Evaluation of different carrier-based PWM methods for modular multilevel converters for HVDC application

The outstanding features of modular multilevel converters (M2C) make it attractive for high voltage direct current (HVDC) systems. In order to achieve high efficiency in HVDC converter stations, the switching frequency and the capacitor voltage ripple of the converter should be minimized. A suitable modulation algorithm should achieve an optimal tradeoff between these two requirements. This paper evaluates different carrier-based PWM algorithms and discusses the most challenging technical aspects of an efficient M2C. It is observed that decoupling the waveform synthesis from the selection of which cell to switch at each instant has beneficial impact on operation performance. The evaluation is done by time-domain simulation considering a grid connected, three-phase M2C converter and an advanced control system. Results of this study can be used for implementing more economical HVDC converters.