The May 25 power outage that affected a portion of the underground network in Downtown Seattle was caused by the failure to block backup circuit breakers during testing of newly installed equipment, Seattle City Light announced today.
The incident started about 11:30 a.m. when crews were testing new relays installed at the Massachusetts Substation to make sure they worked with older, existing relays in the substation’s complex protective system. Relays detect electrical faults, then signal circuit breakers to open in order to protect equipment and customers from damaging power surges.
Such testing is done to check for any wiring errors and correct them before returning the system to service. When testing relays, some circuit breakers are blocked so the test signals won’t cause them to open and cut power.
“During the test, our crews blocked the primary circuit breakers at the substation, but overlooked the backup breakers,” General Manager and CEO Larry Weis said. “When they performed the test, the backup breakers received the test signal and opened, cutting power being delivered to the substation by two high-voltage transmission lines. The disruption also cut the power supply from the Massachusetts Substation to the Union Substation.”
Crews were able to reset all the breakers and restore power in about an hour. The breakers worked as they were designed to and no equipment was damaged.
City Light immediately started and extensive review of the incident to determine what caused the outage.
“We apologize for the inconvenience this caused residents, businesses and visitors Downtown,” Weis said. “And we are taking steps to reduce the risk of this happening again.”
City Light is already half way through an effort to install test switches for relays at all its substations. The utility will continue that strategic investment, which will reduce the complexity involved in setting up this type of testing. Additionally, the utility is reviewing its testing procedures and training. City Light will revise them as necessary to reduce the risk of a similar incident.
Seattle City Light is the 10th largest public electric utility in the United States. It has some of the lowest cost customer rates of any urban utility, providing reliable, renewable and environmentally responsible power to about 750,000 Seattle area residents. City Light has been greenhouse gas neutral since 2005, the first electric utility in the nation to achieve that distinction.
Asymptote says
1) If the crews mistakenly did not block the backup breakers, and they opened when the test signal was sent, why did this cause any power outage at all if the primary circuit breakers remained closed? By definition it seems the backup breakers aren’t backups at all, but primary in some sense. More specifically “the disruption also cut the power supply from the Massachusetts Substation to the Union Substation”, why did the backup breakers opening cause this?
2) Why did it take 1 hour to restore power? Is this a normal time required to restore power? What was the contingency planning for this installation and testing and what was the expected time to recovery before this procedure started?
3) Why are crews installing and testing new equipment at 11:30am? Is there a more appropriate out-of-hours time for such risky installations to be performed? Performing risky installation out of hours could have reduced the number of stuck-in-elevator 911 calls.
4) What is the history of outages during similar installations of new relays?
I’m sure these details are already being discussed but the lack of deeper details in the article leaves me with a lack of confidence in the necessary future work to install new relays.
thomses says
Thank you for your questions.
Backup breakers are in place to open a circuit in case the primaries don’t operate correctly. If either set opens, power is cut off.
Restoring power in this situation involved more than closing the breakers. We also had to prepare for all the energy load that would be added to those circuits as soon as they were closed. This involves a great deal of coordination at our system control center. If that’s not done correctly, the circuit will be overloaded and re-open. Restoring power in one hour was good work by our crews.
The timing of the work was done to coordinate with construction projects underway throughout the city. Those limitations create windows that we have to take advantage of to get this type of work done.
Any maintenance that involves restoring a circuit to operation involves some risk that it will not hold. That’s why we perform testing.
Asymptote says
Thanks for your prompt reply, I appreciate it.
Lonye ("Lny") D. Hall says
Thank you. The non inept/ineptive True Health of Trust must be Addressed about this Issue.
Madge says
Hey, s*%t happens.