But advances in transformer technology ultimately made it possible to transmit DC at higher voltages. The advantages of HVDC then became readily apparent. Compared to AC, HVDC is more efficient—a thousand-mile HVDC line carrying thousands of megawatts might lose 6 to 8 percent of its power, compared to 12 to 25 percent for a similar AC line. And HVDC would require fewer lines along a route. That made it better suited to places where electricity must be transmitted extraordinarily long distances from power plants to urban areas. It also is more efficient for underwater electricity transmission.
In recent years, companies such as ABB and Germany's Siemens have built a number of big HVDC transmission projects, like ABB's 940-kilometer (584-mile) line that went into service in 2004 to deliver power from China's massive Three Gorges hydroelectric plant to Guangdong province in the South. In the United States, Siemens for the first time ever installed a 500-kilovolt submarine cable, a 65-mile HVDC line, to take additional power from the Pennsylvania/New Jersey grid to power-hungry Long Island. (Related: "Can Hurricane Sandy Shed Light on Curbing Power Outages?") And the longest electric transmission line in the world, some 2,500 kilometers (1,553 miles), is under construction by ABB now in Brazil: The Rio-Madeira HVDC project will link two new hydropower plants in the Amazon with São Paulo, the nation's main economic hub. (Related Pictures: "A River People Await an Amazon Dam")
But these projects all involved point-to-point electricity delivery. Some engineers began to envision the potential of branching out HVDC into "supergrids." Far-flung arrays of wind farms and solar installations could be tied together in giant networks. Because of its stability and low losses, HVDC could balance out the natural fluctuations in renewable energy in a way that AC never could. That could dramatically reduce the need for the constant base-load power of large coal or nuclear power plants.
The Need for a Breaker
Until now, however, such renewable energy solutions have faced at least one daunting obstacle. It's much trickier to regulate a DC grid, where current flows continuously, than it is with AC. "When you have a large grid and you have a lightning strike at one location, you need to be able to disconnect that section quickly and isolate the problem, or else bad things can happen to the rest of the grid," such as a catastrophic blackout, explains ABB chief technology officer Prith Banerjee. "But if you can disconnect quickly, the rest of the grid can go on working while you fix the problem." That's where HVDC hybrid breakers—basically, nondescript racks of circuitry inside a power station—could come in. The breaker combines a series of mechanical and electronic circuit-breaking devices, which redirect a surge in current and then shut it off. ABB says the unit is capable of stopping a surge equivalent to the output of a one-gigawatt power plant, the sort that might provide power to 1 million U.S. homes or 2 million European homes, in significantly less time than the blink of an eye.
While ABB's new breaker still must be tested in actual power plants before it is deemed dependable enough for wide use, independent experts say it seems to represent an advance over previous efforts. (Siemens, an ABB competitor, reportedly also has been working to develop an advanced HVDC breaker.)
I'm not going to say I know a whole lot about this. As it's been a few years since I looked into it but to the best of my knowledge; DC has a disadvantage in that conversion to DC then back to AC loses a lot of energy. I believe it still requires over 300 miles to break even. It is only when you get large amounts of uncontrolled energy supply on the grid (renewable energy) that you need a 'Supergrid".
You may need a line to get power from a remote location but a grid? Not really.
If you are going to renewables in a big way then the evaluations I've read say the grid should be finished first. In Germany they have built windfarms offshore and now realize it will be a year or more before they can connect them to the grid. So there they sit rusting away, making no money while interest and other bills pile up.
Use of renewables increases the need for energy storage.
A working supergrid would reduce the need for storage.
Presently a supergrid is beyond 10 years off and not likely to affect the bottom line. I don't see it as affecting ALTI in the next 5 years or so.