Mine operator BHP Billiton decided to switch to underground coal mining at San Juan two years ago. The deeper coal is expected to burn hotter and cleaner, saving the power plant between $US400-$US500 million over the 16-year life of the mine.
While the coal is tested in the Unit 1 boiler, it will be shut down for five days but this will not affect electricity supplies to any customers.
Production from the underground mine was originally expected to begin in June. Delays in the production schedule have pushed the start date to the end of September.
The delays meant the power plants had to buy coal from both San Juan and the nearby La Plata mine for three months.
Estimates indicate that shifting to underground mining will cost about $US8.5 million more than first expected.
San Juan is the seventh largest coal plant in the Western United States. The plant consumes nearly 7 million tons of coal a year.