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.
--
Tim Wescott
Control systems and communications consultinghttp://
www.wescottdesign.com
Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes,
http://www.wescottdesign.com/actfes/actfes.html