[SGVLUG] Off-topic - ac vs. dc

[David Lawyer]

On Fri, Feb 23, 2007 at 06:34:55PM -0800, Matt Wette wrote:
> The reason for using AC is that you can use transformers to hike up the
> voltage for long transmission, then bring it back down to safe operating
> magnitude for home use.  The high voltage reduces line losses,
> the power loss looking like V^2/R.

DC is better because:
1. It doesn't cause cancer, etc. since it doesn't produce alternating
magnetic fields.  AC is not all that dangerous but putting all the
studies together shows a problem.

2. There are less losses in transmission.  Thats why they built the
Columbia River to S. California DC transmission line.  Have you seen
it?  It's just 2 wires.

For the same DC voltage as an AC rms voltage and the same current (rms
for AC) the power loss (I^2R) and power transmitted (VI) are
identical.  So it would erroneously seem that DC has no advantage.

But wait, the AC has a peak voltage is 1.414 times higher than the rms
value.  So for the same insulation we can use a higher DC voltage.
Thus for the same current and same loss as AC we can transmit 1.4
times the power with DC.  This allows us not only to transmit more
power but to do it with less percent of it lost in transmission.

3. It has less interference with communication circuits.

4. It doesn't have the power factor problem where the current and
voltage are not in phase resulting in additional losses in
transmission.

To covert one DC voltage to another you have to use power electronics
and smaller "transformers" since the conversion is essentially done by
chopping DC to get AC which then goes back to DC.  You need to design
voltage conversion that results in low AC ripple on the DC lines.

A disadvantage of DC is that it causes a lot more corrosion of
underground pipe, etc. if it flows in the ground such as an electric
railroad like the Gold line that uses DC.

See my website www.lafn.org/~dave/trans/rail/electric_rr.html#ss4.2
for a discussion of AC vs DC for electric railroads.

			David Lawyer

> AC works well for the power companies because you can control flow in a
> a network by adjusting phase.
> 
> Robert Leyva wrote:
> >I am not an expert, but I believe people chose alternating current over
> >direct current back-in-the-day was that transformers could be used to
> >adjust  current and voltage easily with the technology back then....
> >
> >After all, a transformer is basically low-tech in my opinion : a set of
> >coils.
>