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Transmission line

  1. Feb 8, 2009 #1
    Hello everybody,
    I have a question and I am wondering if you can help me find an answer. The problem I am trying to figure out is:
    I have 400 miles of 345 kV transmission line. An outage happened twice in two different locations on the line caused by two different lightning strikes, a 35kA and 23kA. Although the withstand ability of this line according to T-Flash –a program we use to model existing structure for lightening performance- is 195kA.
    A common factor in both outages is that they were near the splice point (the splice point is where the static wire comes down the pole from both sides to splice the two wires together). We run the static wire down the pole because the static wire we have on this line has fiber optics wiring in it for communications purposes. Also keep in mind, that we have a very good grounding- 20 to 25 Ohms.
    So what I am suspecting is the outage must have something to deal with the static wire going down the structure. I forgot to mention that we have a jumper cable on the top of the pole to connect the static wires together. So do not be confused about what I mentioned earlier regarding the splice point- this is only for the fiber optics connections.
    I appreciate your help.
    Thank you,
     
  2. jcsd
  3. Feb 8, 2009 #2

    dlgoff

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    Science Advisor
    Gold Member

    Last edited by a moderator: Apr 24, 2017
  4. Feb 8, 2009 #3
    Thank you very much dlgoff.
     
  5. Feb 8, 2009 #4

    berkeman

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    Staff: Mentor

    Exactly what I was going to suggest. Great minds think alike....

    BTW, when you're at his Member Profile page, you can also go to his Statistics, and click on Find All Posts by User. Many of the threads m.s.j. participates in relate to high power transmission lines and various failure modes:

    https://www.physicsforums.com/search.php?searchid=1457705&pp=25 [Broken]
     
    Last edited by a moderator: May 4, 2017
  6. Feb 15, 2009 #5
    Excuse me my friends, I have speed problem for accessed to PF, but about your question, It seems you front to high voltage gradients in some location. As you know, there are some means for control of this phenomenon, for example corona rings. The purpose of corona rings is to reduce the electrical field stress at the end fittings. They are required at the line end of all 230KV suspension insulators and at both ends of all 500KV polymer suspension insulators.
    Electrical conductors at high voltage are surrounded by electric fields that are more concentrated, i.e., have higher voltage gradients, in the vicinity of sharp points or small radii of curvature of electrodes, terminals, or conductive components of the circuit. If the voltage gradient is high enough, the air surrounding these points will be ionized, and a corona discharge will develop which may lead to a flashing spark or arc discharge to another conductor at a different potential. The use of toroidal metallic corona rings around these vulnerable points in order to minimize electrical discharging is common practice. However, the metal corona rings frequently are insufficient to prevent flash discharging.

    Also the safety of cable operations will be threatened when the field magnitude is greater than 12kV at the hanging point of the optical cable, caused by the electrical corona of armor rods terminals. To solve this problem some company specially designed a corona coil, which can significantly improve the status of electric field at terminal helical hardware fittings, and can double the corona causing voltage.


    REGARDS
    M.S.J

    ----------------------------------------------------------------------------------
    Creative thinking is enjoyable, Then think about your surrounding things and other thought
     
  7. Feb 15, 2009 #6
    is a "static wire" for lightning protection? i used to install that in cell phone shelters many years ago. related to what msj says above, the wiring for the lightning protection cannot cut any sharp corners. we installed a "halo" in the ceiling with (my memory is a bit fuzzy here) a 2~3 foot radius in the corners, and the "drops" also came down on a curve, from two directions in a "Y". then the drops would exit the building. it was always explained to us that lightning doesn't like to cut corners, but i'm sure it's more like the above explanation.
     
  8. Feb 15, 2009 #7
    Thank you M.S.J. I need to study your answer. I hope I can get back to you if I have more question if you do not mind.
    Thanks again.
     
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