
San Francisco’s Secret DC Grid - jgrahamc
http://spectrum.ieee.org/energy/the-smarter-grid/san-franciscos-secret-dc-grid
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URSpider94
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.

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Gravityloss
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.

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URSpider94
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.

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Gravityloss
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.

