Comparative Study of Small-Signal Stability Under Weak AC System Integration for Different VSCs

Voltage source converters (VSCs) with self-commutation ability are suitable for interconnecting weak ac systems. This article conducts a comparative study of the small-signal stability characteristics for three typical VSCs, namely, the two-level VSCs (TL-VSCs), the half-bridge modular multilevel converters (HB-MMCs), and the hybrid MMCs (HY-MMCs), under weak ac system integration with special consideration on both inverter and rectifier operation. The frequency responses based on impedance models are compared using the frequency-domain analysis. The oscillation frequencies and mainly participated state variables of the unstable modes are compared using root locus and participation factor analysis in the time domain. Proper parameter retuning approaches for stability enhancement are proposed. The above analysis is adequately validated by electromagnetic simulations.