A day after a transformer explosion sent black clouds sweeping through the heart of the city, residents in much of the Back Bay and the Fenway endured a prolonged blackout that left thousands without power, cost local businesses millions of dollars, and forced curtailment of subways and buses, conventions, and college classes.
Hundreds of utility workers from NStar worked through the day to restore electricity, although more than half of the 21,000 businesses and homes that had lost power remained in the dark early Wednesday evening. Utility and government officials pledged that power would be fully restored by 4 a.m. Thursday.
“This clearly is a catastrophic event,’’ said Werner Schweiger, senior vice president of operations at NStar.
The company planned to use as many as 100 generators to provide power until workers make repairs to the local grid, Schweiger said. The fire destroyed one of two transformers in a concrete substation beside the Back Bay Hilton on Scotia Street, costing NStar at least $2.5 million.
Police reported no injuries or arrests as a result of the blackout, and state environmental officials said the three-alarm fire did not release harmful concentrations of toxic pollutants, as was initially feared.
Mayor Thomas M. Menino, speaking at a City Hall news conference, applauded residents for coping gracefully with the power outage. Boston Police Commissioner Edward F. Davis noted that in some other communities where power failed, rioting followed.
“It’s a real testament to the residents that it didn’t happen,’’ Davis said. “We are very happy with the way things have gone so far. . . . Even the driving has been respectful.’’
Neither NStar nor city officials could say what caused the fire.
Boston Fire Department spokesman Steve MacDonald said NStar workers got inside the blown transformer around midnight Wednesday and worked through the night.
“We believe the transformer shorted itself out from the fire and became engulfed in flames,’’ he said, adding it will be up to NStar to determine what happened. “The second transformer worked to pick up the load from the first transformer and that eventually tripped, affecting power in Chinatown and other substations.’’
The Boston Inspectional Services Department dispatched food inspectors across the city, from Kenmore Square to Chinatown, ensuring that spoiled food was not served. Hundreds of restaurants and cafes closed, as required by city codes during a blackout, and brought in dry ice for their food or found off-site storage areas.
“We haven’t found any serious problems,’’ said Lisa Timberlake, a spokeswoman for Inspectional Services. “All the establishments have complied with city codes.’’
Commissioner Kenneth Kimmell of the state Department of Environmental Protection said about 1,000 gallons of mineral oil had burned as a result of the fire, as well as metals and concrete. But a rapid response crew discovered that the plumes of smoke contained limited amounts of potentially harmful chemicals such as benzene, he said.
“There was no concentration of pollutants that would cause a public health concern,’’ Kimmell said.
State Police blocked traffic from entering an eastbound ramp from the Massachusetts Turnpike to Copley Square to allow NStar room to install a replacement generator. Lane restrictions were also imposed on parts of Storrow Drive to ease the flow of traffic out of the Back Bay.
Streets closest to the fire - Scotia, Dalton, and St. Cecilia - remained closed, as did Belvidere Street between Huntington and Massachusetts avenues.
The MBTA used alternate power sources to reopen the Symphony and Prudential stations on the Green Line. Service on the popular 39 bus line was curtailed on Boylston Street, from Massachusetts Avenue to Arlington Street.
“This has been an enormous blow,’’ said Meg Mainzer-Cohen, president of the Back Bay Association, which counts more than 400 businesses as members.
She said the blackout easily cost businesses millions. “Economic activity has not been possible for two-thirds of Back Bay businesses, and when that happens, there’s a serious impact.’’
Officials at the Massachusetts Convention Center Authority estimated the fire cost the city at least $2 million from canceled events at the John B. Hynes Veterans Memorial Convention Center. Authorities canceled the first day of a three-day convention of about 5,000 members of the American Alliance for Health, Physical Education, Recreation, and Dance, most of whom were staying at the Hilton and Sheraton hotels, both of which lost power.
“While the folks are disappointed, they understand it’s beyond anyone’s control,’’ said James Rooney, the convention center authority’s executive director. “I don’t think there’s a lasting blow to the Hynes or the city of Boston from this type of event. If anything, we hope it will be an interesting story folks can tell when they go home.’’
As many businesses operated on auxiliary power, Berklee College of Music canceled all classes and activities, and hundreds of students from five Northeastern University dorms remained displaced.
“In order to ensure student safety, we are requiring that all residents leave these buildings by sundown,’’ Northeastern officials told students in an e-mail, noting that cots had been set up in a school gym. “Affected students are encouraged to find a friend or fellow student in another building who can accommodate a guest.’’
At Trinity Church in Copley Square, officials had to cut short a major program for Lent, and services and other church activities had to be canceled.
“It hit us pretty hard programmatically,’’ said Samuel Lloyd, the priest in charge. “But everyone’s taking it in good spirits.’’
City Councilor Michael P. Ross, who represents the Back Bay, spent the day fielding calls at City Hall from residents and local institutions.
He said he hopes the city can build greater redundancy to avoid similar blackouts.
“This is a tremendous hit for the heart of city to be out of power for this long,’’ he said. “The economic impact of this one event is staggering.’’
Among those affected was Chris Coombs, owner of the restaurant Deuxave on Commonwealth Avenue, which had to evacuate customers as smoke swirled over the neighborhood Tuesday night.
He said his restaurant earned just $135 Tuesday evening, compared with $8,000 to $10,000 on an average night. He had to discard all of the restaurant’s fish, and the business remained closed Wednesday.
Despite the losses, he was optimistic.
“We’ll be just fine,’’ he said.Travis Andersen and Peter Schworm of the Globe staff and Globe correspondent Alli Knothe contributed to this report. David Abel can be reached at email@example.com. Follow him on Twitter @davable.