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Outage cause in Back Bay a mystery to NStar

Offline transformer contributed to woes; upgrades expected to cut failures soon

NStar Electric chief Craig Hallstrom talked about the problem with transformer 110A at the Scotia Street Substation.

Yoon S. Byun/Globe Staff

NStar Electric chief Craig Hallstrom talked about the problem with transformer 110A at the Scotia Street Substation.

NStar officials Monday remained unsure what caused a weekend cable malfunction in a Back Bay substation, leaving thousands without power for several hours. But they said upgrades expected to be complete in about a week at the Scotia Street facility will help minimize future outages.

NStar has spent more than a year — and about $50 million — replacing a transformer and installing other equipment at the 40-year-old substation, which was at the center of a two-day blackout in March 2012. That outage left more than 20,000 customers in one of city’s busiest communities in the dark after a broken connection between a power line and a transformer sparked a fire.

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The power failure on Sunday occurred while one of the Scotia Street substation’s two transformers was out of service for upgrades. That left the facility without backup equipment when a cable connecting the working transformer to a circuit breaker malfunctioned at about 3:15 a.m. About 12,000 people lost power in the substation’s service area, which extends from the Charles River to Columbus Avenue, between Kenmore Square and Clarendon Street.

Several things could have caused Sunday’s cable malfunction, including a larger power load running through a single transformer, said Rich Lordan, a senior technical executive at the nonprofit Electric Research Power Institute.

“Did that contribute? We don’t know. Or was it bad luck? We don’t know,” Lordan said, “and there’s really no way for a utility to do anything about that.”

‘It’s a big mechanical system and it’s under a lot of stress. And at some point, something breaks.’

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If both transformers had been in operation, the electrical system would have rerouted power from one to the other and maintained normal service, said NStar Electric president Craig Hallstrom.

“It would have flopped over seamlessly,” Hallstrom said Monday as he toured the substation where employees worked to finish final upgrades.

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Among the changes: a new transformer to replace one damaged in the 2012 fire and new “gas insulated switch” systems that give NStar more flexibility to reroute power in different directions or isolate problem areas.

While the improvements will reduce the likelihood of problems at the Scotia Street station, Hallstrom said he was unable to guarantee equipment wouldn’t malfunction in the future.

“It’s a big mechanical system and it’s under a lot of [electrical] stress,” Hallstrom said. “And at some point, something breaks.”

The 2012 outage was the first to stem from the Scotia Street substation, which was built in 1973, according to the utility.

Hallstrom said NStar monitors the substation in a variety of ways, including regular inspections with infrared scanners to detect hotspots that could indicate a weak point in the system.

On Sunday, employees were dispatched to Scotia Street immediately to begin a visual inspection of the property’s transformers and roughly two dozen circuit breakers.

Hallstrom said NStar crews are always “very cautious about rushing into a quick repair,” even on a piece of equipment like the substation’s second transformer, which had been taken offline.

“This system ultimately ties into the system of the running transformer, so they have to be painstakingly careful that they’re putting the right wire in the right spot, otherwise there’s going to be a problem,” Hallstrom said.

Erin Ailworth can be reached at eailworth@globe.com. Follow her on Twitter @ailworth.

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