ECN Forum Forums Technical Discussion Electrical Theory and Applications open delta banking concerns

hello there..friends please help me with my problem.
Is it possible to bank 2 transformers in open delta if it has different kva rating?
assuming that both have the same voltage rating,impedance ratio and frequency.
If it is possible, then how would i compute the power contribution of each transformer?
your inputs would truly be appreciated.
thanks in advance,.,.

My answer to your question is yes!
As I know there are 7 condictions should be met when you parallel 2 transformers:

1)P-S voltage must be the same!
2)Proper polarities connetion
3)Percentage impedence of one is between 92.5% and 107.5% of the other
4)Same tap setting
5)Identical phase displacements.
6)Identical frequency
7)Equivalent impedence and X/R ratio

So I think it is possible to bank 2 transformers in open delta even if they have different kva ratings!!!
And the total Apparent Power=(P1+P2)x86.6%!

The utilities do this all the time. You might have a 50 kva and a 10 kva banked together. This is done when the 3 phase load is small. The 3 phase load is 57% on each transformer and the lighting transformer(50kva) will carry 100% of the single phase load.

[This message has been edited by Bob (edited 01-21-2005).]

 
There are some assorted cut-and-paste notes from El Paso Electric on non-symmetrical delta- and open-delta-secondary transformer banks. [It is unnecessary that the pair of transformers be matched.] These configurations are mostly intentionally unsymmetrical to allow for inherently unbalanced 1ø and 3ø loads.

Save file http://6L6.net/bjarn/ecn.DSOexcerpt-7delta.pdf ~1.1MB {available for a short time}

Another [fee-based] reference is §8 in ANSI/IEEE Std C57.105-1978 …Application of Transformer Connections in Three-Phase Distribution Systems.




[This message has been edited by Bjarney (edited 01-22-2005).]

guys thanks a lot. now it is clearer to me.i hope you would still answer mr questions cause i have a lot. btw i am a newly licensed practitioner so your inputs would truly help me...once again thanks..

The seven requirements are important when paralleling transformers.This is when the transformers primarys are connected together and their secondaries feed the same load. When you are banking transformers
like three transformers feeding a three phase load or two transformers in an open delta configuration the impeedance is not important. The only exception is primary delta and secondary delta. Then you want to have your impeedances close. In the case of the open delta with the 50kva and the 10 kva you woutd be just fine. If you bank a transformer with two others the one with the lower impedance will take more of the load. Being that the impedance of the 50kva is most likely lower than the 10 kva you are in good shape.