With all the power lines and substations we designed and built at the
utility, skin effect was never even discussed.  At 60 Hz, it is negligible.
No concern was ever given to radiated losses either.  The capacitance of
the transmission lines to ground was something that had to be dealt with,
especially on lightly loaded or unloaded lines.  An open ended line could
see twice the peak-to-peak voltage of the source at the other end and
equipment had to be designed to accommodate this.  At higher voltages,
switched inductors are used to effectively cancel the capacitance.  Of
course, excessive capacitance results in VAR flow which means more current
is flowing through the lines and transformers than is needed to handle the
real power load.  This all leads to additional heating and losses that age
and limit equipment, and also require more coal at the plant.

AC is not self extinguishing.  The zero crossings make an arc easier to
interrupt.  Much effort has gone into designing are interrupting equipment
for power circuit breakers.  I've been to a substation that had a bus fault
when the downline relaying failed to interrupt the fault.  The arcs
continued for nearly 20 minutes, turning station equipment junk with molten
metal and glass.  Interesting note:  We found the initiator of the fault; a
raccoon.  He was alive, although badly burned, hairless, and blind.

DC lines are cheaper to build because less material is required at each
structure.  A smaller structure is needed because you do not need to have
clearance between three conductors and fewer insulators and associated
hardware are used.  Smaller structures also mean narrower right-of-ways,
again, making the line cheaper.

DC is also used over long distances to eliminate the problem with phase
shift.  Power pools, excluding Texas which stands alone, are connected for
reliability reasons.  If you go across the United States, you are
approaching a power frequency wavelength.  It gets difficult to maintain
stability and control, and to avoid oscillations and swings, therefore,
using DC/AC and AC/DC conversions help lessen these problems.  The cost of
this conversion equipment is extreme.  You should see the SCRs they use at
these facilities.  Incredible.

Shield wires are typically strung over the conductors, whether AC or DC.
If positioned properly, they reduce lightning strikes by a factor of about
1000.

Mark Peterson

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