The dynamic security assessment of power systems needs to scan contingencies in a preselected set through time-domain simulations. With more and more inverter-based-generation, such as wind and solar power generation, integrated into power systems, electro-magnetic transient simulation is adopted. However, the complexity of simulation will increase greatly if inverter-based-generation units are modeled in detail. In order to reduce the complexity of simulation of power systems including large-scale wind farms, it is critical to develop dynamic equivalent methods for wind farms which are applicable to the expected contingency analysis. The dynamic response characteristics of permanent magnet synchronous generator-wind turbine generators (PMSG-WTGs) are not only influenced by their control strategies, but also by the operating wind speeds and the fault severities. Thus, this paper proposes a dynamic equivalent method for PMSG-WTG based wind farms considering the wind speeds and the fault severities. Firstly, this paper analyzes all possible response characteristics of a PMSG-WTG and proposes a clustering method based on the operating wind speed and the terminal voltage of each PMSG-WTG at the end of the fault. Then, a single-machine equivalent method is introduced for each group of PMSG-WTGs. For the group of PMSG-WTGs with active power ramp recovery process, an equivalent model with segmented ramp rate limitation for active current is designed. In order to obtain the clustering indicators, a simulation-based iterative method is put forward to calculate the voltage at point of common connection (PCC) of the wind farm, and a PMSG-WTG terminal voltage calculation method is further presented. Eventually, the efficiency and accuracy of the proposed method are verified by the simulation results.