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[Phys-L] Re: long-distance electric power transmission

On 23/03/2005, at 5:16 PM, John Denker wrote:

On 03/22/05 09:03, Carl Mungan wrote:

2. Google also mentions that dc is sometimes used instead of ac for
long-distance transmission. Can someone give a brief explanation of
the most important advantages and trade-offs involved?

A major incentive for DC is that you don't have the
synchronization problems that come with AC.

On the other side of that coin, you need some
serious equipment at each end to do the AC/DC
and DC/AC conversion. Think about the power
levels involved.

When I migrated to New Zealand some 18 years ago and started teaching
high school Physics I learned of an interesting example of the use of
DC for power transmission. New Zealand comprises of a North and South
Island, separated by the Cook strait which is about 26 km (16 miles)
wide. The South Island is a source of hydro-electric power which is
transmitted to the North Island via a cable on the sea bed.

The power (generated in the south of South Island) is converted from AC
to DC for transmission through the underwater cable and then converted
back to AC for distribution in the North Island grid. A disadvantage of
AC transmission through an underwater cable is that electric currents
are induced in the sea-water and energy is dissipated, presumably by
ohmic heating. I found that this provided an interesting counter
example to the use of AC for long distance power transmission as well
as an example of an inductive effect. The saving in energy costs must
presumably more than offset the losses associated with the AC-DC
conversion. The following reference makes the point that the cabling
system for DC transmission is less then for AC. I do not know why this
is the case.


Kevin Murphy
Retired teacher
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