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[Phys-L] long-distance electric power transmission

Howdy: In the introductory course we have now reached the point in
the syllabus where we discuss transformers. Of course, one important
application is to step-up voltages for long-distance electric power
transmission to reduce Joule heating loss in the lines. A quick check
on google says that typical voltages for long-haul transmission are
115, 230, and 500 kV. Questions:

1. These are round values and are clearly 1000, 2000, and 4000 times
ordinary line voltage. But what is the logic for choosing them: why
not choose even higher values? do other countries in Europe and Asia
use the same values? (ie. are there universal engineering reasons for
choosing these values?)

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?

I'm not looking for exhaustive essays; just some basic ideas for
classroom discussions. Tx, Carl
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
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