On Tue, 6 Oct 2009 17:11:00 -0700, "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.


Only the Navy with their ships and submarines make use of DC well. That
still doesn't mean that it wasn't one hell of a costly implementation.

DC is great... on anything miniature, like a scooter, or model
airplane. :-]

Supplying DC feeds that can push as much power as we are used to with
current AC settings in the home would not be easy, and homes are low
consumption examples.

Even if we had compromised, and made AC to the pole, and DC into the
house, the DC part has a lot of pain in the ass required maintenance that
AC does not suffer from. Galvanic effects being the first one I think
of.

OK, so we drop the HV down to about 600V on the local poles, and then
we rectify that and feed the homes? Sounds like a very high
maintainence/service oriented method.

Maybe if we could make a nice DC chopper that would let us step off DC
highs and Gnd lows.. kind of a psuedo-alternation.