Why HVDC?
I've been led to believe that the problems in efficient DC-DC
conversion outweigh any advantages to DC distribution. (Granted,
some computer power buses use single-voltage DC and downconvert at
the board level, but that's all low-current stuff and efficiency
isn't such a big deal in that case.)
I should clarify that I was referring to long distance transmission
lines required to go from the sun belt to the north, though I think
the cost is dropping to the point smaller distances are economical.
There are economical HVDC lines presently operating, some of which
have operated long term. A couple of HVDC lines on the west coast
are on the order of 1000 mi long if I recall correctly.
Large reductions in cost and reliability of power thyristors have
been occurring, so the price of the electronics has been steadily
dropping, and would drop dramatically if wide spread use occurred.
There are various advantages to DC power transmission. See:
http://en.wikipedia.org/wiki/HVDC
Also google HVDC power transmission.
One of the major advantages is increased grid stability. I expect if
research dollars were available the cost of such systems might be
dropped significantly. I would expect the cost of power conversion
could be limited to about twice that of a transformer station, so the
economics can't be all that far out of reach for even converting
existing transmission systems.
If we're changing the grid, wouldn't it be more reasonable to
switch to higher frequency AC, which would be easier to manage than
60 Hz (and maybe less dangerous)?
Just wondering.
I don't recall if it was Steinmetz or Tesla who first determined the
optimum transmission frequency was about 57 Hz. The standard was
then set at 60 Hz for timing purposes. There are significant
inductive and capacitive losses in transmission lines themselves (in
the form of induced ground currents) and in power conversion in
transformers.
One of the great prospects for HVDC transmission I think is the
possibility of doing it in cables. It is not presently economical to
transmit 60 Hz in insulated cables due to heat losses and to
capacitive impedance. Underground and underwater AC transmission
systems therefore require large gap gas insulation in the cables to
reduce the capacitive linkage. When HVDC transmission is used the
line capacitance can actually increase the stability of the system
because it can be used in a manner similar to line pack in gas
transmission systems. The line acts like a giant power storage
device. Generating capacity can be reduced by managing line voltage
and packing in energy prior to peak hour load. This would require a
variable voltage system though. Managing natural gas transmission
systems is a fairly complicated thing and requires accurate computer
models of the system. Similar capabilities and requirements would be
necessary for an HVDC power grid, and some research and engineering
would likely be needed in that area.
The obvious advantage to underground DC transmission is removing the
blight on the countryside transmission systems represent, and the
legal problems and adversity to building them. There would be a huge
advantages to being able to use existing rail and pipeline and even
possibly highway rights of way to install a new power grid.
At some point I expect superconducting power transmission will be
economically feasible. One source of loss in such a system would
come from the tendency of surface voltage gradients on
superconductors to cause them to radiate. There is a solution to
this problem. That is to enclose the conductor in a thin
nonsuperconducting metal jacket (typically done anyway) which is not
used for power transmission, but rather simply kept at the
transmission voltage in order to keep the SC from radiating.
On major advantage to HVDC might be in the power gathering system for
a large group of windmills. This would reduce generator cost and
power management costs for each windmill. A power conditioning
station could then be used to get the power to the grid.
Making things happen to build a renewable energy power system will
either take a concerted national effort or vertical integration by a
huge monopolistic company with the lobbying budget sufficient to cut
through all the regulatory barriers.
Horace Heffner
http://www.mtaonline.net/~hheffner/
