Unit 1 at Plant Farley returned to service early Friday morning after an unplanned shutdown earlier in the week.The Alabama reactor tripped from full power Tuesday night, when Southern Co. said a lightning arrestor failed on a transformer. According to the company, crews replaced all three of its lightning arrestors before returning the unit to service.The reactor had shut down automatically after losing the startup transformer, according to a report filed with the Nuclear Regulatory Commission. The event de-energized several buses, causing an emergency diesel generator to start. Auxiliary feedwater pumps started automatically, according to the NRC, and steam dumps to the main condenser assisted decay heat removal. Safety systems functioned as designed.Southern Co. announced the plant returned to service in a release Friday, at which point the NRC reported it had reached 15 percent power.