A Modular Multilevel Converter with a Clamping Switch for Quasi-Three-Level Operation

The required module capacitances of modular multilevel converters can be significantly reduced in comparison to conventional operation modes when quasi-two-level operation is applied. Unfortunately, the output voltage distortion becomes similar to those of conventional two-level-converters. Adding a new current path to the topology enables quasi-three-level operation of the modular multilevel converter, which can reduce the harmonic content of the output voltage while still maintaining the advantage of small module capacitances. In this paper, a novel converter topology based on a combination of modular multilevel converter and bidirectional semiconductor clamping switch is presented, which is capable of quasi-three-level operation. This novel topology has all advantages of a quasi-three-level modular multilevel converter topology from literature, while overcoming the requirement for additional full-bridge modules connected between the phase output and the dc center tap. This is expected to reduce costs and losses.