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

Abstract

The AC/AC 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 paper proposes an impedance model of AC/AC MMC with consideration of dominate 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.

Third harmonic injection control for co‐phase traction power supply system using direct AC/AC full‐bridge modular multilevel converter

“…FB-MMC is used in [20] to convert the three-phase voltage directly into single-phase voltage and proposes a corresponding control strategy, but the closed-loop control of the single-phase AC voltage is not achieved. Detailed impedance models of the AC/AC FB-MMC are developed and stability analysis is carried out in [21,22]. However, the single-phase side output frequency with 16.7 Hz is not suitable for the case where the frequencies of both sides are equal (e.g.…”