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 Post subject: Infinite Bus Utility Data
PostPosted: Mon Apr 04, 2016 8:02 am 

Joined: Fri Apr 10, 2015 10:27 am
Posts: 3
How do you know if the fault current provided on a study is infinite bus?

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 Post subject: Re: Infinite Bus Utility Data
PostPosted: Mon Apr 04, 2016 8:37 am 
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Location: North Carolina
Pretty easy. In years past before computer modelling was used, you can calculate short circuit currents on the secondary side of a transformer if you assume a short circuit on the primary side using only the transformer kVA rating, the impedance (%Z), and some adjustments for shielded cables and motor loads using the ANSI method. It can be done with a one page "cheat sheet" and a calculator in a couple minutes, far less time than it takes to construct a computer model. The result is conservative (error is on the high side) but good enough for most engineering purposes. In fact I still do it exactly this way because the time involved in setting up the computer model is not worth it if I just need a quick answer (order of magnitude estimating, result checking, etc.).

With arcing faults however an experienced never do this because it will produce an incident energy estimate that errors to the low side. An inexperienced engineer or more likely someone that has been doing hand calculations for years would make the mistake of applying the same ANSI method with an infinite bus and produce a result that errors to the low side. If you know the available fault current and a few other relatively simple parameters, it is fairly easy to calculate incident energy with a scientific/engineering calculator.

But even with computer modelling unless you have your own on site generation that is not connected to a utility ("off the grid" applications), you do not have control over the fault current from the utility. Most (not all) will estimate fault current on request although it can be challenging finding the right person and/or dealing with bureaucracy to get an answer. The result is that in many cases even with a computer model, the utility may be modeled as an "infinite bus".

Knowing this then there are two places to look:
1. Look for a "hand generated" report or a report where every "bus" is on a separate sheet with symbols for a "utility" on each sheet, indicating that every bus was modeled independently. This is a tell-tale sign. If you don't see this then move on to step 3.
2. Look for the primary transformer available short circuit current. If it's not there, chances are it's infinite bus. This is where you need to ask questions.
3. Look at the input parameters or other details on the utility model. should not be infinite bus.
4. Finally look at the number that is used. A decimal or not "rounded" value such as 13.28 kA is clearly not infinite bus. If it is an even number that is 35, 48, 65, or 100 kA, these are standard interrupting capacities of equipment. It is not "finite" per se but it works exactly the same way, with the same problem (under-rated incident energy). If you check the interrupting capacity of the equipment and it is identical to the short circuit current, this tells you that even though it's not technically "infinite bus", it's "maximum bus"...which is the same thing.

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