Research On Topology Structure Optimization And Modulation Strategy Of Hybrid Multilevel Inverter

With the development of power electronics technology,multilevel inverter has become a hot research topic in academic circles because of its low power level requirements for switching devices,high output voltage waveform quality and small electromagnetic interference.But the traditional multilevel inverter has many disadvantages in the practical application,such as the number of devices,the huge system volume and the complex control.The emergence of the concept of hybrid multilevel inverter brings new ideas to the practical application of traditional multilevel inverter.Under the same output level,the number of switching devices and DC voltage sources needed by hybrid multilevel inverter is less,and the topology is more simple,which is a hot research topic in recent years.The development process and research status of multilevel inverter topology and modulation method have been introduced in this academic dissertation.Then,the working principle and modulation method of the hybrid cascade H-bridge(CHB)and hybrid asymmetric inverter are analyzed.The characteristics and disadvantages of each inverter are summarized.On this basis,two new hybrid multilevel inverter topologies are proposed and the corresponding modulation methods are designed.Firstly,through the analysis of the topology of the hybrid CHB inverter of type II and type III,the paper is committed to solve the current back-filling problems and reduce the switching frequency of the high voltage unit.A new hybrid CHB inverter topology with DC side voltage ratio of 1:1:2:2 is proposed.A hybrid modulation method is designed,which combines step wave modulation and carrier cascade modulation,which makes the high voltage unit work in the basic wave frequency state,and there is no current back-filling problem.The mathematical model is established,and the analytical expression of output voltage of inverter is obtained by using double Fourier series,and its harmonic characteristics are analyzed.In order to solve the power imbalance between the same voltage level units,a power balance method based on logic operation is proposed.The method can realize the power balance between the same voltage level units.Secondly,a new topology of asymmetric five switch six level inverter is obtained by adding an auxiliary switch and DC power supply to the asymmetric four level inverter,which combines the advantages of five switch five level inverter and asymmetric four level inverter.Compared with the traditional cascaded H-bridge structure,18 switching devices and 1 DC power supply are reduced.Using the idea of backward from the target waveform,the corresponding modulation method is designed through the idea of carrier stacking and the logical combination of the initial switch driving signal.The 15 level output and better waveform quality of the inverter are achieved.Finally,the simulation and experimental platform of hybrid CHB inverter with voltage ratio of 1:1:2:2 and cascade asymmetric five switch six level inverter are built,which verify the correctness and feasibility of the proposed topology and modulation strategy.