Mar. 9--The second and last new unit at the Santan Generating Station in Gilbert has been placed in commercial operation by Salt River Project, in time to help meet the peak summer demand for electricity.

The natural-gas-fired unit, the sixth at the Warner and Val Vista roads complex, began commercial service on March 1 after five months of testing, said SRP spokesman Scott Harelson.

It will produce 275 megawatts of additional electricity, enough to provide energy to about 58,000 homes on average, he said.

The controversial $525 million expansion was approved by the Arizona Corporation Commission in April 2001 following a lengthy public review, and the first new unit was placed in service early last year with a capacity of 550 megawatts.

SRP had been operating four units at the site, generating a total of 360 megawatts since the mid-1970s.

With the addition of the second new unit, the plant has a total capacity of 1,185 megawatts.

In comparison, a single unit at the Palo Verde nuclear plant west of Phoenix can produce 1,270 megawatts.

The latest unit at the Santan plant uses combined-cycle technology including a combustion turbine and a steam turbine to extract the maximum amount of electricity from the fuel.

When all units are running at full capacity, the plant consumes about 149,000 cubic feet of natural gas a day.

To win regulatory approval for the project, SRP had to install emission control equipment on the four older units that more than offset the additional emissions coming from the new cleaner-burning turbines.

Also SRP is spending $20 million to improve the appearance of the 120-acre site including planting 1,200 trees and building a 25-foot- high berm with decorative walls and ground-cover plants.