The Nuclear Regulatory Commission said Friday it will increase its oversight of the Catawba Nuclear Station on Lake Wylie after the plant lost off-site power last spring.

The NRC found two violations involving an incorrect modification to a protective relay on a generator for one of the plant’s two reactor units, leading to the April 4 power loss. Plant officials declared an “unusual event,” the lowest of four emergency levels.

Backup generators kicked on and the unit shut down safely. The second unit was already shut down for refueling.

Loss of off-site power is an important incident because it removes the primary energy source needed to operate the plant and circulate vital cooling water.

The NRC categorized one violation as of low to moderate safety significance, meaning the affected unit will get an additional 40-hour inspection, said spokesman Roger Hannah. The second violation was categorized as of very low safety concern.

NRC staff held a regulatory conference with Duke Energy, which runs the plant, to discuss inspection findings on Sept. 11. Investigators traced the relay problem to modifications made in 2011.

Duke presented plant-specific information at the conference that reduced the NRC’s initial conclusions about the event’s safety significance. The differences between Duke and the agency were whether loss of off-site power to one unit could result in power loss to the second unit and affect the likelihood of restoring power, the NRC concluded.

“We are pleased that the NRC’s process provided us an opportunity to present new information,” said spokeswoman Mary Kathryn Green. “We feel like the outcome better represents the significance of the event.”

Duke has changed procedures to ensure there’s no repeat of the error, she said.

Henderson 704-358-5051