Don Kelly wrote:
"The Daring Dufas" wrote in message
...
Tom Horne wrote:
On Oct 4, 5:04 am, The Daring Dufas
wrote:
James Sweet wrote:
My suggestion would be for a 48 volt DC system. Plain
old telephone service uses 48 volts DC for battery and
PoE, power over Ethernet is usually 48 volts DC. The
wire size could be smaller than that for a lower voltage
system and the 78xx type regulators are very inexpensive
and come in a variety of wattage ratings for stepping
down the voltages. The technology to pull it off is not
exotic and can be done with all off the shelf parts.
Solar and wind power could integrate quite easily with
such a system.
TDD
And what would be the advantage? The 78xx series are linear
regulators,
they are in effect a regulated resistor that burns up the excess
voltage
in the form of heat. On top of that, they max out at 37V input at
which
point a substantial heatsink is required to dissipate the heat. The
end
result is FAR less efficient than even the lousy iron class II
transformers found in most wall warts and small appliances.
You could use a switchmode regulator to get decent efficiency, but
once
you've gone that route, you may as well just use 120V or 240VAC since
the additional components required are trivial.
Solar and wind power can easily integrate with the existing grid, with
the additional advantage of being able to sell excess capacity back to
the utility. The cost of the special inverter is low compared to what
the panels cost, and dropping all the time. This proposed DC system is
just reinventing the wheel with something inferior to what we
already have.
I was thinking about it as an off the grid system. I would
imagine that a single high current DC to AC converter in the
battery room putting out standard AC power to a home would
be more practical than trying to reinvent all the appliances
and gadgetry. Tesla won the battle for the power distribution
system and I'm glad of it. There are those very high voltage
DC power transmission lines. I'm going to have to read up on
them and find out why they're using DC. It's been 20 years
since I worked on any high voltage power distribution systems.
Have you ever used a wooden hot stick? Make sure it's dry.
TDD
DC Power transmission lines are used to rid the line of skin effect
and allow the entire cross section of the conductor to carry current.
They are as yet only practical for long haul point to point
circuits.
--
Tom Horne
That's what I read. My only experience with high voltage power
transmission has been installing buried conduit, setting transformers,
making connections and splices on 15kv coaxial underground cable.
Of course there was all of the other wiring on the low voltage side
of the transformer including the facilities wiring. What I find
fascinating about the long haul high voltage DC power transmission
systems has to do with the changes in technology over the years to
handle the conversion of AC to DC then back again. The early mercury arc
valve systems have got to be a sight to behold. I can imagine a mad
scientist wearing super thick lensed glasses cackling in the background.
TDD
Even a 5Kw 6 phase converter was a sight to see- looked like an octopus
with glowing arms and a bright spot dancing on a dish of mercury.
Seriously the advantages of DC transmission has relatively little to do
with skin effect as conductors are typically ACSR with aluminum on the
outside and steel inside- and, at these voltages are grouped in
bundles. The size of the conductor has more to do with mechanical than
electrical properties.
DC transmission at high voltages is economical for long lines where the
reduced cost of the line exceeds the added cost of the terminal
equipment. There are also some other technical advantages . This
breakeven point is at a much shorter distance for underground or
underwater cable. DC back to back terminals are often used where
frequency differences (e.g. in Japan with both 50 and 60 Hz systems) or
stability concerns arise. They do have the disadvantage that reasonable
and economic circuit breakers for DC don't exist and this means that
the system is essentially point to point rather than through an
interconnected grid. In addition, conversion from one voltage level to
the next is bloody expensive, awkward and inefficient compared to the
use of AC transformers.
At low voltages, even for relatively short distances, DC is not a viable
option.
From my reading, the problem of capacitive reactance is also minimized
with the DC transmission lines. When I was at Kwajalein Missile Range
during the late 80's, I got the chance to explore the old phased array
radar installation on Meck Island. It had a room we called the
Frankenstein room which was the power supply for the old radar. From
what I was told, the way they were able to make that monster scan, was
to change the phase angle of the microwave beam. The Frankenstein room
looked just like a prop from a science fiction movie. I wish I still
had the pictures. Here's a link, look for Meck Island an you can see the
big building in the upper right. There are two pictures, one showing a
view of the missile silo or silos. I don't remember if there were two.
http://www.fas.org/spp/military/facility/kwaj.htm
TDD