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Comment by thirdhaf | Hacker News Reader

Zero loss is only true for DC transmission. It's true, loss is lower than copper at 50-60Hz AC but radiative losses don't magically disappear. You're also limited in how much current you can carry since superconductivity breaks down in sufficiently high magnetic fields. For all that room temperature superconductors are revolutionary because of the jump in power density in things like motors and yes power lines.

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But didn't we only use AC because it was more efficient? So we switch back, and turn it back into AC for our houses, until we can switch over completely.

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.

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!

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-...).

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."

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.

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.

>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.

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

No. We are using AC because it is much easier to convert voltages with AC (by using a transformer) than with DC. Generator can't give very high voltage, it is in the range of a few kilovolts, but that voltage can't be efficiently transported over long distances due to heat losses (yes avoidable with superconductivity), and super high (100s of kilovolts) voltage can't be used by consumers, so when delivered, it has to be driven down to 100s of volts (in a few stages).

Superconductivity will change all of that (maybe in a 100 years or so since existing power infrastructure is worth hundreds of billions if not trillions and is a pain to replace).

Ok, so we only use AC because it lets us use more efficient voltages. I think we understand each other.

Also, tying back into what you said about efficiency: AC power can be much more efficient due to this easy transformation. The power dissipated across a resistor (which is a reasonably good model for power lines) is directly proportional to the square of the current passing through it. Thus, by stepping power up to high voltages, you can drastically cut the current and hence resistive losses in the line.

...but this whole thread was prefaced on room temperature superconductors. Hypothetically at least, this high voltage advantage wouldn't matter if you could have 0 transmission losses with DC at low voltages.

The point anovikov was making is that AC is not intrinsically more efficient. In fact, it is less efficient over very long distances dues to radiative losses. We don't use AC because it lets us use more efficient voltages today. We use AC because 100 years ago we didn't have the technology to boost direct current to high voltages.

OK, so if we had been able to just blast DC down the lines at 100kV or whatever, that would have been better?

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