One of my clients has a 138kv service 385 MVA. The client owns the xfm,13.8kv, 20 MVA, 8%
Another has 69KV service, 100MVA, xfm 13.8kv, 20MVA, 8%Z.
These are both really little utilities and the clients are at the end of a long line. The xfm impedance pretty well overwhelms the effect of the utility. So, if the transformer data is available, proceed as you said - infinite bus and have at it.
And I'm thinking you already knew this. Was there some other issue in question?
Aside:
If you don't have it, get the Moon Yen paper on calculating SCC. IEEE will have it. It is from the 70s.
But wait, I have ETAP, why would I need some cobweb paper from the dark ages.
I have ETAP as well. And, there are still some times when it is convenient to do an SCC calc without taking a couple of hours to set it up in ETAP.
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