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Completing the circuit in power lines

  1. Apr 10, 2012 #1
    I am having trouble understanding how a circuit is completed from the AC power plants over the high voltage power lines. In order for current to flow there must be a "neutral" conductor. I can only conclude that the ground is used somehow to make this current flow. In contrast, the wires attached to homes always include a neutral wire.
     
  2. jcsd
  3. Apr 10, 2012 #2
    No neutral needed. Three phase is used for distribution. The 3 phase wires carry the current.

    The earth is used for lightning protection and safety. Every so many feet a stake is driven into earth and connected to the neutral line. This keeps the lines at a known potential wrt earth, and the wires above the hot wires are grounded and provide a path for lightning strokes.

    Did this help.

    Claude
     
  4. Apr 10, 2012 #3
    But why is no neutral needed? For current to flow you must have a difference in potential between two points, no? The three phase wires are all at the same potential as compared to the ground.
     
  5. Apr 11, 2012 #4
    Not at the same time. The potentials will be proportional to: sin(wT), sin(wT+120) and sin(wT+240)
     
  6. Apr 11, 2012 #5

    NascentOxygen

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    The potential difference between any two of the 3-phase conductors is a sinewave, it's amplitude is 73% greater than the potential between any of the conductors and earth.
     
  7. Apr 11, 2012 #6
    I understand that the 3 lines are out of phase, but how does that negate the need for a neutral or grounded wire so that current can flow in any of the wires?
     
  8. Apr 11, 2012 #7

    NascentOxygen

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    For a 3 wire system, the sum of the currents in all 3 wires is .... ZERO! So there's no need for a 4th wire to allow return current, because there isn't any excess current needing a return path. :cool:
     
  9. Apr 11, 2012 #8
    Maybe I don't understand how a 3 phase generator works, but it would appear that somehow the 3 wires are connected so that current flows? Otherwise, I don't understand how the 3 phase wires interact to carry current.
     
  10. Apr 11, 2012 #9

    AlephZero

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    The animation in http://en.wikipedia.org/wiki/Three-phase_electric_power might help.
    There red green and blue arrows represent three alternating currents with different phase angles. The sum of the currents where the three phases are connected is always zero. Joining those two points with a fourth "neutral" wire would not make any difference.
     
  11. Apr 11, 2012 #10
    In 3 phase systems, the objective is to minimize the amount of wire needed. A 3-phase system transmits a given power with only 75% of the wire area compared with 1-phase systems. The transformers used are connected in Y as well as delta configurations. A Y gives a neutral point ideal for grounding, providing lightning protection and safety. A delta allows the use of only 3 wires even when load currents are unbalanced. As long as all transformers have at least 1 winding connected in delta, the phase voltages stay balanced even when the load currents are severely unbalanced.

    In the case of a 3-phase core type transformer, where a 3-legged E core has all 3 phases wound on it, a delta is not needed, phases stay balanced without a delta and without a neutral wire. The theory behind this can be found in any good power systems reference book. It takes time and studying to extend one's thinking from single to 3 phase. It is fascinating to say the least.

    Claude
     
    Last edited: Apr 11, 2012
  12. Apr 12, 2012 #11
    You guys certainly helped me finally realize that the key to understanding this topic was 3 phase power. It took me awhile to study this, but I think I'm getting it!

    Now I have to apply my new understanding to figuring out how this relates to the transformers on the poles and in substations.
     
  13. Apr 12, 2012 #12
    So if the 3-phase wires carry the current, where are they connected together? Is it at the sub-station transformer? If so, are they connected by wye or delta?
     
  14. Apr 15, 2012 #13
    Either or. Sometimes it's a Y, other times a delta. To understand this, I recommend a good book on power systems. "Electric Machinery" by Fitzgerald, Kingsley, & Umans is a good start. I had Dr. Umans for a power class for my Ph.D. requirements spring 2010. He's very good. I would search through the book web sites like Ama***, etc. for good power erferences if you wish to buy a book. If not, check a good library. A uni library especially, can be a good source for reference books.

    Over the web on forums like this it's hard to explain 3-phase in a few paragraphs. Just remember that this subject has been analyzed to the point of exhaustion. No mysteries here. The research started in the late 1800's and wrapped up by the 1920's. Anyway I would advise you to check it out if you need to know about 3 phase.

    Claude
     
  15. Apr 16, 2012 #14
    Thanks for the input. I had no idea that this subject was this complex. No wonder the average guy doesn't understand what the 3 wires on the street are for. I'll check out the library before I spring a couple of hundred dollars for that text book.
     
  16. Apr 16, 2012 #15

    sophiecentaur

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    A real can of worms, actually!! and the net full of mis-information. This isn't helped by the fact that very different systems are used on either side of the Atlantic. (Those Americans can't get anything right. haha)
     
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