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I looked at http://www.amsc.com/pdf/PM3000...

The spec sheet claims "power density of up to 130 W/in. (7.9 W/cm)"

But I also see:
Dimensions 38.2in*19.8in*18.7in = 14100 in^3
AC Power 690V * 750A = 520000 VA
Density: 37 VA/in^3 (also an upper bound on W/in^3)

What is the justification for the 130 W/in^3 claim?

One thing I can think of is that the AC transmission lines for the 3 different interconnect aren't all at the same location. There probably isn't a location where all 3 systems meet, and you need to have access to HVAC lines in order to transfer power to and from each system. So logical location would be somewhere that is closest to all 3 systems.

While you're correct that it's cheaper to have shorter superconducting DC lines, they don't seem to be that expensive. Their website indicates it's on par with HVAC lines: http://www.amsc.com/products/applications/utilities/superconductorpipeline.html

A big part of the cost of having DC line is the high efficiency and high voltage power converters to transfer from AC-DC-AC, which you need regardless where the location is.

And no, you can't get arc that's 5 miles long, that would be quite a sight though. :)

I think the point of building the big superconducting wire triangle is that they couldn't feasibly place the three AC/DC and DC/AC converter setups physically any closer. Looking at the proposed site, there is a pretty limited corridor where setting up a three-grid link would be possible. If they wanted to accomplish this from a single facility (say, where the center of the proposed triangle sits), they would have to extend existing high voltage AC lines to that point, which would have increased resistive losses. Plus, they would still be building power lines- towers, cables, maintenance. American Superconductor, who in addition to being a supplier of materials, is also an equity partner in this project, wants to demonstrate that they can make high voltage superconducting DC lines cost-competitive with high voltage AC transmission lines.

Actually, the article has a link to a map showing the actual location of the site NE of Clovis.

Here's the Tres Amigas design, via the AMSC site.

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Super Conductor Feed @ Feed Distiller

...it is a ceramic, which can hardly be used as a cable conductor.

You mean except for the ceramic cables that are already in use? I think your "information" may be a wee bit out of date.

There's the Enercon E-126 which is rated at 6MW. It is still considered a prototype,
even though there are a couple of them already built, and it'll probably end up more
powerful still: http://www.metaefficient.com/news/new-record-worlds-largest-wind-turbine-7-megawatts.html
I've been told that 8-9MW versions are probably just a couple of years away.

Enercon are the guys with the no-gearbox design by the way.

There's also an American project - with participation of the DoE - to develop a supreconductor-based 10MW generator:
http://www.amsc.com/newsroom/pr.html?id=317

And then there's - admittedly a bit more speculative and potentially vapourware-y - Superwind,
a Danish university project to design a superconducting 10-20MW wind generator: http://www.superwind.dk/

I'm a bit sceptical about those superconducting designs - even very good insulation will not prevent
all icing, and uncontrolled icing isn't a good thing, especially when it is in lousy weather a good distance
off shore. But even conventional (i.e. non-superconducting) technology will probably reach 10MW
not too far in the future. Pretty cool.

Oh, and for the really crazy ideas, have a look at the 1GW Maglev Wind Turbine
At the time being, this is a "looks good on paper" design. However, it might actually be feasible,
but is certainly still a long time away.

Project HYDRA, look into it.

"When cooled and squeezed very hard, the soft metallic element europium turns into a superconductor ... which, if harnessed, could make for more efficient energy transfer." After factoring in the cost of compressing and cooling a big long cable... In other words, not any time soon.

Superconductivity can be harnessed for efficient energy transfer. It's a boilerplate that is attached to any research associated with superconductivity to remind the general public whe they're spending millions of dollars on things which aren't available as direct dividends to their lives.

....hmmmmm, let's see: do you have gym shoes with velcro closure? that was an invention for the apollo program, to avoid having things fly in the capsule. I expect someone at the time said the same thing: "what's the use of sending people to the moon anyway?"
As to High temperature superconducting, the key temperature is the boiling point ofliquid nitrogen, which is relatively cheap and inert. AFAIK, superconducting power cables are in use now.

Found a picture of the units (Static VAR Compensator); I do not remember seeing them on normal poles. So, It looks like I assumed the wrong thing. Tim S http://en.wikipedia.org/wiki/Static_VAR_compensator http://www.amsc.com/products/transmissiongrid/static-VAR-compensators-SVC.html

According to this, this line is just 600 meters long and cost 60 million dollars. That comes out to somewhere around $160 million per mile.