Impact of DFIG based wind penetration on the dynamic performance of future grids is studied in this paper. Both AC and a mix of AC and HVDC transmission are considered. In general, DFIGs are found to contribute positively to system damping although the effect could be opposite depending on the operating condition. For deeper understanding, various effects contributing to the overall system damping (e.g. wind penetration at sending vs receiving end etc.) are studied in isolation keeping the other factors (e.g. tie-flow etc.) constant. Implications of supplying bulk of the future load growth by wind energy with only AC and both AC and HVDC are analyzed and compared. Observed trends are explained in terms of damping torque contribution of synchronous generators which is the primary source of damping in power systems. Power oscillation damping (POD) control action through supplementary modulation of DFIG rotor currents (both direct and quadrature axes) or active and reactive power order of VSC HVDC link is demonstrated.