wrote:

High voltage DC (HVDC) is used to transmit large amounts of power over
long distances or for interconnections between asynchronous grids When
electrical energy is required to be transmitted over very long distances,
it can be more economical to transmit using direct current (An electric
current that flows in one direction steadily) instead of alternating
current (An electric current that reverses direction sinusoidally).
For a long transmission line, the value of the smaller losses, and
reduced construction cost of a DC line, can offset the additional
cost of converter stations at each end of the line. Also, at high AC
voltages significant amounts of energy are lost due to corona discharge
(An electrical discharge accompanied by ionization of surrounding
atmosphere) the capacitance (An electrical phenomenon whereby an
electric charge is stored) between phases or, in the case of buried
cables, between phases and the soil (The part of the earth's surface
consisting of humus and disintegrated rock) or water (Binary compound
that occurs at room temperature as a clear colorless odorless tasteless
liquid; freezes into ice below 0 degrees centigrade and boils above
100 degrees centigrade; widely used as a solvent) in which the cable
s buried. Since the power flow through an HVDC link is directly
controllable, HVDC links are sometimes used within a grid to stabilize
the grid against control problems with the AC energy flow.

Also see
http://www.absoluteastronomy.com/ref...direct_current
http://en.wikipedia.org/wiki/HVDC
http://www.aip.org/tip/INPHFA/vol-9/iss-5/p8.html


I'm quite familiar with HVDC distribution systems, but more
generators are connected via AC than DC and those DO have to be in phase
and have the frequency controlled to keep the rest of the grid happy.

BTW: HVDC distribution has been discussed to death on both the
news:sci.electronics.design and news:alt.electrical.engineering
newsgroups.

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