San Francisco’s Secret DC Grid

spectrum.ieee.org

31 points·jgrahamc··5 comments

Should be marked 2012, this is an old article that I think has been on HN before. Nonetheless, very interesting.

High voltage DC transmission is somewhat scary. It's really easy to create and sustain arcs at much lower voltage than AC, and there are risks from things like electromigration relating to the current always flowing in one direction. Nonetheless, it's a neat curiosity, sort of like the cable car of the electric utility world.

DC is now used for undersea and long distance cables too, making sort of a comeback. Large scale wind power generation facilitates it. Also connecting grids which are not frequency locked.

Wind power doesn't deliver DC. The windmills either generate AC at a variable frequency that is then converted to 60Hz through an inverter, or they have variable speed drive trains that output 60 Hz directly (more common these days).

Rotating generators don't make DC.

Edit: to add, using DC for underwater transmission makes sense because the capacitive coupling loss to water is much larger than for cables in air. If you mean using DC cables to collect power from offshore wind farms, then yes that makes sense.

And also because the power is intermittent / can not be planned, you need long distance transmission lines.

If you have just coal and natural gas plants, you can throttle them according to usage patterns. If you produce a large portion by wind, it might make sense to build a transmission cable to a nearby country with mountains and hydropower.

And there DC is more efficient.

The need for an inverter is another reason for using DC to collect power from multiple windmills. Rectify the power produced at each mill, combine it as DC, then chop it up to put it on the AC grid.

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