Multi-Frequency State-Space Model of MMC-HVDC System for Design-Oriented Stability Analysis

The frequency-coupling dynamics of the modular multilevel converter (MMC) complicate the stability analysis and controller design of MMC-based high-voltage direct current (HVDC) transmission systems. In this paper, a multi-frequency state-space model of MMC with the closed-loop control system is established, which allows to identify the critical oscillation modes and their sensitivities to the control parameters, and thus provides a design-oriented stability analysis. The mathematical relationships between the multi-frequency state-space models in different frames are established, which bridges the gap between the frequency-coupling dynamics of the MMC power stage and that of asymmetrical control loops. Design-oriented stability analysis is then performed based on a generic MMC-HVDC model, which confirms the effectiveness of the modeling techniques.