Research On A Single-stage Three-phase Cuk Three-level Inverter

Compared with the three-phase two-level inverter,the three-phase three-level inverter has the advantages of less output voltage harmonic content,low voltage stress of the switching devices,small switching loss and good electromagnetic compatibility,which made it more suitable for high-voltage and medium-high power applications such as high voltage motor frequency control,reactive power compensation and flexible AC transmission systems.With the continuous development of photovoltaic power generation technology,three-phase three-level inverters with medium and high power processing capabilities are increasingly used as interfaces between power generation devices and power networks,however,the output voltage of the photovoltaic cells fluctuated in a wide range due to changes in light intensity,temperature,etc.,and the inverter is required to ensure stable output when the input voltage varies widely.In order to meet this requirement,this paper proposes a single-stage three-phase Cuk three-level inverter with buck-boost capability,and studies it.Firstly,this paper summarizes the construction principle and development status of three-phase three-level inverters by referring to a large number of relevant domestic and foreign literatures,and makes a comparative study on the structure forms and working characteristics of existing three-phase three-level inverters.A novel single-stage three-phase Cuk three-level inverter is proposed by extending the Cuk flying capacitor three-level DC converter to AC application.The three-level inverter has the characteristics of single-stage power transmission,few number of passive components,simple topological structure and easy control,which is more in line with the needs of medium-high power photovoltaic power generation technology.Then,this paper analyzes the working principle of the three-level inverter in detail,and proposes an improved phase-shifted carrier modulation mode according to the structural characteristics of the inverter,so that the flying-capacitors in the inverter can have the characteristic of self-balancing.At the same time,the steady-state mathematical relationship of the inverter and the selection of circuit parameters are theoretically analyzed and mathematically deduced.In addition,in view of the pre-charging problem of the flying-capacitors during the startup of the inverter,the flying-capacitor pre-charging circuit is designed by using the Cuk loop contained in the inverter without adding other components.Finally,this paper studies the control strategy of the proposed inverter,and performs simulation analysis.Simulation results proved that the proposed inverter has a wide range output voltage gain to realize single-stage voltage Buck/Boost invertion,thus it is suitable for the application of renewable energy power generation field.On the basis of theoretical analysis and simulation verification,this paper builds an experimental prototype and completes experimental research on the proposed inverter,the experimental results prove the correctness of the theoretical analysis and simulation results.