Delta Air Lines Inc. struggled with continued “instability” in its global computer network, a day after a power failure led to a shutdown and the cancellation of about 1,500 flights worldwide. “We’re seeing slowness in a system that airport customer-service agents use to process check-ins, conduct boarding and dispatch aircraft,” Chief Operating Officer Gil West said in a statement Tuesday. A module malfunctioned early Monday morning, “causing a surge to the transformer and a loss of power,” he said. Though electricity was restored quickly, “critical systems and network equipment didn’t switch over to backups. Other systems did. And now we’re seeing instability in these systems.” The disarray at Atlanta-based Delta marked the second time in less than a month that a system failure forced mass flight cancellations at a large U.S. carrier. Southwest Airlines Co., the biggest U.S. discount airline, said a disruption July 20 would cost it “tens of millions” of dollars after more than 2,300 flights were canceled. Flight Cancellations About 530 Delta flights were canceled Tuesday, the second-biggest U.S. airline said in a separate statement. That followed about 1,000 cancellations Monday, the airline said early Tuesday. Delta sent reservations personnel from its headquarters to help customer-service agents process passengers at Hartsfield-Jackson Atlanta International Airport. The carrier continued to waive change fees and fare differentials on ticket prices for passengers whose flights were canceled or delayed. Delta also extended its offer of $200 travel vouchers to passengers whose flights were canceled or delayed more than three hours. Where available, customers are being provided hotel rooms and other accommodations if a cancellation requires an overnight stay. “I’m sure there are a lot of people asking questions. Why didn’t the backup systems perform?” said Bob Mann, president of aviation consultant R.W. Mann & Co. and a former airline executive. “Any high-level central site ought to have not only a fail-over power system, but they ought to be testing it regularly to make sure it actually works as designed and anticipated.” Delta fell less than 1 percent to $37.15 at the close of trading in New York, leaving the shares down 27 percent for this year. Southwest Failure Last month at Southwest, computers were restored after about 12 hours but flights continued to be canceled or delayed for several days as the carrier worked to get crews and planes in the right locations. U.S. carriers experienced technical disruptions to flights late last year and early in 2016, including a connectivity flaw at American Airlines that halted flights at its Chicago, Dallas and Miami hubs in September and a reservations-system glitch at Southwest in October. A United Continental Holdings Inc. computer fault last summer lasted two hours and disrupted travel for thousands of fliers. It began with a router malfunctioning and prevented the carrier from ticketing passengers and dispatching crews.