Transmission, high temperature super conductors

Part of our current transmission system is that we have to use alternating current (AC) for long distance transmission.  The reason for this is that volts push the current, sort of like water pressure, and the pressure needs to be high to push it for long distances.  By high, I mean hundreds of thousands of volts.  To do this a device called a "transformer" increases the voltage from the generation source.  Then, when it arrives at its destination, another transformer lowers to voltage so it can be used in a home or business.  Transformers overheat with direct current, so AC is necessary.

To understand AC, think about putting a battery into a flashlight.  One end is labeled "+,"  for "positive" and the other "-," for negative. The electrons (current) flow from one side to the other in a stream. This is "direct current" (DC), it flows only one way.  With alternating current, it is if the poles switch rapidly (in the US 60 times a second, in Europe, 50 times a second) so the direction of the current flow reverses rapidly. 
DC is just fine for almost any purpose, and in fact, the AC has to be converted to DC o be used.  Those bricks that plug into a wall socket and charge your phone convert AC to DC

One of the things that makes it difficult to use DC for long distances is that when the current goes through the wires it meets "resistance."  The electrons bump into things and lose their energy as heat.  A good example of this where it is done on purpose is an electric heater and you have probably noticed that the power supply on your laptop can get warm as well.  Usually, this heat is wasted.  Your laptop may help keep your house warm on a cold day, but it just gets lost going through wires outside.  If there were no resistance, not only would there be no heat, but there would be no need to increase the voltage or use AC.  That is because the resistance needs to be overcome by pushing the current harder, sort of like pushing a car with the brake on.

There is some good news and bad news about resistance.  The good news is that there are materials with no resistance, the bad news is that the first one discovered, liquid helium, boils at -270°.  It takes a lot of energy to get things that cold. Fortunately, there is more good news: now there are materials that offer no resistance at a balmy -196°, the temperature of liquid nitrogen.   Even though that is colder than Lake Woebegon in January, it does not take nearly as much energy to cool to that temperature as to that of liquid helium.  In fact, it requires significantly less energy that the energy lost by changing the DC to AC, transforming it to high voltages and heating up the power lines.  This study by EPRI talks about the plausibility of doing this, and the verdict is good:

EPRI Shows That Direct Current Superconductor Cable is Feasible for Development Using Today’s Technology