General Electric Co. discovered a flaw in its newest power-plant turbines after a key part failed earlier this month, forcing utility Exelon Corp. to temporarily shut two Texas plants while GE makes repairs.

GE said the problem is an “oxidation issue” related to a metal alloy that could cause distress on the fan blades of its HA gas turbine, hulking machines that generate electricity for thousands of homes. GE spent years developing the HA turbines, and the more-efficient design is central to GE’s efforts to compete with rivals...