Zero Sequence flow of YgYg transformers

[CircuitSherlock]

I need some help developing a mathematical and intuitive understanding of zero sequence current through a YgYg transformer. From previous post I’ve understood that YgYg transformers can pass zero sequence current, i.e if there is a ground fault on the secondary both primary and secondary see the zero sequence current. However, if there is a ground fault on the primary this transformer will not contribute zero sequence current to the fault. If the YgYg transformer was downstream ( say a station service transformer) of a DYg transformer then this means all the fault contribution will be from the DYg transformer even though there is a parallel ground path through the YgYg transformer?

 

[David Castor]

The Yg-Yg transformer simply allows the zero sequence current to flow through the transformer (effectively) and is never a source of zero seq current. If you have a source on the secondary side (such as a delta winding of a transformer or a grounding transformer, then the zero sequence current (ground current) will pass through the Yg-Yg to the fault.

 

[ron]

This may be of help https://library.industrialsolutions...e Papers|GET-3388B|PDF&filename=GET-3388B.pdf

 

[CircuitSherlock]

Thanks @David Castor. I'm attaching a sketch of a scenario. I wonder if the depiction of fault current flow is correct. I see this creating a current divider with the ground current throwing through reach transformer neutral point based on winding impedance of each transformer. This case was ran in software and no current flowed through the YgYg. I'm just having a hard time understanding why. https://drive.google.com/drive/folders/1Mui5iF7AEmHoD0q7PXmTv_nGNuv-07o_?usp=sharing

 

[CircuitSherlock]

Fault Scenario

 

[David Castor]

It depends on what is connected to the transformer on the right. If the winding on the right is just going to loads, then there won't be any ground fault current flowing in that transformer.

With transformers, if current is flowing in one winding, there has to be current flowing in the corresponding winding on the other side of the transformer. This is true for zero sequence currrent as well. In the delta-wye xfmr, zero seq current will flow around the delta so it acts as a ground source for ground faults on the wye side.

 

[JoeStillman]

The Yg-Yg transformer simply allows the zero sequence current to flow through the transformer (effectively) and is never a source of zero seq current. . .

Actually, the Yg primary can feed zero sequence current to an upstream ground fault. The SEI plot in my avatar is an image of exactly that.

 

[kingpb]

Actually, the Yg primary can feed zero sequence current to an upstream ground fault. The SEI plot in my avatar is an image of exactly that.

Yep, been there. The utility switching on their line had enough unbalanced load that it looked like a ground fault, and the transformer zero sequence looked like a fault and would trip the transformer main on ground fault.

 

[David Castor]

The Yg-Yg can pass through ground fault current IF there is a zero sequence source on the other side, but it cannot, on its own, source ground fault current. If there is a Yg-Delta transformer somewhere downstream, then that zero sequence current will flow through the Yg winding back upstream.