Transformer Grounded Wye - Grounded Wye with Delta Tertiary 1

[Uquresh1]

I have a Grounded Wye - Grounded Wye txfmr with delta tertiary (tertiary used for station service). My question is - Do I need a ground relay on the High side? For a Low side ground fault, will my High side see Zero seq current? If it will, would it be the same as what the low side sees?

What is the source for the gnd current? the tertiary? Please explain, and give any other related points that you have think are useful.

Thanks

 

[Kiribanda]

Uquresh1,
For a ground fault on the high side primary,the WYE-WYE trafo is equivalent to a DELTA-WYE trafo with no zero seq current contribution to the ground fault.That means only the overcurrent relays will respond.But for a ground fault on the low side secondary the zero seq currents will (a)pass to the primary (b)circulate in buried tertiary DELTA.Therefore,a calculation is necessary to estimate line currents in all three windings knowing Zps,Zpt & Zst values of the trafo.

 

[davidbeach]

No, a grounded wye-grounded wye (with or without tertiary) never looks like a delta-wye. Look at the sequence connections for both (actually all three) types and you'll see that each is very different.

 

[waross]

With a delta or phantom delta involved the healthy phases will contribute to the fault current. Try modelling the delta with a short on one phase and then double the current contribution for the total delta.(adjusted for ratios of course).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mgtrp]

For an earth fault on the low side of your transformer, you should see some neutral/zero sequence current on the high side (presuming there's a source on the high side), but not all of what you see on the low side. The amount depends largely on the impedances between the various transformer windings.

For a fault on the high-voltage system, you will also see some neutral/zero sequence current on the high side.

Find a text on sequence networks for three-winding transformers and start sketching it out.

Cheers.

 

[waross]

Isn't The primary zero sequence current PU_Pri = PU_Sec + PU_{Delta contribution} + PU_{Transformer losses}?

Bill
--------------------
"Why not the best?"
Jimmy Carter