In the early 1900's, Commonwealth Edison began furnishing Chicago with what at the time, was a modern network of coal fired generating stations. Prior to then, generating stations had reciprocating units, using piston technology similar to traditional steam locomotives. But the new generating stations constructed at the time instead used steam turbines, which proved to be more efficient. And Commonwealth Edison was then in a better position, to be a practical alternative over the electric railways generating their own power.
Converting From AC To DC
From the large centralized generating stations, electricity is best transmitted at high voltage alternating current to substations in neighborhoods throughout Chicago. And then transformed and converted to the 600 volts direct current used by Chicago's streetcars and elevated trains.
In addition, these and newer generating stations supplied 60 hertz AC to the substations, transmitted at 12,000 volts. This 60 hertz AC electricity is now of course standard, including for residences. And in 1964, Commonwealth Edison began phasing out the availability of the 25 hertz AC. And the remaining CTA rotary converters were replaced with more modern rectifiers.
The generating capacity of these stations was expanded over the years. Most notably with the pioneering Fisk station, whose original generator units were 5,000 kilowatts each. But those units were soon replaced with newer 12,000 kilowatt units.
1111 W. Cermak Rd.
10 units 12,000 kilowatts each/total 120,000 kilowatts
25th St. and Quarry St.
6 units 14,000 kilowatts each/total 84,000 kilowatts
Roscoe Ave. and California Ave.
10 units 20,000 kilowatts/total 200,000 kilowatts
As of 1921, Commonwealth Edison still supplied more electricity at 25 hertz than 60 hertz. But as of 1929, public transportation's share of electricity usage had declined to 25 percent, with increased typical consumer use of 60 hertz electricity.
100th St. and Commercial Ave.
3501 S. Pulaski Rd.
State Line and Lake Michigan, Hammond IN
Fisk generating station, as viewed from CTA's Orange Line.
And within Chicago, the coal fired stations were gradually phased out. In 2012, the last three remaining stations (Fisk, Crawford, State Line) were all closed.