UPDATE 3-Exelon NJ Oyster Creek reactor back soon - traders

(Updates with company, NRC and trader comment)

NEW YORK, July 13 (Reuters) - Exelon Corp (EXC.N) started to return the 619-megawatt Oyster Creek nuclear power station in New Jersey to service Monday morning, electricity traders said.

A spokeswoman for the plant said the unit was still off line late Monday morning. She did not comment on any startup activities.

Nuclear reactors usually attach to the grid after reaching 15 percent to 20 percent power.

The unit automatically shut on July 12 from full power due to the loss of offsite power likely caused by a severe electrical storm.

In a report to the U.S. Nuclear Regulatory Commission operators said both startup transformers were de-energized due to the loss of power flow on the distribution lines. The company reenergized the transformers within three hours.

A spokesman at the NRC said the agency was evaluating whether the event qualified as an unexpected shutdown that could lead to increased NRC oversight.

The NRC increases oversight at reactors that have more than three unplanned shutdowns in 7,000 hours of operation. Since November, Oyster Creek shut about three times due to transformer problems.

The electricity traders said an outage related to a severe storm would likely not qualify as an unplanned shutdown for increased NRC scrutiny purposes.

Oyster Creek, which entered service in 1969, making it the oldest operating reactor in the nation, is located in Forked River, in Ocean County, about 60 miles east of Philadelphia. The NRC renewed the plant's original 40-year operating license in April 2009 for another 20 years until 2029.

One MW powers about 800 homes in New Jersey.

Exelon, of Chicago, owns and operates more than 38,000 MW of generating capacity, markets energy commodities, and transmits and distributes electricity to 5.4 million and distributes natural gas to 480,000 customers in Illinois and Pennsylvania. (Reporting by Scott DiSavino; Editing by Walter Bagley)