"Michael A. Terrell" wrote in message
...


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.


You seem to be laboring under the idea that all the AC generators tied to
the grid have to be carefully regulated to stay in sync with each other
through some incredibly precise timing.

That isn't the case. A generator is brought on-line by carefully regulating
the speed and getting it in phase. That is a bit tricky. But once tied to
the grid, 'keeping in sync' is done by the load current and physics. In
fact, base load units don't even have frequency control once on-line. The
speed set-point for the governor is run several hz up out of the way and the
turbine controls are controlled by a 'load' setting. The operator dials in
the amount of MW load they are supposed to carry, and the controls monitor
MW and steam flow. They don't respond at all to frequency changes unless
the frequency rises to the point the unit is in danger of over-speeding.

During grid disturbances, base load units will naturally speed up/slow-down
as grid frequency changes, maintaining their load output based on
'load-set'. Only 'regulating duty' plants monitor generator speed/freq and
make any sort of adjustment based on changes in speed/freq. And
'regulating' units make up a fairly small fraction of all AC units.

The vast majority of AC generators will 'stay in sync' just by virtue of the
physics of synchronous machines. Only if under-excited, or significant
reactance in their output line are they likely to 'pull out' of sync with
the grid. (and that's a *bad thing*)

daestrom