Wye Wye Unbalanced Loads?

[ihatekale]

Hey guys I'm a student taking Electro Mechanical Energy Conversion. Right now we're studying transformers and I had a couple of questions.

Question 1)

Why is it that a Wye Wye transformer cannot sustain an unbalanced load between line and neutral?

Question 2)

I know that a Wye Wye transformer has a hard time with the unbalanced load (although I'm not really clear as to why) between line and neutral, but can it sustain an unbalanced between line to line?

Thanks a lot guys! I know these maybe dumb questions but I'm taking mechanical engineering and electrical stuff has never been a high point for me.

 

[bob]

Question 1. You have some bad information. We try to balance the loads on this type of installation but in the real world YY banks operate in an unbalanced situation all the time. If the unbalance is too great you may experience some problems such as low voltage and transformers are hotter than usual.

Question 2. The balance between the phase and neutral is directly related to the balance between phases. If the phase to neutral load is balanced, the phase loads are also balanced.
Where did you get your information?

 

[ihatekale]

Hey bob,

Thanks for the reply.

These two questions of were asked of me in class and I could not answer them. They were formulated by my professor. According to a handout he gave it states that this sort of connection cannot satisfactory support unbalanced loads and he wanted us to explain why.
The only thing I could possibly think of is that this sort of connection does not have a path for third harmonic current, but I do not know enough about this to say this is correct by any means.
So if I understand you correctly, this connection can support an unbalanced load just not as effectively as a Delta Delta connection?
Sorry I'm kind of new to all of this so I greatly appreciate your input! Thank you.

 

[jim dungar]

In the real world, the vast majority of utility supplied padmount transformers are wye-wye connected.

I believe there is more to the instructor's question.

 

[AdrianWint]

I think the problem here is that you have only caught part of the question...


A Wye/Wye transformer CAN sustain a Line-Neutral load on the secondary, provided that the STAR point of the primary is connected to the supply Neutral.

If the primary star point is not connected to neutral then applying a L-N load on the secondary will result in the primary neutral point shifting & the secondary voltages becoming non-symmetrical.

Adrian

 

[ihatekale]

As far as catching part of the question, I am repeating word for word what he stated on the questions sheet that he passed out in class. So do you guys believe he needs to be more clear in the content of the questions to be able to expect a clear reply? As in it sounds like the questions he asked are very open ended and do not have a clear answer? Thanks!

 

[charlie b]

The problem I have with the question is its use of the word, ?sustain.? I have no clue what that is intended to convey. I can say that if you start with a Y-Y transformer, and connect the secondary to a three phase panel, and connect only one load to that panel, be it a line-to-neutral load or a line-to-line load, making sure that the load does exceed the transformer?s single or three phase rating, the load will get its voltage, and current will flow, and the load will operate, and the transformer will experience no ill effects, and this situation can go on for decades. So in what sense is the transformer supposed to not be able to ?sustain? this condition?

 

[bob]

These two questions of were asked of me in class and I could not answer them. They were formulated by my professor. According to a handout he gave it states that this sort of connection cannot satisfactory support unbalanced loads and he wanted us to explain why.
The only thing I could possibly think of is that this sort of connection does not have a path for third harmonic current, but I do not know enough about this to say this is correct by any means.
So if I understand you correctly, this connection can support an unbalanced load just not as effectively as a Delta Delta connection?
Sorry I'm kind of new to all of this so I greatly appreciate your input! Thank you.

I may not understand just what the Prof is implying. A YY installation supports unbalanced loads just fine as long as you keep the unbalance reasonably close. If the loads due gets too unbalanced you merely just reconnect from the heavy loaded phase to the lightly loaded phase. This can be done in the breaker panel or along the feeder .
This type of installation does have a path for harmonics to flow. The path is the neutral conductor. Suggest you try to get the prof to explain in more detail just what he means. Right now he is not making his point(what ever that is). I would say that the majority of commercial and industrial installations are Wye.