Research On Leakage Current Suppression Of Transformerless Photovoltaic Grid-connected System

Photovoltaic power generation is one of the important ways to use solar energy.Although the transformerless photovoltaic grid-connected system improves the utilization rate of solar energy,it forms an elec trical connection between the inverter system and the grid.The resulting common-mode leakage current will cause serious electromagnetic interference,reduce the power generation quality and conversion efficiency of the system,and increase potential safety hazards.To solve the leakage current problem,this thesis comprehensively considers the leakage current suppression method of the existing literature,on this basis,a novel current source inverter and a common-mode filter loop construction method based on neutral-point capacitance are proposed,which are used for suppressing the leakage current of single-phase and three-phase photovoltaic grid-connected systems.Firstly,this thesis analyzes the mechanism and influencing factors of the leakage current of the transformerless photovoltaic grid-connected system.Starting with the type of grid-connected system and the type of inverter,this thesis analyzes the characteristics of the common-mode model of the grid-connected system and the generation mechanism of leakage current and obtains a feasible solution to suppress the leakage current.The influence of system parameters such as common-mode voltage amplitude,system switching frequency and common-mode loop inductance on leakage current is analyzed to provide reference for suppressing leakage current and parameter design of transformerless grid-connected systems.According to the common-mode characteristics of grid-connected system,this thesis analyzes and summarizes the leakage current suppression mechanis m,advantages and disadvantages of the common-mode voltage constant method,the common-mode loop filtering method and the parasitic capacitance short-circuit method.Secondly,according to the characteristics of the parasitic capacitance short-circuit method,a novel current source inverter is proposed,which can eliminate the leakage current of the transformerless single-phase photovoltaic grid-connected system through the common negative-terminal structure.This thesis analyzes the working principle and leakage current suppression principle of the novel inverter,designs the corresponding single-phase SVPWM modulation strategy according to the working principle and operating mode,and uses the quasi-PR control strategy to design the double closed-loop control system of the proposed novel inverter.Through the establishment of simulation model and experimental prototype,this thesis proves the feasibility of the novel inverter working mode and the effectiveness of the leakage current suppression scheme.Finally,according to the characteristics of the common-mode loop filtering method,this thesis proposes a common-mode filtering loop construction method based on neutral point capacitance to suppress the leakage current of transformerless three-phase photovoltaic grid connected system.This thesis derives the common-mode model of the three-phase three-level inverter after constructing the common-mode filtering loop based on the neutral point capacitance and designs the parameters of the constructed common-mode loop filter.According to the working principle and operating mode of T-type three-phase three-level inverter,this thesis designs the three-phase SVPWM modulation strategy,the midpoint potential balance control strategy based on the generalized distribution factor method,and the double closed-loop control strategy based on dq coordinate transformation.The simulation model and experimental prototype are built,and the simulation analysis and experimental analysis of the leakage current suppression effect of the proposed method are carried out.It is proved that the designed common-mode filtering loop has a good suppression effect on the leakage current of the transformerless three-phase grid-connected system.