Combined size and impedance of Single Phase transformer bank

[philly]

I am told that a given transformer bank has (3) single phase 15kVA transformers connected in a wye-wye 3-phase transformer bank. I am told that each 15kVA single phase transformer has a %Z impedance of 1.2 and an X/R ratio of aproximately 1.1.

I'm pretty sure I know the answer but wanted to confrim when when modeling this arrangement for fault contribution it would be the same as modeling a 45kVA 3-phase transformer with a 1.2% impedance and 1.1 X/R ratio. Is this correct?

Also on a slightly different note why do utility transformers have such a low impedance value? In this case it is 1.2% but I have seen other larger transformers that also have a low %impedance compared to other distribution transofmers of the same size that typically have 5% impedance or there about. Is it simply because there will be less losses across the transofmer that the utility will have to pay for?

 

[jim dungar]

I am told that a given transformer bank has (3) single phase 15kVA transformers connected in a wye-wye 3-phase transformer bank. I am told that each 15kVA single phase transformer has a %Z impedance of 1.2 and an X/R ratio of aproximately 1.1.

I'm pretty sure I know the answer but wanted to confrim when when modeling this arrangement for fault contribution it would be the same as modeling a 45kVA 3-phase transformer with a 1.2% impedance and 1.1 X/R ratio. Is this correct?

Also on a slightly different note why do utility transformers have such a low impedance value? In this case it is 1.2% but I have seen other larger transformers that also have a low %impedance compared to other distribution transofmers of the same size that typically have 5% impedance or there about. Is it simply because there will be less losses across the transofmer that the utility will have to pay for?

Yes to your modeling.

Yes to the reason, utilities have to 'absorb' losses, therefore they want them as low as possible. But the physical size of small oil-filled transformers also factors into their low %Z.

 

[bob]

I agree with Jim. I wanted to know were the X/R figure came from.
Did you get it from a chart or something like that?

 

[philly]

I agree with Jim. I wanted to know were the X/R figure came from.
Did you get it from a chart or something like that?

This was the X/R ratio the utility gave for each of the single phase transformers. Transformes also had an impdance of 1.2%

This X/R ratio does seem low. Would you recommended using an X/R ratio from a standards table instead? What standard is typically used for looking up X/R ratios?

 

[jim dungar]

This was the X/R ratio the utility gave for each of the single phase transformers. Transformes also had an impdance of 1.2%

This X/R ratio does seem low. Would you recommended using an X/R ratio from a standards table instead? What standard is typically used for looking up X/R ratios?

For short circuit device evaluation I user the data provided by the utility, as it is almost always worst case.
For arc flash studies my company has developed internal procedures to judge the 'validity' of any provided values. Typically we end up running at least two different available fault current scenarios during our studies.