Research On θ-type Inverter And Low Frequency Ripple Current Suppression Strategy

Compared with the traditional isolated inverter,the non-isolated inverter has the advantages of high conversion efficiency,low cost,small size and light weight,and provides a good solution for grid-connected applications.However,the main disadvantage of nonisolated inverters is that there is a leakage current problem,which will affect the normal operation of the inverter,reduce the power conversion efficiency,and even endanger human safety.In addition,the low frequency ripple in the input current of the inverter will affect the efficiency and stability of the photovoltaic power generation system.Therefore,this thesis designs a θ-type inverter,which can completely eliminate the leakage current,and proposes two low-frequency ripple current suppression strategies based on the inverter.Firstly,this thesis analyzes the generation mechanism of inverter leakage current and lowfrequency ripple current on the input side.The research results on these two problems in recent years are introduced,and a θ-type inverter is designed on this basis.Through the equivalent circuit analysis of the inverter,it can be known that the branch where the parasitic capacitance of the inverter is located is short-circuited,that is,the leakage current can be eliminated directly by using the circuit structure.In addition,by analyzing the modulation strategy and working mode of the inverter,this thesis expounds the principle of the inverter to suppress the low-frequency ripple of the input current.Then,through the time-averaged equivalent circuit analysis of the θ-type inverter,a lowfrequency ripple current suppression strategy based on inductor current and based on bandpass filter is proposed.By establishing the small-signal equivalent model of the θ-type inverter,two kinds of loop compensators are designed to ensure that the θ-type inverter can operate stably and have good dynamic performance after adding the two low-frequency ripple current suppression strategies proposed in this thesis.The circuit model is built in Matlab/Simulink software,and the simulation results of the θ-type inverter using different control strategies are compared and analyzed.The results show that the low-frequency ripple current suppression strategy proposed in this thesis can not only effectively suppress the low-frequency ripple of the input current,but also greatly reduce the capacity of the bus energy storage capacitor,thereby increasing the power density of the system.Finally,a set of θ-type inverter experimental prototype is designed and built.A series of steady-state and dynamic experimental studies are carried out on the two low-frequency ripple current suppression strategies proposed in this thesis.The experimental results verify the orrectness of analysis and simulation results.