(slightly technical post)
From time to time I have mentioned DC power transmission lines in my posts, and I was discussing this with Mr. X. We both had a very similar question—why is it that AC power transmission won out over DC, back in the 1880s, and yet today there is this new push to use DC for long-distance transmission? We both had a vague idea, and turns out we were both right, but I read more about it, and the details are quite interesting.
Short version—back in the 19th century, Thomas Edison was pushing for the development of a DC grid system, and George Westinghouse and Nikola Tesla were pushing for an AC grid system. Due to one basic fact, the AC system won out over time—it was easier to transform AC current into high voltages, and high voltages suffer exponentially lower losses during long-distance transmission. A simple transformer with a different number of windings on the two sides will step AC current up or down quite efficiently, but to change DC from one voltage to another essentially required the DC current to power an electric motor, the output of which drove another generator which was wound to produce DC output at a different voltage. Losses were higher, and these DC systems, with their moving parts, had much higher maintenance costs. So the AC systems won out, and became standard the world over. Simply put, systems needed high voltages for transmission, and much lower voltages for actual use for power or light, and this was only practical at the time with alternating current.
In all actuality, though, once DC current has been stepped up to those higher voltages, it is actually the more efficient of the two for long distance transmission. This is due to two basic characteristics of AC transmission that cause losses. The first is something called the “skin effect”, whereby the outer surface of an AC conductor actually carries the bulk of the current. The effect is significant with higher currents and voltages and larger wires. As you increase wire size, the mass of the wire goes up faster than the surface area, so you get less and less actual capacity gain with bigger wires. Doubling the weight of an AC conductor does not double its current carrying capacity. (They can get around this by using braided wires, but they aren’t practical for lines that are hundreds or thousands of miles in length). The second problem with alternating current is that every time a current is introduced into a wire, it creates a magnetic field. That magnetic field has to be “charged up” as the power begins. Unfortunately with AC, the power “begins” sixty or more times each second. These losses are called capacitance losses, and get worse as the conductors get closer together. In undersea cables, where the conductors are housed essentially side by side, these losses are so high that AC transmission almost doesn’t work, and most undersea transmission cables are built to use DC. But even with overhead line systems capacitance losses are present, and limit the effective range that AC power can be transmitted. In general, it can be transmitted for hundreds of miles, but not thousands.
Fortunately, much has changed since the 19th century—today it is quite possible to change AC to DC, and vice versa, and to transform DC power into different voltages; mechanical devices are no longer required. Long distance, high voltage DC (HVDC) transmission corridors have already been built; there are several in the U.S., quite a few in Europe, and the longest two in the world, both well over 1,000 miles in length, in China and Brazil.
This modern capability is important, because in the sustainable world that we need to move toward electrical power is going to be far more prominent than it is today. Most renewable power systems generate electricity, be they solar, or wind, or hydroelectric. And much of this power generation is NOT produced where it’s needed. (Just one example—the windiest parts of the U.S., the Great Plains, are not where the big cities tend to be.) So, in the future we will use more electricity as we phase out fossil fuels, increasing amounts of that electricity will be from renewable generation, and it will need to be moved long distances. Thus the need for DC transmission.
Image credit: aarrows / 123RF Stock Photo