A HYBRID MODULATION SCHEME FOR CASCADED H-BRIDGE INVERTER CELLS

ABSTRACT

This work proposes a switching technique for cascaded H-Bridge (CHB) cells. Single carrier Sinusoidal PWM (SCSPWM) scheme is employed in the generation of the gating signals. A sequential switching and base PWM circulation schemes are presented for this fundamental cascaded multilevel inverter topology. With these proposed concepts, it is now possible to generate equal average switching signal patterns in all the constituting power semiconductor switches. This results in equal switching loss dissipation and equal power sharing in CHB multilevel inverter modules; and therefore technically modularizes the cascaded system. A 4-cell cascaded structure has been used to exemplify the proposed switching technique. Outlines with switching functions are given for the proposed modulation strategy. For a modulation index of 0.9, a Total Harmonic Distortion (THD) value of 14.61% has been achieved in the output voltage waveform of the exemplanary 4-cell cascaded configuration. Simulations for single phase cascaded multilevel inverters (five-level, seven-level and nine-level) and their Total Harmonic Distortion (THD) is compared. The THD for the five-level, seven-level and nine-level are 21.92%, 9.51, and 5.30% for different topologies with different modulation techniques for single phase. When compared with the cascaded H-Bridge Cell, there is an improvement compared with the single phase cells. Verification of the performance of the proposed control technique is done through simulations.