On 02/20/08 15:00, Marco Peereboom wrote: > On Wed, Feb 20, 2008 at 02:14:31PM +0100, Henning Brauer wrote: >>> But that >100 year old technology used to be DC earlier, then it was >>> converted to AC because of its inherent benefits.
Marketing blurb. >> way over a hundred years ago, yes (except for some small irrelevant >> isles like parts of new york if memory serves). > > Even new york stopped doing it last year. There is no more DC current > being served. Well http://www.economist.com/science/displaystory.cfm?story_id=9539765 > Put like this, a Europe-wide grid seems an obvious idea. That it has not > yet been built is because AC power lines would lose too much power over > such large distances. Hence the renewed interest in DC. > > Westinghouse won the battle of the currents in the 1880s because it is > easier to transform the voltage of an AC current than of a DC current. (Also debatable with switching power technologies we have now instead of the classical bulkey 50/60Hz transformers, often the first thing we do these days is making the AC DC...) > High voltage is the best way to transmit power (the higher the voltage, > the smaller the loss), but high voltage is not usually what the user > wants. Power is therefore transmitted along high-tension AC lines and > then stepped down to usable voltages in local sub-stations. > > Edison was right, however, to argue that DC is the best way to transmit > electricity of any given voltage. That is because the shifting current of > AC runs to earth more easily than DC does. To avoid this earthing, AC > lines have to be built a long way from the groundand the higher the > voltage, the farther away they need to be. At 400 kilovolts, a standard > value for long-distance transmission, an alternating current 30 metres > (100 feet) from the ground has a fortieth of the loss of a similar cable > at ground level. But even at this height an overhead DC line will beat an > AC line at distances more than 1,000km (600 miles), while ground-level DC > will beat AC at distances as short as 30km. +++chefren
