Research On Topology And Modulation Strategy Optimization Of Hybrid Cascaded H-bridge Inverter

Traditional multilevel inverters have many problems such as switching devices and DC voltage sources,which greatly limit their application in the industrial field.Multi-unit hybrid cascade multilevel inverters can achieve more output levels with fewer switching devices,reduce switch loss,reduce electromagnetic interference,and so on.They have been widely used in high-voltage frequency conversion speed control,renewable energy systems and other fields.Hybrid cascade H-bridge multilevel inverters are developed from traditional CHB multilevel inverters.They are composed of H-bridge units with different DC voltage levels.Without increasing the number of switching devices and DC power sources,the hybrid cascade H-bridge multilevel inverters can increase the output voltage of the inverters,thereby improving the power quality of the output voltage.This paper first introduces the development and research status of several classical multilevel inverter topologies,modulation strategies and power balancing methods.Secondly,the improved type III hybrid cascade CHB multilevel inverter topology with two different structures is taken as the research object,and the specific research contents are as follows:After analyzing the traditional type III inverter’s topology and the causes of its current backfilling,an improved type III hybrid cascade 13-level inverter with a voltage ratio of1:1:1:3 is studied.After combining step-wave modulation with carrier amplitude shift modulation,a hybrid modulation strategy is presented,which avoids the current backfilling of the inverter and makes the high-voltage unit of the inverter work at the base frequency,reducing the switch loss.The modulation wave of the low voltage unit is optimized,which reduces the number of triangular carriers required for the modulation of the low voltage unit and the difficulty of digital control.A new power equalization method for carrier reconfiguration is proposed,which achieves the output power self-balancing among the low voltage units in the same period.The improved type III hybrid cascade 13-level inverters with voltage ratio of 1:1:3require more switching devices.Combining the advantages of the traditional type II and III hybrid cascade CHB multilevel inverters,the traditional type III inverters are optimized.A type III asymmetric 3-unit 13-level inverter topology with voltage ratio of 3:2:1 is presented.The multilevel output is achieved.It reduces the number of required switching devices and DC voltage sources,and solves the problem of current inversion when traditional type III hybrid cascade CHB multilevel inverters work.Then a hybrid modulation strategy based on high frequency modulation is proposed to achieve high quality voltage output of the inverter.A specific harmonic elimination method based on low frequency modulation is proposed to restrict the switching angle of each unit of the inverter,which not only balances the output power among the connected units at all levels in half cycle,but also eliminates some low-order harmonics contained in the output phase voltage.Finally,simulation models and experimental prototypes of hybrid cascade 13-level inverters and asymmetric 3-unit 13-level inverters are built,which verify the correctness and feasibility of the topology,modulation strategy and power balancing method of the inverters mentioned in this paper.