Delta struggled to restart its worldwide operations after a computer failure halted flights for several hours and grounded thousands of passengers. More than 650 flights were scrapped and delays and cancellations continued even as systems came back online, the second-biggest US carrier said Monday on its website. A power outage at Delta’s Atlanta base about 2:30 a.m. interrupted computer operations, the airline said. Flights resumed on a limited basis about 8:30 a.m. Whether Delta had an adequate backup system “is certainly a legitimate question,” said Bob Mann, president of aviation consultant R.W. Mann & Co. “Why didn’t the backup systems perform, or what was it about the systems that if they performed, it still didn’t allow the systems to communicate?” The airline said it had operated about 2,340 of its almost 6,000 scheduled flights as of 3:40 p.m. It carried about 500,000 passengers daily during July, its busiest month, Delta said on its website. Delta waived change fees and fare differentials on ticket prices for passengers whose flights were cancelled or delayed, according to a statement on its website. Passengers must begin their trip by Friday. The company also offered $200 travel vouchers to passengers whose flights were cancelled or delayed by more than three hours. “I apologise for the challenge this has created for you with your travel experience,” Ed Bastian, the airline’s CEO, said in a video message to customers. “The Delta team is working very, very hard to restore and get these systems back as quickly as possible.” The carrier booked inconvenienced travellers on other airlines in select cases but largely accommodated them on its own flights, a spokesman said. No other airlines were affected, according to Hartsfield-Jackson Atlanta International Airport. The likely cause was a failure with Delta’s switch gear, which distributes energy throughout its building, said a spokesman for electric utility Georgia Power.<br/>