Research On Modulation Strategy And Grid-connection Of Novel Multi-level Z-source Inverter

In recent years,the development and utilization of new energy sources such as photovoltaic and wind power have been paid more and more attention to.Z-source inverter has the characteristics of single stage adjustable voltage,it makes up the deficiency of the traditional voltage source and current source inverters,so it gets good graces in the field of new energy grid connection.The traditional Z-source inverter has the disadvantages of poor capability to boost voltage,poor resistance to voltage and low power level.The improved topologies improve the voltage boost capability,but the structures are complex,and that they are mainly concentrated in two-level structure.Recently,three-level and five-level Z-source inverters are proposed to improve the power class,but the voltage boost capability is not enhanced.Aiming at above problems,five new Z-source inverter topologies are proposed,modulation strategies and application for grid connection about the new inverters are studied.The research mainly includes the following aspects:Firstly,the inductances in the traditional Z-source inverter are replaced by boost units,a new Z-source two-level inverter circuit is proposed.The working principle of the new topology is expounded,three PWM methods and one SVPWM method are analyzed.A comparison is made among the new inverter,the traditional inverter and several improved Z-source inverters.Secondly,in order to simplify the topology and improve the efficiency of three-level Zsource inverter,Z-source and quasi Z-source T-type three-level inverter circuits based on boost unit are proposed.The working principle is analyzed,and the relationship between the boost capability and the shoot-through time is deduced.The modulation method of carrier phase disposition PWM and carrier phase opposition PWM are compared.In order to realize neutral point potential balance of the quasi Z-source T-type three-level inverter,the double modulation wave with step wave modulation strategy is designed,and a comparison is made between the new inverter and the traditional Z-source three-level diode clamped inverter.Thirdly,in order to improve the traditional five-level inverter topology,a new T-type five-level inverter circuit is designed.To improve the power grade and quality of output power of the Z-source inverter,a Z-source T-type five-level inverter topology and a high gain Z-source T-type five-level inverter topology are proposed.The principle of the new topologies and the insertion method of the shoot-through state are analyzed,the strategy of carrier phase disposition PWM is designed,and a comparison analysis between the new topology and the traditional Z-source five-level diode clamped topology is made.Finally,aiming at the problem that DC side voltage of the new energy grid connection system is unstable,the grid connection model based on the new Z-source two-level inverter is established,and the parameters of the Z-source network are designed.A lot of simulation experiments have been done and the results show that the new two-level Z-source inverter,the new T-type three-level Z-source inverter and the new T-type five-level Z-source inverter all have a strong ability to boost voltage,reduce capacitor voltage and low switching device voltage.The structure of Z-source T-type three-level inverter and Z-source T-type five-level inverter are simpler and the clamping diode loss is eliminated.The double modulation wave with step wave modulation strategy can effectively restrain the fluctuation of neutral point potential of the Z-source T-type three-level inverter.The carrier phase disposition PWM effectively realizes the stable operation of the T-type five-level inverter,Z-source T-type five-level inverter and the high gain Z-source T-type five-level inverter.The grid connection system based on the new two-level Z-inverter is an effective solution to the problem of the DC side voltage instability in the distributed generation system.