Re: Diesel VS Gas

On Sun, 27 Aug 2006 21:58:31 -0700, Wes Stewart <n7ws*@*yahoo.com>
wrote:

On Mon, 28 Aug 2006 01:59:29 GMT, "JerryD\(upstateNY\)"
<jerry@xxxxxxxxxxxxx> wrote:

Lone Haranguer wrote:
I believe this one is located at Underwood, ND. The DC is changed
back to AC at the Delano, MN end.

I wonder the reasoning behind producing DC electric and then changing it to
AC ?
I have never heard of this before. (but then I am a carpenter, not an
electrician)

Originally, AC replaced DC distribution systems because it is easily
transformed up or down in voltage. DC cannot be run through a
transformer to change its voltage. What was generated was what was
supplied. For long distance distribution a higher voltage is
desirable because power is the product of volts * amps and a higher
voltage results in a lower current for the same amount of power
transmitted. The power lost in the wire is a (square) function of the
current times the resistance. P = I^2 * R.

Extremely high voltage transmission is now the norm, with long
distance systems running hundreds of Kilovolts. As the distribution
lines approach the end user, the voltage is repeatedly stepped down,
with electrical safety being the major concern.

One limitation to using even higher voltages than are currently used
is the performance of the insulators that support the wires and
separate the windings in transformers. Arcing and corona become
serious problems, particularly in salty or polluted locations.

AC has the property that the peak voltage value is higher than the
equivalent DC value, by a factor of 1.414. Insulators are required to
stand off the peak value, thus there is an advantage to the lower DC
voltage. When corona occurs, the energy loss is greater in AC system.

DC systems can also use only one wire for transmission, using the
Earth for the other conductor. This is not without issues, however,
so most DC systems use multiple conductors, but in an emergency should
only one wire break the system can still operate.

Underwater power transmission lines use sealed coaxial cables. The
losses in these for AC are higher than for DC.

The technology that permits modern systems to be viable is the
solid-state switching devices now available. These systems are very
much like the inverters most of us are using to convert 12VDC to
120VAC in our RVs, or the inner workings of the Honda EU series
generators.

The Hondas generate variable frequency AC, convert it to DC, and then
convert that back to fixed frequency AC. This allows the engine to
run only fast enough to supply the load, where a synchronous
alternator has to run a constant speed regardless of load. The Hondas
are also self synchronizing, which allows two (or more) to be
connected in parallel without any hassle.

The same is true for DC transmission systems. Power generated in a
country with 60 HZ as a standard can be delivered to a 50 Hz country.
Likewise the problems of outages and recovery on "grid" systems, were
every alternator has to have the same frequency and phase as every
other, are gone with DC systems.

I think this 'nuff said.

Great post. Thanks.

Consider what is done inside the switching power supply in your
computer.

Take 120 volts AC at 60 Hz and rectify it to 120 volts DC. Use a
couple of transistors to change 120 volts DC to 120 volts AC but at
8000Hz. Use a really small transformer to drop the voltage to 12 and
5 volts AC. Rectify that to DC to power your computer.

Sounds silly, but higher AC frequencies require transformers with much
less massive core to do the same job. Ever wonder why aircraft use
400 Hz? They knew this back in WWII.

- Sandy
.

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