Wye Wye Transformer connection Pros and Cons 1

[sebp7804]

Hi guys...I´m looking for some information (technical or personal) that be related with the pro´s and con´s to use the Y Y transformer connection in comparison with the other ones.......anyone can help

 

[jghrist]

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

deals with transformer connections in relation to connection to IPPs. Also see:
IEEE guide for application of transformer connections in three-phase distribution systems
ANSI/IEEE Std C57.105-1978
Abstract
This guide concerns transformer connections in 3-phase distribution systems. Distribution systems are characterized by primary voltages up to and including 34.5 kV, usually have a preponderance of connected transformers with low-voltage windings below 1000 V and furnish electric service to consumers. The characteristics of the various transformer connections and possible operating problems under normal or abnormal conditions are treated. All combinations of Delta and Y, grounded and ungrounded, T connected, zigzag, and certain special connections are considered. Only two-winding transformers are included.

 

[rbulsara]

Refer to page 43 in the following, for some grounding related information on Wye-wye xfmrs.

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

 

[sebp7804]

I+ve went through that standard but they dont set the pure differrences and advantages of several connections.....it´s a very good standard but in that concern they are not very deep

 

[rbulsara]

You will probaly not find such a clear cut comparisions because pros and cons varies based on application. A pro for one application could be a con for other.

What application do you have? Wye-Wye are mostly used by utiltiy companies mainly for two reasons. One is the groundind as inidcated in IEEE green book, that allows them to ground the grid at more points.

Also utility grid could have generation on either side of the transformers, so wye-wye provides convenient point of grounding the neutral at source, regardless of which direction the power is flowing.

Other is cost. I beleive wye-wye costs little less than a delta-wye or delta-delta? May be due to difference in insulation as wye-wye coils need insulation for L-N voltages. Don't really know.

If you are an end-user and your application is generally unidirectional, I would use dela-wye. Wye being the secondary (user) side. It is most common configuration. I would not use any other configuraiton unless it is somehow required otherwise.

 

[swz202]

I'm reading Handbook of Practrical Electrical Design by J.F. McPartland & Brian J. Mc Partland. Page 445:
Wye-Wye transformer connections should generally be avoided because of the objectionable characteristcis of such connections.
First, a connection of this type generally requires that the primary neutral point of the transformer windings be connectged to the neutral of the source. Without this connection of the prumary neutral, unbalanced loading from phases to neutral on the secondary side produces a " floating neutral" on the primary side, with a serious unbalancing of secondary wye voltages and reduced power handling capacity.......
The second, and major, objection to the general use of wyr-wye connected transfoermers arises from the character of the harmonic current genreated in such a hookup........

 

[jghrist]

A good paper on the subject is Wyes and Wye Nots of Three-phase Distribution Transformer Connections, Robert Rusch and Michael Good, IEEE Transactions on Industry Applications, Vol. 26, No. 4, July/Aug 1990.

Grounded Wye- Grounded Wye: This connection is suitable for three- or four-wire service and prevents the occurrence of ferroresonance. Common voltages are 480Y/277 or 208Y/120 V. Four- or five-legged core construction prevents tank heating which may be common on three-legged core construction. The major disadvantage results when the customer also requires 120/240 single-phase power.

 

[RalphChristie]

The most common use for a star-star transformers is as station transformers in Power station auxillary systems, like stated by rbulsara. You can find more information regarding it in the J&P Transformer Book. (Twelfth edition - Martin J. Heathcote. By the way, this is one of the best, if not the best, transformer books I've seen)

Cost might be an issue, especially on the higher voltages. Normally windings subjected to a voltage of 66kV and higher are connected in star, because, in a star-connection, the winding isolation can be degraded towards the neutral point. But usually the lower voltage side will be connected in delta and a grounding transformer will be used to create a grounding point.

It depends on your application, but I would rather try to stay away from star-star transformers due to protection and harmonic issues. Why would you want to use a star-star transformer?

Regards
Ralph

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[swn1]

Star-star (wye-wye) usually happens around here because the utility connects single-phase loads line-to-ground on the primary side, so they have lots of pole-mount transformers rated for the L-N voltage of their distribution system on hand. So they build 480/277 and 120/208 wye-wye using the same transformers.

The only other reason would be if you need to be able to pass zero-sequence current through the transformer for some reason.

If establishing a ground source for the system is the issue consider adding a separate zig-zag transformer and grounding resistor.

 

[dpc]

Ralph,

In the US, wye-wye pad-mounted transformers have become the norm for a lot of utilities serving commercial customers, at least out west where I am.

The transformers are a little cheaper because the primary winding can be graded and it eliminates supposed risk of ferro-resonance when single-phase switching or fusing is used upstream.

When my client is on the secondary side, I generally push for a delta-wye transformer for all the normal reasons.

 

[RalphChristie]

swn1:

"the utility connects single-phase loads line-to-ground"
Are you refering to SWER-lines?

dpc:

After reading again through the answers I noted jghrist also mentioned ferroresonance. I have not thought about it previously, but it makes sense. Thanks. At least we all agree on using a delta/star connection, if possible, most of the times.

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[RalphChristie]

Dammit! Here should be an edit button. Always see the faults after I've pressed "submit"

"At least we all agree on using a delta/star connection, if possible, most of the times."

I am refering to end users.

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