Virtual Arm Impedance Emulation and Stability Improvement in Modular Multilevel Converters

The arm impedance in a modular multilevel converter (MMC) influences both the internal dynamics and the converter impedance, significantly impacting the overall system stability. This paper proposes a damping method with virtual arm impedance to suppress the small-signal oscillations caused by impedance interactions and improve the MMC stability. The virtual arm impedance, including both a resistance and a reactance component, is emulated based on the common-mode (CM) second-harmonic circulating current, which is injected to optimize the capacitor voltages of MMCs. Different from existing virtual impedance methods, the steady-state operation of the MMC is not influenced by the CM virtual arm impedance. It is shown that the virtual reactance of the same impedance has a better damping effect, compared with the virtual resistance. Selection guidelines of virtual reactance are also provided considering both the stability margin and dynamic performance. Moreover, the stability improvement is quantified by a stability boundary map and validated by real-time grid interaction studies and experimental results