Blue Crab Boulevard

Something - and nobody knows what at this point - appears to have caused a major disturbance on the power grid in Florida. As a result (or maybe the initiating event, nobody knows yet), two nuclear power plants tripped off line while two coal units that share the same sire also tripped. Power was out over wide areas of Florida, although it is starting to be restored at this hour.

MIAMI - A South Florida nuclear plant automatically shut down Tuesday, causing sporadic power outages throughout the state that affected up to 3 million people from Daytona Beach through the Florida Keys.

Authorities did not specify the cause of the midday shutdown of both reactors at Florida Power & Light's Turkey Point plant south of Miami but say there were no safety concerns.

Power was already restored in some places by early afternoon and was estimated to be fully restored by 6 p.m., Florida Power & Light said.

The Nuclear Regulatory Commission said the two reactors automatically shut down. Two other power plants farther north, the Crystal River reactor and St. Lucie twin reactors, continued to operate, although officials at those two facilities noticed the grid disturbance.

Miami-Dade County Mayor Carlos Alvarez said the outages were technical, not criminal.

"It's a matter of just a cascading effect," he said.

The outages have no connection to terrorism, Homeland Security Department spokeswoman Laura Keehner said.

"We don't know whether the grid disturbance caused the units to shut down or that their shut down caused the grid disturbance," said Kenneth Clark, a spokesman at the NRC regional office in Atlanta. He said the two reactors were automatically shut down.

"There are no safety concerns. The reactors shut down as designed," Clark said. Both reactors continued to have offsite electric power, and two coal-burning power plants at Turkey Point also shut down, he said.

It does not take a lot for the grid to collapse - having four big units all go off at once or close together - is a very big blow to the system. I'm surprised they have been able to get most of the area back up this quickly. There will be a lot of people trying to analyze the sequence of events to figure out what happened.

UPDATE: Florida Power and Light says that and equipment malfunction and fire at a substation caused two transmission lines to trip off. It sounds as if several other things that should have operated to mitigate the sudden loss of the lines also may have failed. As a result, the Turkey Point units suddenly had nowhere to send their electricity. The plant's safety systems tripped the units off line to avoid damage. This appears to be tied to some protective relaying problems (that's an educated guess). The system appears to have not responded as the operators expected.

Protective relaying is more of an art than a science (the bible of the industry is still, I believe, The Art and Science of Protective Relaying .) I worked a brief stint in a protective relaying group years back in a familiarization tour. It's very interesting stuff. 

UPDATE: CNN confirms:

Utility workers were still trying to piece together what happened, but he said the "initiating event" was the failure of the disconnect switch.

"These systems are all designed so that you can handle two contingencies," Olivera said. "If you had a switch that failed, protective devices would have isolated the problem. That did not occur today. That's the part we don't have an answer for."

Relaying failure. One fact I may have taken the wrong approach on in the last update, though:

"In a fraction of a second, the demand was far greater than the power plants that were on line generating electricity could handle," he said. "When you have that kind of imbalance, we have a system that kicks in and it starts turning people's lights off, essentially balancing the demand with what's available."

I assumed the plants had nowhere to send power - it may have been that they were asked to meet a demand they could not. Regardless, it is a load mismatch that caused the unit trips and the blackouts. That traces back to a system not responding as expected. That, in turn, points to a relaying problem. And a lot of people are going to lose some stomach lining trying to nail down what happened today.