Impact of Virtual Arm Impedance on Small-Signal Stability of Modular Multilevel Converters

Inclusion of virtual impedance is an effective approach to mitigate small-signal oscillations in power converters without increasing manufacturing cost. This paper utilizes the second-harmonic circulating current in a modular multilevel converter (MMC) to generate virtual arm impedance and enhance system stability. The proposed method includes both resistance and reactance components, which can be combined to emulate a virtual impedance with flexible magnitude and phase. The active damping effect of different virtual impedance magnitude and phase is compared by both theoretical impedance-based stability analysis and experimental results. It is shown that the virtual reactance is the predominant component in improve system stability, of which the optimal value can be derived according to system stability margin. Understanding the stability impact of magnitude and phase of the virtual arm impedance can facilitate the design of active damping schemes of MMCs under specific grid conditions.