PG&E Helms 2 hydro power unit out for planned work

LOS ANGELES, June 16 (Reuters) - The 404-megawatt Unit 2 at the PG&E Corp Helms Pumped Storage hydropower station went offline on Tuesday afternoon, the California Independent System Operator said in a report.

Now, two of the three 404-MW units at the station are offline.

Unit 3 remains offline and has been since last September. It is expected to come back online this coming weekend.

The 1,212 MW Helms station, which entered service in 1984, is located near Fresno in Fresno County.

One MW powers about 700 homes in the PG&E coverage area in California.

Pumped storage plants store and produce electricity to supply peak power demands by moving water between reservoirs at different elevations.

At times of low demand and low power cost -- at night -- the plant uses electricity from the grid to pump water into the higher reservoir. When demand and prices are higher, the station releases the water back into the lower reservoir through a turbine, generating energy.

PG&E's regulated Pacific Gas and Electric Co subsidiary owns and operates the station.

PG&E, of San Francisco, owns and operates more than 6,200 MW of generating capacity, markets energy commodities and transmits and distributes electricity to almost 5.3 million customers and natural gas to 4.2 million customers in northern California. (Reporting by Bernie Woodall)