Cause of Back Bay power outage traced to faulty power supply unit; Mayor Menino calls for explanation of blackouts

By Peter Schworm, Globe Staff

The electric company NStar said today that Tuesday’s outage in the Back Bay was caused when a power supply failed during ongoing repairs to an electrical substation that caught fire in March.

The faulty power supply caused a protective system at the facility to activate, shutting down a transformer and a 115,000-volt line, the power company said. About 12,500 customers lost power for less than an hour, a frustrating reminder of the extended blackout that followed the substation fire in March.

Since last month, crews have been installing a second transformer at the damaged substation, NStar spokesman Michael Durand said.

“It’s a very complex process,” he said. Repairs to the substation involve the installation of several integrated components, he said.

Durand said it was not clear why the power supply unit failed, triggering the shutdown.

“We’re going to analyze that piece of equipment,” he said.

The defective equipment was quickly taken out of service and has since been replaced.

Crews are also working to install additional backup systems to guard against outages, he said. The work should not lead to additional problems, he said.

“We don’t anticipate something like this will happen again,” Durand said.

Boston Mayor Thomas Menino, meanwhile, voiced his frustration with NStar in a letter to the state’s public utilities department. Tuesday’s outage, he wrote, “is just the latest reminder that NStar still has not provided a full explanation about what caused the fire and major outage in March.”

“Boston deserves a thorough explanation of the Back Bay power outage - now outages - urgently,” Menino wrote. “NStar must take whatever steps necessary to prevent outages like the one that occurred March 13 and their lingering aftereffects. I ask that you use the full force of your authority to make sure they do.”

Menino said the company’s report on the incident to state regulators did not detail plans for preventing similar failures.

Durand said the company’s incident report provided the information that state regulators had requested, and said the root cause of the March fire may be elusive.

“Because the fire was so intense and the damage was so catastrophic, we may never know the root cause of the failure,” he said.

In March, the company said the blackout was caused when a break in the connection between a power line and a transformer caused a cooling agent inside the line to spew onto an electrified area, which ignited.