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DC’s revenge: The technology behind the Northern Pass cable shift

Last modified: 9/6/2015 11:31:01 PM
The term HVDC is used because the system involves high voltage – that’s the HV – and uses direct current or DC, compared with the alternating current, or AC, which is used in our homes.

DC is favored for long-distance transmission of electricity because it moves much more efficiently, so that much less of the original electricity dissipates as it moves through the line, especially under very high voltages.

The drawback with DC is that it’s much harder to take the electricity in a transmission line and reduce the voltage so that it can be distributed for local usage. It’s very expensive to make connections to the grid from an HVDC line via transmission stations.

As a result, DC lines are like interstate highways, where you can travel fast and efficiently but only get off at a few exits, whereas AC lines are like Route 202, with many more intersections but more inefficient travel.

There are other technical differences – DC lines can be fatter than AC lines, and it’s difficult to operate high-voltage AC lines underground – but line-loss efficiency is the main reason that DC is increasingly favored as renewable energy becomes more popular. Wind farms are often located far from population centers, including a number of offshore sites in Europe, and the intermittency of solar and wind power means that power grids need to be able to shift a lot of power from one place to another and back again, all of which favors DC.

Fans of the history of technology know how Thomas Edison, an early advocate of DC power, lost out to Nikola Tesla, an AC-power fan, in the development of the world’s electricity systems. Some depict the recent rise of DC as a form of revenge.

EDITOR’S NOTE: The article originally had the positions of Edison and Tesla switched. It has been corrected.

(David Brooks can be reached at 369-3313, dbrooks@cmonitor.com, or on Twitter @GraniteGeek.)


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