Power Line Droop.

Discussion in 'General Electronics Chat' started by BR-549, Apr 27, 2016.

  1. BR-549

    Thread Starter Well-Known Member

    Sep 22, 2013
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    Why do high tension/voltage power lines droop when under heavy loads?
     
  2. crutschow

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    Mar 14, 2008
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    Because the wires get warmer due to I²R losses and this causes thermal expansion of the metal wire, making the wires longer and causing more droop.
     
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  3. Dr.killjoy

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    Apr 28, 2013
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    @crutschow
    +1


    Also remember we have gravity that pulls the wire down stretching it between poles. Plus the kind of poles and anchoring system needed to keep the heavy wire taught would be insanely heavy and huge .. I mean look at the cable holding holding a bridge up and how much tension it's under and still sags..
     
  4. hp1729

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    Nov 23, 2015
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    Even if it is small the wires have some resistance. More current means more voltage loss along the wire.
     
  5. hp1729

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    Interesting. Do you think the no\umber of atoms in diameter or length changes? Resistance is a matter of number of atoms, isn't it?
     
  6. Dr.killjoy

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    Aren't main power feed lines made of aluminum wire ??
     
  7. wayneh

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    Sep 9, 2010
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    Well, the answer does not require any change in resistance, only a change in temperature due to resistive heating, leading to a decreased density and thus an increased length.

    The resistance of a metal wire does change (rise) with temperature, though. I think this effect would be minimal at the temperatures involved in transmission wires. Just a hunch.
     
  8. nsaspook

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  9. crutschow

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    Obviously, if the wire expands due to heat, then the number of atoms per unit volume must decrease, since there are a fixed number of atoms involved.
    This means the space between atoms slightly increases.

    Resistance is more than a simple function of the number of atoms, since a fat wire with more atoms, has less resistance than a thin wire of the same length.
    But a long wire, with more atoms, has more resistance than a short wire, of the same diameter.
     
  10. crutschow

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  11. nsaspook

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    The question was about if the cables were Aluminium.
    https://en.wikipedia.org/wiki/Aluminium-conductor_steel-reinforced_cable
     
  12. hp1729

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    Nov 23, 2015
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    Yes, some wire changes resistance when heated, like tungsten, especially. Does aluminum or copper change resistance notably in this instance? I guess any change effects voltage drop in this instance, doesn't it.
     
  13. wayneh

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    Oh, I think I see the problem. We believe the original question was about the physical drooping of hot wires, and not about any drop in voltage. hp1729 appears to be talking about resistance and voltage sag, another type of droop?
     
  14. crutschow

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  15. crutschow

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    Yes, most common metals increase resistance with temperature.
    But we are talking about heating due to a increase in current (the "heavy load" the op mentioned), which would occur even if the resistance didn't change with temperature.
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
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    Resistance is about how easily an atom can pass an electron on to the next atom.

    Search under valency bands.
     
  17. hp1729

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    Thanks for that observation. Yes, that was the way I originally read the post.
     
  18. MrAl

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    Jun 17, 2014
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    Hello there,

    All metals have resistance at normal temperatures. All metals also have a temperature coefficient. This means that they have some given resistance at a normal temperature like 20 degrees C and then the resistance changes as the temperature increases. This is usually tracked through it's resistivity rho.
    For copper wire, the resistivity in Ohm meters is:
    rho=1.7241e-8(1+.00393(Ta+Tr-20))

    where Ta is the ambient temperature and Tr is the temperature rise, in degrees C.
    In the above we see that when Tr=0 and Ta=20 degrees C the resistivity is the first number about 1.7e-8 Ohm meters. But as the temperature Tr rises the total resistivity goes up and thus the wire will drop more voltage at a given current. As time passes however a given constant current may cause a wire to heat up due to the initial rho and so rho rises more until the rate of cooling equals the rate of heating. This will cause the voltage drop to rise also.

    Physically, there is also a change in length which would play a role in how much the wire sagged when stretched between two rigid poles. The coefficient is about 1.7e-5 per K at 20 degrees C.
     
  19. hp1729

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    1.7e-5 what? mm? inches?
     
  20. MrAl

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    Hello,

    It's a dimensionless factor.
     
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