Helicopter power transmission line repair

Discussion in 'General Electronics Chat' started by RichG, Feb 23, 2006.

  1. RichG

    Thread Starter New Member

    Feb 23, 2006
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    I have a question about live power transmission line repair using a helicopter. If the helicopter is obviously isolated from ground, why is there a visble arc from the wand to a conductor before the repairman clamps on?
    I know the short answer is that the helicopter is brought to the same potential as the conductor, but why doesn't an arc flame a bird about to land on a line? I scoured the web on this, couldn't find anything.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Hi,

    Birds are not metallic, so they don't genetate charge by cutting lines of force. Nor do they accumulate static potentials. It is odd to think how differently birds and aircraft are afffected by flight. But helos do get charged up, and will snap a fat spark to a grounded object as well as a power line.
     
  3. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    In addition to Beenthere's comments:

    Helecopter line work is almost exclusively done on very high voltage, cross country transmission lines. We're talking in the neighborhood of 345,000 volts. You may have noticed that the guys getting out of the helocopters are wearing large, heavy suites. The suits are embedded with wire coils to counter the effects of the extremely high concentrations of magnetic flux found at those voltages. If they weren't wearing the suit, they would get a very uncomfy feeling from the flux passing through their body.

    With this in mind, I doubt you'll ever see a bird perched on 345KV line. I've never seen any animal on an energized line over 14KV.

    In the utility that I work at we only have lines up to 161KV, yet our linemen have to maintain a minimum distance of 12 feet from energized 161 lines, or else risk getting arced.
     
  4. krampuss

    New Member

    Apr 7, 2009
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  5. KL7AJ

    Senior Member

    Nov 4, 2008
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    Yeah, you have to be willing to put on the ol' chain mail jockstrap and dangle from a helicopter. I think I'll stick to safe low-voltage stuff, like the 25KV plate voltage of a broadcast transmitter. :)

    eric
     
  6. davebee

    Well-Known Member

    Oct 22, 2008
    539
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    Because the helicopter has capacitance, and so will absorb some quantity of charge in order to be brought up to a high voltage.

    Usually capacitors are thought of as having two plates, but capacitance is also clearly defined for an individual object, where charges on the object are endpoints of electric field lines to Earth, or to wherever the other charges went when the charge was generated that was applied to the object.

    I guess the bird doesn't have very much capacitance.
     
  7. lmartinez

    Active Member

    Mar 8, 2009
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    To provide an answer to your question, it is imperative to understand that the surrounding air in the vicinity of the conductors is at a higher potential than the metallic parts which make up the helicopter frame. As a result, the air between the conductor and the helicopter is being ionized (it is due to the voltage difference, in other terms the electric field exceeds the dielectric strength of the air) and hence energy is flowing from higher to a lower Potential. Please advice if you would like further explanation on this question. Thank you
     
  8. loosewire

    AAC Fanatic!

    Apr 25, 2008
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    A Point of interest on the subject,In the florida Keys you see one man operated helicopters working on lower voltage lines.With the helicopter
    running in place,the operator attached the helicopter to the pole.The operator then exits the helicopter to work on the lines. This method
    is used because there is no room for large trucks to park on the two
    lane highway It neat to watch the helicopter running by its self,with no one in it.
     
  9. flat5

    Active Member

    Nov 13, 2008
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    loosewire, I wonder what that job pays. Some diverse skills and dangers involved.
     
  10. Darren Holdstock

    Active Member

    Feb 10, 2009
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    Knock me down with a feather.

    Talking of which, there's a reason why you don't see birds on very high voltage lines. I guessed that it was to do with the electric field strength setting up a high potential across the birds body, but apparently this isn't the case, as the field is warped by the bird being slightly conductive. It's to do with corona discharge from the birds feathers - they would get singed around the edges very quickly, and they're not keen on that.
     
  11. b.shahvir

    Active Member

    Jan 6, 2009
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    Hi, :)

    The concept of capacitance is valid for AC voltages. But what if it were an HVDC line? In that case the arcing to the metallic wand would only remain for a few seconds until the helicopter capacitor charges and would then disappear as capacitance blocks DC! However, I do not think this would be the case and the arcing betn line to wand would be continuous w.r.t time, whether line voltage is AC or DC.

