Hi Jim. Concerning high voltage direct current (HVDC) electric power transmission you asked:
"your comment about d.c. transmission was interesting. Would you care to say more on the subject? ... [it] was once preferred, and (as stated in the article) offered lower transmission losses."
The a.c. vs. d.c. debate stems from differences between the two competing power distribution designs of the late 19th Century, primarily those designed by Westinghouse and Edison, respectively. Hence began two separate campaigns to win the public's support, which led to chicanery, skulduggery, public charades and ultimately the folk lore that remains with us to this day.
From Wikipedia, see: ieee-virtual-museum.org
As the foregoing linked item notes, when the carnival and vaudeville acts were finally over, a.c. was accepted as the superior format for hauling electricity over long distances due to its singular ability at the time to undergo transformation from 110 volts to higher voltages (through step-up transformers), whereas d.c. could not. Since higher voltages incurred lower loss over distances, a.c. came out on top.
Since those earlier times, rectifier-, inverter- and even logic- & switching- technologies have been introduced and refined to support the stepping up and down of d.c. voltages, too, so now the question becomes, in part at least, one of backwards compatibility with an existing universe designed for a.c. operation. There are other advantages to using d.c. that transcend improved loss characteristics, however. For example:
In recent years I began noticing an increase in the number of HVDC transmission systems being deployed around the globe in both overland and submarine cable system designs. Sometimes HVDC is used not only for its improved loss characteristic, but also due to its ability to cushion, or isolate, fluctuations and frequency differences between interconnecting systems, such as grids supporting two different frequency standards. While the regulation of phase-angles and center-frequencies for 60 Hz systems (or whatever the normal frequency of a system happens to be) is usually achievable, it's often a cause of great concern, especially on interconnecting systems that use altogether different frequencies (e.g., two adjacent countries using 60Hz and 50 Hz, respectively). In such cases, converting from a.c. to d.c. eliminates the problem of differing frequencies entirely, and even when the same frequencies are employed on interconnecting systems, an all-HVDC transmission design improves overall stability on the interconnect, as well.
A classic example of high-voltage d.c.'s exemplary capabilities over very long distances can be found on the center conductor of many transoceanic submarine cables. In such applications voltages supplied from the shore are often at, or above, 10,000 volts dc feeding repeaters and test apparata along the entire length of the cable, i.e., from one end to the other across thousands of miles. From "Mother Earth Mother Board", Wired Magazine, December 1996:
"The signal coming down the FLAG cable passes through the doped fiber and causes it to lase, i.e., the excited electrons drop back down to a lower energy level, emitting light that is coherent with the incoming signal - which is to say that it is an exact copy of the incoming signal, except more powerful. The amplifiers need power - up to 10,000 volts DC, at 0.9 amperes. Since public 10,000-volt outlets are few and far between on the bottom of the ocean, this power must be delivered down the same cable that carries the fibers. The cable, therefore, consists of an inner core of four optical fibers, coated with plastic jackets of different colors so that the people at opposite ends can tell which is which, plus a thin copper wire that is used for test purposes."
wired.com
The complete article begins here: wired.com
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