Modelling, and Optimization of Grid-Tied Modular Multilevel Converters with a Passive Front-End Rectifier without LC Filter in the DC-Link

Abstract

Due to the critical importance of minimising the cost, size, weight, and component count in applications such as high-power wind turbines, where the size and weight of the required electrical components are restricted by the tower space and capability, this study presents a compact system with low size, weight, and cost that is suitable for high power wind turbine applications. Here, the simplest structure in the rectifier side is used which is the well-known 6-pluse diode bridge rectifier. However, the 6-pluse diode bridge rectifier have substantial drawbacks; namely, unregulated dc-link voltage and significant dc-link ripple. Thus, a dc/dc boost converter is used to regulate the rectifier voltage in the dc-link of the inverter and large LC filter is used to smoothen the rectifier output. These extra parts increase the system size, wight, and cost. Therefore, to overcome these drawbacks, this project presents an optimised design and control of grid-tied full-bridge modular multilevel converters (MMC) with a passive front-end rectifier without passive components in the dc-link. This study shows that the full-bridge modular multilevel converter can act as a boost converter and large dc-link filter by itself.