Power Transmission

I was told by people of learning that power lines carried alot of the power as magnetic waves outside of the actual wire on transmission lines. The lines were supposed to be spaced the correct distance from each each other to maximize this desired effect. This being the transmission of power greater than the individual lines could hold. I believe this to be a falacy of the teachers. I do know that the magnetic lines of force do travel beyond the insulator of the wire, it’s the spacing of lines on the transmission poles I find suspect. Anyone have an enlightened view.

Moving charge does create a magnetic field, but it radiates perpendicular to the charge flow, not along with it. And since we’re talking AC transmission, the waves are occilating which translate to Electro-Magnetic Radiation called power waves.

These power waves are essentaily leakage, and I beleive power lines are hung 120 or 180 degrees out of phase, in which case coupling would like a partial short circuit that would decrease the transmission capactiy.

In any case, the spacing is simply to prevent them from touching in a storm.

I’m not a power engineer, so I can’t speak with authority on the details of implementation, but I do know something of the physics of transmission lines, and can add a few remarks that might count as “enlightened.”

First, power lines most certainly do carry energy outside the conductors themselves in the form of electromagnetic waves. They do not, however, radiate very much of this energy away from the transmission line. In a very real sense, a transmission line is a kind of inside-out waveguide.

At any point on the transmission line, the instantaneous charge on the two conductors is opposite (one is positive, the other is negative) and the current flow is also opposite. The result is an electric field between the conductors, and a magnetic field surrounding each. Since these fields are changing in time (oscillating at 60 Hz) they constitute a radiation field propagating in the direction of the Poynting vector–perpendicular to both the electric and magnetic field lines. It’s a little complicated to describe in detail, but the Poynting vector is not necessarily perpendicular to the direction of charge motion.

Anyway, the result of this configuration of electric and magnetic fields is that each section of conductor is trying to radiate like a radio antenna. But each section of the line is out of phase with neghboring sections in a way that produces an interference pattern that cancels at large distances but does not cancel forward along the line. So the radiation field looks like a train of waves of EM radiation propagating down the line. We know this must be true, because at a large distance from the line the net electic and magnetic fields are vanishingly small. This is because each conductor is doing the opposite thing (opposite charge; opposite current) so at a distance large compared to the spacing the transmission line looks like a single wire with zero current and neutral charge.

Now the wavelength at 60 Hz is about 5,000 km, and typical transmission lines run from a meter or so (your toaster cord) to several hundred km. For runs very short compared to a wavelength, all this transmission line theory is pretty irrelevant because there isn’t enough length to build up the interference pattern I’ve described. The two conductors in your toaster cord pretty much obey dc theory–the voltage and current are essentially the same everywhere on the line. (Jack the frequency up to 100 MHz, though, and it’s a different story.)

At several hundred km, however, these effects begin to come into play in a significant way–and at this point the spacing of the conductors begins to matter. The “impedance” of the line is controlled by the combination of conductor size and spacing. Getting power into and out of the line depends upon matching the impedance of the line with the impedance of the generator and the impedance of the load. The problem for the power companies is that the load impedance is constantly changing–so they have to be constantly rearranging the power network to keep everything as matched as possible.

The other way in which spacing matters is that the closer you make the wires, the less “far-field” radiation there is, and the physically smaller the “near-field” radiation pattern. Keeping the near-field pattern close to the line is important so that you don’t waste power heating up the ground or any passing cows. On the other hand, close spacing corresponds to low imedance–which means high currents and low voltages. This configuration wastes power in ohmic heating of the conductors. You want high voltage, low current for efficiency. So there’s a tradeoff in spacing. Not being a power engineer, I haven’t studied how this is done, but you see this issue: Wide spacing makes the radiation field bigger and reduces the current carried in the conductors, but allows waste due to ground heating. Narrow spacing makes the radiation field smaller and reduces ground heating but increases current in the conductors and allows waste due to conductor heating.

You can tap into the magnetic fields that exist outside power lines. All you need is a conductor that intrudes into the near-field radiation and provides a better path to ground than the line itself, if only for a moment. It’s a good way to electrocute yourself.

The important point is that you don’t have to actually touch high-voltage lines to be shocked by them. Stay well away from them and under no circumstances come anywhere near with a metal pole or pipe or ladder, etc.

“non sunt multiplicanda entia praeter necessitatem”
– William of Ockham

      • I read once in a survivalist magazine that it is possible to “steal” power from high-voltage lines, simply by stringing another conductor parallel to the power lines themselves, and having the necessary electronics to make use of it. The gist of the discussion was that it would work (it does) but it doesn’t really produce much power unless you use hundreds of yards of “antenna” and even then the power you can pull off is still relatively small - not nearly enough to say, stop paying your household electricity bill. Sounds like a lot of trouble to go to just to screw the man, but what the hell. The article noted that a regular livestock electrified fence can be put up right next to suitable power lines and the “live” wires used for this purpose, without attracting any attention. - MC