Modular multilevel converters (MMCs), used in AC/DC power systems, are characterized by complex (trans-)impedances with a frequency dependence never seen before in electromechanical power devices. Since these impedances may influence power system stability, adequate design actions are needed to ensure safety margins. A straightforward derivation of these complex (trans-)impedances requires making numerical tools available to designers. In this paper, we compute them with the periodic small-signal analysis (PAC). After commenting on some impedances computed with this approach, we exploit them to understand how an MMC can impact system stability. Stability issues highlighted through PAC are verified by transient stability simulations.