Research On Topology And Modulaton Method Of Hybrid Asymmetric Multilevel Inverterr

In the power electronics industry,multilevel inverters play an increasingly important role.Compared with two-level inverters,multi-level inverters have more output voltage levels,less output harmonics,less voltage stress on switch devices and lower switch loss.However,with the continuous advancement of industrialization and urbanization,the application of traditional multilevel inverters in new energy power systems is very limited.Hybrid asymmetric multilevel inverters can not only output higher voltage quality with fewer DC power sources and switching devices,but also simplify the topology of inverters,which can greatly reduce costs,and promote the transformation of green energy in the system and the development of energy-saving technology.Therefore,it is of great practical significance to study hybrid asymmetric multilevel inverters.This paper takes the hybrid asymmetric multilevel inverters as the research object.For the purpose of simplifying the inverter structure,reducing switch devices,improving the output voltage level and voltage quality,two optimized hybrid asymmetric topologies are proposed.Based on the analysis of these two topologies,corresponding modulation methods are designed.The specific research contents are as follows:An improved four-unit hybrid asymmetric Cascaded-H Bridge(CHB)multilevel inverter topology is presented by studying the asymmetric CHB inverters of type II and III.A hybrid modulation method combining low-frequency modulation with high-frequency modulation is designed for the topology.This modulation method not only solves the current reversal between the units,but also keeps the high voltage unit of the inverter in the low frequency state,reduces the switch loss and improves the harmonic characteristics.The traditional modulation strategy is complex and the output power between the same voltage level units is not equal.To solve the above problem,this paper optimizes the modulation wave of the hybrid modulation strategy,reduces the complexity of the modulation strategy by half the number of carriers,and proposes a power equalization method based on area equivalence principle to reconstruct the carrier so that the average power output of the same voltage level units of the topology is equal.Due to the limitations of traditional multilevel inverters in practical applications,this paper constructs a new asymmetric multilevel inverter topology on the basis of simplifying the topology and improving the voltage quality,and designs an improved carrier modulation method for the topology to achieve 15-level output.With this modulation method,the number of carriers is reduced,the difficulty of digital modulation is reduced,the harmonic content of the output voltage of the new asymmetric multilevel inverter is low,and there is no problem of current backfilling.Compared with the traditional multilevel inverter topology,this new asymmetric multilevel inverter greatly reduces the number of switching devices and DC voltage sources,and has certain advantages in reducing costs and improving output voltage quality.Finally,the improved four-unit hybrid asymmetric CHB inverters and the new asymmetric multilevel inverters are simulated and experimentally analyzed.The results verify the correctness and feasibility of the two optimized hybrid asymmetric topologies and their modulation methods.