Wye-Delta-Wye Transformer? 4

[NAZ55]

Could anyone explain the function of this transformer circled in red? I have seen delta-wye transformations but never something like wye-delta-wye.

Also, could you think of any situatio where a wye-wye would be a better application than a delta wye?

Thanks for your help

 

[tem1234]

This is a buried delta transformer

you can look at thread238-130030

main function are (taken from the book Power transformer principles and applications) :

-It provides a path for third harmonic currents.
-It helps stabilize voltages in the Y-Y primary-secondary connection.
-It provides a grounding bank action by providing a path for zerosequence currents.
-It provides the necessary voltage taps for regulating the low-side voltage.

These are very commun in wind farm project

 

[waross]

"-It helps stabilize voltages in the Y-Y primary-secondary connection."
The downside of this is that if a primary phase is missing or the primary voltage is more than a few percent out of balance, the delta winding will try so hard to match the primary voltages that it may burn out. This may be detected by an appropriate protection scheme.

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

 

[jghrist]

Wye-wye is better than delta-wye where you need to have no phase displacement from primary to secondary or when you are concerned with ferroresonance when single-phase switching and there is cable between the switch and the transformer.

 

[davidbeach]

Any transformer with a delta winding can be built on a three-legged core. A transformer that is just wye-wye needs to be on a four- or five-legged core to avoid zero-sequence flux circulating in the tank. In many cases it is cheaper to bury a delta tertiary on a 3-legged core than to build the bigger core.

 

[Marmite]

Unless of course you want the tank to provide a return path for zero sequence flux by design to reduce the zero sequence impedance. This type of 3 limb Yy design is common in the UK distribution system.
Regards
Marmite

 

[tem1234]

I think it's less expensive to do it this way instead of a wye-delta with a grounding transformer on the delta side.

 

[NAZ55]

Thank you all for responding.

Now that I understand YYd a little bit better, another couple of items which I found intriguing about windfarm connection are

1- why is the generator connected Y-grnd : delta for windfarms instead of typical delta: Y-grnd

2- What's the purpose of this grounding transformer or zig zag at the location shown i.e. why would you need a connection to ground at the location shown in red and its benefits. Is it for a certain protection scheme. Also can its location be changed to the other two spots as shown in the picture?

Thanks again for all your postings. I really feel like we have a bright bunch here and I tremendously enjoy your insights.

 

[jghrist]

See

http://www.beckwithelectric.com/powerlines/powerlines-37.html#4

for a comparison of transformer connections on IPP (distributed) generation.

 

[prc]

Normally,as Zazmat said generator trfs are delta /star.This is avoid the zerosequence currents entering to generator stator for any LG fault on HV lines.Generators are adversely affected by zerosequence current.

In wind generation stations,small turbine trfs are connected to a main star /star trf where by line fault zerosequence currents will not enter in to generator.

Zig/zag or delta /earthed trfs connected to the unearthed grid to get a neutral for ground fault protection in that unearthed system.Your proposed alternate points also seems OK to me.

The above is my hunch.We may hear from better knowledgeable protection /wind generation gurus in this forum.