UPDATE 1-Calpine/Mitsui Ontario Greenfield natgas plant back

(Updates with company comment)

NEW YORK Oct 6 (Reuters) - All of the units at Calpine Corp (CPN.N)/Mitsui & Co Ltd's (8031.T) Greenfield natural gas-fired power plant in Ontario returned to service by Tuesday afternoon, a spokesman for the plant said.

Units 1, 2 and 4 were out of service for at least a couple hours each Tuesday morning, according to a report by the Independent Electricity System Operator.

The 1,153 MW Greenfield, owned 50-50 by Calpine and Mitsui, is a combined-cycle facility located in Courtright near Sarnia, Ontario about 60 miles northeast of Detroit, Michigan. The plant has three combustion turbines (the 212 MW Units 1-3) and one steam turbine (517 MW Unit 4).

All of the other units remained available for service.

Combined-cycle technology uses natural gas to turn a combustion turbine to generate electricity and then uses the hot gas leaving the combustion turbine to heat water to produce steam to turn a steam turbine and generate more power.

One MW powers about 1,000 homes in Ontario.

Calpine started building Greenfield in 2005. It operates under a 20-year power supply agreement with the Ontario Power Authority.

In April 2007, the companies hired Emera Inc's (EMA.TO) Emera Energy Inc to buy the natural gas the plant burns and sell the power it produces.

Calpine, of Houston and San Jose, California, owns and operates about 24,000 MW of generating capacity in operation and under construction and markets energy commodities in North America.

Mitsui is a Japanese trading house that owns and manages about 3,800 MW of generation in 16 countries, among other things. (Reporting by Scott DiSavino)