MONTICELLO, Minn. (AP) - Operators of the Monticello nuclear power plant are investigating what caused a transformer to lock out, prompting the plant to automatically shut down.

Xcel Energy says the plant shut down safely Friday afternoon. The company says the shutdown was not expected to last long. Xcel Energy spokeswoman Patti Nystuen says the process of getting the plant back online had not started as of 5:25 Saturday evening.

Xcel says the transformer lockout briefly interrupted non-safety-related power from the grid to part of the site. The company says all plant safety systems functioned as designed, and there is no danger to the public or plant workers.

The 600-megawatt plant produces enough electricity to power a half million homes.

(Copyright 2011 by The Associated Press.  All Rights Reserved.)