Short Circuit Calculations

[lile001]

Short circuit current depends heavily on the impedance values of the main utility transformer. In an installation with a short main feeder, the utility transformer may be the only thing between our switchgear and the entire output of hoover dam ending up in a arcing fault in our electrical closet.

I get wildly different values of impedance for utility transformers from different sources. Sometimes I can get this info out of the utility, sometimes I can get part of it. For example, I have used the method outlined in the BUSSMAN EDP-1 HANDBOOK for a long time, as it shows a straightforward method. The old 1984 edition lists impedance of a 300KVA utility transformer as 1.14%R, 4.0%X, 4.16%Z. The newer version online, available here http://www.bussman.com/library/docs/Edp-1.pdf
lists the same transformer as 1.2%Z. The latest utility I called lists their 300KVA transformer as 2.5%Z. With these three numbers, maximum available symmetrical fault current could vary between 8600 and 30,000 amps, according to the quick and dirty calculator available here http://www.bussman.com/apen/software/s_circuit.asp

Is there any consistency in transformer KVA ratings? Does every utility use different transformers?

 

[charlie b]

Re: Short Circuit Calculations

Originally posted by lile001: Does every utility use different transformers?

If not, then I want to buy stock in the one company with the monopoly on building transformers.

Each utility will have its own standards. Fairly early in a given project, they should be able to tell you the available fault current at the point of connection to your equipment. That number can be used, in conjunction with the KVA and voltage ratings, to give you an impedance value.

 

[molotov27]

Re: Short Circuit Calculations

"Is there any consistency in transformer KVA ratings? Does every utility use different transformers? "

The answer the first question is no. The impedance value of a transformer is affected by a couple of factors, one of them being type of conductor used during coil manufacture, the material (copper or aluminum), the shape of the coil, even to some degree how well the coil is manufactured.

A company can manufacture two identical transformer, same primary and secondary voltages, same KVA/MVA ratings and have two completely different impedance. They can also manufacture two identical transformers with the same impedance.

The answer to the second question is usually yes. A utility might specify the impedance value when purchasing a transformer based on their needs, if the transformer is replacing an existing or if it is a new installation, etc. It all depends of the power system study performed at the connection point. Usually, the utility is concerned with the fault levels on their system, and not at the connection point of any customer. There are simply too many customers to take into account...you simply cannot design to make everyone happy, so the utility just specifies and designs impedance based on their needs.....not yours.