Harmonic Stability in MMC-HVDC Applications Controlled as a Virtual Synchronous Machine

Along with the increased requirements of grid-forming assets, a thorough comprehension of the impact of these controls on the system’s harmonic stability gains more and more importance. Therefore, previous research contributed to understanding the admittance-shaping effects of the main control loops related to a converter controlled as a virtual synchronous machine. In extension to previous investigations, this paper focuses on the impact of implementation and design of the ac voltage controller. In light of dynamic performance requirements of power quantities, structural adjustments – namely, a power feed-forward - have been proposed in literature. However, these adjustments might influence the frequency-dependent behavior of modular multilevel converters controlled as a virtual synchronous machine. Therefore, the impact of these dynamics are derived analytically and evaluated by means of their admittance-shaping effects, too. A final summary classifies all admittance-shaping effects of structural and design related dynamics within virtual synchronous machine controlled modular multilevel converters.