Ameren spokeswoman Susan Gallagher said the fire caused an automatic shutdown of the substation, and crews began restoring power around 7 a.m.
"The heat and smoke caused the devices to switch off, as they were designed to do," Gallagher said. "After inspecting and resetting those switches, we're re-energizing the station."
KWMU's transmitter in Shrewsbury, which does not have a backup generator, was off the air for more than two hours.
The outage affected a big part of the metro area, from Richmond Heights and the Central West End in the north to Crestwood and Affton and other points south.
A number of schools in that area are closed for the day or delayed their start times.
© Copyright 2012, St. Louis Public Radio

