MONTICELLO — 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 said the plant shut down safely Friday afternoon. The company said the shutdown was not expected to last long. Xcel Energy spokeswoman Patti Nystuen said the process of getting the plant back online had not started as of 7:15 p.m. Friday.

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

The 600-megawatt plant is located about 40 miles northwest of Minneapolis.