Analysis of Wideband Oscillation of Hybrid MMC Interfacing Weak AC Power System

The wideband oscillation of the hybrid modular multilevel converter (MMC)induced by excessive power infeed under weak ac power system integration is analyzed in this article. A closed-loop state-space-based time-domain small-signal model is firstly established to investigate the instability problem. Different from the findings in two-level voltage-source-converters (VSCs) or half-bridge MMCs, the root locus analysis and participation factor analysis in this article reveal that the oscillation frequency and involved control loops are highly related to the operation status. When hybrid MMC operates as a rectifier, a low-frequency oscillation is observed with the d-channel control loop mainly participated. In contrast, a high-frequency oscillation occurs with a q-channel control loop mainly involved when the hybrid MMC operates as an inverter. This wideband oscillation phenomenon is explored with the aid of two simplified loop-gain-based s-domain models, which are derived referring to the selective modal analysis approach. To suppress the oscillation, sensitivity analysis regarding the impact of parameters on the phase margin is conducted to recognize effective parameter adjustment methods. The analysis results are validated by detailed electromagnetic simulations.