Comparative Optimization Design of a Modular Multilevel Converter Tapping Cells and a 2L-VSC for Hybrid LV ac/dc Microgrids

The paper presents the performance of the modular multilevel converter (MMC) tapping cells associated with an ac filter in terms of efficiency and power density in a hybrid LV ac/dc microgrid application and compares it to the performance of the conventional topology used in LV application, i.e., the two-level voltage source converter (2L-VSC). A biobjective optimization based on the Genetic Algorithm is hence developed, providing details on designing the components of the LCL filter, the MMC, and the 2L-VSC. The MMC reaches an efficiency of 99.4% when the main dc-grid is left floating. However, due to its modularity and scalability, offering multiport connections option, the MMC tapping cells has the disadvantage of low power density. Exploring the filtering capability of the equivalent arm inductance of the MMC seen from the ac grid side, optimization design results show that higher switching frequencies allow a significant volume reduction of the inductive components of the MMC/LCL filter while higher switching frequencies have little impact on the switching losses of the MMC. This has the benefit of reducing the overall footprint of the converter and encouraging the use of the MMC in LV application.