Infinite Bus Calculation

[Brian LeBlanc 913]

Need help with an infinite bus calculation for PEPCO networked utility transformers. We have (2)1000KVA transformers (208V, 3phase secondary) connected to a common bus. I need the fault current at the bus.

 

[Elect117]

Is that all you have? You will need the available fault current from the utility at their point of connection. You also need the impedance of the transformers and a configuration. As in, are the transformer parallel fed or series? Is one a redundancy to the other?

 

[ron]

For the infinite bus calc, take the full load amps of each transformer and dive it by the transformer impedance.

For example, if the transformer impedance from the nameplate was 5.75%, it would be 5552/.0575 = 96555A

 

[jim dungar]

Is that all you have? You will need the available fault current from the utility at their point of connection. You also need the impedance of the transformers and a configuration. As in, are the transformer parallel fed or series? Is one a redundancy to the other?

by definition, you do not need the utility fault current if you are performing an Infinite bus short circuit calculation.
You do need to know the transformer impedance, but guessing low just make the infinite bus value higher. So if you want a SWAG worst case value, use 5.0% for large transformers, like those over 500kVA, and 1.5% for smaller units especially pole top units.

 

[Brian LeBlanc 913]

Is that all you have? You will need the available fault current from the utility at their point of connection. You also need the impedance of the transformers and a configuration. As in, are the transformer parallel fed or series? Is one a redundancy to the other?

Yeah that's all we have at this time, the transformers are not redundant. The contractor want to order gear but the utility fault current letter is not ready.

 

[ggunn]

by definition, you do not need the utility fault current if you are performing an Infinite bus short circuit calculation.

Right. The infinite bus model assumes infinite available fault current (AFC) on the primary side of the transformer. I have made the calculation both ways when I had the numbers from the utility and it made little if any difference to the AFC on the secondary. Maybe if the primary AFC were significantly lower if the xfmr is out on the end of a long MV supply line it would make a difference, but the infinite bus model will always be conservative.