Problems Associated with Star-Star Transformers 4

[Jorgy1]

I attended an interview on Monday and one of the questions asked was that I should tell why star-star transformers are usually not used. According to the questioner, there is a particilar problem with them that makes their use not advisable. I have read a lot about transformers but have not come across any such problem. Could anyone share his/her experience with me.
Thanks.

 

[RalphChristie]

See:

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

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[edison123]

Star/Star connection have the problem known as swinging or unstable neutral


* Basically, I would like a full-time job on part-time basis *

 

[cuky2000]

Some of the disadvantages of Y-Y connections are:
o Voltage can become severely unbalance with unbalance load.
o Large third harmonic voltage.
o Possibility of shifting neutral point.

 

[davidbeach]

edison123 & cuky2000's responses only apply if the transformer is left ungrounded. Grounded and ungrounded wye-wye transformers will respond very differently to various system conditions. One could always ground one side and get something in between the grounded and ungrounded conditions. For what ever reason, I always think of wye-wye transformers as grounded, so their responses would not apply to my first impression. The site referenced by RalphChristie applies ONLY to the grounded-grounded version of the wye-wye transformer.

 

[cuky2000]

Davibeach,
Reviewing the question: “why star-star transformers are usually not used?”. This to me refers to ungrounded configuration since grounded connection is often used in many power systems applications.
Obviously, the solution of the problem mentioned above is resolved grounding the neutral or adding a tertiary winding connected in delta.

 

[waross]

I attended an interview on Monday and one of the questions asked was that I should tell why star-star transformers are usually not used.

Given the number of countries in which this is the preferred connection for three-phase power when using three individual transformers, it sounds like a trick question. This connection is usually used.
There are exceptions with three phase transformers as has been noted.
As far as a floating neutral, this was a problem when delta systems were first being converted to star. This issue has been understood for so many generations now that I thought that the standard response to a situation where a star-star transformer bank is connected with a floating primary neutral is "Uhh fellas, you forgot a wire."

yours

 

[jghrist]

I suspect a culture clash. It's true that in North America, the grd wye - grd wye connection is the most common for utility distribution transformers (MV-LV). This connection is uncommon in the rest of the world, however. I suspect the original poster Jorgy1 is from the rest of the world because of his use of the term star-star.

The main problem with grd wye - grd wye is that in much of the world, the MV system has impedance grounded neutrals that do not work well with phase-neutral connected loads. In North America, the main problem historically has been that the connection provides a path for third harmonic currents that could causes telephone interference. The telephone interference problem is not very significant anymore with very little open wire communication lines around.

 

[cuky2000]

Hello Jgrist,

Even though communication interference with telephone, radio & TV is not a major issue today,as you mentioned, still utilities need to comply with the Federal Communication Commission rules in this matter.

Other reason why Yg-Yg connection is still popular in the US is because is less prone to ferroresonance. In addition, the cost of transformer is lower since do not require full winding insulation.

Exception to this is the Yg primary with a 4 or 5-legged core.

 

[Jorgy1]

Thank you all for the various responses. I am from Africa, specifically from Ghana. The design of our distribution system is based mainly on European standards and as far as I can remember our system has only one star-star 33/11 KV transformer with the neutrals grounded. I believe your various posts have answered the question. The interviewer mentioned the fact that where they are used, a tertiary winding is provided and so I think he is referring to the 3rd harmonic problem. But can someone explain how the presence of 3rd harmonics affect the rest of the power system or the operations of the transformer apart from the telephone interference. I understand harmonics are produced from non-linear loads in the system.

 

[jghrist]

When a Y-Y connection is used and there is a phase unbalance, zero-sequence flux is produced. If the transformer is a 3-legged core design, there is no path for the zero-sequence flux (except through the tank). Distribution transformers usually are of a 5-legged core design to provide a low reluctance path for zero-sequence flux. Power transformers usually use a delta tertiary winding to provide balancing ampere-turns.

 

[peebee]

This is sort of implied in some of the above posts, but just to be a bit more explicit about it:

Any zero-sequence currents (like 3rd-order harmonic currents or single-phase currents) will pass right through a Yy transformer and appear on the primary.

So long as you have a delta winding in there somewhere, though, those zero-sequence currents will be converted into a circulating current in the delta winding. They become "trapped" in the delta winding, and are not passed on back to the primary side.

That "trapping" of currents as delta circulating currents may imply that you need to derate your transformer, but it may well be worth it to keep that "bad" stuff from screwing with your primary-side distribution system.