HVDC power lines, why?

[K6JEK]

I should know this but I don't.  Why are HVDC power lines more efficient that AC?

Is the skin effect significant even at 60 Hz?
Do the AC lines couple to the Earth and lose power inductively?
Should I think of a 60 Hz 100KV line as a transmission line with an air dielectric and capacitance?

Power people, help me out. 

[WB2YGF]

I believe I was once told it was because the inductive reactance of the wire causes a loss for AC, DC being immune to inductive reactance.  Though I can't find anything on the web to corroborate that.  I did find this (can't vouch for the accuracy):

http://www.scribd.com/doc/7360181/HVDC-Transmission-Lines

See page 2.

[k4kyv]

Over long distances, inductive and capacitive losses are greater with a.c.  With DC only two wires are needed, whereas with three phase a.c. a minimum of 3 wires are needed.  The peak-to-average power ratio and peak voltage/current handling capacity must taken into account with a.c.  But the DC converters are much more expensive, and transformers don't work with DC.

For a short distance, the cost of the extra cable and power losses is insignificant compared to the cost of the converters, so the ac system wins.  For a long distance, the cost of the extra cable in the ac system outweighs the cost of the converters at each end, and the power losses become significant.

For overhead lines, the break-even distance for DC transmission is 600-700 KM .

http://oldeee.see.ed.ac.uk/private/ee3/pe/dem/6-hvdc.pdf

http://en.wikipedia.org/wiki/HVDC

[K6JEK]

Thanks

Jon