UPDATE 1-California PG&E Diablo Canyon 2 unit shut for test

* No estimate of when unit will return to service

* Power prices not greatly changed

* Wholesale U.S. West power prices half of year-ago levels

(Recasts, including no estimate of return, background)

LOS ANGELES, Aug 13 (Reuters) - PG&E Corp's 1,150-megawatt Diablo Canyon nuclear power plant shut for unplanned work on Thursday, the company's utility Pacific Gas & Electric Co said.

There is no estimate of when the unit will return to service, said PG&E spokeswoman Emily Christensen. Tests are being conducted to determine if replacement parts will be needed for one of the unit's three transformer banks.

Having a big nuclear power plant go offline unexpectedly in mid-August normally would cause huge price spikes in the wholesale power market. But demand is lower than normal this summer because of the recession, and moderate temperatures that have dampened air-conditioning use, traders said on Thursday.

Prices for California and the Pacific Northwest are less than half of what they were a year ago. Prices for natural gas, which fuels most power plants in California, are depressed.

The unplanned shutdown at Diablo Canyon was completed by 1 p.m. PDT (2000 GMT). The utility said tests will be conducted on "main components in the unit's transformer system."

"Unit 2 will remain offline until the transformer bushings are fully evaluated and, if needed, repairs are completed. Unit 1 continues to operate at full power," PG&E Co said in a statement.

Unit 1 is also of 1,150 megawatts generation capacity.

"During a regularly scheduled equipment inspection, plant personnel identified the need to perform a more in-depth analysis of transformer bushings on transformer bank A of Unit 2," said PG&E's statement. "Each of Diablo Canyon's units has three transformer banks A, B and C, which are used to deliver electricity from the plant to the transmission grid that serves PG&E's customers."

Christensen said that if the brushings needed to be replaced on transformer bank A, replacement parts are on site at Diablo Canyon.

On Thursday, next-day peak hour power prices for northern and central California were about $36 per megawatt-hour, compared to $83 per MWh a year ago.

Diablo Canyon is in Avila Beach, in San Luis Obispo County, about 195 miles northwest of Los Angeles.

A megawatt in PG&E's service territory on average can power 700 households. Unit 2 therefore can serve about 800,000 homes.

PG&E Corp (PCG.N) shut the 1,118-megawatt Unit 2 at the Diablo Canyon nuclear power station in California from full power on June 30 after one of the fuses in the transformer support system failed, a spokeswoman for the company said on Wednesday.

She could not say when the unit would likely return since workers were still putting together the plan to fix the problem.

In a report to the U.S. Nuclear Regulatory Commission, PG&E said the unit shut due to the loss of forced cooling to the main transformer bank.

On June 30, the same unit was taken offline when one of the fuses of the transformer support system failed, which caused the loss of forced cooling to the main transformer bank.

Unit 1 began operation in 1985 and Unit 2 in 1986.

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 (Reporting by Bernie Woodall; Editing by David Gregorio)