Analysis of multi-carrier PWM methods for back-to-back HVDC systems based on modular multilevel converters

This paper analyzes two multi-carrier pulse-width modulation (PWM) techniques (N+1 and 2N+1) of the Modular Multilevel Converter (MMC) used in a back-to-back configuration under conventional dq-frame control. The PWM method applied to the MMC has an effect not only on the output waveforms and number of levels derived out of a given configuration of the MMC but also on the DC-link voltage and arm currents of the converter. As the number of sub-modules in the phase-leg increases, the 2N+1 modulation becomes more attractive as it provides improved waveforms without significantly affecting the operation of the converter. The simulation results show the application and good performance of both methods and the advantages of utilizing the 2N+1 modulation as the number of sub-modules increases.