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High Voltage Powerline 750 KV arc

Discussion in 'Electronic Basics' started by Jack Jet Williams, Jan 5, 2004.

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  1. Does anyone know how far 750 kv ac could arc once it got started ?

    Just wanted to know.
    Thanks.
    Jack Jet Williams
    Hesperia,Ca.
     
  2. DarkMatter

    DarkMatter Guest

    14 feet. That is the minimum safe distance for workers on such
    lines.

    Air ionizes along a path, so a set length is impossible to state. A
    path could propagate several feet further than "thought".

    It also differs for DC and AC with AC less likely to arc.
     
  3. Actually, for a given nominal voltage, AC sparks through a greater
    distance than DC does since AC has a peak voltage about 41% higher than
    nominal.
    But a DC arc is more "stretchable" than AC one is once it gets started.

    Spark gap distance, at 760 mm pressure of dry air at 25 degrees C, is
    13.3 cm between 25 cm spheres and 54.7 cm between needle points at 300KV
    peak (300 KV DC or aprox. 212 KV AC).
    Between electrodes that can form corona due to concentration of electric
    field towards the electrodes, breakdown voltage is approx. 1100 volts per
    meter plus something like 320 volts at lower voltages, but less per unit
    distance when the peak voltage increases past 30-40 KV or so. Between
    large spheres or wherever else the electric field is even throughout the
    spark gap, the breakdown voltage is approx. 3 KV per millimeter plus
    something like 320 volts.
    Where the air density is different from that of 25 degrees C 760 mm,
    breakdown voltage varies directly but maybe not quite proportionately.

    (As far as I know, mostly from a spark gap table in the "CRC Handbook".)

    Humidity has less effect than many would expect, and I am guessing its
    main effect is to cause surface conductivity on insulating materials near
    electrodes which can affect electric field distribution in the spark gap
    in some cases.

    - Don Klipstein ()
     
  4. DarkMatter

    DarkMatter Guest

    NOpe. AC crosses zero too many times to propagate an ion path.

    DC starts one, then finishes it.

    Long before the arc, an ion path must be established.
     
  5. Eric R Snow

    Eric R Snow Guest

    I dunno. But I saw a photo of a high voltage line which had been
    struck by lightning and was arcing to the ground. The ground was
    exploding. The arc must have been at least 25 feet long. What a
    picture.
     
  6. Don Kelly

    Don Kelly Guest

    --------------------
    What you are effectively saying is: "As AC cannot propagate an ion path
    there cannot be ionic conduction between electrodes supplied by AC and that
    big fat and hot arc on a AC system is a figment of imagination. Flourescent
    lights also will not work on AC as they depend on establishment of an
    ionized path."

    Do you want to rethink this?

    AC will produce an ion path and a true arc will follow if the supply
    impedance is low enough to allow this. That nice big AC arc in the photo
    initiating this thread is likely carrying a few KA- this is well past the
    path initiation stage. Sure the arc goes out at the end of each half cycle
    but if the arc products don't dissipate sufficiently near current 0, there
    will still be an ionised path and the arc will restrike.
    Even a relatively high impedance neon transformer will run a Jacob's ladder
    and that flame going up the ladder is past the ion path initiation (Townsend
    discharge) stage and well into the arcing regime (negative impedance ,
    plasma, or however you want to describe it).
    Do we need DC for a fluorescent lamp? It's voltage/current characteristic
    puts it into the arc regime where external impedance is needed to limit the
    current.

    Don Kelly

    remove the urine to answer
     
  7. Don Kelly

    Don Kelly Guest

    --------------

    .. The actual initiation gap depends on a lot of factors such as humidity,
    air pressure, shape of the "electrodes" so there is no single answer.
    Typical conductor to tower flashover (50% probablity) will be in the order
    of 4-5 ft for a 750KV line. At 14 ft- there is a factor of safety to
    account for pointy objects, stupidity, weather conditions, insurance
    requirements, etc. This is meaningless once an arc has been initiated.

    However the original question was "once it started"

    Again there is no single answer. The arc could go well over 100 ft if the
    supply impedance is low. Effectively the current is limited mainly by the
    supply impedance unless the arc is very long. and as long as the supply
    dumps energy into the arc at a higher rate than it is being removed by
    convection, etc, the arc will lengthen. Since it is AC, the current will go
    through 0 every half cycle and, IF the arc products and ionized path
    dissipate sufficiently near current 0 , the arc will go out at this time.
    Needless to say, the length of the arc can be quite large depending on
    current, wind and other factors.
    I have seen a picture of a deliberately struck arc ( wire thrown over the
    line) on a 240KV line and this arc was an estimated 70 -100 ft long.
     
  8. Jo

    Jo Guest

    Jack.
    Check out these movies on Bert Hickmans site at the URL below...

    http://205.243.100.155/frames/longarc.htm

    The 500KV one is awesome and will give you an idea of how far an arc can
    reach.
    Cheers.
    Mark H.
     
  9. Mark,
    I AM IMPRESSED ! That arc was awesome.............thanks !

    Jack Jet Williams
     

  10. Also remember than positive electrodes can launch arcs too, even
    though the massive positive ions in air travel much slower than the
    freed electrons do.

    The "spark stuff" reaches out through the air at a much higher velocity
    than the actual charges travel. A growing spark is more like a wave
    moving through dominos, or a forest fire spreading across trees.
    One group of ions becomes a conductive substance, which causes the
    e-field to distort and concentrate, which ionizes the neighboring dry
    air, and so on in a wave. Also, one group of ions emits UV light,
    which as before can ionize the adjoinging air in a travelling wave.

    I suspect that the physics greatly resembles the growth of thin
    metal-crystal "trees" from solution, where the tips of the crystal
    filaments grow much faster than the sides of the filaments, leading
    to long narrow tree-like shapes.

    With AC, the main issue is whether the ion path "cools off" during
    the zero crossing. If it did, a new spark would have to leap out
    every 120TH of a second, and each spark would take a different
    path. Also, previous sparks could not add new length to existing
    spark filaments, so the sparks would be much shorter. Clearly the
    ionized path does not cool off with hot 60Hz AC sparks, since they
    appear as a single glowing filament which slowly crawls around, not
    as a collection of shorter sparks which flicker around maniacally.


    ((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
    William J. Beaty Research Engineer
    UW Chem Dept, Bagley Hall RM74
    Box 351700, Seattle, WA 98195-1700
    ph206-543-6195 http//staff.washington.edu/wbeaty/
     
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