Utility transformer AIC ratings

[rmiell]

I just got done looking at the thread238-155020 about AIC ratings and utility transformers. I work for a utility, and the lead lineman asked me a question that I was not able to answer.

Is the short-circuit current for transformers available only from the manufacturer, or is there an online calculator which will provide it?

I normally use the Cooper-Bussman SPD book, which contain a table with this info for various size transformers. However, we have a number of different years of these books around, and it was noticed that there are alot of dfferences. The oldest one, 1990, shows a 3-phase 120/208 225KVA transformer having short-circuit amps of around 32,000, while my newest one (2005) shows it as 62000.

I understand the main thrust of short-circuit calcs, but they all start with a utility provided amount (usually high, worst case). I would like to see the chart this is from, or at least the indication that it depends on manufacturer.

Any help here?

Thanks

Rick

 

[dpc]

The maximum short circuit current on the secondary side of a transformer is a function of the transformer impedance (%Z) which is normally stamped on the nameplate of the transformer. So if a transformer has an impedance of 10%, the maximum fault current on the secondary side for a three-phase fault is going to be 1/.10 or ten times the full load current of the transformer - not including secondary side motor contributions, etc.

The actual short circuit current is going to depend of the system impedance for the primary system where the transformer is connected, so it's not possible to get a precise answer without having fault data at the primary side of the transformer.

For ground faults, there are several other factors involved.

 

[waross]

Note that the current calculated with the % impedance is the symmetrical current. Actual fault currents are usually asymmetrical and are usually higher.
respectfully