Markov state transition processes for asynchronized generator（ASG） and conventional synchronous generator（SG） are presented by analyzing their different failure modes of the main functional parts of the excitation systems， and their reliability models are established respectively. In the optimal load shedding model based on the alternative current power flow， different influences on power system reliability are compared incorporating the active and reactive power decoupling control characteristic for ASG and the capacity limits for SG. The system reliability is studied when the practical operational constraints of SG are intensively considered. Finally， case studies using the RBTS and IEEE-RTS79 power systems show that ASG can obviously improve power system reliability.