More than 12,000 people were affected by a power outage in the Back Bay and Fens area of the city early Sunday morning.
NSTAR said residents should expect another controlled power outage overnight.
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"As we work to restore the grid to normal operations we will be taking a planned outage overnight that will affect the customers impacted by this morning's outage. The length of the planned outage will vary depending on where the customers are located and we're taking steps to notify them directly by phone about tonight's outage," said Michael Durand, an NSTAR Spokesman.
NSTAR restored power to approximately 8,000 of the affected customers at about 9 a.m., about 4,000 customers by 10 a.m. and the remaining few around 11 a.m., Durand said. The areas affected included the Charles River to Columbus Avenue, and Copley Square to Fenway.
The problem began around 3 a.m. Sunday at the Scotia Street substation; the same station that was to blame for a blackout in the Back Bay in March of 2012. Durand said the outage was "not a catastrophic fire and failure that occurred previously."
"There are two transformers in that substation. We are in the process of finishing upgrade work on one of the transformers. However, the second transformer is in working order, with the problem instead being in a cable that connects to the transformer," he said.
Mayor Thomas Menino's crisis response team met Sunday to develop a plan to help residents and businesses.
“This morning my team met to ensure we’re deploying all available resources to the residents and businesses of the impacted area,” Menino said in a statement. “We’ll continue to monitor the situation and encourage anyone with questions to call us at our 24-hour hotline.”
Boston Police were stationed at intersections with inoperable signals in the area.
The Prudential Tunnel eastbound and westbound was closed until 11:30 a.m., according to State Police, The Clarendon St. Ramp was also closed, and traffic was backed up on Storrow Drive well into the late morning.
Boston Centers for Youth & Families and the Elderly Commission monitor those most vulnerable, according to a city spokesman. The Blackstone Community Center opened at 10:30 a.m. to provide cool shelter and power to those that need to charge electronic devices.
Representatives from the Department of Neighborhood Development are visiting businesses to offer assistance.
City officials said residents may call the Mayor's Hotline with any questions or concerns 617-635-4500. Customers without service should report outages to NSTAR at 1-800-592-2000.