

Lowering Voltage at Peak Times
Name: Martin
Status: other
Grade: other
Location: N/A
Country: N/A
Date: 9/25/2005
Question:
High voltage lines are used to reduce the losses in
transmission, and to reduce the size of the lines since lower current is
required. Why then do the utility companies lower the voltage
transmitted during peak power periods when they may not have enough
power?
Replies:
Dear Martin,
You are absolutely right in saying that high voltage lines can transmit an
equivalent amount of power (P = IV) with a lower current I when using a
higher voltage V. The power loss in the line is just I^2R = IIR (I squared
R). So if you want to deliver a certain amount of power, if the line
voltage is doubled, the current is halved and the power wasted in the lines
is reduced by a factor of 4.
On the other hand if the line is delivering the power to a system with
resistance R, the power delivered is P = V^2/R so if the voltage is reduced,
the power delivered is reduced, which is exactly what a utility company
wants to do when they are unable to generate sufficient power to meet the
demand.
This analysis is not exact, since R may change when V is changed. For
example, a lower voltage on an incandescent light bulb means the filament is
not so hot, which means the resistance is higher. This effect thus reduces
the power consumed even more than when the resistance is constant. The
argument is otherwise correct for DC (Direct Current), but is a
simplification for AC. AC (alternating current) is, of course, used in
almost all power distribution systems.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012

