Modified hybrid multi-carrier PWM technique for cascaded H-Bridge multilevel inverter

Among the carrier based pulse width modulation (PWM) techniques, Phase Shifted PWM (PS-PWM) technique is preferred for Cascaded H-Bridge (CHB) multilevel inverters. This technique distributes power evenly among the modules and also ensures equal utilization of inverter switches within a module. On the other hand Carrier Disposition PWM (CD-PWM) technique has superior output voltage profile, but results in uneven power distribution and non-uniform switch utilization. Modified CD-PWM and Hybrid-PWM techniques can be used to mitigate the problem related to uneven power distribution. In this paper, the performance comparison of above mentioned modulation techniques is presented for a 7-level CHB multilevel inverter for same device switching frequency. It is found that at low modulation index, the line voltage and line current THDs are less with Hybrid-PWM technique as compared to those with PS-PWM and Modified CD-PWM techniques. But in Hybrid PWM technique, the switches within a module are not equally utilized. A Modified Hybrid-PWM technique is proposed, which retains the power distribution and uniform switch utilization features of PS-PWM technique and results in output voltage profile similar to that with Hybrid-PWM technique.