Banking transformers of differing impedances

[bulllymann]

Are there any issues with banking transformers of differing impedance's. We lost 1 transformer of an existing transformer bank of 3-150 kva transformers 14.4/25 kv connected Wye/Wye 277/480v secondary. Existing bank had transformers with 3.40% impedance and the replacement transformer is a 167 kva with an impedance of 4.19% They went together fine and are in operation but they are lightly loaded. Around 300 amps max. Is the impedance differential low enough to continue operation or should a transformer be found that more closely matches the impedance of the existing transformers?

 

[jghrist]

I don't see a problem as long as the voltage drop does not cause an unacceptable voltage unbalance. This shouldn't be a problem with the transformers lightly loaded. The ohmic impedance of the 167 kVA transformer is not that much different from the 150 kVA transformers.

 

[crshears]

I'll second what jghrist wrote, since load division between the two transformers will be inversely proportional to the dissimilarities in their impedances and not the dissimilarities in their ratings. I'd keep an eye on their respective current flows if the pair becomes more heavily loaded, but beyond that I wouldn't expect it to be an issue.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[bulllymann]

Thanks Gentlemen,
Your feedback is greatly appreciated.

 

[waross]

" 167 kva with an impedance of 4.19%" The impedance of your 167 KVA transformer on a 150 KVA base will be 4.19% x 150/167 = 3.76% That's a lot closer to 3.4%.
Further, the impedance is most important in short circuit conditions when the transformer impedance is the only impedance in the circuit.
Under normal load conditions the voltage regulation is more important than the %impedance. The regulation is related to the impedance by the X:R ratio and is always less than the impedance. with impedances of 3.76% and 3.4% the difference is only 0.36% The difference in regulation will probably be less.
As the transformers become more heavily loaded and the voltage on one transformer drops slightly lower than the others, you may see some motor current unbalance but not much. Many distribution circuits have more than that much voltage unbalance to start with. Particularly long distribution circuits with a lot of single phase loads.
No problem.
Note: "load division between the two transformers will be inversely proportional to the dissimilarities in their impedances and not the dissimilarities in their ratings."
True. But that relationship applies to transformers in parallel, not transformers in a wye:wye bank.

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

 

[crshears]

Good point re your note, Bill, particularly once I re-read the OP.

Sorry about that.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]