NStar officials said they still don’t know how a fire in a Back Bay substation started Tuesday evening, but the blaze set off a chain of events that resulted in the shutdown of two substations and the loss of power to more than 20,000 customers.
The failure also shows how downtown Boston is dependent on six power substations for electricity, and the thin margin of error in the system when one fails.
The problem began when a power line to a transformer at the Scotia Street substation failed; 15 minutes later NStar received a second alert that the transformer had caught fire and was shut down.
At that point, NStar’s system initiated the first of many intricate routing changes to direct electricity to other nodes so that customers did not lose power. The first move was simple: The second transformer in the Scotia Street building picked up the load, separated from its burning neighbor by a firewall.
But the intensity of the blaze burned away at an electrical line connecting the two transformers, forcing the second unit to shut down less than a hour later.
Back Bay businesses and residents began losing power.
Next, NStar’s system automatically began rerouting power demands through another station, on South Charles Street, a smaller substation that serves the Back Bay, Park Square, and the Theatre District.
But within 15 minutes NStar officials shut down the South Charles Street unit out of fear that the fire had spread to the power line that connects the two substations and would further damage the system.
“The strategy was to contain the damage,’’ said Caroline Pretyman, an NStar spokeswoman.
By 7:45 p.m. the blackout extended from the Back Bay to Park Square, a situation that lasted until 3 a.m.
Transformers are designed to reduce high voltage power transmitted from the regional power grid to levels that can be used in homes and businesses. The ones in the Scotia Street station are about the size of a dump truck, and distribute lower voltage electricity to a network of even smaller transformers around the city, many situated on poles and in building basements.
The connected grids are not organized along neighborhood lines, which accounts for the uneven spread of outages across sections of the city that are not contiguous.
“Generally, transformers are very reliable,’’ said Luke van der Zel, technical executive at the Electric Power Research Institute, a nonprofit scientific and technical research organization based in Palo Alto, Calif. “Fires in transformers are very rare.’’
Mark Earley, chief electrical engineer at the National Fire Protection Association located in Quincy, said that transformers such as the ones in Boston “generally last a good long time. Fifty years is not unusual.’’
Earley said the most common reasons for failure include short circuits and a breakdown of the cooling system. Earley added that transformers can become overloaded during times of peak air-conditioning loads, but that would not have been the case on Tuesday night.
The Scotia Street substation was placed into service in 1973. Located near the Back Bay Hilton, the Scotia Street building houses two large transformers, with 24 feeder lines distributing electricity for 13,500 customers. The smaller South Charles Street facility was opened in 1961.
Werner Schweiger, senior vice president of operations at NStar, said that NStar “does not know what precipitated the event’’ at the Scotia Street facility. He said that the company was “entirely focused’’ on the restoration of power, and does not expect to be able to pinpoint the cause until after an investigation.
Shortly after midnight Wednesday, Boston firefighters allowed NStar technicians into the Scotia Street substation. They disconnected the line between Scotia and South Charles streets, and brought the latter substation back online by 3:01 a.m.
At that point, NStar began working to assemble a network of generators and “line jumpers’’ to connect residences and businesses to power. NStar said Wednesday that it expected to restore power to all of its customers in the affected areas by early Thursday morning.
D.C. Denison can be reached at email@example.com.