Fast transfer processes of double power supplies are analyzed by help of ETAP simulation software, when incoming line circuit breaker false tripping fault and incoming line three-phase short-circuit fault happen in a 6 kV substation containing 3-component flue gas expender train. The simulation results show that, the residual voltage of operating bus likely rises more than normal system voltage in the incoming line circuit breaker false tripping fault event, and the phase angle difference is more than the typical 20 degrees. The operating bus voltage drops in the incoming line three-phase short-circuit fault event and is too low to meet the fast transfer criteria, and the transfer time is too long. Thus the operating bus voltage upper limit of 1.1 times rated voltage is added, and the setting values of the phase angle difference is properly 23~30 degrees. The residual pressure switch criterion inspection is added in the incoming line three-phase short-circuit fault, and works under simultaneous switching mode to shorten the transfer time.