1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Short circuit calculation small network: gen motor transformer cables

  1. May 8, 2014 #1

    Luc

    User Avatar

    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:

    Last edited: May 8, 2014
  2. jcsd
  3. May 9, 2014 #2

    Luc

    User Avatar

    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 ;)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted