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Michael A. Terrell wrote:

If all those areas are connected to a single power grid they still
have to stay in sync, even if the control system is broken into
regional centers.

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

So?
Note that Michael said "single" power grid.

You are considering a point to point asynchronous connection between two
systems. A DC link is often used for this purpose even in some cases where
the converter stations are back to back but an asynchronous tie is required
because of differing frequencies (Japan)or simply because otherwise there
are problems maintaining a synchronous tie between two large systems
(Alberta and points west and south-Saskatchewan and points east and south).

It is true that they can be at different frequencies but...

within each system, machines have to be in synchronism. In the case of the
NW power pool, a DC backbone is used, as you suggest indirectly, in order to
maintain stability of the system which implies that it is used to maintain
synchronism in a system which might have problems otherwise.


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Don Kelly @shawcross.ca
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