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 Post subject: Short circuit current calculation
PostPosted: Sun Feb 09, 2014 11:59 pm 

Joined: Sun Feb 09, 2014 11:28 pm
Posts: 2
Hello everybody!

I am new here, my name is Vedran. I am from inland Croatia, working for petrochemical company called Petrokemija. I have one project on my desk and I would need your help regarding calculation of short circuit current with ETAP, primarily.

How to calculate short circuit current(with ETAP) after current transformer FMT STEM-1011 in the configuration shown in the picture? I included and picture where it's shown how I reproduced it in ETAP but I am not sure how to get exact calculation after that FMT transformer.

BTW, I need short circuit current in order to adjust relay in each branch. For I> I got around 305A (278 A(battery current) + 10% = 305A), but I have to calculate and I>> and for that I need short circuit current near that current transformer. Thank you very much for any your input and help.


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PostPosted: Mon Feb 10, 2014 8:34 am 
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Joined: Thu Oct 18, 2007 7:10 pm
Posts: 261
Location: NW USA
Hello Vedran,

what is the problem that does not allow you to calculate the short circuit current?

If ETAP does not accommodate parallel paths as shown, then you might consider modelling a single path of total equivalent impedence to the two conductors for that short length.

If ETAP does not calculate short circuit because there is no bus there, you can put a theoretical bus in the model for that location and have it calculated.

Gary B

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PostPosted: Mon Feb 10, 2014 9:01 am 

Joined: Sun Oct 20, 2013 6:38 am
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Hi Vedran,
It is not clear to me where you measured battery current, but thats ok.

Every component, contact and wire will have an "real" effective series resistance (ESR) and "imaginary" or "reactive impedance" inductance (ESL) that will determine the short circuit current. Once you have a diagram of all these impedances, short circuit current can be calculated. Contact Vishay for ESR values vs Temp. then verify with a non-destructive Impedance test.

You can test parts or fused network using Vector Impedance Analyzer method testing or improvise with small signals at high frequency and measure the impedance of signal required to match Network using the CT to back-drive with a signal. At high frequency (eg >=1MHz) impedance of uF Cap should be lower than ESR of Cap. which would determine the S.C. current. You can back-drive the CT using a suitable variable resistance source or use a current source.and compare with RLC meter)

The rise time of a fault is 1ns so high Freq impedance is needed.

Choosing the breaker limit is a matter of temperature rise, cooling, applied voltage and the values of ESR which may rise with aging or oxidation of connections or loose connections, so setting this breaker level must also protect from loose but dangerous thermal failures, unless thermal sensors are also in critical areas of your design.

Each Capacitor will be composed of many parallel elements each with their own ESR and possibly due to built-in fuse of thin section metal film, which affects the ESR. It should be quite low < 1mOhm.
This improves reliability in case of component fault, if "self-healing". It will also have a temperature coefficient determined by your cooling design and ambient temperature.

Once you have this , the short circuit current is function of voltage when breakdown occurs to cause fault current. I am not sure if fault is likely to occur at peak voltage or peak current, but follow-on S.C. current must be stopped well before next zero crossing.

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PostPosted: Mon Feb 10, 2014 9:11 am 

Joined: Sun Feb 09, 2014 11:28 pm
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Thank you once again for your input and help! :)

@Gary B

I have not seen that in ETAP there is and impedance symbol so I realized I can model capacitor using impedance editor. I have expected there is and two nodes capacitor symbol so that confused me at first. So I did it and I am getting 230 A near that FMT CT. Value is strangely low imho. I am not sure if I made somekind of a mistake or something?

@tony stewart

Current is calculated based on data of the battery, the whole calculation is in the project except SC calculation. I hope I did not make a mistake taking that current into account when I was calculating adjustment of that overcurrent relay? Your answer will be also really valuable!

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