Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: power-grid physics



Thanks for the switching information. I was wondering how they did that!
Those big high-current Frankenstein movie-type knife switches have to be
opened & closed when there's no load, or else they'll just go *poof* or
*BANG* and weld themselves shut, often with substantial damage to the switch
operator. NOT pretty.

Regarding 1908, one major concern of engineers was the mass of copper
required in the transmission lines. There were various schemes for arranging
"mains" and "feeders" to minimize the cost of the copper, as well as the
voltage drop from one end of the system to the other. Longer distances
required thicker lines to keep the ohmic heating manageable, and copper
wasn't cheap even then. With AC, one can use smaller wires because there's a
thermal duty cycle. I'll have to go back & look at the book, since AC vs.
DC was a raging controversy at the time it was written. Mass of copper in
the transmission lines is *still* a major concern of engineers...it's a
non-trivial economic matter.

BTW, it turns out that some of the commercial DC circuits at the time were
*series* so that if the arc or incandescent electric light in your house
(you'd only have one), goes out, so do those of all your neighbors. Some
systems had a 2000V potential drop between generator and ground in order to
supply each arc light with 50V or so. If your light went out, you got the
whole 2000 V in your living room. Yowza! (Caveat...it's been a while since
I read the book, so the numbers quoted are approximate.)

Vickie Frohne

-----Original Message-----
From: John S. Denker [mailto:jsd@AV8N.COM]
Sent: Friday, August 22, 2003 3:19 PM
To: PHYS-L@lists.nau.edu
Subject: Re: power-grid physics

A lot has changed since 1908. At any given voltage,
DC has always been *more* efficient than AC. In 1908
they could deal with high-voltage AC more easily than
high-voltage DC.