Abstract: Crowbar protection and chopper protection operation state during fault have an important influence on the transient process of doubly-fed induction generator（DFIG）. With the different voltage drop depths,the protection operation states are different,and the corresponding transient equivalent models of DFIG are also different. Therefore,for the doubly-fed wind turbines system with different severity of three-phase fault,the coordinated control mode of crowbar protection and chopper protection is comprehensively considered.Firstly,by analyzing the relationship between rotor current,DC bus voltage and terminal voltage drop,a unified protection action analysis criterion is established. Secondly,the operation states of crowbar protection and chopper protection under different voltage drop depths are analyzed,and a unified post fault transient equivalent model of DFIG is established. Finally,based on the standard model of 1 MW DFIG provided by the PSCAD/EMTDC,the accuracy of the DFIG transient equivalent model is verified.