# Homework Help: Short circuit calculation small network: gen motor transformer cables

1. May 8, 2014

### Luc

Hello guys,
I have to make a short circuit calculation for school but I am a bit lost.. I have a copy off the IEC61363 and internet connection as my help lines.

1. The problem statement, all variables and given/known data
The network consists of a generator feeding to switchboard A with a cable in between, on the same switchboard is a motor connected with another cable and there is a transformer connected to the switchboard (see image in attached file). There are also other motors on the switchboard but they can’t feed into the short circuit. I have marked the points where I want to calculate the short circuit current in the image in the attached file.

2. Relevant equations
I have no idea how I am going to put all those calculations in here, I've read the guidelines but I have to put this in a word document or else nobody is going to make sense of it anyway. In the file are the equations I used to calculate what I've done so far.

3. The attempt at a solution
I have calculated the following things:
Generator has a peak current of 28.5kA
Motor has peak current of 8.2kA calculations in attached file.

The 25 meter cable has R=0.206 and X=0.075 per 1000 meters and have 8 conductors in parallel per phase. For 25m this is R=0.00515 and X=0.001875
The 10 meter cable has R=0.206 and X=0.075 per 1000 meters and have 5 conductors in parallel per phase. For 10m this is R=0.00206 and X=0.000750
The 22 meter cable has R=0.206 and X=0.075 per 1000 meters and have 1 conductors in parallel per phase. For 22m this is R=0.004532 and X=0.00165
I assume I have to divide the R and X of each cable by the number of conductors in parallel.

I have tried adding the resistance and reactance of the 25 meter cable to the generators Ra and Xd (I thought I might be able to do this because the cable is parallel with the generator) but the answer I got wasn’t even close to the answer I got with my simulation software (SKM PowerToolsforWindows).
The peak current off generator and cable is 27136A
When I turn on the motor the peak current is 35218A
The current at the transformers secondary’s with both generator and motor on is 5972A.
The answer I got for the generator and cable with hand calculation is 28.4k just a smudge less then without the cable..

What is given about the transformer: 3 phase, S=125kVA, Uprim=690V, Usec=400V, Isec=180,5A, Uk=4%, F=50Hz.

I have also calculated the R and X of the transformer (calculation in attached file). The Rt=0,03 and the Xt=0,149 but same deal as with the cables I do not know how to go on nor can I find it on the internet.

When I started with this problem I thought I’ll have it done in under 1 hour but I was very wrong, I’m not even been able to solve it on my own so far.

The calculations of the transformer, generator and motor are in the attached file.

EDIT: I had been looking for the format for 'how to make a post' in this section for about 10 minutes, when I decided not to go by the format I found it when I opened the window for making a new thread.

#### Attached Files:

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• ###### Small network.png
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Last edited: May 8, 2014
2. May 9, 2014

### Luc

After some trial and error I decided I would try to calculate the drop in short circuit current using a version of (I^2)*R. I used S=(I^2)*Z*Root(3).
Those answers I got with that formula came really close to what my simulation says (error is about 1%).
So now I have calculated:
A=28542A
B=8124A
C=27141A
D=8051A
E=35192A
F=20198A

G I still do not know, so if anyone did not reply because they only knew G then this is your time to shine ;)