HOUSTON (Reuters) -Motiva Enterprises on Monday restarted the second-largest crude distillation unit (CDU) and small coker at its 607,000-barrel-per-day Port Arthur, Texas, refinery, said sources familiar with plant operations.

A Motiva spokesperson declined to comment.

The 195,000-bpd VPS-4 CDU and 54,000-bpd coker were among units shut at the Motiva refinery, the nation’s largest, on Sunday morning by a partial power outage, the sources said. Power was restored later on Sunday.

A 49,000-bpd catalytic reformer 4 and the 25,200-bpd heavy gas oil hydrotreater were also restarting on Monday morning after being knocked offline in the power loss, according to the sources.

The power outage may have been due to a small animal causing a short in a transformer that failed early on Sunday, the sources said.

VPS-4 is the second largest of three CDUs that break down into feedstocks for all other units at the refinery.

Cokers convert residual crude oil from distillation units into either motor fuel feedstocks or petroleum coke, which can be used as a coal substitute.

Reformers convert refining by-products into octane-boosting components added to unfinished gasoline.

Hydrotreaters use hydrogen to remove sulfur from motor fuels in compliance with U.S. environmental rules.