Experimental Validation of the Quasi-Three-Level Operation Mode for a Hybrid Modular Multilevel Converter with Series-Connected Clamping Switches

This paper presents a three-phase hybrid modular multilevel converter prototype for the quasi-three-level operation mode which uses conventional series-connected clamping switches to generate the zero output voltage level. A modular concept for these clamping switches and their threshold voltage detection circuit is presented. Stable operation of the quasi- three-level operation mode is demonstrated by experiment. Moreover, a shutdown procedure for the converter in the case of a failure is shown. The dynamic voltage sharing of the series-connected clamping switches is also measured, showing that an extension of this topology to higher voltages is generally feasible.