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400 kv power line

  1. Jul 1, 2011 #1
    Why do 400 kv power lines have 6 pairs of wires, 3 on each side? Shouldn't there only need to be 4 wires, a ground/neutral wire and 3 wires for 3 phases?
     
  2. jcsd
  3. Jul 1, 2011 #2
    The ground wire comes after the distribution transformer only...
     
  4. Jul 1, 2011 #3

    jambaugh

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    Three phase doesn't need a ground/neutral for power transmission Hence they will come in multiples of 3. Sometimes one or two ground lines are used as lightning protection where there are long spans.

    There are various configurations, from three wires up depending on needs. Multiple circuits give both redundancy and higher power carrying capacity.

    I would speculate that they may also be configured to reduce the small but nonzero radiative losses.
     
  5. Jul 1, 2011 #4
    Here's a link to a picture of a 400 kV pole:

    http://www.emfs.info/Sources+of+EMFs/Overhead+power+lines/Parts+of+a+power+line.htm

    Call the 3 phases A, B, and C. Are the phases transmitted in pairs, AB, BC, and AC? Because in the pictures of the power lines I've seen, they are sent in pairs with spacers preventing them from touching. But why pair them at all? Why not just have 3 wires of A, B, and C?

    Redundancy makes sense, but I'm not sure about power carrying capacity. If more power is needed, then the turbines in the generator just push harder, and more power goes down through the line. I'm not sure why you need multiple lines for that.

    Also if you have twice as many lines don't you get twice the losses?
     
  6. Jul 1, 2011 #5
    No. The two pairs would share the load, and each would have less current. That equals less resistive loss.
     
  7. Jul 1, 2011 #6

    dlgoff

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    The picture is very clear. There are two circuits; one on the left side of the tower and one on the right side of the tower. The spacers between two conductors, say on the left side, is to prevent them from wind damage and are the same phase & potential.
     
  8. Jul 1, 2011 #7

    jambaugh

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    The direct link doesn't work but I cut and pasted the link and did find the pic.
    Note that the spacers between the pairs are not insulators, they are just spacers to keep the wires a fixed distance apart (probably for mechanical reasons.) Note that the spacing is small so even if the spacers are insulating there can't be much potential difference between the pairs. Consider what would happen if a buzzard landed on one of the pairs. Even a moderate 100V would fry it.

    I imagine the pairing with spacers is to keep the cables from swinging in the wind as well as additional carrying capacity from stock wire. It is probably easier to make one gauge of wire and then string pairs than to have custom wire sizes. But I am speculating here.

    BTW, I've seen locally (middle GA) the lines in triple triangular bundles. This would prevent both side to side swing and dampen up and down motion of the wires.

    In the picture shown the pairs are (w.r.t. the electrical circuit) to be treated as a single cable. You thus have six cables, two circuits of 3 phase A1,B1,C1 and A2,B2, and C2.

    Figure the two circuits have independent load control, one may end up supplying East Metropolis and the other supplying West Metropolis, but possibly with switching available at the transformer station allowing all of Metropolis to be powered by one of the circuits should the other fail. Again I speculate... if you are still curious some intense Google-ing should find you some electrical engineering reference which would explain various configurations which are actually used.

    Redundancy makes sense, but I'm not sure about power carrying capacity. If more power is needed, then the turbines in the generator just push harder, and more power goes down through the line. I'm not sure why you need multiple lines for that.

    Also if you have twice as many lines don't you get twice the losses?[/QUOTE]
     
  9. Jul 1, 2011 #8
    Thanks, makes sense now.

    When I see two conductors with a spacer in between them, I usually think they're at different voltages. But I didn't stop to think what type of insulator could withstand 400 kv, so they're not at different voltages.
     
  10. Jul 1, 2011 #9
    Some intersting info from Wiki-

    Bundle conductors

    Bundle conductors are used to reduce corona losses and audible noise. Bundle conductors consist of several conductor cables connected by non-conducting spacers. For 220 kV lines, two-conductor bundles are usually used, for 380 kV lines usually three or even four. American Electric Power[4] is building 765 kV lines using six conductors per phase in a bundle. Spacers must resist the forces due to wind, and magnetic forces during a short-circuit.

    Bundle conductors are used to increase the amount of current that may be carried in a line. Due to the skin effect, ampacity of conductors is not proportional to cross section, for the larger sizes. Therefore, bundle conductors may carry more current for a given weight.

    A bundle conductor results in lower reactance, compared to a single conductor. It reduces corona discharge loss at extra high voltage (EHV) and interference with communication systems. It also reduces voltage gradient in that range of voltage.

    As a disadvantage, the bundle conductors have higher wind loading.

    From Wikipedia on electrical transmission lines.
     
  11. Jul 5, 2011 #10
    Is it true that only AC transmission lines produce a corona? Or am I wrong? I was thinking most transmission line was DC
     
  12. Jul 5, 2011 #11
    Corona can be made with AC, DC, a single electrostatic discharge, or any combination of these. It is the high voltage ionizing the air (ionized air is a conductor) and then supplying an eddy current into the ionized region that makes the light show.

    Electrical transmission lines, however, are AC. DC is impractical due to voltage drops, heat buildup, high cost of DC wiring systems, inability to work through transformers, and a host of other reasons.
     
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