UPDATE 1-Texas grid agency Ike knocks out 114 lines

(Adds damage information outside ERCOT)

HOUSTON, Sept 14 (Reuters) - The Texas electric grid agency said 114 high-voltage transmission lines were knocked out of service in the Houston-Galveston area by Hurricane Ike, which slammed into the Texas coast early on Saturday.

The Electric Reliability Council of Texas (ERCOT), which oversees electric power flow in most of the state, said six 345-kilovolt lines, remained out of service on Sunday.

Also disabled were 89, 138-kv lines and 21, 69-kv lines.

Preliminary reports indicated most of the problems were due to debris flying into the lines, rather than structural damage to transmission towers that could take longer to repair.

ERCOT said 13 power plants, or about 3,000 megawatts of generation, were unable to run due to a lack of transmission capacity.

Lower generating capacity was not an issue because of the loss of electric demand in the state.

ERCOT said more than 2.1 million customers in its territory were in the dark, while more than 390,000 Entergy customers in southeast Texas were also without power. ERCOT does not oversee power flow in the Entergy service territory, which is not connected to Texas primary grid.

Entergy's 2,000-megawatt Sabine Power Station near Bridge City and its Lewis Creek plant near Willis were shut due to the storm. More than 200 substations and 152 transmission lines were knocked out of service after the storm, Entergy said on its website. (Reporting by Eileen O'Grady; Editing by Andre Grenon)