DC power transmission...

Discussion in 'General Electronics Chat' started by antennaboy, Feb 9, 2011.

  1. antennaboy

    Thread Starter Active Member

    Jan 31, 2008
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    Hello Forum,

    I know that electrical power travels AC and high voltage over long
    distances. This serves to reduce ohmic losses. Also AC current is easily
    generated using mechanical methods.

    I have also heard about DC power transmission...does that offer particular
    advantages over AC power transmission?

    thanks,
    antennaboy
     
  2. shortbus

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  3. someonesdad

    Senior Member

    Jul 7, 2009
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    You can also read about the "current wars". The key is that AC can be changed in voltage efficiently (losses of only a percent or two) and easily with low-cost devices with no moving parts. This made the distribution of power easier and more economical than DC.
     
  4. tom66

    Senior Member

    May 9, 2009
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    HVDC is becoming more popular. It relies on electronic conversion of AC to high voltage DC, which is transferred over long distances, then converted back into AC. This has the advantage of lower skin effect losses and lower transformer losses.
     
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    DC is better ( actually any answer so simply stated is wrong ) With AC the average voltage ( absolute value actually ) is much less than the max voltage. So DC can make more efficient use of the transmission line. It also need not be synchronized. DC does not radiate waves the way AC does.

    Solid state method have made converting between DC and AC much more pratical than in the past. Really high tech efficient lines may well be DC.

    And finally as I just recently learned the power does not actually travel in the high tension line, the line guides it, but the power travels in the space around the line.
     
  6. nsaspook

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  7. russ_hensel

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    Just what it says: this is because the electric and magnetic fields carry the energy and they are largely in the space around the wires. You compute the Poynting vector fror that space to see the energy flow. There are several references on the web, one for example is: http://sydney.edu.au/science/uniserve_science/school/curric/stage6/phys/stw2002/sefton.pdf
     
  8. nsaspook

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    Why do wires heat and burn from too much current if this were true in the case of DC current flow?

    Sure there is energy in EM waves but Classical electron current flow theory explains DC power transmission. A energy Poynting vector is about the same as a real vs reactive power flow in a RLC circuit or antenna. It's been a while since I've studied this but I think these links should help.
    http://www.cmmp.ucl.ac.uk/~drb/Teaching/PHAS3201_EnergyFlowNotes.pdf
    http://my.ece.ucsb.edu/bobsclass/201C/Handouts/Chap1.pdf

     
    Last edited: Feb 10, 2011
  9. tom66

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    May 9, 2009
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    A.) because it is not DC, it is AC, which creates a magnetic field
    B.) the current is much larger (hundreds to thousands of amps often)
     
  10. nsaspook

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    A. What? There are no DC magnetic fields? I don't think that's really what you meant to say.

    B. If at 60hz the energy is transmitted around the wire instead of in it what does it matter?

    Don't take a theory of EM energy transfer in space and relate that to simple current flow in a power transmission line. Most of the power lost is from simple capacitive coupling (AC)/IR losses and not from EM fields like from a proper antenna.
    You don't need a solid wire to transmit power, charged ions in a vacuum chamber works just fine to transport electrical charge.
     
    Last edited: Feb 10, 2011
  11. tom66

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    Look - I'm only doing basic physics, but DC has zero net magnetic field.
     
  12. t_n_k

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    Then how do DC electromagnets, relays, etc. operate?
     
  13. tom66

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    Maybe I'm getting confused with something else. I need to learn more about electromagnetism, so I will just say: I don't know.
     
  14. russ_hensel

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    Jan 11, 2009
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    Wires burn because they have resistance which causes an electric field along their length. This with the normal B field caused the Poynting vector to point from the space around the wire into the wire causing the heating.

    I do not get the point of your second comment so I cannot comment. Your references seem to be sound material, but would boost rather than contradict the reference I gave.

    As for the necessity of understanding the physics, it may not be important to know if you are and engineer, but for a scientist it is exactly the point, and I think also interesting.
     
  15. nsaspook

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    My background in this was from the limited view point of understanding EM shielding and signal leakage in secure equipment 30 years ago so I'm far from an expert.

    I just think that saying ...
    misses the point about power losses in DC vs AC power systems. [SIZE=-1][SIZE=+1][SIZE=+1]The flow of energy down a conducting wire is ultimately due to particles coupling the EM fields ([/SIZE][/SIZE][/SIZE]Wave–particle duality?, as a EM wave in space and a electron during matter interaction?) [SIZE=-1][SIZE=+1][SIZE=+1]through the conducting medium (like a energy waveguide that confines the waves passing thru the conductor) not the physical movement of electrons in the wire. [/SIZE][/SIZE][/SIZE]The usage of EM waves and Poynting vectors showing the direction of energy flow down the wire to the load might be correct but is it useful in the real world of power transmission?

    http://www.engr.wichita.edu/ces/sensors/Poynting_naps_v7.pdf
    http://www.guspepper.net/electro/Primer semestre/Poynting vector.pdf


     
    Last edited: Feb 12, 2011
  16. cjdelphi

    New Member

    Mar 26, 2009
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    I have a n00b (me) solution to this problem lol...

    Make all the copper cables solid gold :D - would this not reduce transmission losses? :D
     
  17. cjdelphi

    New Member

    Mar 26, 2009
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    lol, not to mention small motors (12vdc car starter) even the Solenoid uses a coil of copper wire when current is passed the magnetic field is produced, use a secondary magnet to repel of the electromagnetic field and you get motion.

    If DC had zero value electric magnetic field, how would people start their cars :p If you have a Compass, stick it next to a nice large DC current say that of a 12v battery charger you'll see the magnetic field in action.

    oh and one more thing take a nice rare earth magnet (not sure how safe this this is, but it never killed me) and hold the Magnet next to the output of a battery charger (12v+) you can actually feel the magnet vibrating in your hand, it feels like your hand has become part of the charger.. I have no idea how this affects the output nor do i have any idea if it's actually bad for you... but it happens :p
     
  18. t_n_k

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    Apologies for taking this thread even further off topic.

    Yes you are probably getting confused with something else.

    Have you heard of the Biot-Savart Law?

    See this link with special note on the section 5. The Magnetic Field and Electric Currents.

    http://en.wikipedia.org/wiki/Magnetic_field

    Quote: "The magnetic field generated by a steady current I (a constant flow of electric charges in which charge is neither accumulating nor depleting at any point)[nb 13] is described by the Biot–Savart law:" etc.

    You can do a simple experiment with a basic magnetic compass, a battery and suitable length of copper wire wound into a loop. When DC current passes through the wire you should be able to make the magnetic compass deflect off normal position.

    You can even predict what NS orientation the DC magnetic coil will have and therefore which point of the compass will be attracted to the plane of the coil.

    Don't 'wreck' the compass by demagnetizing it with too much magnetic field from the DC coil.
     
  19. tom66

    Senior Member

    May 9, 2009
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    I can see headlines: Another Idiot Grabs 33,000 Volts of Gold from the Lines.
     
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