Abstract Increasing wind power penetration into the grid justifies the requirement of the analysis of wind power dynamics, especially during transient faults. Quantitative transient stability (TS) assessment is required to provide deeper insight into the TS problems for speeding up the operational decision making process. This can be achieved by evaluating transient stability index (TSI) through the assessment of transient energy function. This paper carries out the quantitative insight of the impact of different generator technologies on the grid by comparatively studying the impacts of the fault clearing time, the grid coupling, the inertia constant, the generator terminal voltage sag and the slip on fault responses with the TSI between synchronous generators and wind turbine generators, such as squirrel cage induction generators and doubly fed induction generators.