Delta-Wye vs Wye-Delta-Wye

[zrk]

Looking for advantages/disadvantages of Delta-Wye vs Wye-Delta-Wye windings in medium voltage transformer applications.

 

[Mbrooke]

In wye-delta-wye you will have zero phase shift between the two wye windings. Ferroresonance is usually not a concern with a primary wye-grounded connection. Insulation on the primary and secondary can be reduced as it approaches the neutral point. The delta can also give another usable voltage, ie 132-11-33kv.


Others can chime in on the disadvantes.


Also I assume you mean wye grounded-delta-wye grounded and not floating wye-delta-wye grounded which is a type of animal used "mimick" a delta / zig-zag?

 

[waross]

Wye delta may work well for transmission but it is not so good so good on distribution circuits.
Any voltage or phase angle unbalance on the primary of a four-wire-wye:delta will cause circulating currents in the delta.
In the event of a phase loss on the primary circuit, the delta will transfer power to the open phase and back feed the open phase. It will try to pick any loads on the open phase.
In an area where four-wire-wye:delta transformer banks are commonly used, it is common to see one fused cutout hanging open and the bank running open delta.
In the case of a reduced capacity delta winding, the primary protection may not protect the delta winding.
If you are positive that your supply will be stable and that there will be no single phase switching or fusing the wye-delta-wye will work.

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

 

[cranky108]

I do know of a company that uses a Y-delta-grounded Wye, and the biggest thing is it does not have a phase shift, and does not transfer zero sequence to the transmission system.

But I believe the reason they use it is so another area, which uses delta-wye will be 30 degrees out of phase. This make it almost impossible to connect the distribution systems. But with the distance of the areas, tieing the distribution systems would not be a good idea anyway.

 

[waross]

If all the transformers in a distribution system are delta:wye the phase shift may not be an issue.
The delta winding will provide a lot of voltage and phase angle correction, but at the expense of transformer capacity and heating.

does not transfer zero sequence to the transmission system.

But a single phase fault or unbalanced load on the secondary of a delta wye will cause equal currents to flow in all three primary delta windings.
Eg: A 10:1 transformer ratio with a 1000 Amp load on the secondary (wye or delta) will at first look seem to cause a 100 Amp current on the primary winding.
Not so. It will cause a 50 Amp current in each of the three primary delta windings.






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

 

[zrk]

Thank you for your responses. A quick follow up....

Mbrooke you're correct. I did mean wye grounded-delta-wye grounded and not floating wye-delta-wye grounded. I do not expect that the tertiary will be used for voltage; only for removing the zero sequence.

Another question for anyone out there. Looking at the transformer protection, is one easier to implement over another. Also, can I expect similar cost between these two units(60MVA)?