An Improved Phase-Shifted Carrier Modulation for Modular Multilevel Converter to Suppress the Influence of Fluctuation of Capacitor Voltage

Modular multilevel converter (MMC) is considered to be one of the most promising multilevel topologies from the medium- to high-voltage power applications. However, by reason of energy exchange between the dc side and ac side, the fluctuation of sub-module (SM) capacitor voltage for the MMC system is inevitable. Moreover, this fluctuation would deteriorate the total harmonic distortion (THD) of the output voltage. The influence of the fluctuation of the SM capacitor voltage on the output voltage is analyzed mathematically in detail. In this paper, an improved phase-shifted carrier (PSC) pulse width modulation (PWM) for the MMC is presented to suppress the influence of the fluctuation of SM capacitor voltage. This improved modulation can be regarded as a nonlinear compensation applied in the traditional PSC-PWM method. The THD of the output voltage is improved with this modulation method noticeably. Simulation and experiment results verify the effectiveness of the proposed modulation.