This paper presents a computational technique for conducting assessments of multi-area power system frequency dynamics using reachability analysis theory. To this end, a reduced-order model of the U.S. power system with high renewable penetration is derived. We investigate the control capabilities of HVDC lines to improve the frequency response of asynchronously connected regions, and to transmit the variability of renewable sources within synchronously connected areas. In this modeling framework, disturbances and parameters are treated as belonging to unknown-but-bounded sets. The analysis yields bounds that are guaranteed to contain the trajectories of all possible scenarios.