Modulation Extension Control of Hybrid Cascaded H-bridge Multilevel Converters with 7-level Fund- mental 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 because of its high number of output levels, and has high conversion efficiency and low thermal stress because it uses a fundamental frequency switching scheme. However, one disadvantage of the 7-level fundamental frequency switching scheme is that its modulation index range is too narrow when the capacitor’s voltage balance is maintained. The proposed modulation extension control algorithm can greatly increase the capacitors’ charging time and decrease their discharging time by injecting triplen harmonics to extend the modulation index range of the hybrid cascaded H-bridge multilevel converters to conquer this problem. Therefore, the proposed modulation extension control algorithm has not only a wider modulation index range but also all the advantages of inherent high output power quality, low output switching frequency and high conversion efficiency, and high-speed capability.