The impact of largescale access of wind farms on the transient stability of power grids could not be ignored. Taking the extended twomachine system with doublyfed wind turbines as an example

the equivalent model of doublyfed induction generator was established

and the twomachine system could be equivalent to a singlemachine infinity system. Based on the law of equal area

the analytic formula of critical clearing angle of the system was deduced in detail after wind power accessed. The analytic formula was used to quantitatively analyze the variation trends of the critical clearing angle with wind power ratio

wind turbine grid connection position

fault location and load access position. The influence laws of the above four factors on the stability of transient power angle were summarized. The simulation models of the extended twomachine system with doublyfed induction generator was established in BPA and FASTEST

and the accuracy of the theoretical analysis was verified.