Impedance Of a Transformer

[Hileg]

What would be the ideal impedance for large commercial transformers (750 to 2000 kva). Is there any advantage to having lower impedance tranformers (2%)vs. (5%) transformers. I know the disadvanteges for the lower impedance tranformers are that the AIR rating of switchgear has to withstand the availbale fault current produced by lower impedance transformers. Is there a guideline for utilty companies to provide a certain impedance level so as not to force the building owner to have the expense of larger rated gear. Why do lower impedance tranformers exist?
Thanks

 

[GerH]

IEC Standard for Transformer sets guideline values.

 

[alehman]

Typically higher efficiency transformers have lower impedance. Some utilities purchase high efficiency transformers without consideration to the effect on customer's equipment.

5.75% is "standard" in ANSI regions, but there is a lot of variation.

 

[busbar]

Lower-impedance transformers have lower copper {full-load} losses and less voltage drop with increasing load. For ANSI regions, there are a handful of IEEE C57.12.** standards that typically apply.

 

[jbartos]

Suggestion: The lower impedance transformer has lower losses, creates smaller voltage harmonic distortion as a result of harmonic currents, causes lower voltage drops, etc. However, it causes the secondary short circuit fault levels be higher, costlier, bulkier, etc.
Generally speaking, the trend is to the higher Interrupting Rating equipment, which makes it more affordable. The advantage of such power distribution is in lower voltage drops, higher HP motors can be started with smaller voltage dips, etc. It is a matter of "pay now or pay later."
Usually, some trade offs or optimization is required. Many projects adhere to industry standards to obtain the best price for a transformer.

 

[Hileg]

Are K-rated Tranformers of low impedence design?

Thank you all for your help.

 

[jbartos]

Suggestion: Visit

http://www.olsun.com/Olsun Cap Bro .pdf

for k-rated transformers. The extra low impedance will probably be on a custom-made basis.