HELP: Single phase SC Calculation w/ SKM

[JCHENHI]

Can anyone help me with a single phase short-circuit calculation as follows?

It's a airport runway ILS job. We need to run single phase from a 250KVA 13.8KV-240/120v transformer to a 25KVA 240v-2400v step-up transformer, then about 4000' #6 single phase wire to 25kva 2400-120-240V step-down transformer to feed the load about 20KVA.

We are using SKM software to do the short-circuit calculation, but just cannot get any result. Does anyone have the experience on this?

thanks!

James

 

[davidbeach]

Do you have the single-phase/unbalanced calculation module? If so, it should work quite easily; if not, it will be a PITA. With only the balanced calculations, you would have to make an equivalent circuit, with a different load, to get the same voltage drops.

 

[JCHENHI]

Thanks a lot, David, for your reply. Yes, I do have single-phase/unbalanced calculation module. In fact, I am using the evaluation version.

My problem is that when I start the "Unbalanced/Single Phase Study Setup" dialog window, I cannot find report file name next to "Comprehensive Fault (SC)" item. So how can I start the SC study and view the result?

Also, I am a little confused with the "Compoment Editor" for single-phase transformers. How can I setup the transformer connection correctly? Right now, the following is my configuration of the transformers:

XFMR Primary Secondary TYPE Phase
XF1 13800v(L-L) 120v(L-N) Single Phase Pri:AB
XF2 240v(L-L) 1200v(L-N) Single Phase Pri:AB
XF3 1200v(L-N) 120v(L-N) Single Phase Pri:A

Does it look right to you?

thanks,

James

 

[davidbeach]

For your single phase transformers, use the mid-tap option so that you get two phases out of the transformer rather than one phase and neutral, and leave everything as phase A-B. You won't get an SC.rpt file out of the single phase studies, but you can use the data blocks to put the fault values on the one-line, or you can use Document|Crystal Reports to print a short circuit study (find the Unbalanced - Single Phase folder, not the Short Circuit folders on the top level).

 

[JCHENHI]

Thanks, Dave, for your answer. After changing the circuit as you adviced, I got an error message: "Two Cascade Single Phase Mid Tap Transformer" . Do you know if there is anything I did wrong?

thanks,

James

 

[davidbeach]

Sounds like time to contact SKM directly. I've found them to be very helpful.

 

[waross]

Sorry, can't help you with the software,
However;
25 KVA / 2400 Volts= 10.4 Amps
10.4 Amps / 3% Impedance voltage = 347 amps available fault current symetrical.
25 KVA / 240 Volts= 104 Amps
104 Amps / 3% Impedance voltage = 3472 amps available fault current symetrical.

Check your actual impedance voltages and redo the arithmetic.
Hope this helps

 

[dpc]

Calculation of short-circuit current on the secondary of a 120/240V single-phase transformer is not as straighforward as you might think. In some cases fault current at 120V can be higher than at 240V. I have no idea if SKM software does it correctly or not, but a good reference is Conrad St. Pierre's book on SC calculations. He runs through an example you could use for verify SKM results, if you're able to get a result from the software.

 

[waross]

The short circuit ratings on switches, breakers, panels etc. are based on the calculations I showed. When the manufacturer rates his eqipment, he considers the worst possible stresses of the maximum asymetrical current. He then derates the equipment to the value found by the simple calculations.
For instance, a breaker rated for 5000 Amps available fault current will withstand the maximum stresses imposed by the much greater asymetrical currents that may be present.
It has been my understanding that the codes use the phrase "Available shortcircuit current" to describe the current flowing in a steady state condition, after the asymetrical currents have decayed to zero.
If you are co-ordinating protection for a distribution utility you must calculate the actual fault current.
I do agree with you that calculations of the actual fault current are a lot more involved.
However, as I understand the original post, I think the poster is trying to select panels breakers and/or switches
that are suitable for the available short circuit current.

The last time that I had to submit calculations for available fault current calculations to an inspection authority prior to a permit being issued, the figures I showed were the accepted calculations.