(Reuters) - Duke Energy Corp's (DUK.N: Quote, Profile, Research, Stock Buzz) 846-megawatt Unit 2 at the Oconee nuclear power station in South Carolina returned to service and ramped up to 87 percent power by early Monday, the U.S. Nuclear Regulatory Commission said in a report.

The company took the unit off line by Sept. 24 but left the reactor operating at about 18 percent power to fix an oil leak in the main transformer.

Keeping the reactor hot allows the unit to return to service quickly once the transformer repairs are completed.

The company said the work last week had nothing to do with the upcoming month-long refueling outage expected to start in late October.

The unit last shut for refueling from April 28-May 31, 2007. It is on an 18-month refueling cycle.

The 2,538 MW Oconee station, which entered service in 1973-74, is located in Seneca in Oconee County about 145 miles northwest of Columbia, South Carolina. There are three 846 MW Units 1-3 at the station.

Units 1 and 3 continued to operate at full power.

One MW powers about 700 homes in the Carolinas.

In 2000, the NRC renewed the plant's original 40-year operating licenses for another 20 years until 2033-2034.

Duke, of Charlotte, North Carolina, owns and operates about 39,000 MW of generating capacity in North America and Latin America, markets energy commodities, and transmits and distributes electricity to about 4 million U.S. customers in the Carolinas and the Midwest. (Reporting by Scott DiSavino; Editing by John Picinich)