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DC Power Transmission

Discussion in 'Electronic Design' started by HardySpicer, Aug 28, 2007.

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  1. HardySpicer

    HardySpicer Guest

    I heard recently that the Chinese are transmitting power using dc
    rather than ac. I don't have details about this but I wondered what
    advantages there would be. No inductances to worry about for instance
    at dc. Is there less losses with dc transmission? Of course there can
    be no transformers. I also thought that 3-phase was one of the most
    efficient methods in terms of copper of transmitting power. Also there
    could be no power factor problems if factories used dc. dc on the
    otherhand for motors makes a motor less reliable due to brushes etc.
    So, was Tesla wrong and Edison right after all?

  2. HardySpicer

    HardySpicer Guest

    and I forgot the power factor problems of ac...
  3. Tim Shoppa

    Tim Shoppa Guest

    Lots of places are using DC power transmission in certain spots,
    usually for two reasons of acute economic necessity:

    #1 Reason to use DC at a couple of points in your grid: You're
    connecting two or more AC subgrids that are not synchronized and
    cannot be synchronized.

    #2 Reason is that you have an existing, un-build-outable transmission
    line segment that you need to move more power over. Using DC lets you
    not worry about peak components but only the DC component in terms of
    insulation and current-carrying capacity. Remember, heat losses go
    like I2R but power only goes like IE...

    If you can build a single segment for both reasons #1 and #2, then it
    can be economically feasible, but the converters on both ends are
    massively expensive.

    In fact while DC is used for the long-hauls and choke-points they
    invariably convert to/from AC on both ends except in some very
    specialized applications.

    China is under a lot of political pressure to look like they're doing
    something about urban air polution, and while power plants near large
    cities are certainly a source they probably aren't the biggest one. If
    it results in something useful actually being done, so much the

    Edison didn't have a good way of going up and down in DC voltage on
    any scale. Motor-generator sets dominated for the production of DC
    from AC for a long, long time.
  4. You forgot few 'insignificant' factors like overall efficiency in
    distributing form source to user(s), distances and terrain involved,
    weather, keeping it up and costs.
    BTW the three phases are mathematical solution to an equation which has
    many of those subjects as variants. Simple if you use the right math.
    The DC solution is with us for many years over 'very' long distances as
    the DC creation on one end and AC recreation on the other balances
    favourable against losses in such long conduction line.

    Tesla/Edison each has its technical uses. It is for the designer to come
    with solution to local problem.


  5. Phil Allison

    Phil Allison Guest


    ** See thread:
    " DC used in power distribution "

    here on SED, on the 18th.

    ....... Phil
  6. Fred Bloggs

    Fred Bloggs Guest

    You're having a problem with reading comprehension. The long distance
    transmission at DC does NOT imply that the load end distribution is also
    DC. The HV DC is stepped down and converted to AC at the destination so
    it can plug into the existing infrastructure.
  7. Fred Bloggs

    Fred Bloggs Guest

    It's the same troll...

    Best regards,
    Spehro Pefhany
  9. Tam/WB2TT

    Tam/WB2TT Guest

    There is also the real possibility of radiation loss (line acts like an
    antenna) from 50 and 60 Hz transmission lines. For instance, 1/4 wavelength
    for either will be less than 1000 miles. A high SWR will further increase

  10. Phil Allison

    Phil Allison Guest


    ** Consider that AC lines have 3 phases, running all the way in parallel.

    The EM field at a distance cancels to zero.

    There is no radiation.

    ....... Phil
  11. Bob Myers

    Bob Myers Guest

    For fun, go figure the radiation losses from a line
    even at an exact 1/4 wavelength, taking into consideration
    the fact that the return path is RIGHT THERE alongside
    (virtually along the same path in space, at these
    wavelengths) the "forward" path.

    Bob M.
  12. Tom Bruhns

    Tom Bruhns Guest

    Ah, but if the line is balanced (and they do take some pains to keep
    it balanced, by rotating the wires occasionally and other tricks),
    what _is_ the expected radiation? Does it depend on length of the
    line? What sort of matched-line attenuation do you get in 1000 miles
    of line? And what sort of SWR do the lines run at when operating at
    rated power? Are long lines ever operated at high SWR, or is that
    considered a fault condition? The answers to some of that may
    surprise you.

    Another consideration for DC versus AC transmission is that at DC, the
    current density in a cross-section of a homogeneous conductor is
    practically uniform, but at 60Hz, the skin depth in copper is only
    about 1/3 inch, and only a little more for aluminum; thus the
    resistance of the wire is lower for DC, significantly so for large

  13. Tim Williams

    Tim Williams Guest

    What's the typical characteristic impedance of those, anyway? I would guess
    in the kilohm range at least (which is around 1MV, 1kA).

  14. HardySpicer

    HardySpicer Guest

    But the fields are at 120 degrees spacing.
  15. Bob Myers

    Bob Myers Guest

    And the physical relationship of the conductors
    stays constant over how much distance, ya think?

    Do you think that this sort of problem just might
    have been already thought of by them what design
    power distribution networks?

    Bob M.
  16. Phil Allison

    Phil Allison Guest


    ** Yep - so they cancel exactly to zero, same as neutral current does.

    Dunno much AC theory - do you TROLL ?

    ......... Phil
  17. Tom Bruhns

    Tom Bruhns Guest


    If you assume a two-wire line made from highly conductive round wires
    two inches in diameter and spaced 200 inches apart, well away from any
    ground and essentially air-insulated, the differential mode impedance
    will be pretty nearly 636 ohms resistive. To get to 1000 ohms, they'd
    need to be spaced 4160 inches apart, and kept much further than that
    from ground. It's tough to make a really high impedance RF
    transmission line.

    If you're operating a line at a frequency where the series resistance
    of the conductors is an appreciable fraction of the series inductive
    reactance, or the shunt conductance is an appreciable fraction of the
    parallel capacitive reactance (in other words, at low frequencies),
    the line characteristic impedance may be quite reactive. The calc I
    just did for this hypothetical line at 60Hz and assuming room
    temperature copper conductivity suggests about 636-j10 ohms; assuming
    I didn't mess it up, you can probably forget about the reactive part
    for most calcs.

  18. Tom Bruhns

    Tom Bruhns Guest

    It's not true that there is _no_ radiation, but it's pretty low.
    Consider it as a linear system where each phase consists of a current
    in one direction in one wire and a current of the same magnitude and
    opposite direction in another of the three wires. You can find the
    radiation from that pair and vectorially add it to the radiation of
    the other two phases from two other wire pairs. If you're careful
    about it, you'll discover that there is radiation, but that the total
    radiated power is not so dependent on the length of the line as you
    may have guessed it would be. Even if you added the magnitudes of the
    radiations from each phase (instead of adding them vectorially), you'd
    be dealing with a very small number. Talk with people who actually
    run power through such lines and ask them what they worry about for
    losses. I think you'll find that radiation is pretty far down on the

  19. Jamie

    Jamie Guest

    They make DC brushless motors.
  20. HardySpicer

    HardySpicer Guest

    They are good but more expensive.
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