The June 19 power outage, originally thought to have been caused by a just tree branch, was caused by an entire tree falling into a power line.

Tim McCollough, electric services operations superintendent for the Ames Municipal Power Plant, weighed in on what caused the entire city of Ames and Iowa State to be without power Tuesday afternoon shortly before 3 p.m.

“It was not a tree branch but an entire tree that fell into a high voltage [161,000-volt] high line yesterday,” McCollough said. “It fell into the line and opened the breaker that caused the fault. That caused there to not be enough generation, and we couldn’t get enough power coming from the Ames Municipal Power Plant. That caused an entire blackout throughout the city of Ames.”

Within an hour of the power going out, the tree was discovered to have been the source and within minutes, power was restored to parts of Ames.

“Once we were sure the transmission system was restored and in good condition, we restored power first to the critical operations, such as the hospital, and then to other customers,” McCollough said. “We can only restore power to one section at a time. First we restored power to the sections that were closest to the good source and then went out from there.”

Normally, the university is supplied with electricity from its own power plant, but Tuesday, it had to be supplied power from the Ames Power Plant.

David Miller, associate vice president of facilities planning and management at Iowa State, said via email Tuesday, “Up until [the power outage] MidAmerican has limited the power flow into Ames at 62 [MW] due to capacity limitations of the transmission system in the area. By agreement, Iowa State has access to a portion of that power, currently 10 MW.”

Since Iowa State has access to 10 MW of the imported power, the balance of 52 MW is allocated to the city of Ames’ usage.

Miller said in the email Ames was able to restore power before Iowa State was able, due to their possession of two gas turbines that make up the difference between the power flow the city has and how much the city actually needs.

“Both Ames’ boiler and Iowa State’s boilers [were] off-line due to the power outage,” Miller said. “The university does not have gas turbines and therefore has no way to make up the difference between the transmission system and our actual load, until our boilers and turbines can be re-started.”

McCollough said that, to his knowledge, nothing like this has happened before in Ames within at least a 30-year time span and the city of Ames is looking to get a new transmission line approved by the Iowa Utilities Board so this does not happen again in the future.