Help with 1ph load on 3px TX calc 2

[RichCarnell]

Could someone please explain to me what the primary side currents in a wye-wye unearthed TX would be for a given phase to neutral load on the secondary. I understand that there would be a sharing of current to some degree across all of the primary windings, but how could they be determined?

 

[RichCarnell]

Thanks waross.
I think that it might be time to give up on this one as I am begining to see just how complex this could be.
This isn't an assignment, it's something that was raised in class in the protection training that I am currently doing. The problem could be summed up with a response from marmite:

There's some information on this in the ABB Book Short Circuit Duty Of Power Transformers, Section A13.4. I don't think it's downloadable, but you can order it. Assuming a unit turns ratio, a single phase load current I on the secondary corresponds to a current at the primary side of 2/3 I in the loaded phase and 1/3 I in each of the unloaded phases.
Regards
Marmite

The reasoning for the 1/3 and 2/3 currents that was given in class didn't make sense to me and it was my own curiosity that lead me to start this thread. As I now see, the correct answer to the problem is way beyond the scope of the course and that I should just accept the solution that was given.
Perhaps when I am 1000 years old, am a little bit smarter and have nothing better to do I'll persue it again.
I will see if I can find that book and have a bit of a look.
Thanks again for your trouble.
Rich

 

[waross]

That sounds like a delta:wye rather than a wye:wye
With a wye primary with a floating neutral, Kirkoffs law must be obeyed and the current arriving at the neutral from the loaded phase must equal the current leaving the neutral through the two unloaded phases. That would be "A" phase = 100% and ("B" phase plus "C" phase = 100%).
In a generator connected in delta or double delta to feed a single phase load, there is equal current in all three windings. However the current is in phase with the load so it is in phase on one phase, leading on one phase and lagging on one phase.
The one third/ two third figure may have to do with short circuit conditions, where the phase angles are determined predominantly by the transformer X/R ratios rather than by the load X/R ratio.
AS you say, time to give it up.


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

 

[Marmite]

Gents, I've attached a scan from the ABB book I referred to. It also references another book Transformer Engineering Blume et Alii published by Wiley.
Regards
Marmite