From: hagerman@ece.cmu.edu (John Hagerman)
Subject: Electric Power Transmission (was: History of AC vs DC Power)
Organization: Carnegie Mellon University
Date: Thu, 19 Nov 1992 17:23:52 GMT

In <1992Nov11.154425.8399@speedy.aero.org> steve@pepe.aero.org (Steve Sedlacek) writes:

>A question came up at lunch yesterday: Why is our power system
>completely AC?  What are the historical and physical factors which
>led to the present power grid?  I beleive there are some DC high
>power transmission lines in Europe....  Again, what is the HISTORY
>and WHY should power transmission be AC (I assume it is more than
>just transformers to step voltage up and down being easy for AC).

Here are some excerpts from the Academic American Encyclopedia which
shed some light on these and related questions.  I have not included
the whole article (1) to save bandwidth, (2) to respect copyrights,
and (3) to entice you into a further look at your local encyclopedia.

	ELECTRIC POWER TRANSMISSION

	...

	Overhead Transmission Lines

	Many of the first high-voltage transmission lines in the United
	States were built principally to transmit electrical energy
	from hydroelectric plants to distant industrial locations and
	population centers.  High-voltage transmission lines were
	originally designed to permit the construction of large
	generating units and central stations on attractive, remote
	sites close to fuel sources and supplies of cooling water.
	Today, however, they connect different power networks in order
	to achieve greater economy by exchanges of low-cost power, to
	achieve savings in reserve generating capacity, to improve the
	reliability of the system, and to take advantage of diversity
	in the peak loads of different systems and thereby reduce
	operating costs.

	... Before World War II the highest-voltage lines in the
	United States were 230 kV, with the exception of one 287-kV
	line from Boulder Dam to Los Angeles.  In the early 1950s
	several 345-kV lines were constructed.  By 1964 the first
	500-kV lines in the United States were being completed, and in
	1969 the first 765-kV line was put into service.  All of these
	involved AC systems.

	In 1970 a 1,380-km (856-mi), 800-kV direct-current (DC) line
	was placed in commercial service to connect northwestern U.S.
	hydroelectric sources with the Los Angeles area.  Such systems
	... avoid stability problems sometimes encountered by AC
	systems; DC systems are sometimes used to connect AC systems
	even over short transmission distances.

	Underground Transmission Cables

	...

	One significant problem with underground AC circuits is the
	continuous flow of charging current between the energized
	conductors and the metallic cable sheaths.  Unless expensive
	compensation devices are used, this charging current can
	utilize the entire current-carrying capacity of the cable
	within a few miles of circuit and introduce other operating
	problems as well.  Although these problems do not occur with DC
	cable systems, DC transmission involves the additional cost of
	converters.

	... The most extensive extra-high-voltage (EHV) underground
	cable system at present is the 345 kV network that supplies
	the New York City area.

	The Federal Power Commission reported 760,378 circuit km
	(472,477 mi) of transmission line at voltages of 69 kV and
	above in service in the United States at the end of June 1976,
	and 4,580 circuit km (2,846 mi) of underground transmission
	cables.

	...

	GROWTH OF ELECTRIC ENERGY PRODUCTION IN
	THE UNITED STATES
	---------------------------------------------------------------
	Year              Electric Energy Production Per Capita (kW h)
	---------------------------------------------------------------
	1920                                   540
	1930                                   940
	1940                                 1,380
	1950                                 2,582
	1960                                 4,716
	1970                                 7,950
	1980                                10,245
	---------------------------------------------------------------
	SOURCES: 1970 National Power Survey, Federal Power Commission,
	Washington, D.C.; The World Factbook-1982.

- John
--
hagerman@ece.cmu.edu
Newsgroups: sci.physics