Short Circuit calcs

[Latango]

My boss asked me today to figure out what short circuit capacity a switchboard would need to be designed for given:

2 x gensets with 22kA fault rating each (1 MW 50hz 415V sets, got the kA from manufacturer)
Synced onto a common bus in the switchboard
Each genset will be connected through a circuit breaker to the bus via 3 parallel 100 ft long flex cable at 240sqmm, that according to the book each have an ac resistance at 50hz of aroung .09 ohms/km each.

Note, this won't be used as a definitive because it's been outsourced to a real engineer (I'm only a young foolish electrician) but it's along the lines of a learning experience.

Do I treat the 2 gensets as a 2MW with 44kA, figure out the effective impedance of that using ohms law, then add that impedance with the total impedance of the cables per phase, and again use ohms law to figure out amps = 415v / total z ?

Note - this isn't an assignment. I finished my apprentiship a few years ago now, just not needed to do this sort of thinking before.

 

[waross]

"ac resistance at 50hz"
You may be using the wrong word or the wrong number.
You should be using the impedance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Your thought process is not wrong, but the details are flawed. But having right ideas is a good start.

A proper resource to learn short circuit calculations is to start with IEEE Red Book, assuming you have other basic electrical engineering understanding. Then make friends with a good electrical engineer, who does this for living. It will be very difficult to learn through fora like this. You will get bits and pieces of information but not a whole picture.

There may be other less expensive books, but IEEE is more authentic.

Coming back to details, you need to work with per phase (single phase quantities). So in the end, you need to divide total Z per phase with V/ph or 230V in your case.

Also verify what 22KA is, it may be the AIC rating of the breakers they have provided and not the actual short circuit current deliverable by the generator. Or you can request the sub transient reactance. Use that as your source impedance. There a few other factors that need be considered based on actual system details.


Rafiq Bulsara

http://www.srengineersct.com

 

[Latango]

Bill: That figure comes out of the Australian standard AS 3008 (which is basically the "cable selection" standard. There is also a reactance value for the same cable sizes at varying temperatures (slightly lower). To be honest, I should have probably investigated it more, but I sorta assumed (I know, I know - I'll rectify that stupidity very soon) that they MEANT impedance. Because what does "AC Resistance" mean if not impedance? (I used their terminology exactly - "ac Resistance at 50hz" which is Au's standard power frequency).

Rafiq: I didn't even consider the single phase thing, damn. About the rating, I'll have to ask the boss on Monday, because he was the one who told me (in his exact words pretty much ) "The maximum short circuit current the generators can supply the switchboard is 22kA each".

I will certainly look into into getting hold of the Red Book, because I get the feeling it will be useful in other areas.

 

[rbulsara]

AC resistance is not the same as impedance. AC resistance can be slightly more than DC resistance due to the skin effect. Impedance Z would be the square root of (square of R + square of X). R=resistance, X= Reactance, Z=impedance.

Rafiq Bulsara

http://www.srengineersct.com

 

[Latango]

Damn, that's humiliating. Thanks Rafiq. Back to the books for the weekend it seems.

 

[rbulsara]

Just a correction to my own earlier post, divide V/ph by the total Z (per phase) to get the current.

Rafiq Bulsara

http://www.srengineersct.com

 

[burnt2x]

It's never humiliating trying to learn! We all continue to learn new things everyday.