An underground explosion popped a heavy manhole cover three feet into the air at the busy intersection of Beverly and La Cienega boulevards in West Los Angeles, sparking a small fire and knocking out electrical power in the huge Beverly Center retail complex for 28 minutes. There were no reports of injuries, a Los Angeles Fire Department spokesman said. Five companies of firefighters were dispatched, but the small fire had burned itself out by the time they arrived. The explosion apparently occurred in an underground vault containing electrical equipment. The power outage caused only minor inconvenience in the shopping center, a spokeswoman said. Department of Water and Power spokeswoman Sandy Tanaka said maintenance crews restored power by “alternative switching,” but were still looking for the source of the problem.