Rethinking Linear Modulation Range of Modular Multilevel Converters

The linear modulation problem of modular multilevel converter (MMC) is considerably complicated by capacitor voltage ripples (CVRs) and circulating current suppression control (CCSC). This article examines the linear modulation range of MMC and its influence on parameter design and operating characteristics, taking into account the CVR effect and CCSC. The deviation between the actual voltage output and the reference is revealed and analyzed, and the analytical expression of this deviation is determined. The effects of CCSC on linear modulation are also revealed. Based on the article, a set of equations are proposed to determine the exact reference wave while accounting for the CVR effect and CCSC and precisely assess linear modulation. Subsequently, the linear modulation range of MMC, which is defined as the maximum selectable valve-side voltage that can satisfy whole operating region linear modulation in this article, can be exactly determined. Furthermore, how the CVR effect and CCSC influence the linear modulation range, parameter design, and operating characteristics are analyzed using case studies. The proposed theory and method are also verified through simulation and experimental results.