Does the current in high tension voltage cables have to form a closed circuit?

Discussion in 'General Electronics Chat' started by mydoghasworms, Jan 13, 2015.

  1. mydoghasworms

    Thread Starter New Member

    Jan 13, 2015
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    The high voltage cables I am talking about are the ones that transfer power from a power station to substations, i.e. the ones mounted on pylons that run across the countryside.

    Do these have to form a closed circuit, i.e. is there a flow of electrons back along those lines, or is the electricity "pushed" from one end to the other, as illustrated in the opening section of the DC book?

    [​IMG]
     
  2. #12

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    Nov 30, 2010
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    Simple answer: Yes. There is a transformer on each end.

    In general, all electrical circuits form a loop. When you start talking about the length of the wire being a significant portion of the electrical wavelength, strange things happen. It all still happens in a loop, but there is room to discuss where the energy is at any moment and whether an electron pushed into the wire in San Francisco ever gets to New York. When you start asking questions about that kind of stuff, somebody smarter than I will do the answering.
     
  3. mydoghasworms

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    Jan 13, 2015
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    I take it that means yes :)

    The whole way the book starts, discussing how electrons flow from a surplus point to a deficit point has confused the rest of the book for me, and I am only at the beginning :eek:
     
  4. #12

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  5. WBahn

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    Think of the following analogy:

    Imagine a large body of water, say a large lake. You have a pump at one side that pumps water up out of the lake into a transmission line (a pipe) that runs across the lake say a hundred feet above the surface of the lake. At the other end of the lake is a fan that is powered by a small turbine that is turned by the water in the pipe which is then discharged into the lake. The lake itself is the return conduit and you could run this system forever and it would work just fine. Now consider the case if the pump and the fan were located in two different lakes that were isolated. As time went by one lake would drop and the other lake would rise until eventually either there was no water left to pump or the pump would be unable to force the water up far enough to get it to the other lake, let alone 100 ft above the other lake in order to run the turbine.

    The same is true with long haul transmission lines, particularly high-voltage DC lines. The physical earth itself is the return path (though in most AC transmission situations it is a relatively small amount of current, the residual imbalance between the phases, that is returned via either the neutral or the earth).
     
  6. sirch2

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    Jan 21, 2013
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    Just to emphasize the point WBahn makes; there is no flow of electrons "back along those lines", the flow is into the ground, literally the planet earth. Generally on high tension lines you see 3 or 6 cables. All these cables are positive with respect to earth and carry electrical energy from the power station via sub stations to your house, the return path being the earth.

    There are usually 3 (or multiples of 3) wires because electricity is usually generated in 3 phases each wire carries 1 phase.
     
  7. profbuxton

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    Feb 21, 2014
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    I don't know what sort of distribution system you have but in ALL three phase distribution systems, at least the ones I am familiar with there is NO flow back via "earth" under normal conditions. Not even in single phase systems. Earth currents only flow under fault conditions and usually cause blown fuses and tripped breakers.
    The ONLY place I know of where earth is used as a load carrying conductor is in SWER systems(Single Wire Earth Return) as used in remote location to save copper. Quite widely used in the Aussie outback. But they then only feed a transformer whose secondary is normal household supply and doesn't use a earth return.

    To answer your question, imagine your tube has a U-tube connected at the end and this is connected to another tube leading back to you.
    As you push a marble in the "upper" tube and the tube is completely full of"marbles"(electrons) a marble(electron) will pop out at the tube leading back to you. So you can push that marble into the tube and another will pop out and so on.
    Imagine yourself as the power source(generator) pushing each marble(electron) back into the tube as it pops out. You are now a DC generator. The marbles(electrons) are "flowing" in one direction and keep flowing as long as you can keep it up.
    Now imagine you push a marble(electron) into the "upper" tube and when one pops out of the return tube you push it into the "return" tube. Then a marble(electron) will pop out of the "upper" tube. Push this "electron" back into the "upper tube" and one will pop out of the return tube and you continue to do this till you are worn out.
    Now you are an AC ALTERNATOR. If you can do this at a suitable speed, say 60 times a second you are generating an alternating current of 60Hz as the "electrons" flow backwards and forwards in the tubes.
    Note that I have not mentioned the effect of a "load" on the flow of marbles(electrons) and many other issues. Keep it simple.
     
  8. MagicMatt

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    Sep 30, 2013
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    This will give you a giggle...
     
  9. MagicMatt

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    I've only ever seen pylons which have wires arranged in pairs... I assumed that was essentially the "Live" and "Neutral" for each phase.
     
  10. cmartinez

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    It didn't give me a giggle... it gave the willies... I'm afraid of heights... and I'm afraid of high voltages... and I'm afraid of high voltages on high heights...
     
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  11. ErnieM

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    And to be even more pedantic the electrons don’t flow from the station to your house (or the intermediate transformer).
    Since this is AC they just wiggle about over a short distance.

    But that is not a very importaint thing to know.
     
  12. GopherT

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    ErnieM, could you please make a video of yourself doing the "electron wiggle". I am sure it will be a new viral video and dance craze.
     
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  13. ErnieM

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    Sorry but all I have is a clip of me doing the neutron dance.

    I'm just burnin' doin' the neutron dance.
     
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  14. cmartinez

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    Guess there would also be a trio of quarks doin' the backup singin'....
     
  15. MrChips

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    I think it is a very important thing to know.

    Many folks have a misconceived idea that electrons travel at the speed of light.
    They don't. It is their "wiggle" that travels at the speed of light, or close.
     
  16. ErnieM

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    My sincere apologies to mydoghasworms for kicking off a complete disruption and hijack of his thread.
     
  17. #12

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    OK. You stay off the towers and I'll stay out of the submarines.
     
  18. WBahn

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    They are not positive wrt earth, they are AC and are ground references, so they alternate between positive and negative relative the earth. The intent is for the line currents to balance such that there is no net current into the earth under normal conditions.
     
  19. WBahn

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    Nope. At those voltages you couldn't place the neutral that close to the live wire. It is very common to run phase lines in pairs. Not only does it decrease the I*R losses but it stiffens the lines, making them less susceptible to wind disturbances, and effectively makes the lines bigger in diameter with respect to inductance and corona losses. There are standard arrangements of six and higher multiples of three that are specifically intended to reduce effective inductance and RF losses.
     
  20. profbuxton

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    Feb 21, 2014
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    BR-549, an interesting concept. And how many Megawatts could you generate in this fashion? And what wattage of lightbulb could you light up with your concept.
    I do agree that there are levels of separation between power station and consumer(many tranformers) but I didn't want to complicate the explanation.
    A capacitor is only charged by being connected to a complete circuit. A capacitor connected to a power source via an open switch will not charge up.
    As far as wireless power goes, our local radio station radiates at many KW yet I need a very sensitive reciever to pick a very tiny signal, certainly won't run my toaster!
     
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