Energy Devices – Power systems (Lecture 3)
Eric R. Switzer
(http://kicp.uchicago.edu/∼switzer/)
Oct. 17, 2009
Lecture outline:
•A short history; AC vs. DC.
•Ohm’s law, power in electrical systems, Joule loss.
•Transmission subject to losses; transformers.
•Primary energy sources and generator schematics.
•Some statistics about generation and consumption.
At a fundamental level, AC is convenient because induction (used in generators, trans-
formers, and motors) is due to time-varying currents. In terms of equations, V=−dΦ/dt.
Why do we need high voltages?:
•Consider a 500 kV wire that carries 1kA of current and has 25 Ω resistance. The
total power transmitted at the generating end is P=I V = 500 MW. The voltage
drop across the transmission line is 25 kV, so at the receiving end one has 475 kV (or
475 MW). The total power lost is 25 MW (a 5% loss).
•Take the same generator power, but 125 kV as the transmission voltage. In this case,
the current must be 4 times larger to produce the same power. The Joule loss is
then 400 MW, or (400 MW)/(500 MW) = 80%. Only 20% of the power reaches the
consumer!
Image sources:
Fig. 1 (left) http://en.wikipedia.org/wiki/File:Carl_Saltzmann_Erste_elektrische_Stra%C3%9Fenbeleuchtung.jpg.
Fig. 1 (right) http://en.wikipedia.org/wiki/File:Tour_Eiffel_1878.jpg.
Fig. 3 (top) http://en.wikipedia.org/wiki/File:Court_of_Honor_and_Grand_Basin.jpg.
Fig. 3 (bottom) http://en.wikipedia.org/wiki/File:WorldsFairTeslaPresentation.png.
Fig. 4 http://en.wikipedia.org/wiki/File:Transformer3d_col3.svg.
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