Enhanced Level-Shifted Modulation for a Three-Phase Five-Level Modified Modular Multilevel Converter (MMC)

Abstract

The rapid growth of the penetration level of renewable energy sources (RESs) in low-voltage (LV) networks increases the research interest in more efficient, compact, reliable, and easily expandable energy conversion systems. The long commutation loops of the existing LV multilevel converters (MLCs), such as active neutral point clamped (ANPC) converters, lead to high-voltage overshoots induced during switching due to the significant parasitic inductance. Modular multilevel converters (MMCs) are gaining an increased interest due to their modularity, flexibility, and short commutation loops also for some LV applications. However, several issues related to the number of components, dc capacitor requirements, and the added complexity need to be considered. This article aims to reduce the five-level modified MMC capacitance requirements where the upper and lower submodules (SMs) are connected in parallel. This reduction has been achieved using an improved modulation technique by minimizing the insertion time of the SMs that require higher capacitance. The proposed modulation is effective in reducing the inner SMs’ capacitor voltage ripple at lower modulation indices. Instead, the circulating current injection of the second harmonics effectively reduces the voltage ripple at higher modulation indices. Thus, maximum reduction in the total capacitance requirement for the entire operating range of modulation index can be achieved by a proper combination of the two solutions. Simulation and experimental results on a laboratory-scaled prototype verify the validity of the proposed solution.