UPDATE 1-Calpine/Mitsui Ontario Greenfield natgas units back

(Updates with company comment)

NEW YORK, Nov 12 (Reuters) - Calpine Corp (CPN.N)/Mitsui & Co Ltd's (8031.T) 517-megawatt Unit 4 and 212 MW Unit 3 at the Greenfield natural gas-fired power plant in Ontario returned to service Thursday morning, the Independent Electricity System Operator said in a report.

Both units shut briefly by earlier Thursday.

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 (95 km) northeast of Detroit, Michigan. The plant has three 212 MW combustion turbine Units 1-3 and and one 517 MW steam turbine 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.