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Phase voltages in line-to-neutral fault

  1. Mar 1, 2016 #1
    Hello, I'm a very slow learner! I try to understand every piece of the information that is given. And recently I was solving some problems involving symmetrical components and I couldn't figure this one out:

    Haha.png

    -The neutral is solidly grounded.
    -Phase B and C are open.
    -You're shorting Phase A trough a zero impedance.

    My question is:

    How the he$£ those phase voltages become that?

    I mean, I would expect Phase A and B voltage (Vag, Vbg) to be equal to before the fault, since it is open circuit and the neutral is solidly grounded.

    Plus, a zero-sequence and negative-sequence voltage components are present in that. Wasn't the generator suppose to only supply positive-sequence voltage?

    Despite all that, I'm sure the exercise is correct, but I do not know why is that.

    Hope you can help me :) !
     
  2. jcsd
  3. Mar 1, 2016 #2
    What is normally neglected in the calculation is the current in the healthy phases, hence they are "open" compared to the faulty one. But I agree with you, the phase voltajes are the same (if neutral displacement, voltage drops by homopolar currents, and other phenomena are not considered).
     
  4. Mar 1, 2016 #3
    Sadly, It is not what happen. I saw more exercises like this one (LN fault) and the phase voltages do change, a lot.
    But still don't know why kkkkkkk.
    Pretty interesting stuff.

    If someone has any idea why, it would be very appreciated
     
  5. Mar 2, 2016 #4
    In a L-G fault, when you apply the symmetrical components matrix to the calculation to get the voltages after the currents are calculated, they are affected by the SC in the other phase. One cause is the voltage drops in the zero sequence impedances which in normal balanced conditions have no current.
    The synchronous machines have inverse and zero sequence, of the same magnitude order of the transient direct impedance. When there are unbalanced currents, it is the alternator which generates them, with an inverse and a zero sequence mmf in the airgap, thus with inverse and zero components in the electromotive force also.
     
  6. Mar 2, 2016 #5
    Thanks a lot for your reply!

    I think I understand what you are saying. So, you do have zero sequence and negative sequence voltages created by unbalanced currents in the generator? Is that right? And if yes, would you know how they actually create this voltages? You some things about the mmf in the airgap but I couldn't quite get that.

    Thanks a lot
     
  7. Mar 2, 2016 #6
    Well, as you have negative and zero sequences currents, you have negative and zero sequence voltages and armature reaction, thus mmf in the airgap and electromotive force
     
  8. Mar 2, 2016 #7
    I recommend you the group Power System Analysis in Linkedin for more expertise than I have.
     
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