Problems Associated with Star-Star Transformers
Problems Associated with Star-Star Transformers
(OP)
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.
Thanks.






RE: Problems Associated with Star-Star Transformers
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RE: Problems Associated with Star-Star Transformers
* Basically, I would like a full-time job on part-time basis *
RE: Problems Associated with Star-Star Transformers
o Voltage can become severely unbalance with unbalance load.
o Large third harmonic voltage.
o Possibility of shifting neutral point.
RE: Problems Associated with Star-Star Transformers
RE: Problems Associated with Star-Star Transformers
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.
RE: Problems Associated with Star-Star Transformers
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
RE: Problems Associated with Star-Star Transformers
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.
RE: Problems Associated with Star-Star Transformers
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.
RE: Problems Associated with Star-Star Transformers
RE: Problems Associated with Star-Star Transformers
RE: Problems Associated with Star-Star Transformers
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.