AC Impedance Modeling of Modular Multilevel Converters and Two-Level Voltage-Source Converters: Similarities and Differences

This paper presents a small-signal model for modular multilevel converters (MMCs) by using the harmonic state space (HSS) method. The ac output impedance of the MMC is then derived and compared with that of the commonly used two-level voltage-source converters (TL-VSCs). It turns out that the low frequency ac impedance of the MMC differs from that of the TL-VSC, due to the influences of internal dynamics, especially for the open-loop control and the voltage-mode control. In contrast, for the current-mode control, this difference can be minimized by the current regulator with a high-gain in the low frequency range. Moreover, despite the used control schemes, the high frequency ac impedance of the MMC is similar to that of the TL-VSC. The time-domain simulations are provided to validate the effectiveness of the theoretical analysis.