🚀 go-pugleaf

In article <1992Nov18.204405.29722@mksol.dseg.ti.com> mccall@mksol.dseg.ti.com (fred j mccall 575-3539) writes:
>In <1992Nov11.154425.8399@speedy.aero.org> steve@pepe.aero.org (Steve Sedlacek) writes:
>>A question came up at lunch yesterday:  Why is our power system completely
>>AC?  
>
>Well, actually it isn't.  Isn't there some stupid rule that puts large
>taxes on AC power transmission across state lines?  I seem to remember
>that there is, and that what the power companies do is convert the
>power to DC on the transmission side of the line, send it across (tax
>free), and then change it back to AC on the other side for further
>transmission (a process referred to as 'wheeling', if I'm remembering
>correctly).

Apart from possible taxes, there's a good, physical reason to go to DC
on occasion: grid synchronization.  Take the situation of state A
exporting power to state B; just imagine what would result if the power
fed by A were 180 degrees out of phase with A's grid: big trouble!  If
you don't use AC->DC->AC as a "phase buffer", you need to keep
synchronized the grids of all states that want to be connected
together, which is pretty hard on a continental scale.

To deal with that problem, Hydro-Quebec recently built a high-voltage
DC line from James bay right down to the U.S. border, for the express
purpose of exporting power to New-England.  As far as I know, the
DC->AC converter is north of the border, so that the power is exported
as AC; only this one converter, however, needs to be synchronized with
the New-England grid.

>Even with the power losses involved in the two changes
>and the expense of the 'wheeling' stations, it works out to still be
>cheaper than paying the tax.

For a long line, these losses and costs can be offset by the higher
carrying capacity of the line: the power carried is P = V_rms.I_rms; in
an AC line, the insulators have to be able to stand the peak voltage
V_max = V_rms*sqrt(2); in a DC line, V_max = V_rms, so for the same
line you can jack up the voltage and carry more power, or you can build
a less expensive line to carry the same power.

Disclaimer: I'm no engineer, so if I overlooked some important
practical limitations in the above, please limit your flames to the
mild variety.

-- 
Francois Yergeau (yergeau@phy.ulaval.ca) |  De gustibus et coloribus
Centre d'Optique, Photonique et Laser    |  non disputandum
Departement de Physique                  |          -proverbe scolastique
Universite Laval, Ste-Foy, QC, Canada    |