Impedance Modeling and Stability Analysis of AC/AC Modular Multilevel Converter for Railway System

The ac/ac modular multilevel converter (MMC) application in the electric railway poses new challenges to the stability of the railway system. The impedance analysis method is an effective way to assess the stability of ac/ac MMC-based railway systems. Due to the internal dynamics of ac/ac MMC, there are steady-state harmonic components in the arm currents and capacitor voltages, and these harmonic components have a significant impact on the dynamic characteristics and stability of ac/ac MMCs. To accurately predict the stability of ac/ac MMC-based railway systems, this article proposes an impedance model of ac/ac MMC with consideration of dominant steady-state harmonics. Simulation and experimental results obtained from a prototype are presented to demonstrate the accuracy of the proposed impedance model. Besides, the proposed model is compared with the impedance model ignoring the steady-state harmonics, and the results show the advantage of the proposed model in the description of impedance characteristics of ac/ac MMCs. The stability of an example ac/ac MMC-based railway system is analyzed using the proposed model, and the stability analysis results are validated by simulations. The effect of control parameters on the impedance characteristics is further analyzed, and this provides the guidance for controller design to damp the instability of the system.