[mercuryrising]

AC becomes less efficient (financially and electrically) at more than 1 Megavolt. The big big transmission lines in the United States actually use DC because the AC transformers would have to be so large, and the losses at that high of a voltage are huge (and that 1 MV generators get to be fairly expensive).

NPR made a really awesome visualization of the United States grid (http://www.npr.org/templates/story/story.php?storyId=1109973...) that shows the lines used to transmit power from generating sources.

[dhughes]

Adding to that high voltage more than anything is the benefit of AC.

The weight of the conductor is a big problem for long runs of power lines so if you double the AC voltage the weight of the conductor can decrease by 25%. DC over 1500 V isn't really possible/efficient (some limit of generators) but AC can be many hundreds of thousands of volts.

It appears some of my electronics training is sinking in, I put it to good use!

[mercuryrising]

What do you mean by doubling the AC voltage to decrease the weight by 25%? If I continue doubling, will I eventually not need a conductor?

The true power of AC is the fact that the energy is transmitted through the electric and magnetic fields (Poynting vector). DC needs to push everything down a little copper tube. When you start oscillating things though, the effective area of your conductor increases greatly (you start using the air as a transmission medium). This is why you can do things like this (http://hacknmod.com/hack/field-of-fluorescent-tubes-powered-...).

[dhughes]

I'm just going by my textbook, I'm still new to this.

I understand what you're saying about AC creating an emf and pushing itself through a conductor via magnetism and frequency. The skin effect would play a big role in this too. As I say I'm still new to this, I know enough to be dangerous.

Here's the exact quote in case you are interested:

"The weight of a conductor required to transmit a given amount of power a given distance with a fixed loss varies inversely as the square of the transmission voltage."

[mercuryrising]

Ah ha! That's actually kind of cool.

Power's given as: P = V^2 * R R is given as: R = rhoL/(pir^2) Substituting: and clumping constants: P = (V^2 / r^2) * (rho*L/pi) Weight is proportional to r^2, we can see that voltage and weight are inverses of one another, and increasing the voltage for a fixed amount of power decreases the radius necessary to transmit it, decreasing the weight.

I really wish I still had my lecture notes about the energy flow through free space, this is the best I could find online - http://amasci.com/elect/poynt/poynt.html.

[Retric]

Graph 0.75^(log X) : http://www.google.com/search?q=0.75%5E+log+x

Anyway, DC current is still mostly a magnetic interaction. Actual electrons rarely get above a few cm/s.

[polshaw]

>DC over 1500 V isn't really possible/efficient

High-voltage DC uses voltages in the hundreds of kV. It is also the most efficient way of sending serious amounts of power over long distances (even before superconductors). Just as required cable thickness decreases with AC voltage, it has to increase with current. HVDC gives allows for smaller cabling.

[zrail]

That is an excellent link. Thank you for sharing it.