Power Line Droop.

Dr.killjoy

Joined Apr 28, 2013
1,196
@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..
 

hp1729

Joined Nov 23, 2015
2,304
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.
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?
 

wayneh

Joined Sep 9, 2010
16,398
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?
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.
 

crutschow

Joined Mar 14, 2008
25,122
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?
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.
 

nsaspook

Joined Aug 27, 2009
7,356
The wire material is not really important to this discussion since virtually all metals expand when heated.
The question was about if the cables were Aluminium.
https://en.wikipedia.org/wiki/Aluminium-conductor_steel-reinforced_cable
Steel is higher strength than aluminium which allows for increased mechanical tension to be applied on the conductor. Steel also has lower elastic and inelastic deformation (permanent elongation) due to mechanical loading (e.g. wind and ice) as well as a lower coefficient of thermal expansion under current loading. These properties allow ACSR to sag significantly less than all-aluminium conductors.
 

hp1729

Joined Nov 23, 2015
2,304
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.
 

wayneh

Joined Sep 9, 2010
16,398
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?
 

crutschow

Joined Mar 14, 2008
25,122
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.
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.
 

hp1729

Joined Nov 23, 2015
2,304
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?
Thanks for that observation. Yes, that was the way I originally read the post.
 

MrAl

Joined Jun 17, 2014
7,668
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.
 
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