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Single machine-Infinite bar (SMIB)

  1. Sep 21, 2011 #1
    Greetings ladies and gentleman.
    Well I'm trying to do some small-signal studies, and i'm familiar with the deMello-Concordia linearized model. As far as i know all the constants used in that model (K1-K6) depend on the system, and so they change as the system changes too. I have a large system with several synchronous machines and i want to obtain the SMIB model for one of those machines. So how do i obtain the external reactance connected to the infinite bar, representing the transmission line? I know this reactance corresponds to the Thevenin equivalent, but is there a way to obtain this value by using a power flow simulation software?

    Or is it a good approach to assume this value to be zero?
    Thanks a bunch
     
  2. jcsd
  3. Sep 22, 2011 #2

    jim hardy

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    ""So how do i obtain the external reactance connected to the infinite bar, representing the transmission line? I know this reactance corresponds to the Thevenin equivalent, but is there a way to obtain this value by using a power flow simulation software?""

    what an interesting question.

    I wish i had an answer for you.
    In the utility where i worked we had a whole department dedicated to calculating just such things, and we just asked them. But the number depends on how many lines are in operation and how many units are connected.

    I seem to recall they gave us per-unit numbers like 0.013 so i wouldn't use zero. It was however a lot less than the stepup transformer impedance.

    Here's a link i found with some formulas, perhas you could approximate a reasonable line to get your simulation running then tweak it with real numbers from your utility's system people.

    www.powerworld.com/Document%20Library/version.../TransCalcHelp.pdf[/URL]

    "Power System Analysis" by Charles M Gross is a practical textbook.
    Dr Gross was my instructor for some undergrad courses and he has a God-given talent for clear explanations. I saw his book in the bookcases of many engineers in that department i mentioned, they all spoke highly of it -- so ask among your colleagues, if you find a copy i think it'll have some practical examples worked out that are quite similar to your question.

    Sorry i'm not an expert.

    old jim
     
    Last edited by a moderator: Apr 26, 2017
  4. Sep 23, 2011 #3
    Well thank you Jim! I've been using DigSilent for this purpose, and i guess i'll just have to trust in it. By simulating a three phase short circuit, i can obtain the values for R and X, which i asume are Thevenin equivalent for positive sequence.
     
  5. Sep 23, 2011 #4

    jim hardy

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    for what little this is worth --

    our big steam turbine plant had natural frequency against system of ~ 1 hz
    largely due to 17% main tansformer
    i think that is typical

    it also had main shaft resonance of 7 hz
    so that was a frequency to avoid like the plague in anything connected to it.

    we overdamped voltage regulator to calm system troubles (2/3 hz divergent oscillations) that were solved initially by power system stabilizers , finally by stouter transmission lines.

    wish i could be more help.

    good luck with your studies -
    that is a field i watched "through the fence" like Charly in 'Flowers for Algernon'.

    old jim
     
  6. Oct 5, 2011 #5
    Well thanks Jim, i appreciate your help.
    I'm now trying to find out which type of short circuit i should use to find the thevenin equivalent, because I've found that you can find a 3-phase or1-phase Thevenin equivalent and i'm not so sure if this is true.

    By running a 1-phase short circuit i would find impedances in all 3 sequences. Is it enough to find only the positive sequence impedance by running a 3-phase short circuit in my software for the SMIB equivalent??

    I'm also aware that for big systems Rthev is negligible so i can only consider Xthev in y analysis, and that this value is the inverse of the imaginary part of the short circuit current. But again which short circuit current? 1-phase or 3-phase?

    Thanks again. Would like to get more people involved too. Don't be shy :P
     
  7. Oct 5, 2011 #6

    jim hardy

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    ""Is it enough to find only the positive sequence impedance by running a 3-phase short circuit in my software for the SMIB equivalent??
    ""

    I wish i could answer that question.
    My intuition says use the three phase
    the power system guys i used to observe in 1980's were excited about new relay schemes that could interrupt one phase at a time for single line faults

    so i would think your small signal studies would be focused on balanced three phase.
    but that is a guess.

    our power system oscillations were during steady state operation not fault induced.
    came from interactions of rotating inertia, system impedance and voltage regulators.


    Is there a genuine power system engineer in the house ?


    Dr Gross - are you out there?
    old jim hardy - UMR '69, your student for AC machinery
     
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