UPDATE 2-Dominion Millstone 2 reactor shut in Connecticut

(Updates with company comment)

NEW YORK, July 6 (Reuters) - Dominion Resources Inc's (D.N) 882-megawatt Unit 2 at the Millstone nuclear power plant in Connecticut shut from full power on July 3 due to some grid instability, a spokesman for the company said Monday.

He said the grid instability, potentially caused by a lightning strike on an offsite power line or facility, caused the turbine to shut.

He could not say when the unit would likely return to service.

The event did not cause any damage to plant equipment or harm to workers or the public.

At the time of the trip, the company said in a report to the U.S. Nuclear Regulatory Commission that a 115 kilovolt line went out of service, which may have caused or contributed to the turbine trip.

The 2,037 MW Millstone station is located in Waterford in New London County about 50 miles (80 km) southeast of Hartford. There are two reactors at the station, the 882 MW Unit 2 (which entered service in 1975) and the 1,155 MW Unit 3 (1986). The NRC renewed the plant's original 40-year operating licenses in 2005 for another 20 years until 2035 and 2045.

The NRC in August 2008 approved of Dominion's request to up-rate the generating capacity of Unit 3 by about 7 percent to about 1,230 MW. Dominion has said it would implement the up-rate during the fall 2008 refueling outage.

Unit 3 continued to operate at full power.

One MW powers about 1,000 homes in Connecticut.

Dominion, which wholly owns Unit 2, operates the Millstone station for its owners.

Dominion (93.47 percent), Massachusetts Municipal Wholesale Electric (4.8 percent) and Central Vermont Public Service Corp (CV.N) (1.73 percent) own Unit 3.

Dominion, of Richmond, Virginia, owns and operates about 27,000 MW of generating capacity, markets energy commodities and transmits and distributes electricity to 2.4 million customers in Virginia and North Carolina. (Reporting by Scott DiSavino; Editing by Lisa Shumaker)