Research On Hybrid-carrier PWM Method Of Asymmetric Cascaded H-Bridge Multi-level Inverter

Multilevel power conversion technology has drawn increasing attentions in power electronic research field especially in high voltage, and high power occasions, due to their distinct advantages of superior output voltage quality, reduced voltage derivatives(dv/dt) and low filter expense when compared with two-level inverters. Asymmetric cascaded multilevel inverters have been a hot research topic in recent years, whose interest lies in the advantages compared with conventional cascaded ones of generating more levels with less semiconductor switches and isolated sources.Development of the hybrid cascaded multilevel topologies puts forward new requirements for the research and improvement of traditional multilevel modulation methods. Some common multilevel inverters and multilevel modulation techniques are introduced in this paper firstly. For the asymmetric cascaded H-bridge seven-level inverter researched in this paper, the topology characteristic and operation principle are analyzed, and problems exist in common modulation methods when used for this inverter are illustrated. Among them, the reverse power problem and its reason of the hybrid modulation method is explained in detail.In view of this, a hybrid-carrier PWM method is proposed. The drive logic signals of high voltage cell and low voltage cell are deduced in detail and a simple and easy modulation algorithm is presented. Characteristics of output power distribution among the cascaded cells, switching frequency of the high voltage cell and spectrum analysis of output voltage are analyzed. With the high voltage cell working at a relative low frequency, a double frequency modulation of high frequency serves the low voltage cell, improving the quality of output voltage effectively. Besides, output voltage polarity of cascaded cells is always the same and output active powers of cascaded cells are positive when both power factor and modulation ratio are between 0 and 1. Thus the disadvantage that the power flows backwards to low voltage cell existed in the hybrid modulation is avoided.Simulation and experimental results of the asymmetric cascaded H-bridge seven-level inverter with the hybrid-carrier PWM method are presented at last, which verify correctness of theoretical analysis, and effectiveness of the method analyzed in this paper.