Re: DC used in power distribution


"Tim Wescott" <tim@xxxxxxxxxxxxxxxx> wrote in message
news:DJ-dnZwITKLwj1rbnZ2dnUVZ_gidnZ2d@xxxxxxxxxxxxxxx
On Fri, 17 Aug 2007 23:14:22 -0700, miso wrote:

"With financing in place as expected by the end of the month, the
approximately 53-mile-long high-voltage, direct-current cable is to be
laid beneath the Bay starting in June 2009."

http://www.contracostatimes.com/news/ci_6647828
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I find it odd this power distribution scheme uses high voltage DC.

Long distance power transmission is often done in DC. If it has to go
from point A to point B without being tapped off too many places in
between, if it has to send lots of power and if it doesn't need to get
turned on and off too many times, then DC transmission is a good way to
go.

With AC power transmission you get dielectric, induced conduction and
radiation losses from the AC; these are significant enough on a
dedicated
transmission line that it's worth while to go DC.

What you _don't_ get with DC is the ability to slap the lines onto a
transformer and get easy step-up or step-down, and if you don't design
for
it sudden interruptions in the current can be hazardous because of
inductive discharge. When they cost out one of these lines they have to
balance the cost of the extra equipment to do the AC-DC transform and
back
against the cost of the power lost.
The other thing that is useful in DC transmission, is if the two grids
being linked, are not operating synchronised. This makes the economics of
using a DC-AC converter system far easier to justify.

Best Wishes


.

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