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EM field between conductors

Discussion in 'Electrical Engineering' started by [email protected], Jul 28, 2008.

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  1. Guest

    I always though it was a bad idea to enclose the individual conductors of
    how current electric power distribution/transmission in metallic enclosures,
    and that you needed to enclose everything as a group to cancel out the
    magnetic field. Now I see this:

    http://www.knowledgeproblem.com/archives/002590.html

    Could it be that these tubes really have 2 conductors each at 180 degrees?

    How much current can a major 765kV transmission line carry? Now what voltage
    would be used for the same thing over superconductor "wires"? Is part of the
    idea to allow going lower in voltage, and higher in current? What kinds of
    issues might exist with very high current AC transmission lines (or even DC)?
     
  2. Don Kelly

    Don Kelly Guest

    ----------------------------
    ----------
    These cables are part of a 3 phase system. Each "cable" will have one
    conductor and, yes there will be a magnetic field produced in each. However,
    the external field will be small once you are some relatively small distance
    away from the group. In addition, under balanced conditions, there will be
    no return current in either the pipes or the ground.
    If you look at the connections in the background- you have conventional 3
    phase (2 conductor bundle feeding each cable).

    There also appears to some hype in the story as the attached references
    indicate a total length of a bit under 2000 feet-not 99 miles. The capacity
    is 574MW so the current would be about 2.5KA
    Possibly the mention of 6 system ties and 99 miles means that a future link
    would have roughly 20 mile sections, allowing compensation at each tie.
    ---------

    What a 765KV transmission line will carry is not usually limited by the
    current capability of the conductors except for a short line. Typically
    there will be 4 conductors in a phase bundle and these but sizing for other
    factors generally means that current is not a limit (look at the incoming
    bundle in the picture- which has to handle .

    What an EHV line can carry is given roughly by MW*Km =1.4(KV^2)
    This is a pushing it so about half of that is a typical figure so= 2500MW
    for a 100 mile line is in the ballpark. Roughly 2KA

    For short runs, superconductor "wires" are good but for longer runs, other
    factors do come in. For example 1 mile of underground cable is roughly
    (thumbnail estimate) equivalent to 10 miles of overhead line and 30 miles is
    then a long cable. This is because of the high capacitance of cable. The
    limiting factor of a power line is not resistance but the effects of
    inductance and capacitance. Even for HVDC the resistance is not a big
    issue.

    Where superconducting lines can be effective is in built up, high real
    estate, regions and then the savings in right of way costs can make them
    economic. This is where superconductors promise to be attractive.
     
  3. Guest

    |
    | ----------------------------
    | |>I always though it was a bad idea to enclose the individual conductors of
    |> how current electric power distribution/transmission in metallic
    |> enclosures,
    |> and that you needed to enclose everything as a group to cancel out the
    |> magnetic field. Now I see this:
    |>
    |> http://www.knowledgeproblem.com/archives/002590.html
    |>
    |> Could it be that these tubes really have 2 conductors each at 180 degrees?
    | ----------
    | These cables are part of a 3 phase system. Each "cable" will have one
    | conductor and, yes there will be a magnetic field produced in each. However,
    | the external field will be small once you are some relatively small distance
    | away from the group. In addition, under balanced conditions, there will be
    | no return current in either the pipes or the ground.
    | If you look at the connections in the background- you have conventional 3
    | phase (2 conductor bundle feeding each cable).

    I could not see it all so clearly (a larger sharper picture would have helped).
    But it did look like a simple delta feed.

    What I would be curious about is the effects of magnetic fields very near
    and especially between conductors. I'm assuming the spacing between these
    enclosed cables would remain about the same along the run. Would there be
    continuous vibration between the cables? The reason I am curious about
    this is that I've heard that with superconductors, very high currents
    would be used, like 10kA or more, in lieu of EHV.

    Normally, for service runs to customers, it is not allowed to run each phase
    in separate conduit and that it has to all be in a common conduit.


    | There also appears to some hype in the story as the attached references
    | indicate a total length of a bit under 2000 feet-not 99 miles. The capacity
    | is 574MW so the current would be about 2.5KA
    | Possibly the mention of 6 system ties and 99 miles means that a future link
    | would have roughly 20 mile sections, allowing compensation at each tie.

    Hmmm. So superconducting transmission lines have applicable limitations.


    |> How much current can a major 765kV transmission line carry? Now what
    |> voltage
    |> would be used for the same thing over superconductor "wires"? Is part of
    |> the
    |> idea to allow going lower in voltage, and higher in current? What kinds
    |> of
    |> issues might exist with very high current AC transmission lines (or even
    |> DC)?
    | ---------
    |
    | What a 765KV transmission line will carry is not usually limited by the
    | current capability of the conductors except for a short line. Typically
    | there will be 4 conductors in a phase bundle and these but sizing for other
    | factors generally means that current is not a limit (look at the incoming
    | bundle in the picture- which has to handle .

    So the reactance becomes the limiting factor very quickly with length?


    | What an EHV line can carry is given roughly by MW*Km =1.4(KV^2)
    | This is a pushing it so about half of that is a typical figure so= 2500MW
    | for a 100 mile line is in the ballpark. Roughly 2KA

    So what can you do to boost the capacity of a transmission line? I take it
    you are saying that doubling up the conductors won't do it.

    Is this why the Soviets were working on 1+MV transmission lines to cross
    Siberia?


    | For short runs, superconductor "wires" are good but for longer runs, other
    | factors do come in. For example 1 mile of underground cable is roughly
    | (thumbnail estimate) equivalent to 10 miles of overhead line and 30 miles is
    | then a long cable. This is because of the high capacitance of cable. The
    | limiting factor of a power line is not resistance but the effects of
    | inductance and capacitance. Even for HVDC the resistance is not a big
    | issue.
    |
    | Where superconducting lines can be effective is in built up, high real
    | estate, regions and then the savings in right of way costs can make them
    | economic. This is where superconductors promise to be attractive.

    If we build up a large windmill generating facility on the slope plains east
    of the Rockies, e.g as in "The Pickens Plan", that would mean a very long
    distance transmission of power. How can we most effectively get this power
    to the places it is needed? Lots of compensation points?
     
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