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Superconducting Cables To Carry Power In Detroit
bewert writes: "Check out [this Knight-Ridder wire story.]
This could change electricity distribution economics as
we know it. A project is under way to replace 9 major
copper power distribution cables with 3 smaller ones
made from a high-temperature superconducting material
called BSCCO (pronounced bisco). Pretty interesting technology,
and one that could have huge implications for reduction of
transmission power losses and the need for more generation." Not to mention that it means a 25-fold reduction in the weight of the cables used to carry electricity for a large chunk of Detroit.
Yikes, let's clean this engineering up.
1. One of the main reasons for using the HTS cables is space. Detroit Edison needs to increase the current carrying capability at the station, and using the HTS means they get more capacity with the same underground conduit, so they don't have to excavate to improve the circuit. big savings. Plus, the smaller diameter and weight make it easier to pull the cable through the conduit, and eliminates the need for splices in the cable due to maximum pull weights. Bad splices are a common cause of failure in underground cables. Of course, if you are ComEd in Chicago, you just ignore that until the city goes dark...
2. Cables have higher capacitance than overhead transmission lines, because the conductor is closer to the ground potential. It also has lower inductance for the same reason. There is no external electric field. The sheath is at ground potential, so the field is between the conductor and the sheath.
3. Long distance cables are typically DC because in high voltage AC cables the voltage increases at the sending end due to the high capacitance of the line. That's why underground AC networks have shunt reactors, to keep the voltage down.
Although inductance does reduce the maximum power transfer in a circuit, it's due to its affect on voltage.
4. Although the lossless characteristics of HTS are important, that doesn't by itself make the economics attractive. Avoiding construction costs and pushing more power through the same rights of way due to higher current density is the niche that HTS is currently filling.
Elvis, the power engineer-nerd.
For instance, temperature is just one parameter to look at when you're looking at superconducting cables. Increasing the current density and/or the magnetic field will also tend push you out of the superconducting state. Tc is the temperature when current and field is zero, and the trouble with the high Tc superconductors is that you don't have a lot of clearence between the temperature of liquid N2 (77K) and Tc. When you try and load the cable with current, what you might call the "effective" critical temperature is going to be lower. The easy form of BSSCO is only something like 90K -- can't believe I don't remember more precisely than that, I used to work on this stuff -- anyway, I refuse to believe that they've managed to reliably come up with the 125K form of BSSCO, that's one holy grail that was looking pretty elusive, at least as of ten years ago.
A minute with google turns up what looks like a pretty good technical article about the processing of BSCCO/Ag tape: Is Low Cost BSCCO Tape Just Around the Corner?. (ObGripe: sure would be nice if the slashdot crew would do a teeny bit of background research on these stories, instead of just pointing us at junk news sources). Looks like I might be wrong about the 125K form of the stuff: they talk about working with both the 2223 and 2212 compositions (the numbers there are the main stoichiometries of the compound, e.g. Bi2 Ba2 Ca2 Cu3 Ox... as I remember it they don't usually specify the amount of Oxygen in the mix, because it's a bitch to measure it, and it tends to vary anyway). But then, they wouldn't be talking about both forms if they had the 125K form working really well.
Looks like they've got some decent numbers from direct measurements of current/area, which makes sense, or they wouldn't be announcing projects like this.
(By the way: one of the cool things about BSCCO -- I wonder when they made up this "Bisco" business, that's a new one on me -- but all the components are relatively non-toxic. At least they're not using something really evil like Thallium.)
Actually, no... its resistance does drop to zero, literally. However, the behavior becomes 'curious' when you actually try to drive current through it - especially since this is a T2 superconductor,which means that it forms magnetic whorls at any non-zero magnetic field. As you know, a current creates a magnetic field, and so you get a sort of 'balance' between the amount of current you can drive and the maximum field that the superconductor can support. If you try to drive more current than that, then you will begin to drive the substance out of a superconducting state.
Is it a resistance? Well, no, distinctly not - it's not linear in voltage, for one. It could be thought of as an 'effective' resistance, but it's not resistance.
Incidentally, it's very dangerous to simply show "Look, it's infinity!" and use that as a disproof - several things in nature are extremely curious and are literally infinity: take for instance a superfluid, which has, literally, zero viscosity, or an electron, which has (as far as we know...!) zero volume. Dividing by either of those things would tend towards infinity - the fact that it does not actually get to infinity simply means that another process begins to dominate and damp the previous one.
I really want to read more details about this. But I'm pretty sure that the key advantage is the lack of resistance.
Superconducting wires don't just have less resistance to current flow, they have no resistance at all. A superconducting cable will not have any losses due to resistance. (This means that when you run current through the superconducting cable, the cable won't heat up, so the cable won't be boiling off your liquid nitrogen.) I guess the reduced losses make up for the power needed to keep the cables as cool as liquid nitrogen.
My main worry is whether depending on liquid nitrogen for cooling will make this system more prone to failure. I'm sure they are not replacing all the copper, at least right away!
They wouldn't take risks with this if they were just breaking even. I'm sure that the new cables can carry more electricity than the ones they replace, not just the same amount; and the reduced losses might mean the same power plants can provide more useable power than previously.
steveha
lf(1): it's like ls(1) but sorts filenames by extension, tersely
Really, there aren't any. The stuff is insanely cheap. Like so cheap, you want to start using it as car fuel and stop drinking milk. I purchase 210L for $35.67, which works out to something around 55 cents a gallon!
Nitrogen is cheap, inert, catastrophic leaks have no effect on the world(unless it's in a closed room and someone can't get out before they suffocate), readily availalble (comprises 79% of air), and would only get cheaper to produce as power plants used more of it.
Keeping cables cool is also very easy since LN2 can be easily run through a pressurized system. There is no need to circulate the LN2 since the addition of heat will make some LN2 boil away. Simply allow the vapor to dissapate and replace any lost fluid.
The biggest problem with this project is what happens if the LN2 system fails for some reason. Fortunately, though, they will have an extremely long heads up on a failure and will be able to shut a cable down with plenty of time to spare.
On a side note, the cables use silver because it allows for proper grain growth and flexibility. Otherwise you couldn't make a cable out of the material. A big squarish chunk of it, sure, but not something long, thin, and reasonably flexible like a cable. Science News did an article on it a couple months ago.
www.eissq.com/BandP.html Ball and Plate System. Amuse your friends. Crush your enemies.