Practical issues with Superconductivity Well, Is it really possible to maintain such low temperatures required for super-conductors (taking High-temperature superconductivity into account) over large distances?
What I say is - Even if we were able to pass current through superconductors, we need to constantly cool them for maintaining the zero resistance. Hence to cool, we need power. Then, superconductors wouldn't be necessary in this manner if they don't have an advantage..? Or, are there any new approaches to overcome these disadvantages?
 A: The cooling over large distances is a challenge but the engineers and physicists are working on it. The current record is several km but not enough to connect cities, etc. This basically only works if the cables are in a kind of vacuum  tube to reduce the cooling power.
It is not practical to transmit electric energy if you need liquid helium temperatures. The cooling costs are prohibitive. The current state of the art are cables using thin films of BSCCO (phys.org). They can operate at 77 K without problems. The current world record for such a cable in a vacuum tube is several kilometers but after some distance you need a small building along the cable to cool the liquid nitrogen inside the cable again. 
There is a tremendous research effort to find superconductors with higher critical temperatures and currents but that is not so easy. The usage for practical applications is increasing but the progress is rather slow. In more exotic applications such a CERN or ITER you absolutely need superconducting cables, if it is only for space reasons:

A: In addition to critical temperatures and currents mentioned in the first answer, there is also the consideration of critical magnetic field intensity, above which, like temperature and current, the superconductive phase would transition back to a normal conductor.
The picture from the previous answer shows an excellent application of superconducting technology that is used to mitigate this effect.  Note the flat ribbon shape of the superconductor, as opposed to the (multiple) round shapes for a normal conductor.  This technology was originally patented by an American company, American Superconductor in the 1990s. Ribbon shaped type II superconductors of niobium-titanium are used in the quadrupole superconducting magnets of CERN's Large Hadron Collider in part because the intense magnetic fields induced in the superconducting ribbon can be more evenly distributed across the surface of a ribbon as opposed to a wire with a circular cross section.