    In my opinion, apart from capacitance, the arc current would also consist of air leakage currents. The air in the surrounding area consists of ionized conducting particles which constitute arc currents under the influence of high potential gradients caused by the HT lines. This current would then appear as an arc betn the line & the metallic wand. The circuit is then completed by the leakage currents betn the helicopter and earth below. However, these currents betn helicopter and earth do not appear in the form of an arc as the magnitude of these leakage currents is small and it horizontally distributes itself over a wide area in air.




    Actually, the metallic conducting suits is not to protect against magnetic flux concentrations, rather it is more akin to a Faraday cage which allows the electrostatic leakage currents to flow on it’s surface to the helicopter chassis which acts as a virtual ground. The Faraday cage uses the principle that a charged hollow metallic body acts as an equipotential surface and hence electrical lines of force do not appear inside an equipotential surface. Hence, any object inside an equipotential charged body will not get zapped as no electric lines of force can exist within a charged body, which means no potential gradient (voltage) and hence no current within a charged body! All the current will flow thru the surface of the Faraday cage protecting any object within it from being zapped.

    This is precisely the reason why we are advised to remain inside the car while driving thru a rain storm. If a lightning bolt were to strike the car, the chassis of the car will act as a Faraday cage and all the lightning current will flow thru the surface of the chassis and will dissipate to earth thru the car’s tyres! Since the body of the car acts as an equipotential surface, all the occupants of the car will remain safe as explained above. The same analogy applies to the lineman wearing the protective metallic suit.

    Kind Regards,
    Shahvir
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    :confused: Rather Interesting...since we don't have helo's here. I am some what confused by all these....:confused::confused:.
    It's like how capacitance , helicopters and HV transmission is related. :confused::confused:
     
  13. Mike Mandaville

    Active Member

    May 27, 2009
    81
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    Wow, what a great thread! I don't have much to add, just that I'm glad that I have a satellite dish, because although I have never had the chance to see this kind of work being done live, what I _have_ done is to see it on television, on the history channel. This is really some great stuff!
     
  14. b.shahvir

    Active Member

    Jan 6, 2009
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    Any conducting surface, charged withan electric potential, seperated by a dielectric medium w.r.t. earth possesses capacitance....... Helicopters and HV lines fall into these!
     
  15. radiohead

    Active Member

    May 28, 2009
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    wow. I got educated on that string of replies. I was thinking that perhaps the spinning blades cutting the lines of flux would produce additional electrical potential in the helo in addition to the static electricity it already produces, like a generator. Have you ever seen a helo land at night? You can see the static on the main rotor and tail rotor with the naked eye. very cool. I suppose that's why linemen get paid well, it's a hazardous profession.
     
  16. leftyretro

    Active Member

    Nov 25, 2008
    394
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    No reflection of you personally, but I just don't believe this is true or possible at the present time. Could you show me an article or link showing a single pilot flown helicopter, where the pilot can leave the cockpit to do line repairs while the helicopter continues to hover? Surely such a feat must be documented somewhere being used by utilities if it's indeed true, at least by the helicopter manufacture wishing to sell such capability to utilites.

    Lefty
     
    Last edited: May 30, 2009
  17. mik3

    Senior Member

    Feb 4, 2008
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  18. leftyretro

    Active Member

    Nov 25, 2008
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    Very cool movie, however clearly it's a two person crew, pilot and lineman, NOT what is claimed to be seen by LW, or did you read what he claims? " It neat to watch the helicopter running by its self,with no one in it."

    Lefty
     
    Last edited: May 30, 2009
  19. mik3

    Senior Member

    Feb 4, 2008
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    Then someone else may find such a video. :p
     
  20. Mike Mandaville

    Active Member

    May 27, 2009
    81
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    If I remember correctly, the one-man-helicopter method of working on high power lines was demonstrated on the History Channel, right at the end of the episode of Modern Marvels, or whatever series that epidode was a part of. It might even be possible to find a transcript on the internet. I found a transcript of "Triumoh of the Nerds".
     
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