Impact of Modular Multilevel Converters Impedances on the AC/DC Power System Stability

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