Phase-shifted pulse-width-modulation scheme for modular multilevel converters

Modular Multilevel Converters (MMCs) are widely used in high power industries due to its flexible expandability, transformer-less configuration and suitability in common dc bus applications, etc. This paper presents the triangular carrier based phase-shifted PWM (PS-PWM) scheme normally adopted in MMCs. The carrier harmonics and their sidebands that introduced by the PS-PWM in the output multilevel waveform and the normalized phase leg voltage are analyzed with the help of the double Fourier series method. The harmonic analysis suggests that the presence of carrier harmonics and their sidebands in output and inserted phase leg waveforms can be decided by number of sub-modules connected in each arm and the angle displacement of triangular carriers in the upper arm and lower arm of the MMC. The mathematical carrier harmonics analysis is verified in PLECS simulations. The simulation results and the theoretical analysis agree with each other very well.