Modulation extension control of hybrid cascaded H-bridge multilevel converters with 7-level fundamental frequency switching scheme

This paper presents a modulation extension control algorithm for hybrid cascaded H-bridge multilevel converters. The hybrid cascaded H-bridge multilevel motor drive using only a single DC source for each phase is promising for high power motor drive applications since it can greatly decrease the number of required DC power supplies, has high quality output power due to its high number of output levels, and has high conversion efficiency and low thermal stress by using fundamental frequency switching scheme. But one disadvantage of the 7-level fundamental frequency switching scheme is that its modulation index range is too narrow when capacitor's voltage balance is maintained. The proposed modulation extension control algorithm can greatly increase capacitors' charging time and decrease the capacitors' discharging time by injecting triplen harmonics to extend the modulation index range of the hybrid cascaded H-bridge multilevel converters.