GRAND FORKS (AP) - Xcel Energy Co. is defending the reliability of its system, in the wake of a widespread power outage here that likely will prompt a state investigation.

From 2003 to 2006, Xcel Energy customers in North Dakota experienced on average less than one power outage a year, according to the company.

"Anecdotally, we believe we are in the upper tier as far as what we're doing," said Mark Nisbet, Xcel's principle manager in North Dakota.

About 6,700 Grand Forks customers recently lost power in subzero cold because of a transformer failure at one of Xcel's two electric substations in the city. Most of the power was restored within six hours.

North Dakota Public Service Commission President Susan Wefald plans to recommend an investigation into the reliability of Xcel's electric system.

Brad Sylliaasen, distribution design manager at Xcel, said the Minneapolis-based utility regularly monitors its distribution system and seeks out new technologies. He added, however, that substation technology has not changed much over the years.

"The basic concept of our distribution transformer, substation transformer, is what it is," he said.