When the lights went out across large swaths of Cambridge just as Thursday’s workday ended, it was the latest in a string of outages caused by NStar equipment failures this year, renewing questions about the Boston utility’s ability to keep the power on.
The outage only lasted two hours after a piece of equipment called a relay erroneously cut power to a transmission line, but it knocked out electricity at Harvard University and MIT, left traffic and street lights dark during rush hour, and played havoc with Red Line trains packed with commuters. Roughly 19,000 customers were affected.
The blackout follows cable failures that led to two outages in Allston in August, as well as a fire sparked by a transformer failure at a Scotia Street substation that caused widespread outages in the Back Bay and neighboring areas in March. A problem at the same substation in May caused a smaller outage. The outages prompted NStar to replace equipment and make improvements in both neighborhoods.
David Cash, a commissioner with the Massachusetts Department of Public Utilities, said the outages have regulators keeping close tabs on NStar, which recently merged with Northeast Utilities of Hartford. NStar’s service quality has been no worse than other utilities, Cash said, but his agency will scrutinize it during annual reviews of the state’s utility companies.
“Every time there are events like what we’ve seen at Scotia Street or what we saw in Cambridge, it always raises a level of concern,” Cash said. “Obviously with the last year, we want to look carefully at this.”
Ironically, Thursday’s outage happened while NStar was conducting system upgrades in Cambridge. In the past five years, NStar has invested $24 million in its system, according to a company spokeswoman.
NStar said its data show that from 2009 to 2011 its average time to restore outages, including during major storms, remained below that of other utilities in the Northeast by up to several hours. The data also show NStar had fewer power failures than other utilities in the same time frame, the Boston utility said.
The recent outages could be symptomatic of the power grid’s aging infrastructure and the increasing stress it has come under as electronic devices and other power hungry equipment proliferate, utility specialists said. Under pressure from politicians and regulators to hold down rates, companies may have delayed costly maintenance and upgrades, making the system more vulnerable to failure, said Scott Hempling, a Washington area lawyer who advises state utility regulators.
“Everybody wants great power,” he said, “and nobody wants rate increases.”
Hempling also questioned whether NStar’s nearly $20 billion merger with Northeast Utilities — completed in April — played a role in the spate of outages seen this year.
“In the process of being acquisition-minded, did they remain performance-minded?” he asked.
NStar spokeswoman Caroline Pretyman said the company has always strived to provide customers with quality service. “We’re committed to regular maintenance and upgrade work and clearly our diligence in investments have paid off,” she said, citing NStar’s data.
Ironically, Thursday’s outage happened while NStar was conducting system upgrades in Cambridge. In the past five years, NStar has invested $24 million in its system, Pretyman said.
As part of Thursday’s upgrades in Cambridge, a transmission line was taken out of service while work was underway and electricity was routed through a second line, which would have normally acted as a backup in case of a problem, Pretyman said. As a result, when the relay incorrectly sensed an abnormal power flow and shut down the line, there was no backup available and the lights went out.
Recent investments in switching technology, Pretyman added, helped NStar restore electricity within 20 minutes to more than a quarter of the customers who lost power Thursday, and the rest within two hours.
Jim Fama, vice president of energy delivery at the Edison Electric Institute, a utility trade group, said the outages experienced by NStar are really nothing out of the ordinary for a large power company. NStar has more than 1 million electricity customers in Massachusetts.
“When you’re doing an upgrade or maintenance on something, there’s always a chance you could have a problem like this,” he said. “I would imagine this sort of thing happens in the United States every day.”
Massachusetts Attorney General Martha Coakley,
“We are reaching out to NStar to learn more about the causes of the outage,” said the spokeswoman, Jillian Fennimore. “The frequency and extent of these recent outages are concerning, and that is why we believe the service quality standards these utilities are held to must be updated in order to prevent outages in the future.”
Correction: Because of incomplete information provided to the Globe by NStar, this story did not report the full amount of investments the utility has made in its systems over the last five years. The total is $1.5 billion.