DETROIT (AP) – City crews restored power to parts of downtown Detroit on Friday after days of 90-degree heat caused aging transformer lines to fail, shutting down electricity to traffic signals, municipal and court offices, a convention venue and a college campus.

The Coleman A. Young Municipal Center, Cobo convention center, Wayne State University, fire department administrative offices and traffic signals got power back Friday afternoon with restoration to other buildings expected later in the day, said Karen Dumas, a spokeswoman for Mayor Dave Bing. The municipal system serves public buildings, so the outage didn’t affect most downtown residential power users.

City Hall, Wayne County offices and county circuit courts were closed Friday. Classes were cancelled at Wayne State. Three of five transformer lines at the aging city-owned Mistersky power plant, which provides power to downtown and other parts of Detroit, began to overload and shut down Thursday afternoon.

Workers and visitors at City Hall, Cobo Center and other buildings were forced to leave. Traffic signals through the heart of downtown shut off, snarling traffic heading into the evening rush hour.

Temperatures in Detroit topped 90 degrees earlier this week, peaking at 95 on Tuesday and easily topping the upper 70s average high for this time of year, according to Dave Kook, a National Weather Service meteorologist in Oakland County’s White Lake Township.

City officials had been monitoring the high power output from the transformer lines and on Thursday asked users to limit the amount of electricity they were using, especially for air conditioning, chief operating officer Chris Brown said.

When it became clear the system was going down, a decision was made to shut off power to Cobo Center and city hall to keep the remaining tie lines from overloading, Dumas added.

“It’s an antiquated system,” she said.

Copyright 2011 The Associated Press.