouse/ABB T&D both show a neutral connection. If you were to inject a unidirectional current into one set of a wye connected winding, you could satisfy Ampere's law with nothing coming out the other wye side. Load connected to the delta makes no difference in this network, since zero sequence is confined to delta where it remains unmeasured. I would apply zero sequence filtering to both wye windings to avoid misoperation.