Carrier Overlaping-switch Frequency optional PWM Method for Cascaded Multilevel Inverter

Based on the analysis of SHPWM method, we proposed an improved modulation method suited for cascaded multilevel inverter of three-phase systems, which is called carrier overlapping - switch frequency optional - pulse width modulation (CO-SFO-PWM). The method combines the advantages of carrier overlapping and switch frequency optional, which enables it to increase the fundamental amplitude of the output line voltage and improve the utilization ratio of DC voltage. Besides, this method allows the output line voltage to have better harmonic characteristics, and in particular, significantly improve the distortion ratio of output line voltage at low modulation. This paper applied CO-SFO-PWM modulation to the seven-level cascade inverter, and verified the correctness and feasibility of the CO-SFO-PWM method by MATLAB simulation results.