The higher voltage a power transmission line use, the higher the efficency, correct? According to Wikipedia, the highest voltage in use is 1.15 MV for the
http://en.wikipedia.org/wiki/Powerline_Ekibastuz-Kokshetau line. The problem with high voltage lines, what limits them, is corona discharge, correct? This energy is lost in making plasma of molecules in air, creating ozone, for instance, correct? If we could hypothetically encapsulate the line in a vacuum, we would have removed the possibility of the corona discharge by removing the potential chemical/physical interaction with the air, correct? Could we in that case step up the voltage into.. infinity? What will be the next hinderance for higher voltages when the corona discharge is not a factor anymore?
Even in a vacuum, if the voltages are high enough, electrons can "boil off" a piece of metal. Higher temperatures accelerate this. That's how vacuum tubes work.
Also, if you had wires running through an evacuated tunnel, the wires would probably have to have some sort of supports, and those supports would need to be made of something that could insulate against the voltages involved.
So you couldn't make it all the way to infinite voltage, though you could probably exceed the voltage we currently see in air. I'm not sure what would determine the precise limits. But it would be quite an engineering feat to build the vacuum tunnel required to run long-distance power transmission lines, to keep it evacuated and sealed against leaks, and to maintain it. My hunch is that it's never going to be economically feasible, but my hunches have been wrong before...
It would probably be impossible to create a low enough vacuum for that for any reasonable line length. And if the vacuum isn't low enough, all you have is a giant gas discharge tube! Yay!
Now the opposite may make more sense: running the power line inside a tube/tunnel with high air pressure. Although you'd probably have a similar problem again of maintaining that pressure, eliminating leaks etc.
Far as I know, breakdown in vacuum is much like breakdown in air; keep surfaces smooth rather than pointy; electric field causes atoms to eject and ionize (in a UHV environment, the metal surface itself provides material for the plasma discharge, but once it's going, it's pretty much the same idea).
Breakdown is typically several orders of magnitude better, so for example there are those friggin fancy vacuum variable caps, which might be as wide as your hand -- just to get enough creepage distance along the glass, really -- yet the internal electrodes are mm apart.
A coax conduit would be pretty cool, and bonus points if you make it out of niobium and supercool the whole thing -- then you can shove most any frequency down it, from DC to 100s of MHz. (Handy for collecting the radiation from one of those theoretical solar power satellite microwave downlink things?) 'Course, now we're talking really big bucks, for a list of reasons...
The biggest problem with a long vacuum conductor is the same seen in that TV transmitter video... the inner conductor needs support pegs on it periodically. Those have to withstand breakdown as well, and even in vacuum, creepage is a problem (consider the effect of adsorbed gas molecules). Maybe something ridged and spiraling would do the job?
One big downside is thermal conductivity: the center conductor is cooled almost entirely by radiation alone. Perhaps if the conductors are painted black on the facing surfaces, this wouldn't be too much of a problem, but it's a big hit on losses for sure (it's one thing to have losses, but with the temp rise, it's going to be that much worse, given the large tempco of copper).
It certainly needs to be constructed and sealed well, but for long distance, especially underwater transmission, that kind of goes without saying, anyway!
Tim
It would probably be impossible to create a low enough vacuum for that for any reasonable line length. And if the vacuum isn't low enough, all you have is a giant gas discharge tube! Yay!
Now the opposite may make more sense: running the power line inside a tube/tunnel with high air pressure. Although you'd probably have a similar problem again of maintaining that pressure, eliminating leaks etc.
Not such a big deal -- talk to the people at CERN and other physics facilities. They have
miles of beamline at UHV.
Now, making that maintainable with few pumps, that may prove magnificently difficult. Beam lines are studded with turbomolecular pumps...
Tim
Underground high voltage lines are pressurised with nitrogen pushing on the oil impregnation. The higher pressure keeps voids from forming where low pressure will ionise. reason high voltage lines are up in the air is cost, it is a lot cheaper to have 100km of line hanging from 30m long insulators every 100m than to dig a tunnel, line it with concrete and then run a long thick pipe down it filled with an aluminium cable supported on a spiral insulating spacer and with 100km of low water transformer oil pressurising it, at $30 per litre. You do that for the least distance in a city for aesthetic reasons, but ask the Auckland council how that turned out maintenance wise as they age. Overhead lines may have all bad weather thrown at them, but the faults are mostly self clearing by designing in a massive safety margin, which is very cheap to do. the one tower here that collapsed was due to thieves stealing about 30 percent of the steel structure over a short period to sell as scrap. When they took all the bolts out on 3 legs it finally fell over in a high wind.
Now the opposite may make more sense: running the power line inside a tube/tunnel with high air pressure.
As SeanB says, this seems to be how very high voltage underground cables are insulated. At the hundreds of kilovolts level the state of the art for insulation has always been oil or gas impregnated paper. High pressure nitrogen is a very good insulator. The conductor is wrapped in a special paper, placed in a conduit, and surrounded by high pressure nitrogen. The gas not only provides electrical insulation, but by flowing along the conduit it also removes the heat generated in the cable by the flowing current. (One of the biggest issues with underground power cables is removing the heat produced in an enclosed and confined space.)
Don't forget SF6. Sulphur hexafluoride is a gas with very good isolation properties far exceeding say air. It is extensively used in HV switchgear. Not HV trasmission lines though, AFAIK.
Most of the long runs of HV cables in the UK are directly buried, here is a PDF from the National Grid giving details of the cables and installation of such cables.
There was a few metres of XLPE cable at the scrapyard last week, they get offcuts from cable burial, but this is a lot lower capacity wise than overhead lines. Typical overhead lines run at 1kA per cable, so if there is a bundle of 6 on a single insulator it will be capable of handling 6kA. Easy to upgrade the line as needs change, you just wind out new cables and clip to the existing insulators. Buried you need to first make a new trench........
Don't forget SF6. Sulphur hexafluoride is a gas with very good isolation properties far exceeding say air. It is extensively used in HV switchgear. Not HV trasmission lines though, AFAIK.
I've seen a SF6 distribution line. It was not extremely high voltage, it was just installed between two buildings on a college campus. It was probably in the range 7 to 13 kV.
Dawgie
Yes, pointless considering as room temp superconductors are just around the corner.
Just like fussion power is also just around the corner and has been for 25 years.